Variations in end-expiratory pressure during partial liquid ventilation: impact on gas exchange, lung compliance, and end-expiratory lung volume

STUDY OBJECTIVES: To determine the effects of different levels of positive end-expiratory pressure (PEEP) during partial liquid ventilation (PLV) on gas exchange, lung compliance, and end-expiratory lung volume (EELV). DESIGN: Prospective animal study. SETTING: Animal physiology research laboratory. SUBJECTS: Nine piglets. INTERVENTIONS: Animals underwent saline solution lavage to produce lung injury. Perflubron was instilled via the endotracheal tube in a volume estimated to represent functional residual capacity. The initial PEEP setting was 4 cm H(2)O, and stepwise changes in PEEP were made. At 30-min intervals, the PEEP was increased to 8, then 12, then decreased back down to 8, then 4 cm H(2)O. MEASUREMENTS AND RESULTS: After 30 min at each level of PEEP, arterial blood gases, aortic and central venous pressures, heart rates, dynamic lung compliance, and changes in EELV were recorded. Paired t tests with Bonferroni correction were used to evaluate the data. There were no differences in heart rate or mean BP at the different PEEP levels. CO(2) elimination and oxygenation improved directly with the PEEP level and mean airway pressure (Paw). Compliance did not change with increasing PEEP, but did increase when PEEP was lowered. EELV changes correlated directly with the level of PEEP. CONCLUSIONS: As previously reported during gas ventilation, oxygenation and CO(2) elimination vary directly with PEEP and proximal Paw during PLV. EELV also varies directly with PEEP. Dynamic lung compliance, however, improved only when PEEP was lowered, suggesting an alteration in the distribution of perflubron due to changes in pressure-volume relationships.     

Modifications of plasma concentrations of hormonal and tissue factors during mechanical ventilation with positive end-expiratory pressure.

AIM: The aim of this study is to analyse if the decrease of cardiac performance due to positive end-expiratory pressure (PEEP) application, within low values applied in clinical practice (5 cm H(2)O) is able to trigger a response of the main endogenous factors which control and maintain the mean arterial pressure (MAP). METHODS: This study was applied to 18 patients, admitted to the Intensive Care Unit (ICU) of the University Hospital of Modena, who underwent oro-tracheal intubation and mechanical ventilation. On admission, patients did not suffer from cardiac or lung disease. This study analyses plasma concentrations of epinephrine, norepinephrine, ET-1, NO metabolites, renin, aldosterone at 4 different times: before PEEP application, 60 minutes after the beginning of mechanical ventilation with PEEP, and respectively 30 and 60 minutes after withdrawal of PEEP. At the same time, MAP values and heart rate (HR) have been observed. RESULTS: Results show an increase of epinephrine and norepinephrine after PEEP application and a decrease to basal values at PEEP withdrawal. All variations are statistically significant. After PEEP introduction, ET-1 showed an increased concentration, although it was not statistically significant, while a significant decreasing trend was observed after PEEP withdrawal. A significant increase of NO metabolite values has been observed together with the increase of ET-1, followed by a decrease to basal values after the withdrawal of PEEP. Concentrations of renin increased when PEEP was applied even though they were not significant and decreased significantly when PEEP was withdrawn. A similar trend was revealed by aldosterone even though it underwent constant significant variations. CONCLUSION: The administration of PEEP produces an effective response of endogenous substances whose function is to maintain a proper tissue perfusion.     

Detrimental effects of positive end-expiratory pressure during controlled mechanical ventilation of patients with severe airflow obstruction

Positive end-expiratory pressure (PEEP) in treatment of asthma may be beneficial by dilating airways or detrimental by increasing hyperinflation. Several studies have reported beneficial results but with conflicting effects on lung volume. We studied the effects of PEEP on pulmonary hyperinflation, gas exchange, and circulation in six patients (59 +/- 19 yr, four men, two women) with severe airflow obstruction requiring mechanical ventilation (four with asthma, two with an exacerbation of chronic airflow obstruction). Three levels of PEEP (5, 10, and 15 cm H2O) were studied. All patients were paralyzed and ventilated with a tidal volume of 1.0 L, and respiratory rates (R) of 10, 16, and 22 breaths per min. End-inspiratory lung volume (VEI) or the degree of pulmonary hyperinflation above functional residual capacity (FRC) was quantified by measuring total exhaled gas volume during a period of apnea following steady-state tidal inspiration (1). Two patients were not studied at 15 cm H2O PEEP because of hypotension. Without PEEP, all patients showed gas trapping above FRC that increased progressively as R was increased (i.e., expiratory time decreased). At each R, increases in PEEP progressively increased FRC up to 1.42 +/- 0.43 L (mean +/- SD) at 15 cm H2O PEEP (n = 4) and progressively reduced the degree of gas trapping above the PEEP FRC.(ABSTRACT TRUNCATED AT 250 WORDS)     

低潮气量加呼气末正压通气对急性肺损伤氧代谢的影响

目的 采用低潮气量加呼气末正压(PEEP)通气,观察对急性肺损伤(ALI)氧代谢的影响。方法 对14例ALI患者,最初采用辅助/控制(A/C)通气,并逐渐增加PEEP,每种方式30min,吸入氧浓度(FiO     

Effect of positive end-expiratory pressure on respiratory compliance in children with acute respiratory failure.

We studied the effect of positive end-expiratory pressure (PEEP) on the compliance of the respiratory system (Crs) in 25 children (age, 3 weeks to 10 years) requiring mechanical ventilation. Functional residual capacity (FRC) measurements were performed at 2 cm H2O increments, from 0 to 18 cm H2O of PEEP, and the FRC values were regressed versus PEEP. Static Crs, Crs/kg, and specific compliance (Crs/FRC) were calculated for each PEEP level. When FRC normality was reached Crs/kg improved in 15/25 (60%) patients but decreased in 2/25 (8%). Overall, Crs/kg increased from a mean +/- SE of 0.94 +/- 0.09 to 1.35 +/- 0.13 mL/cm H2O/kg (P = 0.003) and Crs/FRC from a mean +/- SE of 0.067 +/- 0.006 to 0.077 +/- 0.007 mL/cm H2O/mL (P = 0.057). The maximum compliance (mean Max Crs/kg, 1.56 +/- 0.12 mL/cm H2O/kg, and mean Max Crs/FRC, 0.089 +/- 0.005 mL/cm H2O/mL) was significantly higher than the compliance at the clinically chosen PEEP level and the compliance at the PEEP that normalized FRC. Maximum compliance was achieved within 4 cm H2O of the PEEP that normalized FRC. In 14/25 (60%) of cases the PEEP at maximum compliance coincided with the PEEP that resulted in FRC normalization. We concluded that static respiratory compliance improves in most (but not all) children with acute respiratory failure when FRC is normalized. Static respiratory compliance reaches maximum levels at PEEP values that are close (but not equal) to those that result in FRC normalization. Thus, assessment of the effect of PEEP on compliance is required in individual patients.     

Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation. The effect of intrinsic positive end-expiratory pressure

In mechanically ventilated patients with acute respiratory failure, the static compliance of the total respiratory system is conventionally obtained by dividing the tidal volume by the difference between the "plateau" pressure measured at the airway opening (PaO) during an occlusion at end-inspiration and positive end-expiratory pressure (PEEP) set by the ventilator. This analysis is valid only if the elastic recoil pressure of the respiratory system is zero at the end of expiration, indicating that the system has reached its elastic equilibrium point. To test if this is always the case, in 14 mechanically ventilated patients with acute respiratory failure, measurements were made of PaO and of flow and volume changes. In only 4 of the patients did expiratory flow become nil before end-expiration and inspiratory flow started synchronously with the onset of the positive-pressure swing delivered by the ventilator, indicating that in these 4 patients the end-expiratory elastic recoil pressure was indeed zero. By contrast, in the remaining 10 subjects, expiratory flow was still present when the ventilator had already begun to increase PaO, indicating that the end-expiratory elastic recoil pressure was not zero. Indeed, in all these 10 patients, a positive delta PaO (as much as 7.5 cm H2O) had to be applied by the ventilator before the actual onset of inspiratory flow. This delta PaO represents the pressure required to counterbalance the end-expiratory elastic recoil before inspiratory flow will begin, and can be termed intrinsic PEEP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Flow resistance of exhalation valves and positive end-expiratory pressure devices used in mechanical ventilation

We studied the flow-impeding characteristics of the exhalation valves and PEEP attachments commonly used in mechanical ventilation. To characterize these devices, the pressure difference across each mechanism was measured at a series of constant flows (5 to 160 L/min), and resistance-related energy dissipation was measured using mechanical models of passive and active exhalation. At ambient end-expiratory pressure, an inflatable diaphragm (mushroom) design commonly used to valve exhalation presented resistance comparable to that of an endotracheal tube with an internal diameter of 5 mm. The valve's energy dissipation increased further as PEEP was applied. By comparison, the servo-actuated scissor valve we tested presented less resistance during the passive deflation experiment but impeded the early phase of active exhalation. Spring-loaded PEEP attachments were prohibitively resistive in comparison with alternative methods using an underwater tube, a water column, a weighted spirometer, or an inflatable diaphragm to raise end-expiratory pressure. We conclude that the exhalation valves and PEEP attachments currently available for clinical use present significant impedance to air flow. Such resistance within the exhalation pathway may be clinically important for patients supported by mechanical ventilation during the hyperpneic or weaning phases of their illness.     

High inflation pressure pulmonary edema. Respective effects of high airway pressure, high tidal volume, and positive end-expiratory pressure

The respective roles of high pressure and high tidal volume to promote high airway pressure pulmonary edema are unclear. Positive end-expiratory pressure (PEEP) was shown to reduce lung water content in this type of edema, but its possible effects on cellular lesions were not documented. We compared the consequences of normal tidal volume ventilation in mechanically ventilated rats at a high airway pressure (HiP-LoV) with those of high tidal volume ventilation at a high (HiP-HiV) or low (LoP-HiV) airway pressure and the effects of PEEP (10 cm H2O) on both edema and lung ultrastructure. Pulmonary edema was assessed by extravascular lung water content and microvascular permeability by the drug lung weight and the distribution space of 125I-labeled albumin. HiP-LoV rat lungs were not different from those of controls (7 cm H2O peak pressure ventilation). By contrast, the lungs from the groups submitted to high volume ventilation had significant permeability type edema. This edema was more pronounced in LoP-HiV rats. It was markedly reduced by PEEP, which, in addition, preserved the normal ultrastructural aspect of the alveolar epithelium. This was in striking contrast to the diffuse alveolar damage usually encountered in this type of edema. To our knowledge, this constitutes the first example of a protective effect of PEEP during permeability edema.     

Influence of lung injury on pulmonary wedge-left atrial pressure correlation during positive end-expiratory pressure ventilation

The correlation between pulmonary artery wedge pressure (Pw) and left atrial pressure (Pla) requires a continuous fluid column between the catheter tip and the left atrium. We hypothesized that lung injury may protect the fluid column from the collapsing effects of increased airway pressure. Correlation between Pw and Pla would then depend on catheter tip location in injured versus normal lung regions. In 7 anesthetized dogs with unilateral acid pneumonitis, we compared Pla and simultaneous Pw measurements from pulmonary artery catheters located in injured and normal lungs at different levels of positive end-expiratory pressure (PEEP). Studies were repeated in 10 dogs with normal lungs and 5 dogs with bilateral acid pneumonitis. In supine dogs with unilateral lung injury, Pw from the injured lung more accurately reflected Pla than did Pw obtained from the normal lung at PEEP levels of 7 mmHg or higher, in contrast to data from dogs with normal lungs or equally injured lungs. Discrepancies between Pw and Pla at PEEP levels of 7 and 11 mmHg from the normal lung were corrected when that lung was placed in the dependent position to increase venous pressure at the catheter tip. A good Pw-Pla correlation was not guaranteed by catheter tip location below the level of the left atrium during PEEP ventilation. We conclude that the continuity of the fluid column was protected by lung injury. Although Pw-Pla differences from the normal lung were modest at the levels of PEEP that are usually optimal for gas exchange in uneven lung injury, it is recommended that the injured lung should not be avoided during insertion of the balloon-tipped catheter.     

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.     

Positive end-expiratory pressure vs T-piece. Extubation after mechanical ventilation

Because T-piece breathing may impair oxygenation, the best airway pressure from which to extubate ventilated patients is controversial. We compared the effects of extubation after 1 h of either CPAP 5 and T-piece/ZEEP. Once weaned from mechanical ventilation and breathing spontaneously, 106 patients were randomized to 1 h CPAP or 1 h T-piece/ZEEP, following which patients were extubated and mask O2 administered. No significant difference existed between groups in age, sex, HR, BP, FIO2, PaCO2 or PaO2. However, P(A-a)O2 was significantly greater at 120 min in the CPAP group. Within the CPAP group, P(A-a)O2 was also significantly worse at 120 vs 0 min. Nineteen T-piece patients showed improved P(A-a)O2 at 120 min compared with only ten CPAP patients. Three CPAP and two T-piece patients subsequently required reintubation. This study demonstrates that use of a T-piece dose not impair arterial oxygenation and may in fact be superior to direct extubation from CPAP 5.     

Effect of tidal volume on ventilation maldistribution

To examine the effect of changing tidal volume (VT) on ventilation distribution, we studied multiple breath nitrogen washouts in 4 normal subjects breathing with a VT of 0.6, 1.0 or 1.5 L. We used a recently developed technique of analysis (Crawford et al., 1985) that distinguishes inhomogeneity of gas concentrations due to the interaction of convection and diffusion in the lung periphery (DCDI) from ventilation maldistribution among larger units determined at more proximal branchpoints (CDI). The results indicate that increases in VT reduce the inhomogeneity due to DCDI but increase that due to CDI. In view of the VT dependence of physiological dead space, derived from arterial PCO2, we speculate that the increase in CDI has a dominant effect on gas exchange.     

Instantaneous blood flow responses to positive end-expiratory pressure with spontaneous ventilation.

Variable hemodynamic responses to positive end-expiratory pressure (PEEP) with spontaneous ventilation have been reported. To clarify these responses, 15 awake patients were studied using a catheter-tip velocity transducer to record phasic aortic root blood flow continuously before, during and after PEEP (10 cm H2O) applied with a face mask. Central blood volume and effective ventricular filling pressures were measured. Phasic pulmonary artery blood flow was also simultaneously recorded in three of these patients. PEEP produced an acute aortic blood flow reduction, detected within one respiratory cycle. Stroke volume decreased 12%, and since heart rate was unchanged, cardiac output also declined (p less than 0.05). Inspiratory-to-expiratory aortic flow changes were less during PEEP. In contrast, inspiratory-to-expiratory pulmonary artery flow alterations were exaggerated due to a marked flow decline during expiration. Central blood volume and effective left ventricular filling pressure decreased 9% and 19%, respectively (p less than 0.05 in all patients). The decrease in pulmonary artery flow was associated with a decrease in central blood volume in the three patients in whom pulmonary flow was measured. PEEP promptly reduces cardiac output during spontaneous ventilation, related to a decrease in pulmonary flow in expiration.     

Distribution of ventilation and perfusion during positive end-expiratory pressure in the adult respiratory distress syndrome

The response of respiratory gas exchange to incremental increases in positive end-expiratory pressure (PEEP) was studied in patients with the adult respiratory distress syndrome (ARDS). Fifty total changes in PEEP were studied in 19 PEEP trials performed in 16 patients. The initial patterns of ventilation-perfusion distribution as measured by the multiple inert gas elimination technique showed a large shunt flow (32 +/- 14% of total cardiac output), which was accompanied in half of the patients by perfusion to a region of low ventilation-perfusion ratio (VA/Q ratio less than 0.1). In 17 PEEP trials, there was an improvement in PaO2 (increase in PaO2 greater than 10 mmHg over control value) with at least one level of PEEP tested. In the 38 PEEP increments in these trials where PaO2 did improve, there was either a reduction in shunt alone, a reduction in ventilation-perfusion regions alone, or a redistribution in blood flow from shunt to regions of low or normal ventilation-perfusion ratio. In the increments where no increase was observed in PaO2, this reduction in blood flow to shunt or low VA/Q regions did not occur. In some instances, there was an increase in ventilation to unperfused alveoli and evidence of high ventilation-perfusion ratio (VA/Q greater than 10) as the level of PEEP increased. Because patients had an adequate pulmonary artery wedge pressure at the start of the PEEP trial (mean wedge pressure, 12.8 +/- 1.5 mmHg) improvements in oxygenation could usually be attained with only mild decreases in cardiac output.