Sigh in acute respiratory distress syndrome

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.

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.

Adjusting positive end-expiratory pressure and tidal volume in acute respiratory distress syndrome according to the pressure-volume curve

BACKGROUND

Management of acute respiratory distress syndrome (ARDS) patients implies the selection of the adequate ventilatory parameters, essentially PEEP and tidal volume (Vt), to prevent ventilator-induced lung injury. These parameters should be reset as the lung injury evolves. Among the different methods proposed for the adjustment of the ventilator, the measurement of the P-V curve has emerged as a useful, although debated, tool. Our aim has been to study the relationship between the different inflection points of the P-V curve in ARDS patients, and to assess the changes in the empiric PEEP and Vt (PEEP(emp), V(temp) following its use.

METHODS

P-V curves were measured in 27 patients (lung injury score [LIS] >or= 2, 69 measurements) by means of the low-flow continuous inflation method.

RESULTS

A lower inflection point (LIP) was found in all patients and, although it correlated with the PEEP(emp), there was only a fair concordance, so the PEEP was modified in 80% of the cases. The expiratory inflection point (EIP) was significantly lower than the LIP (6.3 +/- 1.7 vs. 8.1 +/- 3.2, P = 0.008). An upper inflection point was observed in 16 measurements (23%) and the Vt was reset in 20% of the cases. Both PEEP and Vt were readjusted on 10 occasions (14%). Only the EIP was significantly higher on the first 3 days of mechanical ventilation. The LIS was correlated with all the inflection points. There were no differences for any parameter independent of the cause of the ARDS (pulmonary/extrapulmonary).

CONCLUSIONS

The quasi-static measurement of the P-V curve is a simple method, easy to interpret, for objective adjustment of the ventilatory parameters in ARDS patients as the lung injury evolves. The implementation of this strategy may vary the empiric clinical practice. The role of the EIP for the evaluation of the severity of lung injury deserves further investigation.

Atelectasis formation during anesthesia: causes and measures to prevent it

Pulmonary gas exchange is regularly impaired during general anaesthesia with mechanical ventilation. This results in decreased oxygenation of blood. A major cause is collapse of lung tissue (atelectasis), which can be demonstrated by computed tomography but not by conventional chest x-ray. Collapsed lung tissue is present in 90% of all subjects, both during spontaneous breathing and after muscle paralysis, and whether intravenous or inhalational anaesthetics are used. There is a correlation between the amount of atelectasis and pulmonary shunt. Shunt does not increase with age. In obese patients, larger atelectatic areas are present than in lean ones. Finally, patients with chronic obstructive lung disease may show less or even no atelectasis. There are different procedures that can be used in order to prevent atelectasis or to reopen collapsed lung tissue. The application of positive end-expiratory pressure (PEEP) has been tested in several studies. On the average, arterial oxygenation does not improve markedly, and atelectasis may persist. Further, reopened lung units re-collapse rapidly after discontinuation of PEEP. Inflation of the lungs to an airway pressure of 40 cm H2O, maintained for 7-8 seconds (recruitment or "vital capacity" manoeuvre), re-expands all previously collapsed lung tissue. During induction of anaesthesia, the use of a gas mixture, that includes a poorly absorbed gas such as nitrogen, may prevent the early formation of atelectasis. During ongoing anaesthesia, pulmonary collapse reappears slowly if a low fraction of oxygen in nitrogen is used for the ventilation of the lungs after a previous VC-manoeuvre. On the other hand, ventilation of the lungs with pure oxygen results in a rapid reappearance of atelectasis. Thus, ventilation during anaesthesia should be done if possible with a moderate fraction of inspired oxygen (FIO2, e.g. 0.3-0.4). Alternatively, if the lungs are ventilated with a high inspiratory fraction of oxygen, the use of PEEP may be considered. In summary, atelectasis is present in most humans during anaesthesia and is a major cause of impaired oxygenation. Avoiding high fractions of oxygen in inspired gas during induction and maintenance of anaesthesia may prevent formation of atelectasis. Finally, intermittent "vital capacity"-manoeuvres together with PEEP reduces the amount of atelectasis and pulmonary shunt.

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.

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.

Lung computed tomography during a lung recruitment maneuver in patients with acute lung injury

OBJECTIVE

To assess the acute effect of a lung recruitment maneuver (LRM) on lung morphology in patients with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS).

PATIENTS

Ten patients with ALI/ARDS on mechanical ventilation.

DESIGN

Prospective clinical study.

SETTING

Computed tomography (CT) scan facility in a teaching hospital.

INTERVENTIONS

An LRM performed by stepwise increases in positive end-expiratory pressure (PEEP) of up to 30-40 cm H(2)O. Lung basal CT sections were taken at end-expiration (patients 1 to 5), and at end-expiration and end-inspiration (patients 6 to 10). Arterial blood gases and static compliance (C(st)) were measured before, during and after the LRM.

MEASUREMENTS AND MAIN RESULTS

Poorly aerated and non-aerated tissue at PEEP 10 cm H(2)O accounted for 60.0+/-29.1% of lung parenchyma, while only 1.1+/-1.8% was hyperinflated. Increasing PEEP to 20 and 30 cm H(2)O, compared to PEEP 10 cm H(2)O, decreased poorly aerated and non-aerated tissue by 16.2+/-28.0% and 33.4+/-13.8%, respectively ( p<0.05). This was associated with an increase in PaO(2) and a decrease in total static compliance. Inspiration increased alveolar recruitment at all PEEP levels. Hyperinflated tissue increased up to 2.9+/-4.0% with PEEP 30 cm H(2)O, and to a lesser degree with inspiration. No barotrauma or severe hypotension occurred.

CONCLUSIONS

Lung recruitment maneuvers improve oxygenation by expanding collapsed alveoli without inducing too much hyperinflation in ALI/ARDS patients. An LRM during the CT scan gives morphologic and functional information that could be useful in setting ventilatory parameters.

Lung recruitment manoeuvres are effective in regaining lung volume and oxygenation after open endotracheal suctioning in acute respiratory distress syndrome

INTRODUCTION

Lung collapse is a contributory factor in the hypoxaemia that is observed after open endotracheal suctioning (ETS) in patients with acute lung injury and acute respiratory distress syndrome. Lung recruitment (LR) manoeuvres may be effective in rapidly regaining lung volume and improving oxygenation after ETS.

MATERIALS AND METHOD

A prospective, randomized, controlled study was conducted in a 15-bed general intensive care unit at a university hospital. Eight consecutive mechanically ventilated patients with acute lung injury or acute respiratory distress syndrome were included. One of two suctioning procedures was performed in each patient. In the first procedure, ETS was performed followed by LR manoeuvre and reconnection to the ventilator with positive end-expiratory pressure set at 1 cmH2O above the lower inflexion point, and after 60 min another ETS (but without LR manoeuvre) was performed followed by reconnection to the ventilator with similar positive end-expiratory pressure; the second procedure was the same as the first but conducted in reverse order. Before (baseline) and over 25 min following each ETS procedure, partial arterial oxygen tension (PaO2) and end-expiratory lung volume were measured.

RESULTS

After ETS, PaO2 decreased by 4.3(0.9-9.7)kPa (median and range; P < 0.005). After LR manoeuvre, PaO2 recovered to baseline. Without LR manoeuvre, PaO2 was reduced (P = 0.05) until 7 min after ETS. With LR manoeuvre end-expiratory lung volume was unchanged after ETS, whereas without LR manoeuvre end-expiratory lung volume was still reduced (approximately 10%) at 5 and 15 min after ETS (P = 0.01).

DISCUSSION

A LR manoeuvre immediately following ETS was, as an adjunct to positive end-expiratory pressure, effective in rapidly counteracting the deterioration in PaO2 and lung volume caused by open ETS in ventilator-treated patients with acute lung injury or acute respiratory distress syndrome.

New modes of mechanical ventilation: proportional assist ventilation, neurally adjusted ventilatory assist, and fractal ventilation

Increased knowledge of the mechanisms that determine respiratory failure has led to the development of new technologies aimed at improving ventilatory treatment. Proportional assist ventilation and neurally adjusted ventilatory assist have been designed with the goal of improving patient-ventilator interaction by matching the ventilator support with the neural output of the respiratory centers. With proportional assist ventilation, the support is continuously readjusted in proportion to the predicted inspiratory effort. Neurally adjusted ventilatory assist is an experimental mode in which the assistance is delivered in proportion to the electrical activity of the diaphragm, assessed by means of an esophageal electrode. Biologically variable (or fractal) ventilation is a new, volume-targeted, controlled ventilation mode aimed at improving oxygenation; it incorporates the breath-to-breath variability that characterizes a natural breathing pattern.

Influence of low tidal volumes on gas exchange in acute respiratory distress syndrome and the role of recruitment maneuvers

BACKGROUND Use of a low tidal volume (V(T)) strategy in the treatment of acute respiratory distress syndrome can lead to a decrease in oxygenation. This study evaluated the safety and efficacy of a recruitment maneuver (RM) in this setting.

METHODS

Twelve patients with acute respiratory distress syndrome were studied within 48 hours of diagnosis. Baseline gas exchange, hemodynamics, and respiratory mechanics were determined and patients were placed on a V(T) of 6 mL/kg. Measurements were repeated and an RM of 30 cm H20 for 40 seconds was performed. Measurements were repeated at 30 minutes and 2 hours post-RM.

RESULTS

Decreasing V(T) resulted in a decrease in arterial oxygenation (from 91 +/- 9 mm Hg to 75 +/- 9 mm Hg, p < 0.01), an increase in shunt (from 19 +/- 3.7% to 23 +/- 5%, p < 0.01), and a decrease in lung compliance (from 37 mL/cm H2O to 33 mL/cm H2O, < 0.05). At 30 minutes post-RM, oxygenation improved to 99 +/- 16 mm Hg, shunt decreased to 17 +/- 3%, and lung compliance improved to 39 mL/cm H2O (p < 0.05). Two hours later, oxygenation fell (86 +/- 12 mm Hg), shunt increased (20 +/- 3%), and compliance diminished (36 mL/cm H2O). There were no hemodynamic or barotraumatic complications.

CONCLUSION

An RM transiently improves gas exchange during low V(T)ventilation. RMs are well tolerated and no hemodynamic consequences were seen.

Recruitment and retention of lung volume

Both a reduction in tidal volume and alveolar recruitment may be necessary to prevent ventilator-induced lung injury in the management of patients with acute respiratory distress syndrome. The lung collapse associated with endotracheal suctioning produces hypoxaemia, but it also causes de-recruitment, potentially aggravating lung injury. A study conducted by Dyhr and colleagues demonstrates the benefit of lung recruitment manoeuvres after suctioning, which help to improve oxygenation and restore lung volume more rapidly. Although this intervention appears safe and beneficial, the precise role of lung volume recruitment manoeuvres remains to be elucidated.

A comparison of the effects of manual and ventilator hyperinflation on static lung compliance and sputum production in intubated and ventilated intensive care patients

BACKGROUND AND PURPOSE

Lung hyperinflation is a technique used by physiotherapists to mobilize and remove excess bronchial secretions, reinflate areas of pulmonary collapse and improve oxygenation. Hyperinflation may be delivered by the ventilator or manually, by use of a manual resuscitation circuit, depending upon the respiratory and cardiovascular status of the patient. The effects of manual hyperinflation, with respect to excess bronchial secretions and static lung compliance, have been well-established. There is, however, only limited evidence as to the efficacy of ventilator hyperinflation as a physiotherapy treatment technique. The purpose of the present study was to compare the effects of manual hyperinflation and ventilator hyperinflation on static pulmonary compliance and sputum clearance in stable intubated and ventilated patients.

METHOD

Twenty patients who met the inclusion criteria were studied. This was a double crossover study where all patients were randomly allocated to one of two treatment sequences over two days. The first sequence involved manual hyperinflation followed two hours later by ventilator hyperinflation and the order was reversed on the second day. In the second sequence, ventilator hyperinflation preceded manual hyperinflation. The variables of static pulmonary compliance and sputum wet weight were analysed by use of an analysis of variance (ANOVA) for repeated measures. Statistical significance was set at p < 0.05.

RESULTS

There was no significant difference in sputum wet weight production between either technique or on either day of treatment. Static pulmonary compliance improved with both hyperinflation techniques (p < 0.05).

CONCLUSIONS

Hyperinflation as part of a physiotherapy treatment can be performed with equal benefit using either a manual resuscitation circuit or a ventilator. Both methods of hyperinflation improve static pulmonary compliance and clear similar volumes of pulmonary secretions.

Ventilatory care in myasthenia gravis crisis: assessing the baseline adverse event rate

OBJECTIVE

Myasthenic patients who require mechanical ventilation often develop pneumonia or atelectasis. Although there are differences in the prevalence of these complications among various institutions, there is no evidence that aggressive treatment shortens the course of the myasthenic crisis. We have quantified the severity of lung injury and aggressiveness of respiratory intervention in myasthenic patients admitted to the neuro-critical care unit.

DESIGN

We retrospectively identified all mechanically ventilated myasthenic patients admitted in our unit between 1990 and 1998.

SETTING

Neuro-critical care unit of a tertiary care center in an urban area with a large, established, regional neuromuscular disease program.

PATIENTS

Eighteen myasthenia gravis patients with 24 episodes of respiratory failure requiring mechanical ventilation.

INTERVENTIONS

A novel respiratory intervention index, comprising the use of suction, intermittent positive-pressure breathing or bronchodilator treatments, sighs, and chest physiotherapy represented the aggressiveness of the respiratory treatment. The respiratory intervention index was correlated with the lung injury score, used as a measure of lung involvement and other respiratory variables. MEASUREMENTS AND MAIN RESULTS Our patients had less atelectasis and pneumonia than previously published series (46% vs. 91%), leading to shorter mechanical support and neuro-critical care unit stay. The mean respiratory intervention index correlated with lung injury score and inversely with forced vital capacity.

CONCLUSIONS

This study presents an estimate for both severity of pulmonary complications and intensity of respiratory therapy in the severe myasthenic patient with mechanical ventilatory compromise. Our results suggest that aggressive respiratory treatment should be used in myasthenic patients in crisis to diminish the risk for prolonged respiratory complications. These observations should be validated in a prospective study.

Psychophysiologic effects of Hatha Yoga on musculoskeletal and cardiopulmonary function: a literature review

Yoga has become increasingly popular in Western cultures as a means of exercise and fitness training; however, it is still depicted as trendy as evidenced by an April 2001 Time magazine cover story on "The Power of Yoga." There is a need to have yoga better recognized by the health care community as a complement to conventional medical care. Over the last 10 years, a growing number of research studies have shown that the practice of Hatha Yoga can improve strength and flexibility, and may help control such physiological variables as blood pressure, respiration and heart rate, and metabolic rate to improve overall exercise capacity. This review presents a summary of medically substantiated information about the health benefits of yoga for healthy people and for people compromised by musculoskeletal and cardiopulmonary disease.

Transient mechanical benefits of a deep inflation in the injured mouse lung

The lasting effects of a recruitment maneuver (RM) in the injured lung are not well characterized. We speculated that the reduction in respiratory elastance (H) after a deep inflation (DI) is transient in nature and should be sustained longer at higher positive end-expiratory pressure (PEEP). Thirteen ventilated mice were given 2 DIs at various levels of PEEP before and after saline lavage. Forced oscillations were used to measure H periodically over 7 min after the DIs. Time constants (tau) were estimated for the post-DI recovery in H. Values for tau before lavage (80-115 s) were reduced after lavage (13-30 s) at all levels of PEEP (P = 0.0001). PEEP did not significantly influence tau before or after lavage. The plateau level and total recovery in H after a DI were significantly influenced by PEEP and lavage (P < 0.0001). Our results suggest that for a DI to be beneficial in the injured mouse lung, it may have to be applied several times a minute.

Effects of short-term pressure-controlled ventilation on gas exchange, airway pressures, and gas distribution in patients with acute lung injury/ARDS: comparison with volume-controlled ventilation

STUDY OBJECTIVES

The potential clinical benefits of pressure-controlled ventilation (PCV) over volume-controlled ventilation (VCV) in patients with acute lung injury (ALI) or ARDS still remain debated. We compared PCV with VCV in patients with ALI/ARDS with respect to the following physiologic end points: (1) gas exchange and airway pressures, and (2) CT scan intrapulmonary gas distribution at end-expiration.

DESIGN

Prospective, observational study.

SETTING

A multidisciplinary ICU in a nonuniversity, acute-care hospital.

PATIENTS

Ten patients with ALI or ARDS (9 men and 1 woman; age range, 17 to 80 years).

INTERVENTIONS

Sequential ventilation in PCV and VCV with a constant inspiratory/expiratory ratio, tidal volume, respiratory rate, and total positive end-expiratory pressure; measurement of gas exchange and airway pressures; and achievement of CT sections at lung base, hilum, and apex for the quantitative analysis of lung densities and of aerated vs nonaerated zones.

RESULTS

PaO(2), PaCO(2), and PaO(2)/fraction of inspired oxygen ratio levels did not differ between PCV and VCV. Peak airway pressure (Ppeak) was significantly lower in PCV compared with VCV (26 +/- 2 cm H(2)O vs 31 +/- 2 cm H(2)O; p < 0.001; mean +/- SEM). The surface areas of the nonaerated zones as well as the total areas at each section level were unchanged in PCV compared with VCV, except at the apex level, where there was a significantly greater nonaerated area in VCV (11 +/- 2 cm(2) vs 9 +/- 2 cm(2); p < 0.05). The total mean CT number of each lung (20 lungs from 10 patients) was similar in the two modes, as were the density values at the basal and apical levels; the hilum mean CT number was - 442 +/- 28 Hounsfield units (HU) in VCV and - 430 +/- 26 HU in PCV (p < 0.005).

CONCLUSIONS

These data show that PCV allows the generation of lower Ppeaks through the precise titration of the lung distending pressure, and might be applied to avoid regional overdistension by means of a more homogeneous gas distribution.

Recruitment maneuver: does it promote bacterial translocation?

OBJECTIVE

High peak airway opening pressures (Pao) are used routinely during recruitment maneuvers to open collapsed lung units. High peak Pao, however, can cause lung injury as evidenced by translocation of intratracheally inoculated bacteria. In this study we explored whether recruitment maneuvers that used high Pao could cause translocation of the intratracheally inoculated from the alveoli into the systemic circulation.

DESIGN

Prospective, randomized, animal study.

SETTING

Experimental animal care laboratory.

SUBJECTS

Eighteen male Sprague Dawley rats. INTERVENTIONS Rats were anesthetized, tracheostomized, and ventilated with 14 cm H2O peak Pao and 0 cm H2O positive end-expiratory pressure (PEEP) in pressure-controlled ventilation (frequency, 30 bpm; inspiratory/expiratory ratio, 1:2; Fio, 1). Intratracheal inoculation of 500 microL of saline containing 1 x 10 colony forming units/mL was performed before randomization into three groups (n = 6 in each): a low-pressure group (14 cm H2O peak Pao, 0 cm H2O PEEP), a high-pressure group (45 cm H2O peak Pao, 0 cm H2O PEEP), and a recruitment maneuver group (14 cm H2O peak Pao, 0 cm H2O PEEP, and a recruitment maneuver sustained inflation of 45 cm H2O continuous positive airway pressure for 30 secs every 15 mins). Blood samples for blood gas analysis were obtained before intratracheal instillation of bacteria and at the end of the experimental protocol (2 hrs). Blood cultures were obtained before and after bacterial instillation at 30-min intervals during the experiment. Blood samples were cultured directly in sheep blood, MacConkey, and Iso-Sensitest agars and were observed on the second day. Bacteremia was defined as the presence of one or more colonies of in 1 mL of blood.

MEASUREMENTS AND MAIN RESULTS

The blood cultures were positive for in only six rats in the high-pressure group and remained negative throughout the study period in the low-pressure and recruitment maneuver groups. Oxygenation deteriorated in all groups after intratracheal instillation of bacteria. In the high-pressure group, oxygenation decreased from 417 +/- 67 mm Hg to 79 +/- 20 mm Hg ( p=.004), whereas in the low-pressure and recruitment maneuver groups PaO2 decreased from 410 +/- 98 mm Hg and 383 +/- 78 mm Hg to 287 +/- 105 mm Hg ( p=.031) and 249 +/- 59 mm Hg (p =.11), respectively.

CONCLUSION

Intermittent recruitment maneuvers applied as a sustained inflation superimposed on low-pressure ventilation with 0 cm H2O PEEP did not cause translocation of intratracheally inoculated.

The pulmonary physician in critical care. 8: Ventilatory management of ALI/ARDS

Current data relating to ventilation in ARDS are reviewed. Recent studies suggest that reduced mortality may be achieved by using a strategy which aims at preventing overdistension of lungs.

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.

Neonatal respiratory failure

The classic entity of neonatal distress syndrome, as a lung disease expressing predominant surfactant deficiency, is currently changing to a more complex disease of the developing lung as a result of the number of extremely immature preterm infants. Prenatal factors, such as the fetal inflammatory response syndrome influence short- and long-term outcome in these premature infants presenting with respiratory distress syndrome at birth. Therefore, various previously dismissed treatment strategies, such as surfactant prophylaxis or newer anti-inflammatory approaches have to be reinvestigated in this emerging population. Despite the resurrection of a new picture of bronchopulmonary dysplasia, lung injury induced by mechanical ventilation remains a major issue in neonatal intensive care. With the advance in understanding of mechanical ventilation, it is becoming evident, that improvement in outcome can not be achieved by restoring normal lung physiology in the diseased lung using sophisticated ventilators and ventilation modes. A more disease specific ventilator strategy that will target as early as possible homogenous lung opening while at the same time avoiding overdistention of the lung, has the potential to affect outcome. The possible antiinflammatory properties of surfactant-proteins, nitric oxide and corticosteroids, despite some drawbacks, may show to have a synergistic effect. However, this needs further exploration.

Mechanical properties of the lung and upper airways in patients with sleep-disordered breathing

We studied the changes in lung and upper airway mechanics in adult human subjects with obstructive sleep apnea/hypopnea syndrome (OSAHS) during wakefulness, sleep, and at arousal from sleep. We used two numerical methods that we have previously developed specifically for dealing with inspiratory flow limitation during sleep: the modified Mead-Whittenberger method, and information-weighted histograms obtained using recursive least squares. Full polysomnography including esophageal pressure and airflow measurements was performed in seven men with OSAHS (respiratory disturbance index: 55.8 +/- 23.2 events/h). Pharyngeal pressure was recorded in four of the subjects to partition lung mechanics into its upper airway and lower lung components. Both techniques showed that total lung resistance and elastance increased significantly (p < 0.05) during obstructed breathing and that this increase was reversed at the end of the obstruction. The partitioning of mechanics showed that upper airway collapse was primarily responsible for the increase in lung resistance. Our results suggest that OSAHS may lead to transient abnormalities in the recruitment of lung units and the gas exchanging capacity of the lungs.

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.

Variable tidal volume ventilation improves lung mechanics and gas exchange in a rodent model of acute lung injury

Random variations in breath rate and tidal volume during mechanical ventilation in the setting of acute lung injury have been shown to improve arterial oxygen tension. To test whether this improvement occurs over a specific range of variability, we examined several ventilation protocols in guinea pigs with endotoxin-induced lung injury. In Group I (n = 10), after 30 min of conventional volume-cycled ventilation, animals were ventilated with variable ventilation for 30-min intervals, during which time tidal volume was randomly varied by 10, 20, 40, and 60% of the mean, while simultaneously adjusting the frequency to maintain constant minute ventilation. In a second group of animals (Group II, n = 4), conventional volume-cycled ventilation was administered for 3 h. Variable ventilation significantly improved lung function over conventional volume-cycled ventilation. In Group I, lung elastance decreased, and blood oxygenation increased significantly during periods of 40 and 60% variable ventilation (p < 0.05) compared with conventional ventilation. These data indicate that variable ventilation is effective in improving lung function and gas exchange during acute lung injury.

Pulmonary versus extrapulmonary acute respiratory distress syndrome: different diseases or just a useful concept?

The acute respiratory distress syndrome may complicate both pulmonary and extrapulmonary conditions. There is a growing belief that the predisposition to, and clinical course of, the syndrome may be influenced by the extent to which the lung is directly involved in the precipitating pathologic changes. Several studies have highlighted differences in morphology and respiratory physiology between the two subgroups in the early stages of acute respiratory distress syndrome. Further, preliminary reports have suggested that the effects of therapeutic interventions such as alterations in positive end-expiratory pressure, prone ventilation, and the use of inhaled vasoactive agents may differ between pulmonary and extrapulmonary acute respiratory distress syndrome. There are, however, inconsistencies between various studies addressing these issues, which may relate in part to differences in etiologic case mix. There are also practical difficulties in assigning certain cases to one of these two groups. Finally, there are as yet no outcome data to support any modification of clinical management on the basis of this distinction.

Recruitment maneuvers during lung protective ventilation in acute respiratory distress syndrome

The objective was to analyze the physiologic effects of recruitment maneuvers (RM) in 17 patients with acute respiratory distress syndrome (ARDS) ventilated with a lung protective strategy. RM consisted of 2 min of pressure-controlled ventilation at a peak pressure of 50 cm H(2)O and a positive end-expiratory pressure (PEEP) above the upper inflection point of the respiratory pressure-volume curve obtained at zero PEEP. In eight patients, RM were repeated in the late phase of ARDS. Oxygenation did not change 15 min after RM in the early and late phase of ARDS. When Pa(O(2))/fraction of inspired oxygen (FI(O(2))) increased during RM, venous admixture (Q VA/Q T) decreased. The opposite occurred in patients in whom Pa(O(2))/FI(O(2)) decreased during RM. RM-induced changes in cardiac output were not observed. A significant correlation was found between RM-induced changes in Pa(O(2))/FI(O(2)) during the RM and changes in respiratory system compliance at 15 min (r = 0.66, p < 0.01) and RM-induced changes in Q VA/Q T (r = -0.85; p < 0.01). The correlation between RM-induced changes in Pa(O(2))/FI(O(2)) in responders (improvement in Pa(O(2))/FI(O(2)) of greater than 20% during the RM) and the inspired oxygen fraction was also significant. In ARDS patients ventilated with a lung protective strategy we conclude that RM have no short-term benefit on oxygenation, and regional alveolar overdistension capable of redistributing blood flow can occur during RM.

Inspiratory pressure-volume curves at different positive end-expiratory pressure levels in patients with ALI/ARDS

BACKGROUND

In lung protective strategy, positive end-expiratory pressure (PEEP) slightly higher than the Pflex (the airway pressure corresponding to the lower inflection point (LIP) on the inspiratory pressure-volume (P-V) curve measured with ZEEP) is generally recommended. However, this method to determine optimal PEEP lacks a theoretical background and there is no clinical report that investigated how the P-V relationship would be with such PEEP. Therefore, we measured inspiratory P-V curves at different PEEP levels to increase our knowledge about the inspiratory P-V curve with PEEP.

METHODS

In eight consecutive patients with ALI/ARDS, inspiratory P-V curves were repeatedly measured at different PEEP levels by low flow inflation technique and LIP was assessed in all inspiratory P-V curves. Afterwards, the minimum PEEP level at which LIP was not identifiable (PEEP(LIP)(-)) was determined and the relationship between Pflex and PEEP(LIP)(-) was investigated.

RESULTS

Pflex and PEEP(LIP)(-) could be determined in all patients. Pflex was 9.4+/-2.0 cmH2O (range: 7 to 12 cmH2O) and PEEP(LIP)(-) was 7.9+/-1.6 cmH2O (range: 5 to 10 cmH2O) (mean+/-SD, P=0.0877). PEEP(LIP)(-) was lower than the Pflex in five patients, and significantly lower than the Pflex + 2 cmH2O (P=0.0024).

CONCLUSION

From the analysis of inspiratory P-V curves at different PEEP levels, PEEP 2 cmH2O higher than the Pflex may not be necessary to prevent cyclic collapse and reopening of alveoli, at least in some ALI/ARDS patients. Further studies are needed to confirm this preliminary result.

Strategies to optimize alveolar recruitment

Low tidal volume (4-8 mL/kg) during mechanical ventilation in adult respiratory distress syndrome is the standard of care. However, there are questions regarding the approach to setting positive end-expiratory pressure and the use of recruitment maneuvers in patients with adult respiratory distress syndrome. Animal data and preliminary patient data suggest that lung recruitment maneuvers can markedly improve arterial oxygen tension (PaO2). Prone positioning has also become established a method of recruiting lung and improving PaO2 in those with adult respiratory distress syndrome. The data suggest that recruitment maneuvers in the prone position are most effective in improving PaO2 and that the positive end-expiratory pressure level required to sustain the improved PaO2 is less in the prone position than in the supine position.

Effects of different continuous positive airway pressure devices and periodic hyperinflations on respiratory function

OBJECTIVE

To compare the effect on respiratory function of different continuous positive airway pressure systems and periodic hyperinflations in patients with respiratory failure.

DESIGN

Prospective

SETTING

Hospital intensive care unit.

PATIENTS

Sixteen intubated patients (eight men and eight women, age 54 +/- 18 yrs, PaO2/FiO2 277 +/- 58 torr, positive end-expiratory pressure 6.2 +/- 2.0 cm H2O).

INTERVENTIONS

We evaluated continuous flow positive airway pressure systems with high or low flow plus a reservoir bag equipped with spring-loaded mechanical or underwater seal positive end-expiratory pressure valve and a continuous positive airway pressure by a Servo 300 C ventilator with or without periodic hyperinflations (three assisted breaths per minute with constant inspiratory pressure of 30 cm H2O over positive end-expiratory pressure).

MEASUREMENTS AND MAIN RESULTS

We measured the respiratory pattern, work of breathing, dyspnea sensation, end-expiratory lung volume, and gas exchange. We found the following: a) Work of breathing and gas exchange were comparable between continuous flow systems; b) the ventilator continuous positive airway pressure was not different compared with continuous flow systems; and c) continuous positive airway pressure with periodic hyperinflations reduced work of breathing (10.7 +/- 9.5 vs. 6.3 +/- 5.7 J/min, p <.05) and dyspnea sensation (1.6 +/- 1.2 vs. 1.1 +/- 0.8 cm, p <.05) increased end-expiratory lung volume (1.6 +/- 0.8 vs. 2.0 +/- 0.9 L, p <.05) and PaO2 (100 +/- 21 vs. 120 +/- 25 torr, p <.05) compared with ventilator continuous positive airway pressure.

CONCLUSIONS

The continuous flow positive airway pressure systems tested are equally efficient; a ventilator can provide satisfactory continuous positive airway pressure; and the use of periodic hyperinflations during continuous positive airway pressure can improve respiratory function and reduce the work of breathing.

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.

Influence of tidal volume on alveolar recruitment. Respective role of PEEP and a recruitment maneuver

Both reduction in tidal volume (VT) and alveolar recruitment may be important to limit ventilator-associated lung injury during mechanical ventilation of patients with the acute respiratory distress syndrome (ARDS). The aim of this study was to assess the risk of alveolar derecruitment associated with VT reduction from 10 to 6 ml/kg. Whether this VT-related derecruitment could be reversed, either by a recruitment maneuver or by an increase in positive end-expiratory pressure (PEEP) level, was also investigated. Fifteen patients with ARDS were successively ventilated using conventional VT (CVT = 10 +/- 1 ml/kg) and low VT (LVT = 6 +/- 1 ml/ kg); total PEEP (PEEPtot) was individually set at the lower inflection point (Plip) of the pressure-volume curve (PEEPtot = 11 +/- 4 cm H(2)O). Pressure-volume curves were recorded from zero PEEP (ZEEP) and from PEEP, and recruited volume (Vrec) was calculated as the volume difference between the two curves for a given pressure. Despite a similar PEEPtot, Vrec was significantly lower with LVT than with CVT, indicating low VT-induced alveolar derecruitment. Reduction in VT was associated with a reduced Sa(O(2)). In 10 patients, Vrec was also measured before and after a recruitment maneuver (two sustained inflations at 45 cm H(2)O), and after an increase in PEEP (by 4 cm H(2)O). Low VT-induced derecruitment was reversed by a recruitment maneuver and by increasing PEEP. We conclude that a reduction in VT could be responsible for alveolar derecruitment, which may be transiently reversed by a reexpansion maneuver or prevented by a PEEP increase above Plip.

Mechanistic scheme and effect of "extended sigh" as a recruitment maneuver in patients with acute respiratory distress syndrome: a preliminary study

OBJECTIVE

To devise a new form of sigh ("extended sigh") capable of providing a sufficient recruiting pressure x time, and to test it as a recruitment maneuver in patients with acute respiratory distress syndrome.

DESIGN

Prospective uncontrolled clinical trial.

SETTING

Medical intensive care unit of a university-affiliated hospital.

PATIENTS

Twenty consecutive patients diagnosed with acute respiratory distress syndrome (18 men, 2 women, age 59 +/- 10 yrs).

INTERVENTIONS

From baseline settings of tidal volume (Vt) 8 mL/kg and positive end-expiratory pressure (PEEP) 10 cm H2O on volume control mode with the high pressure limit at 40 cm H2O, the Vt-PEEP values were changed to 6-15, 4-20, and 2-25, each step being 30 secs (inflation phase). After Vt-PEEP 2-25, the mode was switched to continuous positive airway pressure of 30 cm H2O for a duration of 30 secs (pause), after which the baseline setting was resumed following the reverse sequence of inflation (deflation phase). This extended sigh was performed twice with 1 min of baseline ventilation between.

MEASUREMENTS AND RESULTS

Airway pressures and hemodynamic parameters were traced at each step during the extended sigh. Arterial blood gases and physiologic parameters were determined before the extended sigh (pre-extended sigh), at 5 mins after two extended sighs (post-extended sigh), and then every 15 mins for 1 hr. In our average patient, the recruiting pressure x time of the inflation phase was estimated to be 32.8-35.4 cm H2O x 90 secs. Compared with the inflation phase, inspiratory pause pressure of the deflation phase was lower at Vt-PEEP 6-15 (28.9 +/- 2.7 cm H2O vs. 27.3 +/- 2.8 cm H2O) and 4-20 (31.8 +/- 2.9 cm H2O vs. 31.1 +/- 2.9 cm H2O; both p <.05). Compared with pre-extended sigh, Pao2 (81.5 +/- 15.3 mm Hg vs. 104.8 +/- 25.0 mm Hg; p <.001) and static respiratory compliance both increased post-extended sigh (27.9 +/- 7.9 mL/cm H2O vs. 30.2 +/- 9.7 mL/cm H2O; p =.009). Improvement in these parameters was sustained above pre-extended sigh for the duration of the study. Major hemodynamic or respiratory complications were not noted during the study.

CONCLUSION

We present a new form of sigh (i.e., extended sigh) capable of achieving an augmented recruiting pressure x time through a prolonged inflation on a gradually increased end-expiratory pressure. In view of the sustained effect and absence of major complications in our patients, extended sigh could be a useful recruitment maneuver in acute respiratory distress syndrome.

Permissive hypercapnia

The term permissive hypercapnia defines a ventilatory strategy for acute respiratory failure in which the lungs are ventilated with a low inspiratory volume and pressure. The aim of permissive hypercapnia is to minimize lung damage during mechanical ventilation; its limitation is the resulting hypoventilation and carbon dioxide (CO2) retention. In this article we discuss the rationale, physiologic implications, and implementation of permissive hypercapnia. We then review recent clinical studies that tested the effect of various approaches to permissive hypercapnia on the outcome of patients with acute respiratory failure.

Compliance is nonlinear over tidal volume irrespective of positive end-expiratory pressure level in surfactant-depleted piglets

Between the lower and the upper inflection point of a quasistatic pressure-volume (PV) curve, a segment usually appears in which the PV relationship is steep and linear (i.e., compliance is high, with maximal volume change per pressure change, and is constant). Traditionally it is assumed that when positive end-expiratory pressure (PEEP) and tidal volume (V T) are titrated such that the end-inspiratory volume is positioned at this linear segment of the PV curve, compliance is constant over VT during ongoing ventilation. The validity of this assumption was addressed in this study. In 14 surfactant-deficient piglets, PEEP was increased from 3 cm H(2)O to 24 cm H(2)O, and the compliance associated with 10 consecutive volume increments up to full VT was determined with a modified multiple-occlusion method at the different PEEP levels. With PEEP at approximately the lower inflection point, compliance was minimal in most lungs and decreased markedly over VT, indicating overdistension. Compliance both increased and decreased within the same breath at intermediate PEEP levels. It is concluded that a PEEP that results in constant compliance over the full VT range is difficult to find, and cannot be derived from conventional respiratory-mechanical analyses; nor does this PEEP level coincide with maximal gas exchange.