Recruitment and derecruitment during acute respiratory failure: an experimental study

Fluid transport in branched structures with temporary closures: a model for quasistatic lung inflation

We analyze the problem of fluid transport through a model system relevant to the inflation of a mammalian lung, an asymmetric bifurcating structure containing random blockages that can be removed by the pressure of the fluid itself. We obtain a comprehensive description of the fluid flow in terms of the topology of the structure and the mechanisms which open the blockages. We show that when calculating averaged flow properties of the fluid, the tree structure can be partitioned into a linear superposition of one-dimensional chains. In particular, we relate the pressure-volume P-V relationship of the fluid to the distribution Pi(n) of the generation number n of the tree's terminal branches, a structural property. We invert this relation to obtain a statistical description of the underlying branching structure of the lung, by analyzing experimental pressure-volume data from dog lungs. The Pi(n) extracted from the experimental P-V data agrees well with available data on lung branching structure. Our general results are applicable to any physical system involving transport in bifurcating structures with removable closures.

Sigh in supine and prone position during acute respiratory distress syndrome

Interventions aimed at recruiting the lung of patients with acute respiratory distress syndrome (ARDS) are not uniformly effective. Because the prone position increases homogeneity of inflation of the lung, we reasoned that it might enhance its potential for recruitment. We ventilated 10 patients with early ARDS (PaO2/FIO2, 121 +/- 46 mm Hg; positive end-expiratory pressure, 14 +/- 3 cm H2O) in supine and prone, with and without the addition of three consecutive "sighs" per minute to recruit the lung. Inspired oxygen fraction, positive end-expiratory pressure, and minute ventilation were kept constant. Sighs increased PaO2 in both supine and prone (p < 0.01). The highest values of PaO2 (192 +/- 41 mm Hg) and end-expiratory lung volume (1840 +/- 790 ml) occurred with the addition of sighs in prone and remained significantly elevated 1 hour after discontinuation of the sighs. The increase in PaO2 associated with the sighs, both in supine and prone, correlated linearly with the respective increase of end-expiratory lung volume (r = 0.82, p < 0.001). We conclude that adding a recruitment maneuver such as cyclical sighs during ventilation in the prone position may provide optimal lung recruitment in the early stage of ARDS.

Effect of alveolar recruitment maneuver in early acute respiratory distress syndrome according to antiderecruitment strategy, etiological category of diffuse lung injury, and body position of the patient

OBJECTIVE

To assess how the level of positive end-expiratory pressure (PEEP) (antiderecruitment strategy), etiological category of diffuse lung injury, and body position of the patient modify the effect of the alveolar recruitment maneuver (ARM) in acute respiratory distress syndrome (ARDS).

DESIGN

Prospective clinical trial.

SETTING

Medical intensive care unit at a tertiary hospital.

PATIENTS

Forty-seven patients with early ARDS, including 19 patients from our preliminary study.

INTERVENTION

From baseline ventilation at a tidal volume of 8 mL/kg and PEEP of 10 cm H2O, the ARM (a stepwise increase in the level of PEEP up to 30 cm H2O with a concomitant decrease in the magnitude of tidal volume down to 2 mL/kg) was given with (ARM + PEEP, n = 20) or without (ARM only, n = 19) subsequent increase of PEEP to 15 cm H2O. In eight other patients, PEEP was increased to 15 cm H2O without a preceding ARM (PEEP only).

MEASUREMENTS AND RESULTS

In all three groups, Pao2 was increased by the respective intervention (all p<.05). In the ARM-only group, Pao2 at 15 mins after intervention was lower than Pao2 immediate after intervention (p =.046). In the ARM + PEEP group, no such decrease in Pao2 was observed, and Pao2 at 15, 30, 45, and 60 mins after intervention was higher than in the ARM-only group (all p<.05). Compared with the PEEP-only group, Pao2 of the ARM + PEEP group was higher immediately after intervention and at the later time points (all p <.05). Compared with patients with ARDS associated with direct lung injury (pulmonary ARDS), patients with ARDS associated with indirect lung injury (extrapulmonary ARDS) showed a greater increase in Pao2 (27 +/- 21% vs. 130 +/- 112%; p=.002) and a greater decrease in radiologic scores (1.0 +/- 2.4 vs. 3.4 +/- 1.5; p=.005) after the ARM. The increase in Pao2 induced by the ARM was greater for patients in the supine position than for patients in the prone position (61 +/- 82% vs. 21 +/- 14%; p=.028). Consequently, Pao immediately after the ARM was similar in the two groups of patients in different positions.

CONCLUSIONS

After the ARM, a sufficient level of PEEP is required as an antiderecruitment strategy. Pulmonary ARDS and extrapulmonary ARDS may be different pathophysiologic entities. An effective ARM may obviate the need for the prone position in ARDS at least in terms of oxygenation.

Where are we with recruitment maneuvers in patients with acute lung injury and acute respiratory distress syndrome?

Reduction of tidal volume to limit plateau pressure currently is recommended for the ventilatory management of acute respiratory distress syndrome. However, sufficient evidence now exists to support the fact that excessive reduction in tidal volume may result in harmful alveolar derecruitment depending on the level at which positive end-expiratory pressure is set. The use of recruitment maneuvers has been proposed as an adjunctive lung-protective strategy to reverse low tidal volume-related derecruitment. Many questions remain regarding the basic physiologic principles of recruitment, and, therefore, the optimal way to perform recruitment maneuvers remains unknown. Moreover, apart from physiologic studies suggesting a potential benefit of recruitment maneuver in terms of recruitment and gas exchange, no data are yet available that demonstrate the ability of such a maneuver to improve outcome. In this article, we discuss the physiologic rules governing recruitment and derecruitment and review articles that provide new insights in the field of recruitment maneuver.

Effects of respiratory rate, plateau pressure, and positive end-expiratory pressure on PaO2 oscillations after saline lavage

One of the proposed mechanisms of ventilator-associated lung injury is cyclic recruitment of atelectasis. Collapse of dependent lung regions with every breath should lead to large oscillations in PaO2 as shunt varies throughout the respiratory cycle. We placed a fluorescence-quenching PO2 probe in the brachiocephalic artery of six anesthetized rabbits after saline lavage. Using pressure-controlled ventilation with oxygen, ventilator settings were varied in random order over three levels of positive end-expiratory pressure (PEEP), respiratory rate (RR), and plateau pressure minus PEEP (Delta). Dependence of the amplitude of PaO2 oscillations on PEEP, RR, and Delta was modeled by multiple linear regression. Before lavage, arterial PO2 oscillations varied from 3 to 22 mm Hg. After lavage, arterial PO2 oscillations varied from 5 to 439 mm Hg. Response surfaces showed markedly nonlinear dependence of amplitude on PEEP, RR, and Delta. The large PaO2 oscillations observed provide evidence for cyclic recruitment in this model of lung injury. The important effect of RR on the magnitude of PaO2 oscillations suggests that the static behavior of atelectasis cannot be accurately extrapolated to predict dynamic behavior at realistic breathing frequencies.

Tidal volume increases do not affect alveolar mechanics in normal lung but cause alveolar overdistension and exacerbate alveolar instability after surfactant deactivation

OBJECTIVE

We utilized microscopy to measure the impact of increasing tidal volume on individual alveolar mechanics (i.e., the dynamic change in alveolar size during tidal ventilation) in the living porcine lung.

DESIGN

In three anesthetized, mechanically ventilated pigs, we observed normal alveoli (n = 27) and alveoli after surfactant deactivation by Tween 20 lavage (n = 26) at three different tidal volumes (6, 12, and 15 mL/kg). Alveolar area was measured at peak inspiration (I) and at end expiration (E) by image analysis and I minus E was calculated as an index of alveolar stability (I-Edelta).

MEASUREMENTS AND MAIN RESULTS

In normal alveoli, increasing tidal volume did not change alveolar area at I (6 mL/kg = 9726 +/- 848 microm; 15 mL/kg = 9,637 +/- 884 microm ), E (6 mL/kg = 9747 +/- 800 microm; 15 mL/kg = 9742 +/- 853 microm ), or I-Edelta (6 mL/kg = -21 +/- 240 microm; 15 mL/kg = -105 +/- 229 microm ). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at I (6 mL/kg = 11,413 +/- 1032 microm; 15 mL/kg = 13,917 +/- 1214 microm ), at E (6 mL/kg = 10,462 +/- 906 microm; 15 mL/kg = 12,000 +/- 1066 microm ), and I-Edelta (6 mL/kg = 825 +/- 276 microm; 15 mL/kg = 1917 +/- 363 microm ). Moreover, alveolar instability (increased I-Edelta) was significantly increased at all tidal volumes with altered surface tension when compared with normal alveoli.

CONCLUSIONS

We conclude that high tidal volume ventilation does not alter alveolar mechanics in the normal lung; however, in the surfactant-deactivated lung, it causes alveolar overdistension and exacerbates alveolar instability.

Effect of ventilator-induced lung injury on the development of reperfusion injury in a rat lung transplant model

OBJECTIVE

Although mechanical ventilation can potentially worsen preexisting lung injury, its importance in the setting of lung transplantation has not been explored. This study was undertaken to examine the effect of 2 ventilatory strategies on the development of ischemia-reperfusion injury after lung transplantation.

METHODS

In a rat lung transplant model animals were randomized into 2 groups defined by the ventilatory strategy during the early reperfusion period. In conventional mechanical ventilation the transplanted lung was ventilated with a tidal volume equal to 50% of the inspiratory capacity of the left lung and a low positive end-expiratory pressure. In minimal mechanical stress ventilation the transplanted lung was ventilated with a tidal volume equal to 20% of the inspiratory capacity of the left lung, and positive end-expiratory pressure was adjusted according to the shape of the pressure-time curve to minimize pulmonary stress.

RESULTS

After 3 hours of reperfusion, oxygenation from the transplanted lung was significantly higher with minimal mechanical stress ventilation than with conventional ventilation. In addition, elastance, cytokine levels, and morphologic signs of injury were significantly lower in the group with minimal mechanical stress ventilation.

CONCLUSIONS

This study demonstrates that the mode of mechanical ventilation used in the early phase of reperfusion of the transplanted lung can influence ischemia-reperfusion injury, and a protective ventilatory strategy on the basis of minimizing pulmonary mechanical stress can lead to improved lung function after lung transplantation.

Time dependence of recruitment and derecruitment in the lung: a theoretical model

Recruitment and derecruitment (R/D) of air spaces within the lung is greatly enhanced in lung injury and is thought to be responsible for exacerbating injury during mechanical ventilation. There is evidence to suggest that R/D is a time-dependent phenomenon. We have developed a computer model of the lung consisting of a parallel arrangement of airways and alveolar units. Each airway has a critical pressure (Pcrit) above which it tends to open and below which it tends to close but at a rate determined by how far pressure is from Pcrit. With an appropriate distribution of Pcrit and R/D velocity characteristics, the model able to produce realistic first and second pressure-volume curves of a lung inflated from an initially degassed state. The model also predicts that lung elastance will increase transiently after a deep inflation to a degree that increases as lung volume decreases and as the lung becomes injured. We conclude that our model captures the time-dependent mechanical behavior of the lung due to gradual R/D of lung units.

Effects of lung recruitment maneuver and positive end-expiratory pressure on lung volume, respiratory mechanics and alveolar gas mixing in patients ventilated after cardiac surgery

BACKGROUND

It is unclear whether positive end-expiratory pressure (PEEP) is needed to maintain the improved oxygenation and lung volume achieved after a lung recruitment maneuver in patients ventilated after cardiac surgery performed in the cardiopulmonary bypass (CPB).

METHODS

A prospective, randomized, controlled study in a university hospital intensive care unit. Sixteen patients who had undergone cardiac surgery in CPB were studied during the recovery phase while still being mechanically ventilated with an inspired fraction of oxygen (FiO2) 1.0. Eight patients were randomized to lung recruitment (two 20-s inflations to 45 cmH2O), after which PEEP was set and kept for 2.5 h at 1 cmH2O above the pressure at the lower inflexion point (14+/-3 cmH2O, mean +/-SD) obtained from a static pressure-volume (PV) curve (PEEP group). The remaining eight patients were randomized to a recruitment maneuver only (ZEEP group). End-expiratory lung volume (EELV), series dead space, ventilation homogeneity, hemodynamics and PaO2 (oxygenation) were measured every 30 min during a 3-h period. PV curves were obtained at baseline, after 2.5 h, and in the PEEP group at 3 h.

RESULTS

In the ZEEP group all measures were unchanged. In the PEEP group the EELV increased with 1220+/-254 ml (P<0.001) and PaO2 with 16+/-16 kPa (P<0.05) after lung recruitment. When PEEP was discontinued EELV decreased but PaO2 was maintained. The PV curve at 2.5 h coincided with the curve obtained at 3 h, and both curves were both steeper than and located above the baseline curve.

CONCLUSIONS

Positive end-expiratory pressure is required after a lung recruitment maneuver in patients ventilated with high FiO2 after cardiac surgery to maintain lung volumes and the improved oxygenation.

Ventilation with negative airway pressure induces a cytokine response in isolated mouse lung

We tested the hypothesis that, under relatively low tidal volume (VT) mechanical ventilation, continuing lung decruitment induced by negative end-expiratory pressure (NEEP) would increase the lung cytokine response, potentially contributing to lung injury. Mouse lungs were excised and randomly assigned to one of 3 different ventilatory strategies: 1) the zero end-expiratory pressure group served as a control, 2) the NEEP7 group received a NEEP of -7.5 cm H(2)O, and 3) the NEEP15 group received a NEEP of -15 cm H(2)O. In all 3 groups, a VT of 7 mL/kg was used. After 2 h of ventilation, lung lavage fluid was collected for measurements of tumor necrosis factor-alpha, monocyte chemoattractant protein-1, and lactate dehydrogenase. Increases in plateau pressure before and after mechanical ventilation were significantly greater in the NEEP15 group compared with the zero end-expiratory pressure group or NEEP7 group. Lung compliance was decreased in the NEEP15 compared with the other two groups. Concentrations of tumor necrosis factor-alpha, monocyte chemoattractant protein-1, and lactate dehydrogenase in lung lavage were larger in the NEEP15 group than in the other groups. Atelectatic lung during repeated collapse and reopening of lung units accentuates the lung cytokine response that may contribute to lung injury even during relatively low VT mechanical ventilation.

IMPLICATIONS

Repeated closing and reopening of lung units induced by negative end-expiratory pressure resulted in lung inflammation and cell injury even under mechanical ventilation using a normal tidal volume. This finding may have clinical relevance in certain patients who are prone to atelectasis during mechanical ventilation.

Improved arterial oxygenation with biologically variable or fractal ventilation using low tidal volumes in a porcine model of acute respiratory distress syndrome

We compared biologically variable ventilation (V (bv); n = 9) with control mode ventilation (V (c); n = 8) at low tidal volume (VT)--initial 6 ml/kg--in a porcine model of acute respiratory distress syndrome (ARDS). Hemodynamics, respiratory gases, airway pressures, and VT data were measured. Static P-V curves were generated at 5 h. Interleukin (IL)-8 and IL-10 were measured in serum and tracheal aspirate. By 5 h, higher Pa(O(2)) (173 +/- 30 mm Hg versus 119 +/- 23 mm Hg; mean +/- SD; p < 0.0001 group x time interaction [G x T]), lower shunt fraction (6 +/- 1% versus 9 +/- 3%; p = 0.0026, G x T) at lower peak airway pressure (21 +/- 2 versus 24 +/- 1 cm H(2)O; p = 0.0342; G x T) occurred with V (bv). IL-8 concentrations in tracheal aspirate and wet:dry weight ratios were inversely related; p = 0.011. With V (c), IL-8 concentrations were 3.75-fold greater at wet:dry weight ratio of 10. IL-10 concentrations did not differ between groups. In both groups, ventilation was on the linear portion of the P-V curve. With V (bv), VT variability demonstrated an inverse power law indicating fractal behavior. In this model of ARDS, V (bv) improved Pa(O(2)) at lower peak airway pressure and IL-8 levels compared with V (c).

Reduced tidal volumes and lung protective ventilatory strategies: where do we go from here?

Three major determinants of lung injury associated with mechanical ventilation have been clearly identified: high pressure/high volume, the shear forces caused by intratidal collapse and decollapse leading to barotrauma/volotrauma/biotrauma. The lung protective strategy aims to reduce the impact of all three determinants. A groundbreaking study showed that reduced tidal volume is less dangerous than high tidal volume, but the researchers did not apply "full" lung protective strategy and did not take into account the shear forces. "Full" protective lung strategy was tested in only one study and in a limited number of patients. Several physiologic studies strongly suggest the advantages of the lung protective strategy.

Reinterpreting the pressure-volume curve in patients with acute respiratory distress syndrome

New evidence requires a reinterpretation of the inflation pressure-volume curve and suggests that neither the lower nor the upper inflection point provides reliable information to determine safe ventilator settings in the acute respiratory distress syndrome. Recruitment probably continues throughout the inflation pressure-volume curve, and studies of the deflation pressure-volume curve, reinflations after partial deflation, or decremental positive end-expiratory pressure trials after a recruitment maneuver are probably needed to determine open-lung positive end-expiratory pressure.

Gas exchange in the ventilated patient

Increased knowledge of the pathophysiologic mechanisms of impaired gas exchange during acute respiratory failure during recent years has stimulated many studies that evaluate different treatments to improve oxygenation and outcome. Changes in body position (mainly prone positioning) can significantly improve gas exchange in patients with acute respiratory distress syndrome and acute lung failure, with few complications related to the maneuver; however, no survival advantage has yet been detected. A correlation between aerated lung tissue and oxygenation also confirms the importance of recruitment maneuvers in improving gas exchange. Recent suggestions that recruitment of alveoli proceeds during most of the inspired vital capacity and not only around the lower inflection point of the pressure-volume curve raises the question how to best perform recruitment maneuvers. New data support the hypothesis that maintenance of even small amount of spontaneous breathing during mechanical ventilation (with airway pressure release ventilation or biphasic positive airway pressure) can improve gas exchange, whereas other unconventional ventilatory modes have not yet proved advantageous. Some mechanisms responsible for the high percentage of nonresponse to inhaled nitric oxide have recently been proposed, and combinations of inhaled nitric oxide with other therapies have been tested. Increased knowledge in this area may, in the future, make inhaled nitric oxide more attractive in the treatment of adult respiratory failure as well as in neonatal intensive care.

Optimal peep in ARDS. Changing concepts and current controversies

From many recently performed studies, it is clear that a criterion standard for determining the optimal positive end-expiratory pressure (PEEP) level in patients with acquired respiratory distress syndrome (ARDS) does not exist. What is evident and consistent, however, are several points such the optimal PEEP level ultimately represents a balance between regional areas of overstretching and regional derecruitment; higher levels of PEEP may be required early in ARDS, independent of oxygenation requirements; and the exact method for titrating PEEP in patients with ARDS remains to be determined. These points and others are delineated and discussed in this article.

Recruitment and derecruitment during acute respiratory failure: a clinical study

In a model of acute lung injury, we showed that positive end-expiratory pressure (PEEP) and tidal volume (VT) are interactive variables that determine the extent of lung recruitment, that recruitment occurs across the entire range of total lung capacity, and that superimposed pressure is a key determinant of lung collapse. Aiming to verify if the same rules apply in a clinical setting, we randomly ventilated five ALI/ARDS patients with 10, 15, 20, 30, 35, and 45 cm H2O plateau pressure and 5, 10, 15, and 20 cm H2O of PEEP. For each PEEP-VT condition, we obtained computed tomography at end inspiration and end expiration. We found that recruitment occurred along the entire volume-pressure curve, independent of lower and upper inflection points, and that estimated threshold opening pressures were normally distributed (mode = 20 cm H2O). Recruitment occurred progressively from nondependent to dependent lung regions. Overstretching was not associated with hyperinflation. Derecruitment did not parallel deflation, and estimated threshold closing pressures were normally distributed (mode = 5 cm H2O). End-inspiratory and end-expiratory collapse were correlated, suggesting a plateau-PEEP interaction. When superimposed gravitational pressure exceeded PEEP, end-expiratory collapse increased. We concluded that the rules governing recruitment and derecruitment equally apply in an oleic acid model and in human ALI/ARDS.