呼气末正压对急性呼吸窘迫综合征肺复张容积及氧合影响的临床研究

目的　评价呼气末正压 (PEEP)对急性呼吸窘迫综合征 (ARDS)肺复张容积的影响 ,探讨ARDS患者 PEEP的选择方法。方法　以 11例血流动力学稳定、接受机械通气的 ARDS患者为研究对象 ,采用压力容积曲线法分别测定 PEEP为 5、10、15 cm H2 O(1cm H2 O=0 .0 98k Pa)时的肺复张容积 ,观察患者动脉血气、肺机械力学和血流动力学变化。结果　 PEEP分别 5、10和 15 cm H2 O时肺复张容积分别为 (4 0 .2±15 .3) ml、 (12 3.8± 4 3.1) ml和 (178.9± 4 3.5 ) m l,随着 PEEP水平的增加 ,肺复张容积亦明显增加 (P均 <0 .0 5 )。动脉氧合指数也随着 PEEP水平增加而增加 ,且其变化与肺复张容积呈正相关 (r=0 .4 83,P<0 .0 1)。不同 PEEP条件下 ,患者的肺静态顺应性无明显变化 (P>0 .0 5 )。将患者按有无低位转折点 (L IP)分为有 L IP组与无 L IP组 ,两组患者的肺复张容积都随着 PEEP水平的增加而增加 ,其中 PEEP15 cm H2 O时 L IP组患者的肺复张容积大于无 L IP组 (P<0 .0 5 )。结论　 PEEP水平越高 ,肺复张容积越大 ,肺复张容积增加与动脉氧合指数的变化呈正相关     

Respective effects of end-expiratory and end-inspiratory pressures on alveolar recruitment in acute lung injury

OBJECTIVE: A low tidal volume can induce alveolar derecruitment in patients with acute lung injury. This study was undertaken to evaluate whether this resulted mainly from the decrease in tidal volume or from the reduction in end-inspiratory plateau pressure and whether there is any benefit in raising the level of positive end-expiratory pressure (PEEP) while plateau pressure is kept constant. DESIGN: Prospective crossover study. SETTING: Medical intensive care unit of a university teaching hospital. PATIENTS: Fifteen adult patients ventilated for acute lung injury (PaO2/FiO2, 158 +/- 34 mm Hg; lung injury score, 2.7 +/- 0.6). INTERVENTIONS: Three combinations were tested: PEEP at the lower inflection point with 6 mL/kg tidal volume, PEEP at the lower inflection point with 10 mL/kg tidal volume, and high PEEP with tidal volume at 6 mL/kg, keeping the plateau pressure similar to the preceding condition. MEASUREMENTS AND MAIN RESULTS: Pressure-volume curves at zero PEEP and at set PEEP were recorded, and recruitment was calculated as the volume difference between both curves for pressures ranging from 15 to 30 cm H2O. Arterial blood gases were measured for all patients. For a similar PEEP at the lower inflection point (10 +/- 3 cm H2O), tidal volume reduction (10 to 6 mL/kg) led to a significant derecruitment. A low tidal volume (6 mL/kg) with high PEEP (14 +/- 3 cm H2O), however, induced a significantly greater recruitment and a higher Pao than the two other strategies. CONCLUSION: At a given plateau pressure (i.e., similar end-inspiratory distension), lowering tidal volume and increasing PEEP increase recruitment and PaO2.     

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 cmH₂O 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 cmH₂O above the inspiratory lower inflection point (15.5±3.1 cmH₂O) and a PEEP equal to the expiratory point of maximum curvature (23.5±4.1 cmH₂O).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 PaCO₂, 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.This work was supported by a grant from Fondo de Investigación Sanitaria (PI03/0833) and Red GIRA (G03/063)     

Dynamic versus static respiratory mechanics in acute lung injury and acute respiratory distress syndrome

OBJECTIVES: It is not clear whether the mechanical properties of the respiratory system assessed under the dynamic condition of mechanical ventilation are equivalent to those assessed under static conditions. We hypothesized that the analyses of dynamic and static respiratory mechanics provide different information in acute respiratory failure. DESIGN: Prospective multiple-center study. SETTING: Intensive care units of eight German university hospitals. PATIENTS: A total of 28 patients with acute lung injury and acute respiratory distress syndrome. INTERVENTIONS: None. MEASUREMENTS: Dynamic respiratory mechanics were determined during ongoing mechanical ventilation with an incremental positive end-expiratory pressure (PEEP) protocol with PEEP steps of 2 cm H2O every ten breaths. Static respiratory mechanics were determined using a low-flow inflation. MAIN RESULTS: The dynamic compliance was lower than the static compliance. The difference between dynamic and static compliance was dependent on alveolar pressure. At an alveolar pressure of 25 cm H2O, dynamic compliance was 29.8 (17.1) mL/cm H2O and static compliance was 59.6 (39.8) mL/cm H2O (median [interquartile range], p < .05). End-inspiratory volumes during the incremental PEEP trial coincided with the static pressure-volume curve, whereas end-expiratory volumes significantly exceeded the static pressure-volume curve. The differences could be attributed to PEEP-related recruitment, accounting for 40.8% (10.3%) of the total volume gain of 1964 (1449) mL during the incremental PEEP trial. Recruited volume per PEEP step increased from 6.4 (46) mL at zero end-expiratory pressure to 145 (91) mL at a PEEP of 20 cm H2O (p < .001). Dynamic compliance decreased at low alveolar pressure while recruitment simultaneously increased. Static mechanics did not allow this differentiation. The decrease in static compliance occurred at higher alveolar pressures compared with the dynamic analysis. CONCLUSIONS: Exploiting dynamic respiratory mechanics during incremental PEEP, both compliance and recruitment can be assessed simultaneously. Based on these findings, application of dynamic respiratory mechanics as a diagnostic tool in ventilated patients should be more appropriate than using static pressure-volume curves.     

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.     

Positive end-expiratory pressure above lower inflection point minimizes influx of activated neutrophils into lung

OBJECTIVES: To compare the effects of low vs. high tidal volume (Vt) with three positive end-expiratory pressure (PEEP) strategies on activated neutrophil influx into the lung. DESIGN: Prospective, randomized controlled animal study. SETTING: Animal laboratory in a university hospital. SUBJECTS: Newborn piglets. INTERVENTIONS: Surfactant-depleted piglets were randomized in littermate pairs; to PEEP of either 0 (zero end-expiratory pressure [ZEEP]; n = 6), 8 cm H2O (PEEP 8; n = 5), or 1 cm H2O above the lower inflection point (LIP) (PEEP>LIP; n = 6). Within each pair piglets were randomized to a low VT (5-7 mL/kg) or high VT strategy (17-19 mL/kg). After 4 hrs of mechanical ventilation, 18-fluorodeoxyglucose (18FDG) was injected and positron emission tomography scanning was performed. MEASUREMENTS AND MAIN RESULTS: VT and PEEP changes on influx constants of 18FDG were assessed by analysis of variance. A within-litter comparison of Vt was nonsignificant (p = .50). A between-litter comparison, ordered in linear trend rank, from ZEEP, to PEEP 8, to PEEP>LIP, showed a strong effect of PEEP on influx constant (p = .019). CONCLUSIONS: PEEP set above the LIP on the inspiratory limb of the pressure-volume curve affords a stronger lung protection than VT strategy.     

Time required for equilibration of arterial oxygen pressure after setting optimal positive end-expiratory pressure in acute respiratory distress syndrome

OBJECTIVE: To evaluate the time course of Pao2 change following the setting of optimal positive end-expiratory pressure (PEEP) in patients with acute respiratory distress syndrome (ARDS). DESIGN: Prospective clinical study. SETTING: Multidisciplinary intensive care unit of a university hospital. PATIENTS: Twenty-five consecutive patients with ARDS. INTERVENTIONS: ARDS was diagnosed during pressure-regulated volume control ventilation with tidal volume of 7 mL/kg actual body weight, respiratory rate of 12 breaths/min, inspiratory/expiratory ratio of 1:2, Fio2 of 1, and PEEP of 5 cm H2O. A critical care attending physician obtained pressure volume curves and determined the lower inflection point. Following a rest period of 30 mins with initial ventilation variables, PEEP was set at 2 cm H2O above the lower inflection point, and serial blood samples were collected during 1-hr ventilation with optimal PEEP. Arterial blood gas analyses were performed at 1, 3, 5, 7, 9, 11, 15, 20, 30, 45, and 60 mins. MEASUREMENTS AND MAIN RESULTS: Twenty-five patients were found eligible for the study. Three patients were excluded due to deterioration of oxygen saturation and hemodynamic instability following the initiation of optimal PEEP. Eight cases (36%) were considered to be of pulmonary origin and 14 cases (64%) of extrapulmonary origin. Optimal PEEP levels were 14 +/- 3 cm H2O and 14 +/- 4 cm H2O in pulmonary and extrapulmonary ARDS, respectively. Pao2 demonstrated a 130 +/- 101% increase at the end of 1-hr period in total study population. This improvement did not differ significantly between pulmonary and extrapulmonary forms of ARDS (135 +/- 118% vs. 127 +/- 95%, p = .8). Mean 90% oxygenation time was found to be 20 +/- 19 mins. In the subset of patients with ARDS of pulmonary origin, 90% oxygenation time was 25 +/- 26 mins, whereas it was 17 +/- 15 mins in patients with ARDS of extrapulmonary origin (p = .8). CONCLUSIONS: Our data showed that 20 mins would be adequate for obtaining a blood gas sample in ARDS patients with pulmonary and extrapulmonary origin after application of optimal PEEP 2 cm H2O above the lower inflection point.     

急性呼吸窘迫综合征绵羊肺复张容积测定方法的比较

目的　比较压力容积 (P V)曲线法与等压法测定肺复张容积的差异。方法　内毒素持续静脉注射复制绵羊急性呼吸窘迫综合征模型 ,分别用 P V曲线法与等压法测定相同呼气末正压 (PEEP)的肺复张容积。结果　等压法测定肺复张容积可以立即得出结果 ;而 P V曲线法测定肺复张容积所需时间为5～ 6 min。随着 PEEP从 5 cm H2 O(1cm H2 O=0 .0 98k Pa)增至 15 cm H2 O,两种方法测定的肺复张容积均显著增加 (P均 <0 .0 5 ) ;PEEP为 5 cm H2 O时 ,等压法与 P V曲线法所测的肺复张容积分别为 (2 5 .79±2 0 .4 8) m l和 (6 3.2 6± 5 4 .5 7) m l,两组比较无明显差异 (P>0 .0 5 ) ;当 PEEP为 10和 15 cm H2 O时 ,等压法所测肺复张容积分别为 (4 8.6 4± 30 .5 1) m l、(71.5 0± 5 8.0 9) ml,P V曲线法测得肺复张容积分别为 (14 8.14±85 .4 2 ) m l、(32 2 .86± 14 8.4 2 ) m l,等压法所测值明显小于 P V曲线所测值 (P均 <0 .0 5 )。结论　虽然等压法较为简便 ,但由于准确性较差 ,因此不能代替 P V曲线法来测定肺复张容积     

Use of recruitment maneuvers and high-positive end-expiratory pressure in a patient with acute respiratory distress syndrome

OBJECTIVE: To present the use of a novel high-pressure recruitment maneuver followed by high levels of positive end-expiratory pressure in a patient with the acute respiratory distress syndrome (ARDS). DESIGN: Observations in one patient. SETTING: The medical intensive care unit at a tertiary care university teaching hospital. PATIENT: A 32-yr-old woman with severe ARDS secondary to streptococcal sepsis. INTERVENTIONS: The patient had severe gas exchange abnormalities because of acute lung injury and marked lung collapse. Attempts to optimize recruitment based on the inflation pressure-volume (PV) curve were not sufficient to avoid dependent lung collapse. We used a recruitment maneuver using 40 cm H2O of positive end-expiratory pressure (PEEP) and 20 cm H2O of pressure controlled ventilation above PEEP for 2 mins to successfully recruit the lung. The recruitment was maintained with 25 cm H2O of PEEP, which was much higher than the PEEP predicted by the lower inflection point (P(Flex)) of the PV curve. MEASUREMENTS AND MAIN RESULTS: Recruitment was assessed by improvements in oxygenation and by computed tomography of the chest. With the recruitment maneuvers, the patient had a dramatic improvement in gas exchange and we were able to demonstrate nearly complete recruitment of the lung by computed tomography. A PV curve was measured that demonstrated a P(Flex) of 16-18 cm H2O. CONCLUSION: Accumulating data suggest that the maximization and maintenance of lung recruitment may reduce lung parenchymal injury from positive pressure ventilation in ARDS. We demonstrate that in this case PEEP alone was not adequate to recruit the injured lung and that a high-pressure recruitment maneuver was required. After recruitment, high-level PEEP was needed to prevent derecruitment and this level of PEEP was not adequately predicted by the P(Flex) of the PV curve.     

Effects of positive end-expiratory pressure on gas exchange and expiratory flow limitation in adult respiratory distress syndrome

OBJECTIVE: To assess the effects of different positive end-expiratory pressure (PEEP) levels (0, 5, 10, and 15 cm H2O) on tidal expiratory flow limitation (FL), regional intrinsic positive end-expiratory pressure (PEEPi) inhomogeneity, alveolar recruited volume (Vrec), respiratory mechanics, and arterial blood gases in mechanically ventilated patients with acute respiratory distress syndrome (ARDS). DESIGN: Prospective clinical study. SETTING: Multidisciplinary intensive care unit of a university hospital. PATIENTS: Thirteen sedated, mechanically ventilated patients during the first 2 days of ARDS. INTERVENTIONS: Detection of tidal FL and evaluation of total dynamic PEEP (PEEPt,dyn), total static PEEP (PEEPt,st), respiratory mechanics, and Vrec from pressure, flow, and volume traces provided by the ventilator. The average (+/-sd) tidal volume was 7.1 +/- 1.5 mL/kg, the total cycle duration was 2.9 +/- 0.45 secs, and the duty cycle was 0.35 +/- 0.05. MEASUREMENTS: Tidal FL was assessed using the negative expiratory pressure technique. Regional PEEPi inhomogeneity was assessed as the ratio of PEEPt,dyn to PEEPt,st (PEEPi inequality index), and Vrec was quantified as the difference in lung volume at the same airway pressure between quasi-static inflation volume-pressure curves on zero end-expiratory pressure (ZEEP) and PEEP. RESULTS: On ZEEP, seven patients exhibited FL amounting to 31 +/- 8% of tidal volume. They had higher PEEPt,st and PEEPi,st ( p<.001) and lower PEEPi inequality index ( p<.001) than the six nonflow-limited (NFL) patients. Two FL patients became NFL with PEEP of 5 cm H2O and five with PEEP of 10 cm H2O. In both groups, PaO2 increased progressively with PEEP. In the FL group, there was a significant correlation of PaO2 to PEEPi inequality index ( p=.002). For a given PEEP, Vrec was greater in NFL than FL patients, and a significant correlation of Pao to Vrec ( p<.001) was found only in the NFL group. CONCLUSIONS: We conclude that on ZEEP, tidal FL is common in ARDS patients and is associated with greater regional PEEPi inhomogeneity than in NFL patients. With PEEP of 10 cm H2O, flow limitation with concurrent cyclic dynamic airway compression and re-expansion and the risk of "low lung volume injury" were absent in all patients. In FL patients, PEEP induced a significant increase in PaO2, mainly because of the reduction of regional PEEPi inequality, whereas in the NFL group, arterial oxygenation was improved satisfactorily because of alveolar recruitment.     

Static pressure-volume curves of the respiratory system: were they just a passing fad?

PURPOSE OF REVIEW: The aim of this article is to describe the physiologic utility, correlation with lung morphology, difficulties in interpretation and current clinical applications of static respiratory system pressure-volume curves at the bedside in patients with acute lung injury or acute respiratory distress syndrome. RECENT FINDINGS: Complex interpretation of pressure-volume curves indicates that alveolar reopening continues past the lower inflection point on the linear part of the curve and suggests the presence of homogeneous lung disease in which recruitment is still possible by positive end-expiratory pressure application. Setting positive end-expiratory pressure above the lower inflection point and tidal ventilation (approximately 6 ml/kg) in the linear portion of the respiratory system pressure-volume curve improved mortality and ameliorated lung and plasma inflammatory mediators compared with ventilation with the lowest positive end-expiratory pressure at traditional tidal volumes. Recent studies have found that regular use of pressure-volume curves provides useful physiological data that help to optimize mechanical ventilation at the bedside. SUMMARY: The physiologic data obtained by measuring the static pressure-volume curves have helped clinicians to better understand the behavior of the respiratory system when positive-pressure ventilation is applied. The advanced technology incorporated into modern ventilators allows routine measurement of pressure-volume curves under sedation without paralysis, with acceptable variability and no serious adverse effects.     

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.     

Positive end-expiratory pressure titration in acute respiratory distress syndrome patients: impact on right ventricular outflow impedance evaluated by pulmonary artery Doppler flow velocity measurements

OBJECTIVE: Positive end-expiratory pressure (PEEP) titration in acute respiratory distress syndrome patients remains debatable. We used two mechanical approaches, calculation of the compliance of the respiratory system and determination of the lower inflexion point of the pressure-volume curve of the respiratory system, to identify specific PEEPs (PEEPS and PEEPA) whose impact on right ventricular (RV) outflow was compared with Doppler analysis of pulmonary artery flow velocity. DESIGN: Prospective, open, clinical study. SETTING: Medical intensive care unit of a university hospital. PATIENTS: Sixteen consecutive ventilator-dependent acute respiratory distress syndrome patients. INTERVENTIONS: Two PEEPs were determined: PEEPS was the highest PEEP associated with the highest value of respiratory compliance, and PEEPA was the coordinate of the lower inflexion point of the inspiratory pressure-volume curve on the pressure axis plus 2 cm H2O. MEASUREMENTS AND MAIN RESULTS: We observed a large difference between the two PEEPs, with PEEPA (13 + 4 cm H2O) > PEEPS (6 + 3 cm H2O). Changes in RV outflow impedance produced by tidal ventilation with zero end-expiratory pressure (ZEEP) and after application of these two PEEPs were assessed by Doppler study of pulmonary artery flow velocity obtained by a transesophageal approach, with particular reference to the end-expiratory and end-inspiratory pulmonary artery velocity-time integral, as reflecting RV stroke output, and mean acceleration as reflecting RV outflow impedance during an unchanged flow period. A significant inspiratory reduction in pulmonary artery velocity-time integral (from 11.8 + 0.3 to 10.0 + 0.3 cm) and mean acceleration (from 11.9 + 0.9 to 8.0 + 0.9 m/sec2) was observed with ZEEP, showing a reduction in RV stroke index (from 29.0 + 0.9 to 26.0 + 0.6 cm3/m2) by a sudden increase in outflow impedance during tidal ventilation. Application of PEEPA, which improved Pao2 (102 + 40 vs. 65 + 18 torr with ZEEP), worsened the inspiratory drop in RV stroke index (21.6 + 0.8 cm3/m2), resulting in a significant reduction in cardiac index compared with ZEEP (from 3.0 + 1.0 to 2.7 + 1.1). Application of PEEPS, which also significantly improved Pao2 (81 + 21 torr), was associated with a lesser impact on RV outflow impedance (inspiratory mean acceleration: 9.5 + 1 m/sec2) and cardiac index (3.2 + 1.0) than PEEPA. CONCLUSION: RV outflow impedance evaluated by the Doppler technique appeared sensitive to PEEP titration. Application of PEEPA worsened RV systolic function impairment produced by tidal ventilation. Conversely, application of PEEPS reduced RV systolic function impairment, suggesting an association with a lower pulmonary vascular resistance.     

Pulmonary pressure-volume relationship in acute respiratory distress syndrome in adults: Role of positive end expiratory pressure

During the early phase of the acute respiratory distress syndrome in adults (ARDS), pulmonary pressure-volume (P-V) curves exhibit a peculiar pattern, with increased hysteresis and an inflection in the ascending limb (Matamis et al, Chest 86:58-661984). End-expiratory lung volume is also markedly reduced. We traced P-V curves using a 2-L syringe in six patients with ARDS (group 2) and five patients without ARDS (group 1). End-expiratory lung volume, measured using the closed circuit helium dilution technique, was markedly reduced in both groups (39% ± 7% predicted in group 1, and 27% ± 7% in group 2). In the ARDS group, P-V curve was grossly abnormal, with an inflection at low lung volume and increased hysteresis: lung volume difference during inflation and deflation at a pressure 10 cm H₂O higher than end-expiratory pressure was 803 ± 127 mL in group 2 and was only 450 ± 189 mL in group 1. Compliance measured during deflation was only slightly reduced in group 2. Application of first positive end-expiratory pressure 10, then 20 cm H₂O, restored end-expiratory lung volume in all patients, and, in group 2 (ARDS), suppressed the inflection of the ascending limb, reduced hysteresis, and shifted the P-V trace upward and to the left. We conclude that, in ARDS patients, an abnormal pattern of P-V curve is explained by loss of volume, and by increased surface tension, since lung volume was similarly reduced in both groups. Increasing the level of end-expiratory pressure restores the normal pulmonary P-V relationship by suppressing the airway closure.     

Determinants of tidal volume during high-frequency oscillation

OBJECTIVE: High-frequency oscillation has been proposed for use in adult acute respiratory distress syndrome. However, limited data are available on the effect of pressure amplitude and rate (Hz) on tidal volumes delivered during high-frequency oscillation in adults. DESIGN: Prospective, animal model, lung injury study. SETTING: Large-animal laboratory of a university-affiliated medical center. SUBJECTS: Nine sheep (29.2 +/- 2.4 kg). INTERVENTIONS: Severe lung injury was induced by repeated saline lung lavage. After stabilization, high-frequency oscillation was initiated at a mean airway pressure equal to the point of maximum curvature on the deflation limb of the pressure-volume curve (26 +/- 1.9 cm H2O). Tidal volume at all combinations of rates of 4, 6, 8, and 10 Hz, pressure amplitudes of 30, 40, 50, and 60 cm H2O, and inspiratory/expiratory ratios of 1:1 and 1:2 (using the Sensormedics 3100B oscillator) were measured. Flow was measured by a pneumotachometer, amplified and digitized at 1000 Hz. Three breaths were analyzed at each setting. MEASUREMENTS AND MAIN RESULTS: At both inspiratory/expiratory ratios, tidal volume was directly proportional to pressure amplitude and inversely proportional to frequency. During an inspiratory/expiratory ratio of 1:1, at 60 cm H2O pressure amplitude and 4 Hz, a tidal volume of 129.1 +/- 34.8 mL (4.4 +/- 1.2 mL/kg) was delivered. CONCLUSIONS: At low rates and high-pressure amplitudes in this model, tidal volumes approaching conventional mechanical ventilation can be delivered during high-frequency oscillation.     

Multiple pressure-volume loops recorded with sinusoidal low flow in a porcine acute respiratory distress syndrome model

OBJECTIVES: To develop a method for automatic recording of multiple dynamic elastic pressure-volume (P(el)/V) loops. To analyse the relationship between multiple dynamic P(el)/V loops and static P(el)/V loops in a porcine model of acute lung injury/acute respiratory distress syndrome (ALI/ARDS). To test the hypothesis that increasing lung collapse and re-expansion with decreasing positive end expiratory pressure (PEEP) can be characterized by hysteresis of the P(el)/V loops. MATERIAL AND INTERVENTIONS: In eight anaesthetized and paralysed pigs, ALI/ARDS was induced by inhalation of dioctyl sodium sulfosuccinate and large tidal volume ventilation. MEASUREMENTS AND RESULTS: The physiological and histopathological findings indicated a status mimicking an early stage of ALI/ARDS. Automatically, a series of dynamic P(el)/V loops from different PEEP levels were recorded with the sinusoidal flow modulation method using a computer-controlled ventilator. During expiration, resistance increased more than twofold. For each step of lower starting pressure, the inspiratory limb was displaced towards lower volume indicating derecruitment. Recruitment occurred between 20 and 40 cm H(2)O. The expiratory curves, all starting from 50 cm H(2)O, overlapped. Hysteresis increased significantly in loops recorded from lower PEEP levels. Viscoelasticity explained differences between static and dynamic P(el)/V loops. CONCLUSIONS: Automated multiple P(el)/V loop determination is feasible and provides comprehensive information on lung derecruitment and recruitment. It requires determination of volume dependence of expiratory resistance. An expiratory curve serves as a reference to inspiratory curves and provides information about hysteresis.     

Using pressure-volume curves to set proper PEEP in acute lung injury

The evolution of respiratory care on patients with acute respiratory distress syndrome (ARDS) has been focused on preventing the deleterious effects of mechanical ventilation, termed ventilator-induced lung injury (VILI). Currently, reduced tidal volume is the standard of ventilatory care for patients with ARDS. The current focus, however, has shifted to the proper setting of positive end-expiratory pressure (PEEP). The whole lung pressure-volume (P/V) curve has been used to individualize setting proper PEEP in patients with ARDS, although the physiologic interpretation of the curve remains under debate. The purpose of this review is to present the pros and cons of using P/V curves to set PEEP in patients with ARDS. A systematic analysis of recent and relevant literature was conducted. It has been hypothesized that proper PEEP can be determined by identifying P/V curve inflection points. Acquiring a dynamic curve presents the key to the curve's bedside application. The lower inflection point of the inflation limb has been shown to be the point of massive alveolar recruitment and therefore an option for setting PEEP. However, it is becoming widely accepted that the upper inflection point (UIP) of the deflation limb of the P/V curve represents the point of optimal PEEP. New methods used to identify optimal PEEP, including tomography and active compliance measurements, are currently being investigated. In conclusion, we believe that the most promising method for determining proper PEEP settings is use of the UIP of the deflation limb. However, tomography and dynamic compliance may offer superior bedside availability.     

Static pressure volume curves and body posture in acute respiratory failure

Objective In acute respiratory distress syndrome the body posture effects on pressure-volume (PV) curves are still unclear. We examined the effects of prone position on inflation PV curves and their potential relationships with postural alterations in gas exchange.Design and setting Prospective study with patients serving as their own controls in a university-affiliated 30-bed intensive care unit.Patients and participants Thirteen anesthetized, paralyzed, semirecumbent, mechanically ventilated patients with early/severe/diffuse ARDS.Interventions Sequential body posture changes: preprone semirecumbent, prone, and postprone semirecumbent.Measurements and results In each posture hemodynamics, gas exchange, and lung volumes were determined before/during removal and after restoration of positive end-expiratory pressure (PEEP=10.2±0.6 cmH₂O). At zero PEEP PV curves of respiratory system, lung, and chest wall were constructed. Prone position vs. preprone semirecumbent resulted in significantly reduced pressure at lower inflection point of lung PV curve (2.2±0.2 vs. 3.7±0.5 cmH₂O) and increased volume at upper inflection point (0.87±0.03 vs. 0.69±0.05 l). Postural reduction in lower inflection point pressure of lung PV curve was the sole independent predictor of pronation-induced increases in PaO₂/FIO₂ (R²=0.76). PaO₂/FIO₂ increases were also significantly related with increases in functional residual capacity (R²=0.60).Conclusions In early/severe/diffuse ARDS prone position reduces lower inflection point pressure and increases upper inflection point UIP volume of the lung PV curve. Lower inflection point pressure reductions explain oxygenation improvements, which are also associated with a postural increase in functional residual capacity.Electronic Supplementary Material Electronic supplementary material to this paper can be obtained by using the Springer Link server located at .This work was funded solely by the Department of Intensive Care Medicine of Evaggelismos General Hospital, Athens, Greece     

Successful determination of lower inflection point and maximal compliance in a population of patients with acute respiratory distress syndrome

OBJECTIVE: To compare the ease and efficacy of two commonly used methods for choosing optimal positive end-expiratory pressure (PEEP) in patients with acute respiratory distress syndrome: a static pressure-volume curve to determine the lower inflection point (P(flex)) and the "best PEEP" (PEEP(best)) as determined by the maximal compliance curve. DESIGN: Prospective study. SETTING: Medical and respiratory intensive care units of university-associated tertiary care hospital. PATIENTS: Twenty-eight patients on mechanical ventilation with acute respiratory distress syndrome. INTERVENTIONS: A critical care attending physician or fellow and an experienced respiratory therapist attempted to obtain both static pressure-volume curves and maximal compliance curves on 28 patients with acute respiratory distress syndrome by using established methods that were practical to everyday use. The curves then were used to determine both P(flex) and PEEP(best), and the results were compared. MEASUREMENT AND MAIN RESULTS: Our results showed at least one value for optimal PEEP was obtained in 26 of 28 patients (93%). P(flex) was determined in 19 (68%), a PEEP(best) in 24 (86%), and both values in 17 (61%). In patients who had both P(flex) and PEEP(best) determined, there was a close concordance (+/-3 cm H2O) in 60%. When the values of P(flex) and PEEP(best) were interpreted by two additional investigators, there was unanimous agreement on the P(flex) (+/-3) only 64% of the time. There was agreement on the value of PEEP(best) 93% of the time. CONCLUSIONS: Our data show that optimal PEEP, as determined by a pressure-volume curve and a maximal compliance curve, are sometimes unobtainable by practical means but, when obtained, often correspond. A maximal compliance is more often identified, has less interobserver variability, and poses less risk to the patient. We conclude that determining optimal PEEP by maximal static compliance may be easier to measure and more frequently obtained at the bedside than by using a static pressure-volume curve.     

Mean airway pressure vs. positive end-expiratory pressure during mechanical ventilation

To investigate the effects of both positive end-expiratory pressure (PEEP) and mean airway pressure (Paw) on gas exchange, we used lung lavage to induce severe respiratory insufficiency in six lambs. The animals were then mechanically ventilated at constant tidal volume, respiratory rate, and inspired O2 fraction. PEEP levels were varied -5, +5 and +10 cm H2O around the pressure (Pflex) corresponding to a major change in slope of the inspiratory limb of the respiratory volume-pressure curve. In each animal the effects of the three PEEP levels were studied at two Paw levels, differing by 5 cm H2O. Increasing Paw significantly improved PaO2 and reduced venous admixture. A 5-cm H2O PEEP increase from +5 to +10 did not affect oxygenation; however, oxygenation was significantly better when PEEP was greater than Pflex. Both PaCO2 and anatomic dead space were higher at higher PEEP, and decreased with increasing Paw. Hence, Paw was a major determinant of oxygenation, although a PEEP greater than Pflex appeared necessary to optimize oxygenation at a constant Paw.