Recruitment and derecruitment during acute respiratory failure: an experimental study

Clinical review: Positive end-expiratory pressure and cardiac output

In patients with acute lung injury, high levels of positive end-expiratory pressure (PEEP) may be necessary to maintain or restore oxygenation, despite the fact that 'aggressive' mechanical ventilation can markedly affect cardiac function in a complex and often unpredictable fashion. As heart rate usually does not change with PEEP, the entire fall in cardiac output is a consequence of a reduction in left ventricular stroke volume (SV). PEEP-induced changes in cardiac output are analyzed, therefore, in terms of changes in SV and its determinants (preload, afterload, contractility and ventricular compliance). Mechanical ventilation with PEEP, like any other active or passive ventilatory maneuver, primarily affects cardiac function by changing lung volume and intrathoracic pressure. In order to describe the direct cardiocirculatory consequences of respiratory failure necessitating mechanical ventilation and PEEP, this review will focus on the effects of changes in lung volume, factors controlling venous return, the diastolic interactions between the ventricles and the effects of intrathoracic pressure on cardiac function, specifically left ventricular function. Finally, the hemodynamic consequences of PEEP in patients with heart failure, chronic obstructive pulmonary disease and acute respiratory distress syndrome are discussed.

Multislice spiral computed tomography to determine the effects of a recruitment maneuver in experimental lung injury

Although recruitment of atelectatic lung is a common aim in acute respiratory distress syndrome (ARDS), the effects of a recruitment maneuver have not been assessed quantitatively. By multislice spiral CT (MSCT), we analyzed the changes in lung volumes calculated from the changes in the CT values of hyperinflated (V(HYP)), normally (V(NORM)), poorly (V(POOR)) and nonaerated (V(NON)) lung in eight mechanically ventilated pigs with saline lavage-induced acute lung injury before and after a recruitment maneuver. This was compared to single slice analysis near the diaphragm. The increase in aerated lung was mainly for V(POOR) and the less in V(NORM). Total lung volume and intrathoracic gas increased. No differences were found for tidal volumes measured by spirometry or determined by CT. The inspiratory-expiratory volume differences were not different after the recruitment maneuver in V(NON) (from 62+/-18 ml to 43+/-26 ml, P = 0.114), and in V(NORM) (from 216+/-51 ml to 251+/-37 ml, P = 0.102). Single slice analysis significantly underestimated the increase in normally and poorly aerated lung. Quantitative analysis of lung volumes by whole lung MSCT revealed the increase of poorly aerated lung as the main mechanism of a standard recruitment maneuver. MSCT can provide additional information as compared to single slice CT.

Comparison of prone positioning and high-frequency oscillatory ventilation in patients with acute respiratory distress syndrome*

OBJECTIVE

Both prone position and high-frequency oscillatory ventilation (HFOV) have the potential to facilitate lung recruitment, and their combined use could thus be synergetic on gas exchange. Keeping the lung open could also potentially be lung protective. The aim of this study was to compare physiologic and proinflammatory effects of HFOV, prone positioning, or their combination in severe acute respiratory distress syndrome (ARDS).

DESIGN

: Prospective, comparative randomized study.

SETTING

A medical intensive care unit.

PATIENTS

Thirty-nine ARDS patients with a Pao2/Fio2 ratio <150 mm Hg at positive end-expiratory pressure > or =5 cm H2O.

INTERVENTIONS

After 12 hrs on conventional lung-protective mechanical ventilation (tidal volume 6 mL/kg of ideal body weight, plateau pressure not exceeding the upper inflection point, and a maximum of 35 cm H2O; supine-CV), 39 patients were randomized to receive one of the following 12-hr periods: conventional lung-protective mechanical ventilation in prone position (prone-CV), HFOV in supine position (supine-HFOV), or HFOV in prone position (prone-HFOV).

MEASUREMENTS AND MAIN RESULTS

Prone-CV (from 138 +/- 58 mm Hg to 217 +/- 110 mm Hg, p < .0001) and prone-HFOV (from 126 +/- 40 mm Hg to 227 +/- 64 mm Hg, p < 0.0001) improved the Pao2/Fio2 ratio whereas supine-HFOV did not alter the Pao2/Fio2 ratio (from 134 +/- 57 mm Hg to 138 +/- 48 mm Hg). The oxygenation index ({mean airway pressure x Fio2 x 100}/Pao2) decreased in the prone-CV and prone-HFOV groups and was lower than in the supine-HFOV group. Interleukin-8 increased significantly in the bronchoalveolar lavage fluid (BALF) in supine-HFOV and prone-HFOV groups compared with prone-CV and supine-CV. Neutrophil counts were higher in the supine-HFOV group than in the prone-CV group.

CONCLUSIONS

Although HFOV in the supine position does not improve oxygenation or lung inflammation, the prone position increases oxygenation and reduces lung inflammation in ARDS patients. Prone-HFOV produced similar improvement in oxygenation like prone-CV but was associated with higher BALF indexes of inflammation. In contrast, supine-HFOV did not improve gas exchange and was associated with enhanced lung inflammation.

Ventilator strategies for posttraumatic acute respiratory distress syndrome: airway pressure release ventilation and the role of spontaneous breathing in critically ill patients

PURPOSE OF REVIEW

Patients who experience severe trauma are at increased risk for the development of acute lung injury and acute respiratory distress syndrome. The management strategies used to treat respiratory failure in this patient population should be comprehensive. Current trends in the management of acute lung injury and acute respiratory distress syndrome consist of maintaining acceptable gas exchange while limiting ventilator-associated lung injury.

RECENT FINDINGS

Currently, two distinct forms of ventilator-associated lung injury are recognized to produce alveolar stress failure and have been termed low-volume lung injury (intratidal alveolar recruitment and derecruitment) and high-volume lung injury (alveolar stretch and overdistension). Pathologically, alveolar stress failure from low- and high-volume ventilation can produce lung injury in animal models and is termed ventilator-induced lung injury. The management goal in acute lung injury and acute respiratory distress syndrome challenges clinicians to achieve the optimal balance that both limits the forms of alveolar stress failure and maintains effective gas exchange. The integration of new ventilator modes that include the augmentation of spontaneous breathing during mechanical ventilation may be beneficial and may improve the ability to attain these goals.

SUMMARY

Airway pressure release ventilation is a mode of mechanical ventilation that maintains lung volume to limit intra tidal recruitment /derecruitment and improves gas exchange while limiting over distension. Clinical and experimental data demonstrate improvements in arterial oxygenation, ventilation-perfusion matching (less shunt and dead space ventilation), cardiac output, oxygen delivery, and lower airway pressures during airway pressure release ventilation. Mechanical ventilation with airway pressure release ventilation permits spontaneous breathing throughout the entire respiratory cycle, improves patient comfort, reduces the use of sedation, and may reduce ventilator days.

Inspiratory vs. expiratory pressure-volume curves to set end-expiratory pressure in acute lung injury

OBJECTIVE

To study the effects of two levels of positive end-expiratory pressure (PEEP), 2 cm H(2)O above the lower inflection point of the inspiratory limb and equal to the point of maximum curvature on the expiratory limb of the pressure-volume curve, in gas exchange, respiratory mechanics, and lung aeration.

DESIGN AND SETTING

Prospective clinical study in the intensive care unit and computed tomography ward of a university hospital.

PATIENTS

Eight patients with early acute lung injury.

INTERVENTIONS

Both limbs of the static pressure-volume curve were traced and inflection points calculated using a sigmoid model. During ventilation with a tidal volume of 6 ml/kg we sequentially applied a PEEP 2 cm H(2)O above the inspiratory lower inflection point (15.5+/-3.1 cm H(2)O) and a PEEP equal to the expiratory point of maximum curvature (23.5+/-4.1 cmH(2)O).

MEASUREMENTS AND RESULTS

Arterial blood gases, respiratory system compliance and resistance and changes in lung aeration (measured on three computed tomography slices during end-expiratory and end-inspiratory pauses) were measured at each PEEP level. PEEP according to the expiratory point of maximum curvature was related to an improvement in oxygenation, increase in normally aerated, decrease in nonaerated lung volumes, and greater alveolar stability. There was also an increase in PaCO(2), airway pressures, and hyperaerated lung volume.

CONCLUSIONS

High PEEP levels according to the point of maximum curvature of the deflation limb of the pressure-volume curve have both benefits and drawbacks.

Pulmonary impedance and alveolar instability during injurious ventilation in rats

The mechanical derangements in the acutely injured lung have long been ascribed, in large part, to altered mechanical function at the alveolar level. This has not been directly demonstrated, however, so we investigated the issue in a rat model of overinflation injury. After thoracotomy, rats were mechanically ventilated with either 1) high tidal volume (Vt) or 2) low Vt with periodic deep inflations (DIs). Forced oscillations were used to measure pulmonary impedance every minute, from which elastance ( H) and hysteresivity (η) were derived. Subpleural alveoli were imaged every 15 min using in vivo video microscopy. Cross-sectional areas of individual alveoli were measured at peak inspiration and end exhalation, and the percent change was used as an index of alveolar instability (%I-EΔ). Low Vt never led to an increase in %I-EΔ but did result in progressive atelectasis that coincided with an increase in H but not η. DI reversed atelectasis due to low Vt, returning H to baseline. %I-EΔ, H, and η all began to rise by 30 min of high Vt and were not reduced by DI. We conclude that simultaneous increases in both H and η are reflective of lung injury in the form of alveolar instability, whereas an isolated and reversible increase in H during low Vt reflects merely derecruitment of alveoli.

Mechanical ventilation in acute respiratory failure: recruitment and high positive end-expiratory pressure are necessary

PURPOSE OF REVIEW

To review as best the critical care clinicians can recruit the acute respiratory distress syndrome (ARDS) lungs and keep the lungs opened, assuring homogeneous ventilation, and to present the experimental and clinical results of these mechanical ventilation strategies, along with possible improvements in patient outcome based on selected published medical literature from 1972 to 2004 (highlighting the period from June 2003 to June 2004 and recent results of the authors' group research).

RECENT FINDINGS

In the experimental setting, repeated derecruitments accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate lung injury. In the clinical setting, recruitment maneuvers improve short-term oxygenation in ARDS patients. A recent prospective clinical trial showed that low versus intermediate positive end-expiratory pressure (PEEP) levels (8 vs 13 cm H2O) associated with low tidal ventilation had the same effect on ARDS patient survival. Nevertheless, both conventional and electrical impedance thoracic tomography studies indicate that stepwise PEEP recruitment maneuvers increase lung volume and the recruitment percentage of lung tissue, and higher levels of PEEP (18-26 cm H2O) are necessary to keep the ARDS lungs opened and assure a more homogeneous low tidal ventilation.

SUMMARY

Stepwise PEEP recruitment maneuvers can open collapsed ARDS lungs. Higher levels of PEEP are necessary to maintain the lungs open and assure homogenous ventilation in ARDS. In the near future, thoracic CT associated with high-performance monitoring of regional ventilation (electrical impedance tomography) may be used at the bedside to determine the optimal mechanical ventilation of ARDS patients.

Respiratory compliance but not gas exchange correlates with changes in lung aeration after a recruitment maneuver: an experimental study in pigs with saline lavage lung injury

INTRODUCTION

Atelectasis is a common finding in acute lung injury, leading to increased shunt and hypoxemia. Current treatment strategies aim to recruit alveoli for gas exchange. Improvement in oxygenation is commonly used to detect recruitment, although the assumption that gas exchange parameters adequately represent the mechanical process of alveolar opening has not been proven so far. The aim of this study was to investigate whether commonly used measures of lung mechanics better detect lung tissue collapse and changes in lung aeration after a recruitment maneuver as compared to measures of gas exchange

METHODS

In eight anesthetized and mechanically ventilated pigs, acute lung injury was induced by saline lavage and a recruitment maneuver was performed by inflating the lungs three times with a pressure of 45 cmH2O for 40 s with a constant positive end-expiratory pressure of 10 cmH2O. The association of gas exchange and lung mechanics parameters with the amount and the changes in aerated and nonaerated lung volumes induced by this specific recruitment maneuver was investigated by multi slice CT scan analysis of the whole lung.

RESULTS

Nonaerated lung correlated with shunt fraction (r = 0.68) and respiratory system compliance (r = 0.59). The arterial partial oxygen pressure (PaO2) and the respiratory system compliance correlated with poorly aerated lung volume (r = 0.57 and 0.72, respectively). The recruitment maneuver caused a decrease in nonaerated lung volume, an increase in normally and poorly aerated lung, but no change in the distribution of a tidal breath to differently aerated lung volumes. The fractional changes in PaO2, arterial partial carbon dioxide pressure (PaCO2) and venous admixture after the recruitment maneuver did not correlate with the changes in lung volumes. Alveolar recruitment correlated only with changes in the plateau pressure (r = 0.89), respiratory system compliance (r = 0.82) and parameters obtained from the pressure-volume curve.

CONCLUSION

A recruitment maneuver by repeatedly hyperinflating the lungs led to an increase of poorly aerated and a decrease of nonaerated lung mainly. Changes in aerated and nonaerated lung volumes were adequately represented by respiratory compliance but not by changes in oxygenation or shunt.

Monitoring of pulmonary mechanics in acute respiratory distress syndrome to titrate therapy

PURPOSE OF REVIEW

This paper reviews recent findings regarding the respiratory mechanics during acute respiratory distress syndrome as a tool for tailoring its ventilatory management.

RECENT FINDINGS

The pressure-volume curve has been used for many years as a descriptor of the respiratory mechanics in patients affected by acute respiratory distress syndrome. The use of the sigmoidal equation introduced by Venegas for the analysis of the pressure-volume curve seems to be the most rigorous mathematical approach to assessing lung mechanics. Increasing attention has been focused on the deflation limb for titration of positive end-expiratory pressure. Based on physiologic reasoning, a novel parameter, the stress index, has been proposed for tailoring a safe mechanical ventilation, although its clinical impact has still to be proved. Evidence has confirmed that a variety of underlying pathologies may lead to acute respiratory distress syndrome, making unrealistic any attempt to unify the ventilatory approach. Although extensively proposed to tailor mechanical ventilation during acute respiratory distress syndrome, there is no evidence that the pressure-volume curve may be useful in setting a lung-protective strategy in the presence of different potentials for recruitment.

SUMMARY

The Venegas approach should be the standard analysis of pressure-volume curves. In any patient, the potential for recruitment should be assessed, as a basis for tailoring the most effective mechanical ventilation. Further studies are needed to clarify the potential use of the pressure-volume curve to guide a lung-protective ventilatory strategy.

Computed tomography scan assessment of lung volume and recruitment during high-frequency oscillatory ventilation

OBJECTIVE

This review describes how computed tomography has increased our understanding of the pathophysiology of acute respiratory distress syndrome. It summarizes current knowledge about lung volume changes and alveolar recruitment during high-frequency oscillatory ventilation (HFOV) assessed by computed tomography (CT), outlines potential problems when comparing HFOV with conventional ventilation (CV) as a result of the different pressure-time profiles, and describes future research directions.

DATA SOURCE

CT allows accurate assessment of total lung volumes and differentiation between overinflated, normally aerated, poorly aerated, and nonaerated lung regions. It allows for classification of different patterns of consolidation and may be predictive for the potential for recruitment.

DATA SUMMARY

Experimental data suggest that HFOV at mean airway pressures (mPaw) set according to a static PV curve leads to effective lung recruitment but results in overall lung volumes that are considerably higher than those predicted from the PV relationship. In saline-lavaged sheep, similar changes in total lung volumes and subvolumes were observed during HFOV and CV. One single study specifically assessed lung volume recruitment during HFOV as compared with CV in eight patients with acute respiratory distress syndrome from pneumonia or sepsis. After 48 hrs on HFOV, total ventilated lung volume was significantly increased, whereas only a minor increase in overinflated lung volume was observed. These changes correlated with a significant improvement in gas exchange.

CONCLUSION

CT is a valuable tool to quantify recruitment and overinflation during HFOV. Additional studies are needed to better characterize the specific effects of HFOV on lung volume and morphology.

The concept of "baby lung"

BACKGROUND

The "baby lung" concept originated as an offspring of computed tomography examinations which showed in most patients with acute lung injury/acute respiratory distress syndrome that the normally aerated tissue has the dimensions of the lung of a 5- to 6-year-old child (300-500 g aerated tissue).

DISCUSSION

The respiratory system compliance is linearly related to the "baby lung" dimensions, suggesting that the acute respiratory distress syndrome lung is not "stiff" but instead small, with nearly normal intrinsic elasticity. Initially we taught that the "baby lung" is a distinct anatomical structure, in the nondependent lung regions. However, the density redistribution in prone position shows that the "baby lung" is a functional and not an anatomical concept. This provides a rational for "gentle lung treatment" and a background to explain concepts such as baro- and volutrauma.

CONCLUSIONS

From a physiological perspective the "baby lung" helps to understand ventilator-induced lung injury. In this context, what appears dangerous is not the V(T)/kg ratio but instead the V(T)/"baby lung" ratio. The practical message is straightforward: the smaller the "baby lung," the greater is the potential for unsafe mechanical ventilation.

Other approaches to open-lung ventilation: Airway pressure release ventilation

OBJECTIVE

To review the use of airway pressure release ventilation (APRV) in the treatment of acute lung injury/acute respiratory distress syndrome.

DATA SOURCE

Published animal studies, human studies, and review articles of APRV.

DATA SUMMARY

APRV has been successfully used in neonatal, pediatric, and adult forms of respiratory failure. Experimental and clinical use of APRV has been shown to facilitate spontaneous breathing and is associated with decreased peak airway pressures and improved oxygenation/ventilation when compared with conventional ventilation. Additionally, improvements in hemodynamic parameters, splanchnic perfusion, and reduced sedation/neuromuscular blocker requirements have been reported.

CONCLUSION

APRV may offer potential clinical advantages for ventilator management of acute lung injury/acute respiratory distress syndrome and may be considered as an alternative "open lung approach" to mechanical ventilation. Whether APRV reduces mortality or increases ventilator-free days compared with a conventional volume-cycled "lung protective" strategy will require future randomized, controlled trials.

Differential effects of sustained inflation recruitment maneuvers on alveolar epithelial and lung endothelial injury

OBJECTIVE

The role of recruitment maneuvers in mechanical ventilation for patients with the acute respiratory distress syndrome and acute lung injury remains uncertain in part due to a lack of data on the effects of specific recruitment maneuvers on lung injury severity. The primary objective of this study was to determine the effect of one type of recruitment maneuver--sustained inflation--on alveolar epithelial and lung endothelial injury in experimental acute lung injury.

DESIGN

Randomized experimental study.

SETTING

Academic research laboratory.

SUBJECTS

Forty-nine Sprague-Dawley rats.

INTERVENTIONS

Lung injury was induced in anesthetized, ventilated rats by instillation of acid (pH 1.5) into the airspaces. Rats were ventilated with a tidal volume of 6 mL/kg and a positive end-expiratory pressure of 5 cm H(2)O with or without a sustained inflation recruitment maneuver repeated every 30 mins. Each recruitment maneuver consisted of two 30-sec inflations to total lung capacity (30 cm H(2)O) 1 min apart.

MEASUREMENTS AND MAIN RESULTS

The use of recruitment maneuvers significantly improved oxygenation, compliance, end-expiratory lung volume, functional residual capacity, and deadspace fraction. Recruitment maneuvers reduced extravascular lung water and lung endothelial injury as measured by protein permeability (217 +/- 28 vs. 314 +/- 70 extravascular plasma equivalents [microL], p < .05). However, recruitment maneuvers did not prevent alveolar epithelial injury. Epithelial permeability and bronchoalveolar lavage RTI40 levels, a marker of type I cell injury, were similar with or without recruitment maneuvers. Recruitment maneuvers decreased epithelial fluid transport, a functional marker of epithelial injury. Recruitment maneuvers did not reduce markers of airspace inflammation.

CONCLUSIONS

Sustained inflation recruitment maneuvers improve respiratory mechanics and oxygenation and may protect the lung endothelium but do not reduce alveolar epithelial injury. Because of the differential effects of recruitment maneuvers on the lung endothelium and alveolar epithelium, the net effect in clinical acute lung injury may not be beneficial. Additional clinical studies will be needed to assess the net impact of recruitment maneuvers in patients with acute lung injury.

Recruitment maneuvers during prone positioning in patients with acute respiratory distress syndrome

OBJECTIVE

To evaluate the interaction of recruitment maneuvers and prone positioning on gas exchange and venous admixture in patients with early extrapulmonary acute respiratory distress syndrome ventilated with high levels of positive end-expiratory pressure. We hypothesized that a sustained inflation performed after 6 hrs of prone positioning would induce sustained improvement in oxygenation (Pao2/Fio2) and venous admixture.

DESIGN

Prospective, interventional study.

SETTING

Tertiary care, postoperative intensive care unit.

PATIENTS

Fifteen patients with early extrapulmonary acute respiratory distress syndrome.

INTERVENTIONS

After 6 hrs of prone positioning, a sustained inflation was performed with 50 cm H2O maintained for 30 secs. Data were recorded in supine position, after 6 hrs of prone positioning, at 3, 30, and 180 mins following the sustained inflation.

MEASUREMENTS AND MAIN RESULTS

A response to prone positioning was observed in nine of 15 patients leading to an improvement of Pao2/Fio2 (147 +/- 37 torr vs. 225 +/- 77 torr, p = .005) and venous admixture (35.4 +/- 8.3% vs. 28.9 +/- 9.8%, p = .001). Six patients did not respond to prone positioning. Following the sustained inflation, the responders to prone positioning showed a further increase of Pao2/Fio2 and decrease of venous admixture at 3 mins (Pao2/Fio2, 225 +/- 77 torr vs. 368 +/- 90 torr, p = .018; venous admixture, 28.9 +/- 9.8% vs. 18.9 +/- 6.7%, p = .05). In all six nonresponders to prone positioning, an improvement of Pao2/Fio2 and venous admixture occurred at 3 mins following the sustained inflation (128 +/- 18 torr vs. 277 +/- 59 torr, p = .03; venous admixture, 34.2 +/- 6.0% vs. 23.8 +/- 6.3%, p = .05). The beneficial effects of the sustained inflation remained significantly elevated over 3 hrs in responders and nonresponders to prone positioning.

CONCLUSION

In patients with early extrapulmonary acute respiratory distress syndrome, a sustained inflation performed after 6 hrs of prone positioning induced further and sustained improvement of oxygenation and venous admixture in both responders and nonresponders to prone positioning.

Elastic pressure-volume curves in acute lung injury and acute respiratory distress syndrome

BACKGROUND

The principal features of elastic pressure-volume curves of lungs or the respiratory system (P(el)/V curves) recorded during reexpansion of collapsed lungs and subsequent deflation have been known since the 1950s. In acute respiratory failure and acute respiratory distress syndrome such curves have recently attracted increasing interest because new knowledge can be acquired from them, and because such curves may be useful as guidelines in setting the ventilator so as to avoid ventilator-induced lung injury.

DISCUSSION

This article reviews recording methods, underlying physiology and utility of P(el)/V curves in research and clinical work.

Repeated generation of the pulmonary pressure-volume curve may lead to derecruitment in experimental lung injury

OBJECTIVE

Measurements from the pulmonary pressure-volume (PV) curve have been proposed to adjust ventilator settings. We investigated the effects of repeated construction of an inflation PV curve implemented in a standard ventilator on recruitment or derecruitment in acutely injured lungs.

DESIGN AND SETTING

Prospective experimental animal study in eight anesthetized and mechanically ventilated pigs.

INTERVENTIONS

Acute lung injury was induced by lung lavage and animals were ventilated in volume controlled mode with PEEP 10 cmH(2)O. The PV curve was constructed five times repeatedly by constant pressure rise, after which ventilation with the preset PEEP was resumed immediately. Studies of hemodynamics, lung mechanics, blood gases and computed tomography were carried out before and after maneuvers.

MEASUREMENTS AND RESULTS

Derecruitment was assessed as an increase in nonaerated lung volume (V(NON)), and V(PEEP) was the end-expiratory volume difference between PEEP and ZEEP. There was a significant decrease in PaO(2) from 90.4+/-33.3 to 70.9+/-36.3 mmHg and a rise in venous admixture from 47.8+/-12.7 to 59.1+/-16.6%. V(PEEP) was reduced from 244 to 202 ml. A corresponding decrease in normally aerated lung volume was observed, while regression analysis revealed increase in V(NON) depending on the amount of preexisting atelectasis.

CONCLUSIONS

Repeated generation of the PV curve with a readily available tool resulted in worsened oxygenation. Derecruitment of the lungs occurred with loss of PEEP at the start of the maneuver, which could not be recovered by a maximum inflation pressure of 40 cmH(2)O. Repeated use of the investigated tool should be cautioned, and users should consider measures to preserve aerated lung volumes.

Intercomparison of recruitment maneuver efficacy in three models of acute lung injury*

OBJECTIVE

To compare the relative efficacy of three forms of recruitment maneuvers in diverse models of acute lung injury characterized by differing pathoanatomy.

DESIGN

We compared three recruiting maneuver (RM) techniques at three levels of post-RM positive end-expiratory pressure in three distinct porcine models of acute lung injury: oleic acid injury; injury induced purely by the mechanical stress of high-tidal airway pressures; and pneumococcal pneumonia.

SETTING

Laboratory in a clinical research facility.

SUBJECTS

Twenty-eight anesthetized mixed-breed pigs (23.8 +/- 2.6 kg).

INTERVENTIONS

The RM techniques tested were sustained inflation, extended sigh or incremental positive end-expiratory pressure, and pressure-controlled ventilation.

PRIMARY MEASUREMENTS

Oxygenation and end-expiratory lung volume.

MAIN RESULTS

The post-RM positive end-expiratory pressure level was the major determinant of post-maneuver PaO2, independent of the RM technique. The pressure-controlled ventilation RM caused a lasting increase of PaO2 in the ventilator-induced lung injury model, but in oleic acid injury and pneumococcal pneumonia, there were no sustained oxygenation differences for any RM technique (sustained inflation, incremental positive end-expiratory pressure, or pressure-controlled ventilation) that differed from raising positive end-expiratory pressure without RM.

CONCLUSIONS

Recruitment by pressure-controlled ventilation is equivalent or superior to sustained inflation, with the same peak pressure in all tested models of acute lung injury, despite its lower mean airway pressure and reduced risk for hemodynamic compromise. Although RM may improve PaO2 in certain injury settings when traditional tidal volumes are used, sustained improvement depends on the post-RM positive end-expiratory pressure value.

Transient hemodynamic effects of recruitment maneuvers in three experimental models of acute lung injury*

OBJECTIVE

Elevated lung volumes and increased pleural pressures associated with recruitment maneuvers (RM) may adversely affect pulmonary vascular resistance and cardiac filling or performance. We investigated the hemodynamic consequences of three RM techniques after inducing acute lung injury.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

Hospital research laboratory.

SUBJECTS

Thirteen anesthetized, mechanically ventilated pigs.

INTERVENTIONS

We induced three types of acute lung injury: oleic acid injury (n = 4); ventilator-induced lung injury (n = 4); and pneumonia (n = 5). All three models were designed to initiate a similar severity of oxygenation impairment. RM methods tested were sustained inflation, incremental positive end-expiratory pressure (PEEP) with a limited peak pressure, and pressure-controlled ventilation with increased PEEP and a fixed driving pressure. From a baseline PEEP of 8 cm H2O, all interventions were tested using post-RM PEEP levels of 8, 12, and 16 cm H2O. Cardiac output by thermodilution and systemic and pulmonary artery pressures were measured frequently during the RM and for 15 mins after its completion.

MEASUREMENTS AND MAIN RESULTS

During the RM, cardiac output decreased to a greater extent in the pneumonia model (0.49 of baseline cardiac output) than in the oleic acid injury (0.67 of baseline) or ventilator-induced lung injury (0.79 of baseline) models. Cardiac output recovered to the baseline value by 5 mins post-RM in oleic acid injury and ventilator-induced lung injury models. However, cardiac output remained decreased 15 mins post-RM in the pneumonia model. There were no differences in hemodynamic parameters among RM methods in oleic acid injury and ventilator-induced lung injury models. In the pneumonia model, however, cardiac output decreased to a greater extent during the RM with sustained inflation (to 0.33 of baseline cardiac output) compared with pressure-controlled ventilation (to 0.68 of baseline).

CONCLUSIONS

We conclude that RM transiently but profoundly depressed cardiac output in three models of acute lung injury. The results imply that a lung recruiting maneuver should be used with caution, especially when using sustained inflation in the setting of pneumonia.

Tomographic Study of the Inflection Points of the Pressure–Volume Curve in Acute Lung Injury

The inflection points of the pressure-volume curve have been used for setting mechanical ventilation in patients with acute lung injury. However, the lung status at these points has never been specifically addressed. In 12 patients with early lung injury we traced both limbs of the pressure-volume curve by means of a stepwise change in airway pressure, and a computed tomography (CT) scan slice was obtained for every pressure level. Although aeration (increase in normally aerated lung) and recruitment (decrease in nonaerated lung) were parallel and continuous along the pressure axis during inflation, loss of aeration and derecruitment were only significant at pressures below the point of maximum curvature on the deflation limb of the pressure-volume curve. This point was related to a higher amount of normally aerated tissue and a lower amount of nonaerated tissue when compared with the lower inflection point on both limbs of the curve. Aeration at the inflection points was similar in lung injury from pulmonary or extrapulmonary origin. There were no significant changes in hyperinflated lung tissue. These results support the use of the deflation limb of the pressure-volume curve for positive end-expiratory pressure setting in patients with acute lung injury.

Bench-to-bedside review: chest wall elastance in acute lung injury/acute respiratory distress syndrome patients

The importance of chest wall elastance in characterizing acute lung injury/acute respiratory distress syndrome patients and in setting mechanical ventilation is increasingly recognized. Nearly 30% of patients admitted to a general intensive care unit have an abnormal high intra-abdominal pressure (due to ascites, bowel edema, ileus), which leads to an increase in the chest wall elastance. At a given applied airway pressure, the pleural pressure increases according to (in the static condition) the equation: pleural pressure = airway pressure x (chest wall elastance/total respiratory system elastance). Consequently, for a given applied pressure, the increase in pleural pressure implies a decrease in transpulmonary pressure (airway pressure - pleural pressure), which is the distending force of the lung, implies a decrease of the strain and of ventilator-induced lung injury, implies the need to use a higher airway pressure during the recruitment maneuvers to reach a sufficient transpulmonary opening pressure, implies hemodynamic risk due to the reductions in venous return and heart size, and implies a possible increase of lung edema, partially due to the reduced edema clearance. It is always important in the most critically ill patients to assess the intra-abdominal pressure and the chest wall elastance.

Oleic acid lung injury: a morphometric analysis using computed tomography

BACKGROUND

The oleic acid-induced lung injury (OAI) model is considered to represent the early phase of acute respiratory distress syndrome (ARDS). Its inherent properties are important for the design and the interpretation of interventional studies. The aim of this study was to describe the evolution of morphometric lung changes during OAI using computed tomography (CT) analysis. Furthermore, the effect of a temporary change in positive end-expiratory pressure (PEEP) was evaluated.

METHODS

Fifteen anaesthetized pigs were ventilated in volume-controlled mode with a baseline PEEP of 5 cm H(2)O. Helical CT scans were taken at baseline and 1 h after oleic acid injection. The PEEP was then either increased to 10 cm H(2)O (n = 5), decreased to 0 cm H(2)O (n = 5) or kept constant (n = 5) for 30 min. For the next 30 min, the baseline PEEP level was applied in all animals before the final CT scans 2 h after the induction of OAI. Dimensional and volumetric changes were determined from radiographical attenuation values.

RESULTS

There was a major decrease in gas volume and an increase in tissue volume within the first hour. A net increase in total lung volume, with a larger transverse area but no displacement of the diaphragm, was manifest after 2 h. A minor increase in volume of non-aerated lung, located to the caudal region, was observed during the second hour. The tidal volume was redistributed to the middle and apical regions. The temporary change in PEEP did not influence the morphological progress of OAI.

CONCLUSION

Decreased gas volume and increased tissue volume are the dominating morphometric characteristics of oleic acid lung injury, occurring mainly within the first hour. With these changes manifest, the course of injury is not affected by a limited period of moderately changed PEEP during the second hour. The net increase of total lung volume suggests a predominance of oedema formation over airway and alveolar collapse.

Lung volumes and alveolar expansion pattern in immature rabbits treated with serum-diluted surfactant

In acute respiratory distress syndrome, mechanical ventilation often induces alveolar overdistension aggravating the primary insult. To examine the mechanism of overdistension, surfactant-deficient immature rabbits were anesthetized with pentobarbital sodium, and their lungs were treated with serum-diluted modified natural surfactant (porcine lung extract; 2 mg/ml, 10 ml/kg). By mechanical ventilation with a peak inspiration pressure of 22.5 cm H2O, the animals had a tidal volume of 14.7 ml/kg (mean), when 2.5 cm H2O positive end-expiratory pressure was added. This volume was similar to that in animals treated with nondiluted modified natural surfactant (24 mg/ml in Ringer solution, 10 ml/kg). However, the lungs fixed at 10 cm H2O on the deflation limbs of the pressure-volume curve had the largest alveolar/alveolar duct profiles (> or =48,000 microm2), accounting for 38% of the terminal air spaces, and the smallest (<6,000 microm2), accounting for 31%. These values were higher than those in animals treated with nondiluted modified natural surfactant (P <0.05). We conclude that administration of serum-diluted surfactant to immature neonatal lungs leads to patchy overdistension of terminal air spaces, similar to the expansion pattern that may be seen after dilution of endogenous surfactant with proteinaceous edema fluid in acute respiratory distress syndrome.

Inflammation and the acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a clinical syndrome of non-cardiogenic pulmonary oedema associated with bilateral pulmonary infiltrates, stiff lungs and refractory hypoxaemia. ARDS is characterized by an explosive acute inflammatory response in the lung parenchyma, leading to alveolar oedema, decreased lung compliance and, ultimately, hypoxaemia. Although our understanding of the causes and pathophysiology of ARDS has increased, the mortality rate remains in the range of 30-50%. No major advances in pharmacological therapy have been achieved. Mechanical ventilation is the main therapeutic intervention in the management of ARDS. The only approach that has been shown to reduce the inflammatory response and mortality is the use of lung-protective ventilatory strategy with a low tidal volume and high positive-end expiratory pressure. This chapter will review the current state of the literature on the pathogenesis of ARDS and ventilatory and pharmacotherapy approaches to its management.

Acute respiratory distress syndrome, the critical care paradigm: what we learned and what we forgot

In the last several years, we definitely learned that the acute respiratory distress syndrome lung is small, nonhomogeneous, and that mechanical ventilation in this baby lung may cause physical damage as well as inflammatory reaction. The clinical benefit of the gentle lung treatment, based on a decrease of global/regional stress and strain into the lung, has been finally proved. However, we forgot the importance of lung perfusion and its distribution in this syndrome and, besides a low tidal volume, we still do not know how to handle the other variables of mechanical ventilation. Measurements of variables as transpulmonary pressure and end expiratory lung volume, for a rational setting of mechanical ventilation, should be introduced in routine clinical practice.

Peak volume history and peak pressure-volume curve pressures independently affect the shape of the pressure-volume curve of the respiratory system

OBJECTIVE

To determine the specific effect of peak volume history pressure on the inflation limb of the pressure-volume curve and peak pressure-volume curve pressure on the deflation limb of the pressure-volume curve.

DESIGN

Prospective assessment of pressure-volume curves in saline, lung lavage injured sheep.

SETTING

Large animal laboratory of a university-affiliated hospital.

SUBJECTS

Eight female Dorset sheep.

INTERVENTIONS

: The effect of two volume history pressures (40 and 60 cm H2O) and three pressure-volume curve peak pressures (40, 50, and 60 cm H2O) were randomly compared.

MEASUREMENTS AND MAIN RESULTS

Peak volume history pressure affected the inflation curve beyond the lower inflection point but did not affect the inflection point (Pflex). Peak pressure-volume curve pressure affected the deflation curve. Increased peak volume history pressure increased inflation compliance (p <.05). Increased peak pressure-volume curve pressure increased the point of maximum compliance change on the deflation limb and deflation compliance and decreased compliance between peak pressure and the point of maximum curvature on the deflation limb (p <.05).

CONCLUSION

Peak volume history pressure must be considered when interpreting the inflation limb of the pressure-volume curve of the respiratory system beyond the inflection point. The peak pressure achieved during the pressure-volume curve is important during interpretation of deflation compliance and the point of maximum compliance change on the deflation limb.

[Treatment of acute respiratory distress syndrome in a treatment center. Success is dependent on risk factors]

SUBJECT

Mortality rates remain high for the acute respiratory distress syndrome (ARDS) despite standardised treatment algorithms. Little is known about prognostic factors and exclusion criteria for advanced treatment including extracorporeal membrane oxygenation (ECMO).

METHODS

In an observational study design a cohort of 93 patients with severe ARDS admitted to a referral centre were analysed according to ventilatory and vital parameters.

RESULTS

Overall survival rate was 70% and in patients who received ECMO treatment it was 67%. In patients exhibiting relevant co-morbidity the odds ratio for fatal outcome increased to 4.7 (95% CI: 3.3-24.9), and patients with multiple organ failure had a 7.5-fold increase (95% CI: 2.3-25.2) for risk of death. Survivors demonstrated a more pronounced improvement in oxygenation ( p<0.05) and CO(2) removal ( p<0.05) than non-survivors.

CONCLUSIONS

Advanced treatment of ARDS including ECMO represents a therapeutic option if none of the currently considered contraindications are present. An improvement in gas exchange parameters, but not a defined value per se may be useful as a prognostic factor for favourable outcome.

Airway pressure-time curve profile (stress index) detects tidal recruitment/hyperinflation in experimental acute lung injury

OBJECTIVE

To evaluate whether the shape of the airway pressure-time (Paw-t) curve during constant flow inflation corresponds to radiologic evidence of tidal recruitment or tidal hyperinflation in an experimental model of acute lung injury.

DESIGN

Prospective randomized laboratory animal investigation.

SETTING

Department of Clinical Physiology, University of Uppsala, Sweden.

SUBJECTS

Anesthetized, paralyzed, and mechanically ventilated pigs.

INTERVENTIONS

Acute lung injury was induced by lung lavage. During constant inspiratory flow, the Paw-t curve was fitted to a power equation: airway pressure =a x time + c, where coefficient b (stress index) describes the shape of the curve:b = 1, straight curve; b < 1, progressive increase in slope; and b > 1, progressive decrease in slope. Tidal volume (Vt) was 6 mL/kg, and positive end-expiratory pressure was set to obtain a b value between 0.9 and 1.1 before (b = 1) and after (b = 1 after recruiting maneuver) application of a recruiting maneuver. Positive end-expiratory pressure was decreased and Vt increased to obtain 0.9 >b > 0.8 and 0.8 >b > 0.6, whereas positive end-expiratory pressure and Vt were both increased to obtain 1.3 >b > 1.1 and 1.5 >b > 1.3. Experimental conditions sequence was random.

MEASUREMENTS AND MAIN RESULTS

Pulmonary computed tomography was obtained during end-expiratory and end-inspiratory occlusions. Tidal recruitment was quantified as nonaerated (between -100 and +100 Hounsfield units) lung area at end-expiration minus end-inspiration. Tidal hyperinflation was quantified as hyperinflated (between -900 and -1000 Hounsfield units) lung area at end-inspiration minus end-expiration. Computed tomography images showed that tidal recruitment and tidal hyperinflation corresponded to b < 1 and b > 1, respectively. Stress index values and tidal recruitment and tidal hyperinflation values were significantly correlated (R =.917 and R =.911, p <.0001, respectively).

CONCLUSIONS

Shape of the Paw-t curve detects tidal recruitment and tidal hyperinflation.

Functional Residual Capacity and Respiratory Mechanics as Indicators of Aeration and Collapse in Experimental Lung Injury

Increased functional residual capacity (FRC) and compliance are two desirable, but seldom measured, effects of positive end-expiratory pressure (PEEP) in mechanically ventilated patients. To assess how these variables reflect the morphological lung perturbations during the evolution of acute lung injury and the morphological changes from altered PEEP, we correlated measurements of FRC and respiratory system mechanics to the degree of lung aeration and consolidation on computed tomography (CT). We used a porcine oleic acid model with FRC determinations by sulfur hexafluoride washin-washout and respiratory system mechanics measured during an inspiratory hold maneuver. Within the first hour, during constant volume-controlled ventilation with PEEP 5 cm H(2)O, FRC decreased by 45% +/- 15% (P = 0.005) and compliance decreased by 35% +/- 12% (P = 0.005). Resistance increased by 60% +/- 62% (P = 0.005). Only the FRC changes correlated significantly to the decreased aeration (R(2) = 0.56; P = 0.01) and the increased consolidation (R(2) = 0.43; P = 0.04) on CT. When the PEEP was changed to either 10 or 0 cm H(2)O, there were larger changes in FRC than in compliance. We conclude that, in our model, FRC was a more sensitive indicator of PEEP-induced aeration and recruitment of lung tissue and that FRC may be a useful adjunct to PaO(2) monitoring.

IMPLICATIONS

Lung injury was quantified on computed tomography and related to monitored values of functional residual capacity and mechanical properties of the respiratory system. We found the functional residual capacity to be a more sensitive marker of the lung perturbations than the compliance. It might be of value to include functional residual capacity in the monitoring of acute lung injury.

Ventilation with biphasic positive airway pressure in experimental lung injury

OBJECTIVE

We investigated whether improvement in ventilation perfusion (.V(A)/.Q) distribution during mechanical ventilation using biphasic positive airway pressure (BIPAP) with spontaneous breathing may be attributed to an effectively increased transpulmonary pressure (P(TP)) and can also be achieved by increasing P(TP) during controlled ventilation.

DESIGN

In 12 pigs with saline lavage-induced lung injury we compared the effects of BIPAP to pressure-controlled ventilation with equal airway pressure (PCV(AW)) or equal transpulmonary pressure (PCV(TP)) on V(A)/.Q distribution assessed by the multiple inert gas elimination technique (MIGET).

SETTING

Animal laboratory study.

MEASUREMENTS AND RESULTS

Intrapulmonary shunt was 33+/-11% during BIPAP, 36+/-10% during PCV(AW) and 33+/-15% during PCV(TP) ( p= n.s.). BIPAP resulted in higher PaO(2) than PCV(AW) (188+/-83 versus 147+/-82 mmHg, p < 0.05), but not than PCV(TP) (187+/-139 mmHg). Oxygen delivery was significantly higher during BIPAP (530+/-109 ml/min) versus 374+/-113 ml/min during PCV(AW) and 353+/-93 ml/min during PCV(TP) ( p < 0.005). Tidal volume with PCV(TP) increased to 11.9+/-2.3 ml/kg, compared to 8.5+/-0.8 with BIPAP and 7.6+/-1.4 with PCV(AW) ( p <0.001) and cardiac output decreased to 3.5+/-0.6 l/min (BIPAP 4.9+/-0.8 and PCV(AW) 3.9+/-0.8, p<0.006).

CONCLUSIONS

In experimental lung injury, BIPAP with preserved spontaneous breathing was effective in increasing regional P(TP), since pressure-controlled ventilation with the same P(TP) resulted in similar gas exchange effects. However, PCV(TP) caused increased airway pressures and tidal volumes, whereby, with BIPAP, less depression of oxygen delivery and cardiac output were observed. BIPAP could be useful in maintaining pulmonary gas exchange and slightly improving oxygenation without interfering with circulation as strongly as PCV does.

Dynamic elastic pressure-volume loops in healthy pigs recorded with inspiratory and expiratory sinusoidal flow modulation. Relationship to static pressure-volume loops

OBJECTIVE

The objective was to analyse relationships between inspiratory and expiratory static and dynamic elastic pressure-volume (P(el)/V) curves in healthy pigs.

DESIGN

The modulated low flow method was developed to allow studies also of the expiratory P(el)/V curves. Static P(el)/V (P(el,st)/V) and dynamic P(el)/V (P(el,dyn)/V) loops were studied in healthy pigs.

SETTING

Animal research laboratory in a university hospital.

MATERIAL

Ten healthy anaesthetised and paralysed pigs.

INTERVENTIONS AND MEASUREMENTS

A computer controlled a Servo Ventilator 900C with respect to respiratory rate, inspiratory flow and expiratory pressure to achieve a sinusoidal modulation of inspiration and expiration for determination of P(el,dyn)/V loops from zero end-expiratory pressure (ZEEP) and from a positive end-expiratory pressure (PEEP) of 6 cmH(2)O to 20, 35 and 50 cmH(2)O. The same system was used for studies of P(el,st)/V loops with the flow-interruption method from ZEEP and PEEP to 35 cmH(2)O. Recordings were analysed with an iterative technique.

RESULTS

The feasibility of automated determination of P(el,dyn)/V loops was demonstrated. Differences between P(el,dyn)/V and P(el,st)/V loops were explained by viscoelastic behaviour. P(el,st)/V loops recorded from PEEP to 35 cmH(2)O showed no significant hysteresis, indicating a non-significant surface tension hysteresis. P(el,dyn)/V loops from PEEP and both P(el,st)/V and P(el,dyn)/V loops from ZEEP to 35 cmH(2)O showed hysteresis. This indicates that lung collapse/re-expansion caused P(el)/V loop hysteresis which, in P(el,dyn)/V loops, was augmented by viscoelastic behaviour.

CONCLUSIONS

Viscoelasticity influences P(el,dyn)/V curves. Hysteresis caused by surface tension merits re-evaluation. Lung collapse and re-expansion may be indicated by hysteresis of P(el)/V loops.

Ventilatory management of acute respiratory distress syndrome: a consensus of two

OBJECTIVE

To synthesize the emerging body of experimental, observational, and clinical trial data into a practical guideline for safe and effective ventilatory management of acute respiratory distress syndrome.

DATA SOURCES

Relevant, peer-reviewed, scientific literature and personal observations from clinical practice.

STUDY SELECTION

Relevant experimental studies and high-impact observational and clinical trials of acute respiratory distress syndrome management.

DATA EXTRACTION

Detailed review of information contained in published scientific work.

DATA SYNTHESIS

Interactive discussions between the authors that culminated in our consensus view of appropriate management.

CONCLUSIONS

Prevention of ventilator-induced lung injury while accomplishing the essential life-supporting roles of mechanical ventilation is a complex undertaking that requires application of principles founded on a broad experimental and clinical database and on the results of well-executed clinical trials. At the bedside, execution of an effective lung-protective ventilation strategy remains an empirical process best guided by integrated physiology and a readiness to revise the management approach depending on the individual's response.

Lung volume in mechanically ventilated patients: measurement by simplified helium dilution compared to quantitative CT scan

OBJECTIVE

We describe a simplified helium dilution technique to measure end-expiratory lung volume (EELV) in mechanically ventilated patients. We assessed both its accuracy in comparison with quantitative computerized tomography (CT) and its precision.

DESIGN AND SETTING

Prospective human study.

PATIENTS

Twenty-one mechanically ventilated ALI/ARDS patients.

INTERVENTIONS

All patients underwent a spiral CT scan of the thorax during an end-expiratory occlusion. From the CT scan we computed the gas volume of the lungs (EELVCT). Within a few minutes, a rebreathing bag, containing a known amount of helium, was connected to the endotracheal tube, and the gas mixture diluted in the patient's lungs by delivering at least ten large tidal volumes. From the final helium concentration, EELV could be calculated by a standard formula (EELVHe).

MEASUREMENT AND RESULTS

The results obtained by the two techniques showed a good correlation (EELVHe=208+0.858xEELV(CT), r=0.941; P<0.001). Bias between the two techniques was 32.5+/-202.8 ml (95% limits of agreement were -373 ml and +438 ml), with a mean absolute difference of 15%. The amount of pathological tissue did not affect the difference between the two techniques, while the amount of hyperinflated tissue did. Bias between two repeated helium EELV measurements was -24+/-83 ml (95% limits of agreement were -191 ml and +141 ml), with a mean absolute difference of 6.3%.

CONCLUSIONS

The proposed helium dilution technique is simple and reproducible. The negligible bias and the acceptable level of agreement support its use as a practical alternative to CT for measuring EELV in mechanically ventilated ARDS patients.

Effects of end-inspiratory and end-expiratory pressures on alveolar recruitment and derecruitment in saline-washout-induced lung injury -- a computed tomography study

BACKGROUND

Lung protective ventilation using low end-inspiratory pressures and tidal volumes (VT) has been shown to impair alveolar recruitment and to promote derecruitment in acute lung injury. The aim of the present study was to compare the effects of two different end-inspiratory pressure levels on alveolar recruitment, alveolar derecruitment and potential overdistention at incremental levels of positive end-expiratory pressure.

METHODS

Sixteen adult sheep were randomized to be ventilated with a peak inspiratory pressure of either 35 cm H2O (P35, low VT) or 45 cm H2O (P45, high VT) after saline washout-induced lung injury. Positive end-expiratory pressure (PEEP) was increased in a stepwise manner from zero (ZEEP) to 7, 14 and 21 cm of H2O in hourly intervals. Tidal volume, initially set to 12 ml kg(-1), was reduced according to the pressure limits. Computed tomographic scans during end-expiratory and end-inspiratory hold were performed along with hemodynamic and respiratory measurements at each level of PEEP.

RESULTS

Tidal volumes for the two groups (P35/P45) were: 7.7 +/- 0.9/11.2 +/- 1.3 ml kg(-1) (ZEEP), 7.9 +/- 2.1/11.3 +/- 1.3 ml kg(-1) (PEEP 7 cm H2O), 8.3 +/- 2.5/11.6 +/- 1.4 ml kg(-1) (PEEP 14 cm H2O) and 6.5 +/- 1.7/11.0 +/- 1.6 ml kg(-1) (PEEP 21 cm H2O); P < 0.001 for differences between the two groups. Absolute nonaerated lung volumes during end-expiration and end-inspiration showed no difference between the two groups for given levels of PEEP, while tidal-induced changes in nonaerated lung volume (termed cyclic alveolar instability, CAI) were larger in the P45 group at low levels of PEEP. The decrease in nonaerated lung volume was significant for PEEP 14 and 21 cm H2O in both groups compared with ZEEP (P < 0.005). Over-inflated lung volumes, although small, were significantly higher in the P45 group. Significant respiratory acidosis was noted in the P35 group despite increases in the respiratory rate.

CONCLUSION

Limiting peak inspiratory pressure and VT does not impair alveolar recruitment or promote derecruitment when using sufficient levels of PEEP.

An increase of abdominal pressure increases pulmonary edema in oleic acid-induced lung injury

Increased abdominal pressure is common in intensive care unit patients. To investigate its impact on respiration and hemodynamics we applied intraabdominal pressure (aIAP) of 0 and 20 cm H(2)O (pneumoperitoneum) in seven pigs. The whole-lung computed tomography scan and a complete set of respiratory and hemodynamics variables were recorded both in healthy lung and after oleic acid (OA) injury. In healthy lung, aIAP 20 cm H(2)O significantly lowered the gas content, leaving the tissue content unchanged. In OA-injured lung at aIAP 0 cm H(2)O, the gas content significantly decreased compared with healthy lung. The excess tissue mass (edema) amounted to 30 +/- 24% of the original tissue weight (455 +/- 80 g). The edema was primarily distributed in the base regions and was not gravity dependent. Heart volume, central venous, pulmonary artery, wedge, and systemic arterial pressures significantly increased. At aIAP 20 cm H(2)O in OA-injured lung, the central venous and pulmonary artery pressures further increased. The gas content further decreased, and the excess tissue mass rose up to 103 +/- 37% (tissue weight 905 +/- 134 g), with homogeneous distribution along the cephalocaudal and sternovertebral axis. We conclude that in OA-injured lung, the increase of IAP increases the amount of edema.

Differences in the deflation limb of the pressure-volume curves in acute respiratory distress syndrome from pulmonary and extrapulmonary origin

OBJECTIVE

To assess the differences in the deflation pressure-volume (PV) curves between acute respiratory distress syndrome from pulmonary (ARDSp) and extrapulmonary (ARDSe) origin.

DESIGN

. Prospective study.

SETTING

Twenty-bed intensive care unit in an university hospital.

PATIENTS

Ten patients within the first 24 h from meeting ARDS criteria, classified as ARDSp or ARDSe in a clinical basis.

INTERVENTIONS

A deflation PV curve was recorded by means of decreasing steps of continuous positive airway pressure (CPAP) from 35 to 0 cmH(2)O.

RESULTS

The simultaneous recording of pressure at the airway opening (Pao), esophageal pressure (Pes) and volumes (V) allows us to trace the Pao-V, Pes-V and transpulmonary pressure (Ptp)-V curves. These data were fitted to a sigmoid model and ARDSp and ARDSe groups were compared. ARDSp has lower lung compliance and higher chest wall compliance than ARDSe (35.9+/-11.3 vs. 77.2+/-50.6 and 199.6+/-44.4 vs. 125.5+/-16.5 ml/cmH(2)O, respectively, P<0.05). The Pao-V curve in ARDSp is shifted down and right with respect to ARDSe. The Ptp-V curve shows a similar displacement. The Pes-V curve in the ARDSp group is, however, shifted to the left. When relative values (percentage to the maximum volume achieved at 35 cmH(2)O) are considered, these differences persist, but, in the Ptp-V curves, are only significant in the low-pressure range.

CONCLUSIONS

Differences between ARDSp and ARDSe PV curves are present all along the pressure axis and are related to differences not only in the Pes-V curve, but also in the Ptp-V curve.

Clinical review: bedside assessment of alveolar recruitment

Recruitment is a dynamic physiological process that refers to the reopening of previously gasless lung units. Cumulating evidence has led to a better understanding of the rules that govern both recruitment and derecruitment during mechanical ventilation of patients with acute respiratory distress syndrome. Therefore not only the positive end-expiratory pressure, but also the tidal volume, the inspired oxygen fraction, repeated tracheal suctioning as well as sedation and paralysis may affect recruitment of acute respiratory distress syndrome lungs that are particularly prone to alveolar instability. In the present article, we review the recently reported data concerning the physiological significance of the pressure–volume curve and its use to assess alveolar recruitment. We also describe alternate techniques that have been proposed to assess recruitment at the bedside. Whether recruitment should be optimized remains an ongoing controversy that warrants further clinical investigation.

Application of continuous positive airway pressure to trace static pressure-volume curves of the respiratory system

OBJECTIVE

To evaluate a new technique for pressure-volume curve tracing.

DESIGN

Prospective experimental study.

SETTING

Animal research laboratory.

SUBJECTS

Six anesthetized rats.

INTERVENTIONS

Two pressure-volume curves were obtained by means of the super-syringe method (gold standard) and the continuous positive airway pressure (CPAP) method. For the CPAP method, the ventilator was switched to CPAP and the pressure level was raised from 0 to 50 cm H2O in 5 cm H2O steps and then decreased, while we measured lung volume using respiratory inductive plethysmography. Thereafter, lung injury was induced using very high-volume ventilation. Following injury, two further pressure-volume curves were traced. Pressure-volume pairs were fitted to a mathematical model.

MEASUREMENTS AND MAIN RESULTS

Pressure-volume curves were equivalent for each method, with intraclass correlation coefficients being higher than.75 for each pressure level measured. Bias and precision for volume values were 0.46 +/- 0.875 mL in basal measurements and 0.31 +/- 0.67 mL in postinjury conditions. Lower and upper inflection points on the inspiratory limb and maximum curvature point on the deflation limb obtained using both methods and measured by regression analysis also were correlated, with intraclass correlation coefficients (95% confidence interval) being.97 (.58,.99),.85 (.55,.95), and.94 (.81,.98) (p <.001 for each one). When inflection points were estimated by observers, the correlation coefficient between methods was.90 (.67,.98) for lower inflection points (p <.001). However, estimations for upper inflection points and maximum curvature point were significantly different.

CONCLUSIONS

The CPAP method for tracing pressure-volume curves is equivalent to the super-syringe method. It is easily applicable at the bedside, avoids disconnection from the ventilator, and can be used to obtain both the inspiratory and the deflation limbs of the pressure-volume curve. Use of regression techniques improves determination of inflection points.

Temporal change, reproducibility, and interobserver variability in pressure-volume curves in adults with acute lung injury and acute respiratory distress syndrome

OBJECTIVES

To assess the reproducibility of the static pressure-volume curve of the respiratory system by using a "mini-syringe" technique; to assess the temporal change in upper (UIP) and lower inflection points (LIP) measured from pressure-volume curves of the respiratory system; to assess the inter- and intraobserver variability in detection of the UIP and LIP in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS); and to compare the syringe and multiple occlusion techniques for determining LIP and UIP.

DESIGN

Prospective observational study.

SETTING

Academic medical-surgical critical care unit.

PATIENTS

Consecutive patients with ALI or ARDS.

INTERVENTIONS

Static inspiratory pressure-volume curves of the respiratory system were determined twice on day 1 of diagnosis of ALI/ARDS and then once daily for up to 6 days by using the syringe technique. Pressure-volume curves were determined from zero positive end-expiratory pressure. At each time point, three separate measurements of the pressure-volume curve were made to determine reproducibility. A 100-mL graduated syringe was used to inflate patients' lungs with 50- to 100-mL increments up to an airway pressure of 45 cm H2O or a total volume of 2 L; each volume step was maintained for 2-3 secs until a plateau airway pressure was recorded. On day 1, the static pressure-volume curve also was determined by using the multiple occlusion technique. In a random and blinded sequence, the pressure-volume curves were examined visually by three critical care physicians on three different occasions, to determine the intra- and interobserver variability in visual detection of the LIP and UIP. Observers were given objective instructions to visually identify LIP and UIP.

MEASUREMENTS AND MAIN RESULTS

Eleven patients were enrolled, with a total of 134 pressure-volume curves generated. LIP and UIP could be detected in 90-94% and 61-68% of curves, respectively. When the three successive pressure-volume curves were compared, both the LIP and UIP were within 3 cm H2O in >65% of curves. The index of reliability (intraclass correlation coefficient) in LIP and UIP was 0.92 and 0.89 for interobserver variability and 0.90 and 0.88 for intraobserver variability. Daily variability was as high as 7 cm H2O for LIP and 5 cm H2O for UIP. When pressure-volume curves obtained by using the multiple occlusion and syringe techniques were compared, LIP was within 2 cm H2O, and UIP was within 4 cm H2O with the two techniques.

CONCLUSIONS

The static pressure-volume curve of the respiratory system is reasonably reproducible, thus avoiding the need for multiple measurements at a single time. We found excellent interobserver and intraobserver correlation in manual identification of the LIP and UIP. Both LIP and UIP show appreciable daily variability in patients with ALI/ARDS. The multiple occlusion and syringe techniques generate similar values for LIP and UIP.

Positive end-expiratory pressure after a recruitment maneuver prevents both alveolar collapse and recruitment/derecruitment

We tested the hypothesis that collapsed alveoli opened by a recruitment maneuver would be unstable or recollapse without adequate positive end-expiratory pressure (PEEP) after recruitment. Surfactant deactivation was induced in pigs by Tween instillation. An in vivo microscope was placed on a lung area with significant atelectasis and the following parameters measured: (1) the number of alveoli per field and (2) alveolar stability (i.e., the change in alveolar size from peak inspiration to end expiration). We previously demonstrated that unstable alveoli cause lung injury. A recruitment maneuver (peak pressure = 45 cm H2O, PEEP = 35 cm H2O for 1 minute) was applied and alveolar number and stability were measured. Pigs were then separated into two groups with standard ventilation plus (1) 5 PEEP or (2) 10 PEEP and alveolar number and stability were again measured. The recruitment maneuver opened a significant number of alveoli, which were stable during the recruitment maneuver. Although both 5 PEEP and 10 PEEP after recruitment demonstrated improved oxygenation, alveoli ventilated with 10 PEEP were stable, whereas alveoli ventilated with 5 PEEP showed significant instability. This suggests recruitment followed by inadequate PEEP permits unstable alveoli and may result in ventilator-induced lung injury despite improved oxygenation.

Alveolar inflation during generation of a quasi-static pressure/volume curve in the acutely injured lung

OBJECTIVE

Lower and upper inflection points on the quasi-static curve representing a composite of pressure/volume from the whole lung are hypothesized to represent initial alveolar recruitment and overdistension, respectively, and are currently utilized to adjust mechanical ventilation in patients with acute respiratory distress syndrome. However, alveoli have never been directly observed during the generation of a pressure/volume curve to confirm this hypothesis. In this study, we visualized the inflation of individual alveoli during the generation of a pressure/volume curve by direct visualization using in vivo microscopy in a surfactant deactivation model of lung injury in pigs.

DESIGN

Prospective, observational, controlled study.

SETTING

University research laboratory.

SUBJECTS

Eight adult pigs.

INTERVENTIONS

Pigs were anesthetized and administered mechanical ventilation, underwent a left thoracotomy, and were separated into two groups: control pigs (n = 3) were subjected to surgical intervention, and Tween lavage pigs (n = 5) were subjected to surgical intervention plus surfactant deactivation by Tween lavage (1.5 mL/kg 5% solution of Tween in saline). The microscope was then attached to the lung, and the size of each was alveolus quantified by measuring the alveolar area by computer image analysis. Each alveolus in the microscopic field was assigned to one of three types, based on alveolar mechanics: type I, no visible change in alveolar size during ventilation; type II, alveoli visibly change size during ventilation but do not totally collapse at end expiration; and type III, alveoli visibly change size during tidal ventilation and completely collapse at end expiration. After alveolar classification, the animals were disconnected from the ventilator and attached to a super syringe filled with 100% oxygen. The lung was inflated from 0 to 220 mL in 20-mL increments with a 10-sec pause between increments for airway pressure and alveolar confirmation to stabilize. These data were utilized to generate both quasi-static pressure/volume curves and individual alveolar pressure/area curves.

MEASUREMENTS AND MAIN RESULTS

The normal lung quasi-static pressure/volume curve has a single lower inflection point, whereas the curve after Tween has an inflection point at 8 mm Hg and a second at 24 mm Hg. Normal alveoli in the control group are all type I and do not change size appreciably during generation of the quasi-static pressure/volume curve. Surfactant deactivation causes a heterogenous injury, with all three alveolar types present in the same microscopic field. The inflation pattern of each alveolar type after surfactant deactivation by Tween was notably different. Type I alveoli in either the control or Tween group demonstrated minimal change in alveolar area with lung inflation. Type I alveolar area was significantly (p <.05) larger in the control as compared with the Tween group. In the Tween group, type II alveoli increased significantly in area, with lung inflation from 0 mL (9666 +/- 1340 microm2) to 40 mL (12,935 +/- 1725 microm2) but did not increase further (220 mL, 14,058 +/- 1740 microm2) with lung inflation. Type III alveoli initially recruited with a relatively small area (20 mL lung volume, 798 +/- 797 microm2) and progressively increased in area throughout lung inflation (120 mL, 7302 +/- 1405 microm2; 220 mL, 11,460 +/- 1078 microm2)

CONCLUSION

The normal lung does not increase in volume by simple isotropic (balloon-like) expansion of alveoli, as evidenced by the horizontal (no change in alveolar area with increases in airway pressure) pressure/area curve. After surfactant deactivation, the alveolar inflation pattern becomes very complex, with each alveolar type (I, II, and III) displaying a distinct pattern. None of the alveolar pressure/area curves directly parallel the quasi-static lung pressure/volume curve. Of the 16, only one type III atelectatic alveolus recruited at the first inflection point and only five recruited concomitant with the second inflation point, suggesting that neither inflection point was due to inflection point was due to massive alveolar recruitment. Thus, the components responsible for the shape of the pressure/volume curve include all of the individual alveolar pressure/area curves, plus changes in alveolar duct and airway size, and the elastic forces in the pulmonary parenchyma and the chest wall.

Acute respiratory distress syndrome: lessons from computed tomography of the whole lung

OBJECTIVE

This review aims to show how computed tomography of the whole lung has modified our view of acute respiratory distress syndrome, and why it impacts on the optimization of the ventilatory strategy.

DATA SOURCES

Computed tomography allows an accurate assessment of the volumes of gas and lung tissue, respectively, and lung aeration. If computed tomographic sections are contiguous from the apex to the lung base, quantitative analysis can be performed either on the whole lung or, regionally, at the lobar level. Analysis requires a manual delineation of lung parenchyma and is facilitated by software, including a color-coding system that allows direct visualization of overinflated, normally aerated, poorly aerated, and nonaerated lung regions. In addition, lung recruitment can be measured as the amount of gas that penetrates poorly aerated and nonaerated lung regions after the application of positive intrathoracic pressure.

DATA SUMMARY

The lung in acute respiratory distress syndrome is characterized by a marked increase in lung tissue and a massive loss of aeration. The former is homogeneously distributed, although with a slight predominance in the upper lobes, whereas the latter is heterogeneously distributed. The lower lobes are essentially nonaerated, whereas the upper lobes may remain normally aerated, despite a substantial increase in regional lung tissue. The overall lung volume and the cephalocaudal lung dimensions are reduced primarily at the expense of the lower lobes, which are externally compressed by the heart and abdominal content when the patient is in the supine position. Two opposite radiologic presentations, corresponding to different lung morphologies, can be observed. In patients with focal computed tomographic attenuations, frontal chest radiography generally shows bilateral opacities in the lower quadrants and may remain normal, particularly when the lower lobes are entirely atelectatic. In patients with diffuse computed tomographic attenuations, the typical radiologic presentation of "white lungs" is observed. If these patients lie supine, lung volume is preserved in the upper lobes and reduced in the lower lobes, although the loss of aeration is equally distributed between the upper and lower lobes. This observation does not support the "opening and collapse concept" described as the "sponge model." In fact, interstitial edema, alveolar flooding, or both, not collapse, are histologically present in all regions of the lung in acute respiratory distress syndrome. Compression atelectasis is observed only in caudal parts of the lung, where external forces (such as cardiac weight, abdominal pressure, and pleural effusion) tend to squeeze the lower lobes. When a positive intrathoracic pressure is applied to patients with focal acute respiratory distress syndrome, poorly aerated and nonaerated lung regions are recruited, whereas lung regions that are normally aerated at zero end-expiratory pressure tend to be rapidly overinflated, increasing the risk of ventilator-induced lung injury.

CONCLUSION

Selection of the optimal positive end-expiratory pressure level should not only consider optimizing alveolar recruitment, it should also focus on limiting lung overinflation and counterbalancing compression of the lower lobes by maneuvers such as appropriate body positioning. Prone and semirecumbent positions facilitate the reaeration of dependent and caudal lung regions by partially relieving cardiac and abdominal compression and may improve gas exchange.

Lung recruitment maneuvers in acute respiratory distress syndrome and facilitating resolution

OBJECTIVES

To summarize the possible ways that acute respiratory distress syndrome (ARDS) lungs can be recruited and to present the experimental and clinical results of these maneuvers, along with the possible effects on patient outcome.

DATA SOURCES

Selected published medical literature from 1972 to 2002 and personal observations.

DATA SUMMARY

In the experimental setting, repeated derecruitments accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate lung injury. In the clinical setting, recruitment maneuvers that use a continuous positive airway pressure of 40 cm H(2)O for 40 secs improve oxygenation in patients with early ARDS who do not have an impairment in the chest wall. High intermittent positive end-expiratory pressure (PEEP), intermittent sighs, or high-pressure controlled ventilation improves short-term oxygenation in ARDS patients. Both conventional and electrical impedance thoracic tomography studies indicate that high airway pressures increase the lung volume and recruitment percentage of lung tissue in ARDS patients. To sustain the recruited ARDS lungs, it is important to maintain adequate PEEP levels. High PEEP/low tidal volume ventilation was seen to reduce inflammatory mediators in both bronchoalveolar lavage and plasma, compared with low PEEP/high tidal volume ventilation, after 36 hrs of mechanical ventilation in ARDS patients. Recruitment maneuvers that used continuous positive airway pressure levels of 35-40 cm H(2)O for 40 secs, with PEEP set at 2 cm H(2)O above the Pflex (the lowest inflection point on the pressure-volume curve), and tidal volume <6 mL/kg were associated with a 28-day intensive care unit survival rate of 62%. This contrasted with a survival rate of only 29% with conventional ventilation (defined as the lowest PEEP for acceptable oxygenation without hemodynamic impairment with a tidal volume of 12 mL/kg), without recruitment maneuvers (number needed to treat = 3; p <.001).

CONCLUSIONS

High airway pressures can open collapsed ARDS lungs and partially open edematous ARDS lungs. High PEEP levels and low tidal volume ventilation decrease bronchoalveolar and plasma inflammatory mediators and improve survival compared with low PEEP/high tidal volume ventilation. In the near future, thoracic computed tomography associated with high-performance monitoring of regional ventilation (electrical impedance tomography) may be used at the bedside to determine the optimal mechanical ventilation of ARDS patients.

Physiologic rationale for ventilator setting in acute lung injury/acute respiratory distress syndrome patients

OBJECTIVES

To review the physiologic approach to setting mechanical ventilation in acute lung injury/acute respiratory distress syndrome.

DATA SOURCES

MEDLINE search from 1979 to the present.

DATA SELECTION

Personal selection of some articles we believe relevant for understanding acute lung injury/acute respiratory distress syndrome physiopathology and its physiologic management.

DATA SUMMARY

Knowing the underlying pathology is key to estimating the potential for recruitment. The potential for recruitment is rather low when the consolidation of pulmonary units exceeds collapse, as in diffuse pneumonia. In contrast, when pulmonary unit collapse exceeds consolidation, as in acute lung injury/acute respiratory distress syndrome from extrapulmonary origin, the potential for recruitment may be high. To exploit the potential for recruitment, a transpulmonary pressure greater than the opening pressure must be applied to the lung. To do so, chest wall elastance must be measured or estimated. To avoid collapse after recruitment, a positive end-expiratory pressure greater than the compressive forces operating on the lung and an alveolar ventilation sufficient to prevent absorption atelectasis must be provided. Indeed, avoidance of stretch (low airway plateau pressure) and prevention of cyclic collapse and reopening (adequate positive end-expiratory pressure and alveolar ventilation) are the physiologic cornerstones of mechanical ventilation in acute lung injury/acute respiratory distress syndrome. When considering all the randomized clinical trials reported so far, it is tempting to speculate that transpulmonary pressure and stresses, rather than tidal volume per se, are the key factors that may have an impact on mortality.

CONCLUSIONS

The majority of physiologic, experimental, and clinical trial data converge on one simple concept: treat the lung gently.

Effects of sustained inflation and postinflation positive end-expiratory pressure in acute respiratory distress syndrome: focusing on pulmonary and extrapulmonary forms

OBJECTIVE

To investigate whether the response to sustained inflation and postinflation positive end-expiratory pressure varies between acute respiratory distress syndrome with pulmonary (ARDS(exp)) and extrapulmonary origin (ARDS(exp)).

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary intensive care unit in a university hospital.

PATIENTS

A total of 11 patients with ARDS and 13 patients with ARDS.

INTERVENTIONS

A 7 ml/kg tidal volume, 12-15 breaths/min respiratory rate, and an inspiratory/expiratory ratio of 1:2 was used during baseline ventilation. Positive end-expiratory pressure levels were set according to the decision of the primary physician. Sustained inflation was performed by 45 cm H2O continuous positive airway pressure for 30 secs. Postinflation positive end-expiratory pressure was titrated decrementally, starting from a level of 20 cm H2O to keep the peripheral oxygen saturation between 92% and 95%. Fio2 was decreased, and baseline tidal volume, respiratory rate, inspiratory/expiratory ratio were maintained unchanged throughout the study period.

MEASUREMENTS AND MAIN RESULTS

Blood gas, airway pressure, and hemodynamic measurements were performed at the following time points: at baseline and at 15 mins, 1 hr, 4 hrs, and 6 hrs after sustained inflation. After sustained inflation, the Pao2/Fio2 ratio improved in all of the patients both in ARDS(p) and ARDS(exp). However, the Pao2/Fio2 ratio increased to >200 in four ARDS(p) patients (36%) and in seven ARDS(p) patients (54%). In two of those ARDS patients, the Pao2/Fio2 ratio was found to be <200, whereas none of the ARDS(p) patients revealed Pao2/Fio2 ratios of <200 at the 6-hr measurement. Postinflation positive end-expiratory pressure levels were set at 16.7 +/- 2.3 cm H O in ARDS(p) and 15.6 +/- 2.5 cm H2O in ARDS. The change in Pao /Fio ratios was found statistically significant in patients with ARDS(p) (p =.0001) and with ARDS(p) (p =.008). Respiratory system compliance increased in ARDS patients (p =.02), whereas the change in ARDS was not statistically significant.

CONCLUSIONS

Sustained inflation followed by high levels of postinflation positive end-expiratory pressure provided an increase in respiratory system compliance in ARDS; however, arterial oxygenation improved in both ARDS forms.