Cardiovascular effects of positive end-expiratory pressure in dogs

High-CPAP Does Not Impede Cardiovascular Changes at Birth in Preterm Sheep

Objective: Continuous positive airway pressures (CPAP) used to assist preterm infants at birth are limited to 4-8 cmH₂O due to concerns that high-CPAP may cause pulmonary overexpansion and adversely affect the cardiovascular system. We investigated the effects of high-CPAP on pulmonary (PBF) and cerebral (CBF) blood flows and jugular vein pressure (JVP) after birth in preterm lambs. Methods: Preterm lambs instrumented with flow probes and catheters were delivered at 133/146 days gestation. Lambs received low-CPAP (LCPAP: 5 cmH₂O), high-CPAP (HCPAP: 15 cmH₂O) or dynamic HCPAP (15 decreasing to 8 cmH₂O at ~2 cmH₂O/min) for up to 30 min after birth. Results: Mean PBF was lower in the LCPAP [median (Q1-Q3); 202 (48-277) mL/min, p = 0.002] compared to HCPAP [315 (221-365) mL/min] and dynamic HCPAP [327 (269-376) mL/min] lambs. CBF was similar in LCPAP [65 (37-78) mL/min], HCPAP [73 (41-106) mL/min], and dynamic HCPAP [66 (52-81) mL/min, p = 0.174] lambs. JVP was similar at CPAPs of 5 [8.0 (5.1-12.4) mmHg], 8 [9.4 (5.3-13.4) mmHg], and 15 cmH₂O [8.6 (6.9-10.5) mmHg, p = 0.909]. Heart rate was lower in the LCPAP [134 (101-174) bpm; p = 0.028] compared to the HCPAP [173 (139-205)] and dynamic HCPAP [188 (161-207) bpm] groups. Ventilation or additional caffeine was required in 5/6 LCPAP, 1/6 HCPAP, and 5/7 dynamic HCPAP lambs (p = 0.082), whereas 3/6 LCPAP, but no HCPAP lambs required intubation (p = 0.041), and 1/6 LCPAP, but no HCPAP lambs developed a pneumothorax (p = 0.632). Conclusion: High-CPAP did not impede the increase in PBF at birth and supported preterm lambs without affecting CBF and JVP.

The right ventricle: interaction with the pulmonary circulation

The primary role of the right ventricle (RV) is to deliver all the blood it receives per beat into the pulmonary circulation without causing right atrial pressure to rise. To the extent that it also does not impede left ventricular (LV) filling, cardiac output responsiveness to increased metabolic demand is optimized. Since cardiac output is a function of metabolic demand of the body, during stress and exercise states the flow to the RV can vary widely. Also, instantaneous venous return varies widely for a constant cardiac output as ventilatory efforts alter the dynamic pressure gradient for venous return. Normally, blood flow varies with minimal changes in pulmonary arterial pressure. Similarly, RV filling normally occurs with minimal increases in right atrial pressure. When pulmonary vascular reserve is compromised RV ejection may also be compromised, increasing right atrial pressure and limiting maximal cardiac output. Acute increases in RV outflow resistance, as may occur with acute pulmonary embolism, will cause acute RV dilation and, by ventricular interdependence, markedly decreased LV diastolic compliance, rapidly spiraling to acute cardiogenic shock and death. Treatments include reversing the causes of pulmonary hypertension and sustaining mean arterial pressure higher than pulmonary artery pressure to maximal RV coronary blood flow. Chronic pulmonary hypertension induces progressive RV hypertrophy to match RV contractility to the increased pulmonary arterial elastance. Once fully developed, RV hypertrophy is associated with a sustained increase in right atrial pressure, impaired LV filling, and decreased exercise tolerance. Treatment focuses on pharmacologic therapies to selectively reduce pulmonary vasomotor tone and diuretics to minimize excessive RV dilation. Owning to the irreversible nature of most forms of pulmonary hypertension, when the pulmonary arterial elastance greatly exceeds the adaptive increase in RV systolic elastance, due to RV dilation, progressive pulmonary vascular obliteration, or both, end stage cor pulmonale ensues. If associated with cardiogenic shock, it can effectively be treated only by artificial ventricular support or lung transplantation. Knowing how the RV adapts to these stresses, its sign posts, and treatment options will greatly improve the bedside clinician’s ability to diagnose and treat RV dysfunction.

The application of esophageal pressure measurement in patients with respiratory failure

This report summarizes current physiological and technical knowledge on esophageal pressure (Pes) measurements in patients receiving mechanical ventilation. The respiratory changes in Pes are representative of changes in pleural pressure. The difference between airway pressure (Paw) and Pes is a valid estimate of transpulmonary pressure. Pes helps determine what fraction of Paw is applied to overcome lung and chest wall elastance. Pes is usually measured via a catheter with an air-filled thin-walled latex balloon inserted nasally or orally. To validate Pes measurement, a dynamic occlusion test measures the ratio of change in Pes to change in Paw during inspiratory efforts against a closed airway. A ratio close to unity indicates that the system provides a valid measurement. Provided transpulmonary pressure is the lung-distending pressure, and that chest wall elastance may vary among individuals, a physiologically based ventilator strategy should take the transpulmonary pressure into account. For monitoring purposes, clinicians rely mostly on Paw and flow waveforms. However, these measurements may mask profound patient-ventilator asynchrony and do not allow respiratory muscle effort assessment. Pes also permits the measurement of transmural vascular pressures during both passive and active breathing. Pes measurements have enhanced our understanding of the pathophysiology of acute lung injury, patient-ventilator interaction, and weaning failure. The use of Pes for positive end-expiratory pressure titration may help improve oxygenation and compliance. Pes measurements make it feasible to individualize the level of muscle effort during mechanical ventilation and weaning. The time is now right to apply the knowledge obtained with Pes to improve the management of critically ill and ventilator-dependent patients.

Effect of Positive End-Expiratory Pressure on Oxygen Delivery During 1-Lung Ventilation for Thoracoscopy in Normal Dogs

OBJECTIVE

To evaluate the effect of positive end-expiratory pressure (PEEP) on oxygen delivery (DO(2)) with 1-lung ventilation during thoracoscopy in normal anesthetized dogs.

STUDY DESIGN

Prospective, controlled experimental study.

ANIMALS

Eight, adult, intact Walker Hound dogs weighing 25.6-29.2 kg.

METHODS

Anesthetized dogs had 1-lung ventilation during an open-chest condition. A Swan-Ganz catheter was used to measure pulmonary hemodynamic variables and to obtain mixed venous blood samples for blood gas analysis. A dorsal pedal catheter was used for measurement of systemic arterial pressure and to obtain arterial blood samples for blood gas analysis. Oxygen delivery was calculated and used to assess the effect of 0, 2.5, and 5 cm H(2)O PEEP during 1-lung ventilation on cardiopulmonary function. Each dog was its own control at 0 cm H(2)O PEEP. A randomized block ANOVA for repeated measures was used to evaluate the effect of the treatment on hemodynamic and pulmonary variables.

RESULTS

Use of 5 cm H(2)O PEEP induced a significant augmentation in the arterial partial pressure of oxygen (PaO(2)). Shunt fraction (Q(s)/Q(t)), physiologic dead space (V(D)/V(T)), and the alveolar-arterial oxygen difference (P(A-a)O(2)) decreased significantly after 5 cm H(2)O PEEP, compared with 1-lung ventilation without PEEP. Use of 2.5 cm H(2)O PEEP had no significant effect on cardiopulmonary variables. Use of PEEP had no significant effect on arterial oxygen saturation (SaO(2)), DO(2), and hemodynamic variables in normal dogs.

CONCLUSIONS

PEEP had no effect on DO(2) in normal dogs undergoing open-chest 1-lung ventilation because it had no adverse effect on hemodynamic variables.

CLINICAL RELEVANCE

PEEP in normal dogs during open-chest 1-lung ventilation for thoracoscopy is not detrimental to cardiac output and can be recommended in clinical patients.

Evaluation of hemodynamic measurements, including lithium dilution cardiac output, in anesthetized dogs undergoing ovariohysterectomy

OBJECTIVE

To measure cardiac output in healthy female anesthetized dogs by use of lithium dilution cardiac output and determine whether changes in mean arterial pressure were caused by changes in cardiac output or systemic vascular resistance.

DESIGN

Prospective clinical study.

ANIMALS

20 healthy female dogs.

PROCEDURE

Dogs were anesthetized for ovariohysterectomy. Ten dogs breathed spontaneously throughout anesthesia, and 10 dogs received intermittent positive-pressure ventilation. Cardiovascular and respiratory measurements, including lithium dilution cardiac output, were performed during anesthesia and surgery.

RESULTS

Mean arterial pressure and systemic vascular resistance index were low after induction of anesthesia and just prior to surgery and increased significantly after surgery began. Cardiac index (cardiac output indexed to body surface area) did not change significantly throughout anesthesia and surgery.

CONCLUSIONS AND CLINICAL RELEVANCE

Results provide baseline data for cardiac output and cardiac index measurements during clinical anesthesia and surgery in dogs. Changes in mean arterial pressure do not necessarily reflect corresponding changes in cardiac index.

Influence of expiratory loading and hyperinflation on cardiac output during exercise

Patients with obstructive lung disease are exposed to expiratory loads (ELs) and dynamic hyperinflation as a consequence of expiratory flow limitation. To understand how these alterations in lung mechanics might affect cardiac function, we examined the influence of a 10-cmH2O EL, alone and in combination with voluntary hyperinflation (ELH), on pulmonary pressures [esophageal (Pes) and gastric (Pg)] and cardiac output (CO) in seven healthy subjects. CO was determined by using an acetylene method at rest and at 40 and 70% of peak work. At rest and during exercise, EL resulted in an increase in Pes and Pg (7-18 cmH2O; P < 0.05) and a decrease in CO (from 5.3 ± 1.8 to 4.5 ± 1.4, 12.2 ± 2.2 to 11.2 ± 2.2, and 16.3 ± 3.3 to 15.2 ± 3.2 l/min for rest, 40% peak work, and 70% peak work, respectively; P < 0.05), which remained depressed after an additional 2 min of EL. With ELH, CO increased at rest and both exercise loads (relative to EL only) but remained below control values. The changes in CO were due to a reduction in stroke volume with a tendency for stroke volume to fall further with prolonged EL. There was a negative correlation between CO and the increase in expiratory Pes and Pg with EL ( R = -0.58 and -0.60; P < 0.01), whereas the rise in CO with subsequent hyperinflation was related to a more negative Pes ( R = 0.72; P < 0.01). In conclusion, EL leads to a reduction in CO, which appears to be primarily related to increases in expiratory abdominal and intrathoracic pressure, whereas ELH resulted in an improved CO, suggesting that lung inflation has little impact on cardiac function.

Changes in BP induced by passive leg raising predict response to fluid loading in critically ill patients

OBJECTIVE

To test the hypothesis that passive leg raising (PLR) induces changes in arterial pulse pressure that can help to predict the response to rapid fluid loading (RFL) in patients with acute circulatory failure who are receiving mechanical ventilation.

DESIGN

Prospective clinical study.

SETTING

Two medical ICUs in university hospitals.

PATIENTS

Thirty-nine patients with acute circulatory failure who were receiving mechanical ventilation and had a pulmonary artery catheter in place.

INTERVENTIONS

PLR for > 4 min and a subsequent 300-mL RFL for > 20 min.

MEASUREMENTS AND MAIN RESULTS

Radial artery pulse pressure (PPrad), heart rate, right atrial pressure, pulmonary artery occlusion pressure (PAOP), and cardiac output were measured invasively in a population of 15 patients at each phase of the study procedure (i.e., before and during PLR, and then before and after RFL). PPrad, PAOP, and stroke volume (SV) significantly increased in patients performing PLR. These changes were rapidly reversible when the patients' legs were lowered. Changes in PPrad during PLR were significantly correlated with changes in SV during PLR (r = 0.77; p < 0.001). Changes in SV induced by PLR and by RFL were significantly correlated (r = 0.89; p < 0.001). Finally, PLR-induced changes in PPrad were significantly correlated to RFL-induced changes in SV (r = 0.84; p < 0.001). In a second population of 24 patients, we found the same relationship between PLR-induced changes in PPrad and RFL-induced changes in SV (r = 0.73; p < 0.001).

CONCLUSION

The response to RFL could be predicted noninvasively by a simple observation of changes in pulse pressure during PLR in patients with acute circulatory failure who were receiving mechanical ventilation.

Dynamic effects of positive-pressure ventilation on canine left ventricular pressure-volume relations

Positive-pressure ventilation (PPV) may affect left ventricular (LV) performance by altering both LV diastolic compliance and pericardial pressure (Ppc). We measured the effect of PPV on LV intraluminal pressure, Ppc, LV volume, and LV cross-sectional area in 17 acute anesthetized dogs. To account for changes in lung volume independent of changes in Ppc and differences in contractility, measures were made during both open- and closed-chest conditions, during closed chest with and without chest wall binding, and after propranolol-induced acute ventricular failure (AVF). Apneic end-systolic pressure-volume relations (ESPVR) were generated by inferior vena caval occlusions. With the open chest, PPV had no effects. With the chest closed, PPV inspiration decreased LV end-diastolic volume (EDV) along its diastolic compliance curve and decreased end-systolic volume (ESV) such that the end-systolic pressure-volume domain was shifted to a point left of the LV ESPVR, even when referenced to Ppc. The decrease in EDV was greater in control than in AVF conditions, whereas the shift of the ESV to the left of the ESPVR was greater with AVF than in control conditions. We conclude that the hemodynamic effects of PPV inspiration are due primarily to changes in intrathoracic pressure and that the inspiration-induced decreases of LV EDV reflect direct effects of intrathoracic pressure on LV filling. The decreases in LV ESV exceed the amount explained solely by a reduction in LV ejection pressure.

Ventricular interaction: from bench to bedside

Decreased right ventricle (RV) output results in decreased left ventricle end-diastolic volume (LVEDV) and output by series interaction. Direct ventricular interaction may also have a major effect on LV function. Thus, decreased LVEDV caused by reduced RV output may be further reduced by a leftward septal shift and pericardial constraint. This has been shown to be true in acute and chronic pulmonary hypertension and is now also apparent in severe congestive heart failure. The use of intracavitary LV end-diastolic pressure (LVEDP) to assess LVEDV is inappropriate if pressure surrounding the LV is increased: the surrounding pressure should be subtracted from LVEDP to calculate the effective distending (transmural) pressure which governs preload. If the surrounding pressure increases more than LVEDP, transmural LVEDP and LVEDV will decrease despite the increased LVEDP. Thus, the use of filling pressure to reflect changes in LVEDV has led to erroneous conclusions regarding changes in myocardial compliance and contractility. It is now clear that volume loading may reduce LVEDV and stroke work in pulmonary embolism, chronic lung disease and severe congestive heart failure despite increased LVEDP. The decreased stroke work is a result of reduced LV preload, not decreased contractility as would be suggested if filling pressure is used to reflect preload.

Effects of positive intrathoracic pressure on pulmonary and systemic hemodynamics

The Frank-Starling Law accounts for many changes in cardiac performance previously attributed to changes in contractility in that changes in contractility might have been incorrectly inferred from changing ventricular function curves (i.e. systolic performance plotted against filling pressure) if diastolic compliance also changed. To apply the Frank-Starling Law in the presence of changing diastolic compliance, it is necessary to measure end-diastolic volume directly or to calculate end-diastolic transmural pressure, which requires that pericardial pressure be known. Under most normal circumstances, increased intrathoracic pressure (and other interventions, such as vasodilators or lower-body negative pressure, that decrease central blood volume) decreases the transmural end-diastolic pressures of both ventricles, their end-diastolic volumes and stroke work. However, when ventricular interaction is significant, the effects of these interventions might be quite different; this may be important in patients with heart-failure. Although these interventions decrease RV transmural pressure, they may increase LV transmural pressure, end-diastolic volume, and thus stroke work by the Frank-Starling mechanism.

The role of the pulmonary afferent receptors in producing hemodynamic changes during hyperinflation and endotracheal suctioning in an oleic acid-injured animal model of acute respiratory failure

The purpose of this study was to examine the role of the pulmonary afferent receptors in producing hemodynamic changes during hyperinflation and endotracheal suctioning (ETS) in an oleic acid-injured animal model of acute respiratory failure. Previous investigations of hyperinflation as a method to prevent hypoxia-induced sequelae of ETS have demonstrated unrecognized hemodynamic consequences. In this within-subject, repeated-measures study, instrumented, oleic acid-injured dogs had continuous measurements of heart rate (HR), mean aortic blood pressure (MAP), left ventricular pressure (Plv), pulmonary artery pressure (Ppa), right ventricular afterload (Ppa(tm)), right atrial pressure (Pra), and right ventricular filling pressure (Pra(tm)) during hyperinflation and ETS when the vagi were intact and after the pulmonary branches of the vagus nerves had been severed. After severing the vagi, MAP and Plv were decreased and HR and Ppa were increased. With the vagi severed, there was less variation in MAP and Ppa but increased variation in HR. These findings suggest that vagally mediated reflexes from the lungs produce some, but not all, of the hemodynamic effects associated with hyperinflation and ETS. Continued research is necessary to discover a method of hyperoxygenation and suctioning that does not produce potentially harmful hemodynamic effects.

Pulmonary complications of mechanical ventilation

Although life-saving, mechanical ventilation may be associated with many complications, including consequences of positive intrathoracic pressure, the many aspects of volutrauma, and adverse effects of intubation and tracheostomy. Optimal ventilatory care requires implementing mechanical ventilation with attention to minimizing adverse hemodynamic effects, averting volutrauma, and effecting freedom from mechanical ventilation as quickly as possible so as to minimize the risk of airway complications.

Cardiac arrest as a possible sequela of critical airway management and intubation

Immediate cardiac arrest may occur as a result of the physiological consequences of critical airway management, which may include one or all of the following: (1) sedation and/or paralysis, (2) tracheal intubation, and (3) positive pressure ventilation. Two patients are reported, both with myocarditis, who developed cardiac arrest within minutes of simple intubations. Their arrests were not related to technical difficulties of critical airway management. Any disease process that creates a preload-dependent cardiovascular system also creates a situation wherein critical airway management may cause cardiac decompensation. All medications administered to sedate patients and facilitate intubation, as well as mechanical ventilation itself, can cause a decrease in preload. This may be a significant mechanism through which immediate decompensation occurs. Potential conditions that cause preload-dependent cardiovascular systems, as well as alternate therapeutic considerations, are outlined. In these patients intubations should not be delayed, but should be done with extreme caution in anticipation of possible cardiac arrest.

Does positive end-expiratory pressure ventilation improve left ventricular function? A comparative study by transesophageal echocardiography in cardiac and noncardiac patients

STUDY OBJECTIVES

Positive end-expiratory pressure (PEEP) has been proposed to improve cardiac output in patients with left ventricular (LV) dysfunction. This study was designed to compare quantitative global and regional LV performance in response to PEEP in patients with normal and poor LV function.

DESIGN

A prospective clinical trial.

SETTING

Adult medical ICU in a university hospital.

PATIENTS

Twelve critically ill patients requiring respiratory support and divided into two groups according to baseline transesophageal echocardiographic (TEE) measurements: normal LV dimensions and fractional area of contraction (FAC=61+/-5%) (n=7) and dilated cardiomyopathy with reduced FAC (21+/-1%) (n=5).

MEASUREMENTS AND RESULTS

All patients were studied when two successive levels of PEEP (best PEEP as the highest value of respiratory compliance and high PEEP as best PEEP+10 cm H2O) were applied. Global systolic LV performance and quantitative regional wall motion analysis performed by the centerline method were assessed on the TEE transgastric short-axis view. End-systolic wall stress (ESWS) was used as a reliable indication of LV afterload. PEEP reduced LV dimensions asymmetrically in both groups of patients and septolateral diameter significantly decreased without affecting global LV systolic performance. Additionally, high PEEP produced a significant impairment in septal kinetics as evidenced by the centerline method. High PEEP also decreased ESWS for all patients (-27% in normal group and -23% in cardiac group, p<0.05) without significant improvement in global systolic LV performance (FAC: +2% in normal group and +0% in cardiac group; not significant).

CONCLUSIONS

PEEP cannot be recommended routinely to improve LV performance in patients with severe dilated cardiomyopathy.

Cardiopulmonary interactions in healthy children and children after simple cardiac surgery: the effects of positive and negative pressure ventilation

OBJECTIVE

To investigate the effects of cuirass negative pressure ventilation on the cardiac output of a group of anaesthetised children after occlusion of an asymptomatic persistent arterial duct, and a group of paediatric patients in the early postoperative period following cardiopulmonary bypass.

DESIGN

Prospective study.

SETTING

The paediatric intensive care unit and catheter laboratory of a tertiary care centre.

PATIENTS

16 mechanically ventilated children were studied: seven had undergone surgery for congenital heart disease, and nine cardiac catheterisation for transcatheter occlusion of an isolated asymptomatic persistent arterial duct.

INTERVENTIONS

Cardiac output was measured using the direct Fick method during intermittent positive pressure ventilation and again after a short period of negative pressure ventilation. In five of the postoperative patients a third measurement was made following reinstitution of positive pressure ventilation.

RESULTS

Negative pressure ventilation was delivered without complication, with no significant change in systemic arterial oxygen and carbon dioxide tension. The mixed venous saturation increased from 74% to 75.8% in the healthy children, and from 58.9% to 62.3% in the postoperative group. Negative pressure ventilation increased the cardiac index from 4.0 to 4.5 l/min/m2 in the healthy children, and from 2.8 to 3.5 l/min/m2 in the surgical group. The increase was significantly higher in the postoperative patients (28.1%) than the healthy children (10.8%).

CONCLUSIONS

While offering similar ventilatory efficiency to positive pressure ventilation, cuirass negative pressure ventilation led to a modest improvement in the cardiac output of healthy children, and to a greater increase in postoperative patients. There are important cardiopulmonary interactions in normal children and in children after cardiopulmonary bypass, and by having beneficial effects on these interactions, negative pressure ventilation has haemodynamic advantages over conventional positive pressure ventilation.

Venous and arterial reflex responses to positive-pressure breathing and lower body negative pressure

We examined the relative importance of arteriolar and venous reflex responses during reductions in cardiac output provoked by conditions that increase [positive end-expiratory pressure (PEEP)] or decrease [lower body negative pressure (LBNP)] peripheral venous filling. Five healthy subjects were exposed to PEEP (10, 15, 20, and 25 cmH2O) and LBNP (-10, -15, -20, and -25 mmHg) to induce progressive but comparable reductions in right atrial transmural pressure (control to minimum): from 5.9 +/- 0.4 to 1.8 +/- 0.7 and from 6.5 +/- 0.6 to 2.0 +/- 0.2 mmHg with PEEP and LBNP, respectively. Cardiac output (impedance cardiography) fell less during PEEP than during LBNP (from 3.64 +/- 0.21 to 2.81 +/- 0.21 and from 3.39 +/- 0.21 to 2.14 +/- 0.24 l.min-1.m-2 with PEEP and LBNP, respectively), and mean arterial pressure increased. We observed sustained increases in forearm vascular resistance (i.e., forearm blood flow by venous occlusion plethysmography) and systemic vascular resistance that were greater during LBNP: from 19.7 +/- 2.91 to 27.97 +/- 5.46 and from 20.56 +/- 2.48 to 50.25 +/- 5.86 mmHg.ml-1.100 ml tissue-1.min (P < 0.05) during PEEP and LBNP, respectively. Venomotor responses (venous pressure in the hemodynamically isolated limb) were always transient, significant only with the greatest reduction in right atrial transmural pressure, and were similar for LBNP and PEEP. Thus arteriolar rather than venous responses are predominant in blood volume mobilization from skin and muscle, and venoconstriction is not intensified with venous engorgement during PEEP.

Maintained cardiac output during positive end-expiratory pressure ventilation in open-chest pigs

BACKGROUND

Does ventilation with positive end-expiratory pressure (PEEP) act to reduce cardiac output (CO) not only by impeding venous return but also by inducing myocardial depression? The present study was aimed to demonstrate the possible existence of this latter mechanism.

METHODS

Eight pigs of Swedish native breed weighing 20-25 kg and 10-12 weeks old were anaesthetized, tracheotomized and connected to a volume-controlled ventilator. To prevent intrathoracic pressure from interfering with venous return, the heart and juxtacardiac vessels were exposed to atmospheric pressure by opening and retracting the chest and pericardium. Heart rate (HR), CO, stroke volume (SV), mean arterial (MAP), mean right (MRAP) and left (MLAP) atrial pressures were recorded before and after retransfusion of 500 ml of autologous blood. This procedure was carried out twice in each animal-during ventilation with zero and with 15 cm H2O of PEEP.

RESULTS

Comparison of the two ventilation modes before volume load revealed negligible differences in HR, CO, SV, MAP, MRAP and MLAP. Moreover, the changes evoked by volume load were practically identical.

CONCLUSIONS

Addition of PEEP to regular positive pressure ventilation does not induce any haemodynamically detectable myocardial depression in the piglet.

Hemodynamics and positive end-expiratory pressure in critically ill patients

This article examines the factors that affect the transmission of airway pressures to intrathoracic structures. The effects of positive end-expiratory pressure on central venous pressures, cardiac filling pressures, and right and left ventricular function are discussed. Various techniques for estimating intrathoracic pressures and their limitations are reviewed.

Effects of positive end-expiratory pressure on right ventricular function in COPD patients during acute ventilatory failure

OBJECTIVE

To examine the effects of external positive end-expiratory pressure (PEEP) on right ventricular function in chronic obstructive pulmonary disease (COPD) patients with intrinsic PEEP (PEEPi).

DESIGN

Prospective study.

SETTING

General intensive care unit in a university teaching hospital.

PATIENTS

Seven mechanically ventilated flow-limited COPD patients (PEEPi = 9.7 +/- 1.3 cmH2O, mean +/- SD) with acute respiratory failure.

INTERVENTION

Hemodynamic and respiratory mechanic data were collected at four different levels of PEEP (0-5-10-15 cmH2O).

MEASUREMENTS AND RESULTS

Hemodynamic parameters were obtained by a Swan-Ganz catheter with a fast response thermistor. Cardiac index (CI) and end-expiratory lung volume (EELV) reductions started simultaneously when the applied PEEP was approximately 90% of PEEPi measured on 0 cmH2O (ZEEP). Changes in transmural intrathoracic pressure (PEEPi,cw) started only at a PEEP value much higher (120%) than PEEPi. The reduction in CI was related to a decrease in the right end-diastolic ventricular volume index (RVEDVI) (r = 0.61; p < 0.001). No correlation between CI and transmural right atrial pressure was observed. The RVEDVI was inversely correlated with PEEP-induced changes in EELV (r = -55; p < 0.001), but no with PEEPi,cw (r = -0.08; NS). The relationship between RVEDVI and right ventricular stroke work index, considered an index of contractility, was significant in three patients, i.e., PEEP did not change contractility. In the other patients, an increase in contractility seemed to occur.

CONCLUSIONS

In COPD patients an external PEEP exceeding 90% of PEEPi causes lung hyperinflation and reduces the CI due to a preload effect. The reduction in RVEDVI seems related to changes in EELV, rather than to changes in transmural pressures, suggesting a lung/heart volume interaction in the cardiac fossa. Thus, in COPD patients, application of an external PEEP level lower than PEEPi may affect right ventricular function.

Electrocardiogram changes during positive pressure breathing in rabbits

Positive pressure breathing produced by mechanical ventilation with an expiratory threshold load (ETL) may modify electrocardiogram (ECG) complexes independently of any recording artefact due to lung volume changes. Anaesthetized, paralyzed rabbits were treated for about 2 h, then killed. In intact then vagotomized animals two situations were studied successively. Firstly, positive inspiratory pressure breathing, and secondly, positive inspiratory plus expiratory pressure breathing by adding ETL to mechanical ventilation. Arterial blood gases were measured and held constant throughout the challenge. Oesophageal pressure, giving indirect measurement of intrathoracic pressure, arterial blood pressure, blood flows in abdominal aorta and inferior vena cava and standard ECG recordings were made at baseline condition during mechanical ventilation, then at the end of a 10-min period of ETL breathing. The ETL breathing decreased arterial blood pressure significantly and reduced arterial and venous blood flows in the same proportion. No change in the duration of ECG complexes was noticed. However, ETL markedly reduced the amplitude of P- and T-waves, but not that of R-wave, an effect significantly accentuated after vagotomy. The ETL breathing increased the T-vector angle, with no associated change in QRS vector angle. The present animal investigations revealed that positive pressure breathing modifies the ECG independently of the consequences of ETL-induced lung volume changes. We speculate that the changes in P- and T-wave amplitude may have resulted from a reduced transmural pressure gradient between the epicardium and endocardium.

Effect of jet ventilation on heart failure: decreased afterload but negative response in left ventricular end-systolic pressure-volume function

OBJECTIVE

To examine the mechanism of cardiac assist with systolic jet ventilation, specifically effects on loading conditions and left ventricular pressure-volume function. Both systolic and diastolic jet ventilation were compared in the absence and presence of heart failure.

DESIGN

Prospective, two-factor, repeated-measures study.

SETTING

Animal laboratory.

SUBJECTS

Ten anesthetized, closed-chest dogs.

INTERVENTIONS

The measurement protocol consisted of two phases: a) apnea, randomized jet ventilation (systole- and diastole-synchronized); b) postjet ventilation apnea, before and after heart failure, induced with a propranolol-imipramine-plasma expansion treatment.

MEASUREMENT AND MAIN RESULTS

Systolic and diastolic jet ventilation was associated with mean airway pressures of approximately 7 mm Hg and intrapleural pressures of approximately 3 mm Hg in both heart conditions. In normal hearts, jet ventilation (either mode) decreased transmural left ventricular end-diastolic pressure by 40% to 60% (p < .05), left ventricular end-diastolic volume 25 +/- 8%, and stroke volume by 28% to 30%. Heart failure was associated with decreases (41 +/- 6%) in end-systolic pressure-volume function (i.e., pressure change/volume change or elastance), transmural left ventricular end-systolic pressure (22 +/- 3%), and stroke volume (16 +/- 4%), and increased transmural left ventricular end-diastolic pressure (139 +/- 6%). Application of jet ventilation (either mode) during heart failure did not affect stroke volume but significantly (p < .05) attenuated transmural left ventricular end-diastolic pressure by 30% to 40%, left ventricular end-diastolic volumes by 33 +/- 9%, and transmural left ventricular end-systolic pressure by 11% to 19% (p < .05). After jet ventilation, left ventricular elastance was decreased 36 +/- 8% in normal hearts and 35 +/- 11% in failing hearts. Stroke volume, however, returned to baseline levels because of increases in transmural left ventricular end-diastolic pressure in both heart conditions, and also in failing hearts, because transmural left ventricular end-systolic pressure remained decreased approximately 30% (p < .05).

CONCLUSIONS

Jet ventilation did not decrease stroke volume in failing hearts because of the afterload-reducing benefit (decreased transmural left ventricular end-systolic pressure) of increased intrapleural pressure in dilated ventricles. Moreover, jet ventilation did not have positive effects on myocardial function and had negative effects on left ventricular elastance in the postjet ventilation period in both normal and failing hearts. Cardiac assist by jet ventilation was not cycle specific, suggesting no selective benefit of jet ventilation over conventional positive-pressure ventilation during heart failure. These studies demonstrate a negative inotropy associated with jet ventilation that, during heart failure, may compromise the general benefit of positive-pressure-mediated increases in intrapleural pressure.

A potential clinical method for calculating transmural left ventricular filling pressure during positive end-expiratory pressure ventilation: an intraoperative study in humans

OBJECTIVES

This study sought to investigate whether right atrial pressure could be used to estimate pericardial pressure during positive end-expiratory pressure (PEEP).

BACKGROUND

Because of elevated intrathoracic pressure during PEEP, pulmonary capillary wedge pressure may not accurately reflect left ventricular preload. An estimate of pericardial pressure during PEEP would allow assessment of transmural filling pressure.

METHODS

In eight patients, at the start of cardiac surgery, pericardial and pleural pressures were recorded by balloon transducers placed over the anterolateral left ventricular wall. We also recorded intravascular pressures and left ventricular short-axis area by transesophageal echocardiography.

RESULTS

A stepwise increase in PEEP from 0 to 15 cm H2O caused a linear increase in pleural pressure from 0.3 +/- 0.6 (mean +/- SEM) to 6.1 +/- 0.8 mm Hg (p < 0.01). Pericardial pressure increased from 2.3 +/- 0.5 to 5.9 +/- 0.6 mm Hg (p < 0.01). The correlation between right atrial (Pra) and pericardial pressure (Pperic) was good: Pra = 0.85 x Pperic + 1.8, r = 0.77. The correlation between changes in right atrial pressure and in pericardial pressure was better: delta Pra = 0.96 x delta Pperic -0.2, r = 0.97. Pulmonary capillary wedge pressure increased with PEEP (p < 0.05), whereas left ventricular area decreased (p < 0.05). However, there was a progressive reduction in transmural pressure, calculated as wedge pressure minus pericardial pressure (p < 0.05), and in transmural pressure, estimated as wedge pressure minus right atrial pressure (p < 0.05). The estimated transmural filling pressure correlated (r = 0.86) with end-diastolic area.

CONCLUSIONS

The present observations suggest that right atrial pressure may be used to estimate changes in pericardial pressure with PEEP and that pulmonary capillary wedge pressure minus right atrial pressure is a potentially clinically useful approximation of transmural filling pressure.

Effects of (nCPAP) on cardiac function awake and asleep

Continuous positive airway pressure (CPAP) leads to a fall in cardiac output (CO) when applied to individuals with normal cardiac function. However, some reports indicate that CPAP improves CO in selected patients with congestive heart failure, although other reports disagree. Nasal CPAP effectively reverses obstructive sleep apnoea, a condition in which vigorous inspiratory efforts against an occluded upper airway can induce falls in CO. The cardiovascular effects of CPAP in such patients will depend on the balance between the indirect cardiac benefits resulting from relief of apnoeas, and the direct effects of positive pressure on the heart itself.

Risks and hazards of mechanical ventilation: a collective review of published literature

A collective, analytic review was undertaken of all available published scientific papers that reported data about risks, hazards, adverse effects, or complications from augmentation of blood gas exchange by means of intensive closed system positive pressure mechanical ventilation. On the basis of the data collected, the adverse effects of intensive positive pressure mechanical ventilation were classified into the following groups: oxygen toxicity; adverse effects from excessive ventilatory pressures, volumes, and flow rates; adverse effects from tracheal intubation; dangers from adjuvant drugs; stress-related sequelae; altered enzyme and hormone systems; nutritional problems; and psychologic trauma. A bibliography pertaining to each group of adverse effects has been prepared. In addition, the reported incidence of adverse effects resulting from intensive mechanical ventilation in patients in clinical intensive care is shown. Clinical and laboratory observations of patients who receive intensive positive pressure mechanical ventilation in respiratory intensive care units have yielded some data, and findings from experimental studies in normal volunteers and laboratory animals have also been collected and reviewed. Tables, charts, and graphs that summarize the pertinent findings are presented and discussed. The following conclusions are drawn from critical evaluation of the collected data: (1) Closed system positive pressure mechanical ventilation applied at mild to moderate levels of intensity is a safe and effective method for augmenting deficient blood gas exchange in most patients who are in acute respiratory failure. (2) On the other hand, intensive levels of mechanical ventilator support or inappropriate methods of applying mechanical ventilation may be accompanied by a variety of risks, hazards, adverse effects, and complications that may further injure the failing lungs or may add significantly to the morbidity and mortality rates of patients in whom it is applied. (3) Because of the unfavorable risk/benefit ratio of intensive positive pressure mechanical ventilation, physicians should consider the use of alternative methods that are now available for augmenting blood gas exchange in patients in acute respiratory failure who are not adequately treated by safe (mild to moderate) levels of positive pressure mechanical ventilation instead of electing to increase the intensity of positive pressure mechanical ventilation to more dangerous (intensive) levels.

Doppler evaluation of right ventricular outflow impedance during positive-pressure ventilation

Positive-pressure ventilation has often been advocated to increase oxygen delivery. This ventilation mode itself, however, can impair right ventricular ejection and, thus, diminish cardiac output. In this study, alterations of right ventricular outflow impedance were evaluated after stepwise increases of positive end-expiratory pressure (PEEP). Different pulmonary artery flow characteristics were evaluated with transesophageal echocardiography in mechanically ventilated postoperative coronary artery bypass surgery patients without pulmonary hypertension. A progressive decrease of pulmonary artery flow velocity and time velocity integrals was found with increasing PEEP levels. No changes in acceleration time or pre-ejection period were observed. In order to decrease the influence of heart rate, the ratios of the different pulmonary artery flow characteristics were calculated. At end-inspiration, both the ratio of acceleration time to right ventricular ejection period and the ratio of pre-ejection period to right ventricular ejection period showed progressive increases above 10 cmH2O positive end-expiratory pressure (13.3% at the level of 15 cmH2O and 8.5% at the level of 20 cmH2O). In this study, acceleration time appears not to be of importance in ventilated patients. These data strongly support the hypothesis that intermittent squeezing of the pulmonary arterial tree during inspiration, rather than positive end-expiratory pressure, creates an increase of right ventricular outflow impedance.

Juxtacardiac pleural pressure during positive end-expiratory pressure ventilation: an intraoperative study in patients with open pericardium

OBJECTIVES

This study was conducted to measure the cardiac constraining effect of the lungs during positive end-expiratory pressure and relate extracardiac pleural pressure (radial stress) to airway pressure, right atrial pressure and left ventricular filling.

BACKGROUND

During positive end-expiratory pressure ventilation, the extracardiac pressure is elevated, and therefore intracavitary filling pressure does not reflect ventricular preload. Estimates of this pressure might be useful clinically to assess left ventricular preload.

METHODS

In eight patients who had undergone coronary or valvular surgery and whose pericardium was left widely open, a flat pleural balloon transducer was placed over the anterolateral left ventricular wall. We recorded pulmonary capillary wedge pressure, right atrial pressure and left ventricular short-axis end-diastolic area by transesophageal echocardiography. Incremental positive end-expiratory pressure was applied.

RESULTS

Extracardiac pleural pressure increased (p < 0.01) from 0.6 +/- 1.8 (+/- SD) to 2.4 +/- 1.8, 5.3 +/- 1.5 and 8.2 +/- 1.5 mm Hg at a positive end-expiratory pressure of 5, 10 and 15 cm H2O, respectively. The slope relating extracardiac pleural pressure to positive end-expiratory pressure (in mm Hg) was 0.70 +/- 0.10, and the intercept was zero. Increasing extracardiac pleural pressure was associated with a progressive increase in pulmonary capillary wedge pressure and a decrease in left ventricular end-diastolic area. Consequently, although pulmonary capillary wedge pressure and left ventricular area changed in opposite directions, the value of pulmonary capillary wedge pressure minus extracardiac pleural pressure correlated positively with left ventricular area (r = 0.95, p < 0.001). Changes in right atrial pressure (Pra) correlated with changes in extracardiac pleural pressure (Ppleural): delta Pra = -0.3 + 0.56. delta Ppleural (r = 0.89, p < 0.001).

CONCLUSIONS

In postoperative patients with open pericardium, pulmonary capillary wedge pressure minus extracardiac pleural pressure predicts left ventricular end-diastolic area during positive end-expiratory pressure. Further studies should be done to determine whether the observed relations between airway pressure and extracardiac pleural pressure and between right atrial pressure and extracardiac pleural pressure may give clinically useful estimates of left ventricular preload during positive end-expiratory pressure.

The effect of sustained positive airway pressure on derived cardiac output in children

It is well established that the improvement in gas exchange that occurs with positive pressure ventilation may come at the expense of a decrease in cardiac output and oxygen delivery. Clinical observation suggests that children and infants may be more resistant than adults to the falls in cardiac output induced by positive airway pressure. The aims of this study were to quantify the effect of a sustained increase in intrathoracic pressure on cardiac output and stroke volume, and to determine whether this change is age-related. Twenty-eight children undergoing general anaesthesia were studied. Cardiac output was derived using pulsed wave Doppler techniques at four different levels of sustained positive airway pressure, and stroke volume was calculated. The relationship between airway pressure and both cardiac output and stroke volume was examined using a general linear model which included age as a continuous variable. Cardiac output decreased with increasing levels of sustained positive airway pressure (P = 0.001). The fall in SV for a given airway pressure increased with increasing age (P = 0.02). The mechanisms responsible for the increase of the magnitude of the fall in stroke volume with age remain to be elucidated.

The effect of peep-ventilation on cardiac function in closed chest pigs

OBJECTIVE

Does ventilation with positive end-expiratory pressure (PEEP) depress myocardial contractility?

DESIGN

Ten piglets were anaesthetized and prepared for the measurement of cardiac output (SV) and right (MRAPtm) and left (MLAPtm) mean transmural atrial pressure, the latter serving as indices of preload. 500 ml of autologous blood was re-transfused during intermittent positive pressure ventilation without PEEP (IPPV) and continuous positive pressure ventilation with 15 cm H2O PEEP (CPPV).

MEASUREMENTS AND RESULTS

Right and left ventricular function curves were drawn by plotting MRAPtm and MLAPtm respectively versus the corresponding strokevolumes before and after re-transfusion. Similar inclinations were obtained during IPPV and CPPV on either side of the heart.

CONCLUSIONS

Although the ventricular function curves during IPPV and CPPV covered partially different preload levels, the results suggest that CPPV i.e. PEEP does not affect myocardial contractility.

Effects of continuous negative extrathoracic pressure ventilation on left ventricular dimensions and hemodynamics in dogs

It has been reported that continuous negative extrathoracic pressure ventilation (CNETPV) depresses cardiac output less than continuous positive pressure ventilation (CPPV) does, and this difference may be related to the different effects of two ventilatory modes on preload. We performed simultaneous measurements of hemodynamics and left ventricular short axis dimensions by transesophageal echocardiography (TEE) to evaluate left ventricular preload and function during CNETPV and CPPV in normal dogs. Hemodynamic measurements and simultaneous TEE recording were performed at 5 successive periods; 1) the first control period of intermittent positive pressure ventilation (IPPV1), 2) CNETPV with negative end-expiratory pressure (NEEP) of -10 cmH2O (CNET10), 3) CNETPV with NEEP of -15 cmH2O (CNET15), 4) the second control period of IPPV (IPPV2), and 5) CPPV with PEEP of 15 cmH2O (CPPV15). Left ventricular end-systolic and end-diastolic dimension (LVESD and LVEDD), ejection fraction (EF) and fractional shortening (FS) were measured from TEE recordings. Both CNET10 and CNET15 induced no significant changes in hemodynamics and left ventricular dimensions, compared with those during IPPV1. However, CPPV15 reduced cardiac output and stroke volume (SV) and increased heart rate significantly, compared with IPPV2. CPPV15 significantly decreased LVEDD compared with IPPV2. Neither EF nor FS showed any significant change throughout the experiment. These results indicate that CNETPV preserved cardiac output because it maintained the preload and the left ventricular function.

Haemodynamic effects of pressure support and PEEP ventilation by nasal route in patients with stable chronic obstructive pulmonary disease

BACKGROUND

Intermittent positive pressure ventilation applied through a nasal mask has been shown to be useful in the treatment of chronic respiratory insufficiency. Pressure support ventilation is an assisted mode of ventilation which is being increasingly used. Invasive ventilation with intermittent positive pressure, with or without positive end expiratory pressure (PEEP), has been found to affect venous return and cardiac output. This study evaluated the acute haemodynamic support ventilation by nasal mask, with and without the application of PEEP, in patients with severe stable chronic obstructive pulmonary disease and hypercapnia.

METHODS

Nine patients with severe stable chronic obstructive pulmonary disease performed sessions lasting 10 minutes each of pressure support ventilation by nasal mask while undergoing right heart catheterisation for clinical evaluation. In random order, four sessions of nasal pressure support ventilation were applied consisting of: (1) peak inspiratory pressure (PIP) 10 cm H2O, PEEP 0 cm H2O; (2) PIP 10 cm H2O, PEEP 5 cm H2O; (3) PIP 20 cm H2O, PEEP 0 cm H2O; (4) PIP 20 cm H2O, PEEP 5 cm H2O.

RESULTS

Significant increases in arterial oxygen tension (Pao2) and saturation (Sao2) and significant reductions in arterial carbon dioxide tension (PaCO2) and changes in pH were observed with a PIP of 20 cm H2O. Statistical analysis showed that the addition of 5 cm H2O PEEP did not further improve arterial blood gas tensions. Comparison of baseline values with measurements performed after 10 minutes of each session of ventilation showed that all modes of ventilation except PIP 10 cm H2O without PEEP induced a small but significant increase in pulmonary capillary wedge pressure. In comparison with baseline values, a significant decrease in cardiac output and oxygen delivery was induced only by the addition of PEEP to both levels of PIP.

CONCLUSIONS

In patients with severe stable chronic obstructive pulmonary disease and hypercapnia, pressure support ventilation with the addition of PEEP delivered by nasal mask may have short term acute haemodynamic effects in reducing oxygen delivery in spite of adequate levels of SaO2.

An alarm for monitoring CPAP

We have built a device for use within the hospital and at home that is designed to warn of circuit disconnection when used in conjunction with continuous positive airway pressure (CPAP) therapy delivered via ventilators or CPAP generating systems. The Royal Children's Hospital CPAP alarm is a compact, battery operated alarm and monitor of circuit pressure. The device includes intrinsic safety features including a safety blow-off valve, a high pressure alarm and design features that make the device practical, safe and easy to use by both trained hospital personnel and home care attendants with limited training.

Physiologic effects of positive end-expiratory pressure in patients with chronic obstructive pulmonary disease during acute ventilatory failure and controlled mechanical ventilation

Dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEPi) are observed in patients with chronic obstructive pulmonary disease (COPD) and flow limitation. Several reports suggest that PEEP levels approaching PEEPi reduce inspiratory load due to PEEPi, without further hyperinflation. Hence PEEP should not increase intrathoracic pressure or affect hemodynamics and gas exchange. To verify this hypothesis, the effects of PEEP (0 to 15 cm H2O) on respiratory mechanics, hemodynamics, and gas exchange were studied in nine COPD patients during controlled mechanical ventilation. PEEP levels approaching PEEPi (9.8 +/- 0.5 cm H2O) did not affect the expiratory flow/volume relationship, confirming the presence of flow limitation. PEEP levels of 5 and 10 cm H2O did not change lung volume and PEEPi in the respiratory system (PEEPtot,rs) and chest wall (PEEPtot,cw) or affect hemodynamics and gas exchange. When applied PEEP overcame PEEPi, changes in lung volume and the expiratory flow/volume relationship were observed. PEEPtot,rs and PEEPtot,cw also increased. Under these circumstances, PEEP increased static elastance in both the respiratory system and the chest wall, reducing cardiac index and affecting hemodynamics and gas exchange. Our data show that in mechanically ventilated COPD patients with PEEPi due to flow limitation, PEEP levels exceeding the 85% of PEEPi (Pcrit) caused further hyperinflation and compromised hemodynamics and gas exchange.

Transesophageal Echo-doppler evaluation of the hemodynamic effects of positive-pressure ventilation after coronary artery surgery

Transesophageal echocardiography was used to extend knowledge about the impact of positive end-expiratory pressure (PEEP) during mechanical ventilation on right and left ventricular function and right ventricular impedance. At 20 cmH2O PEEP, a progressive increase of right ventricular end-diastolic area was seen (27%) that coincided with a reduction of early left ventricular filling velocity (25%) across the mitral valve, and a decrease of both pulmonary artery flow velocity (end-expiration 27% and end-inspiration 42%) and time-velocity index (end-inspiration 25%). As these changes were not accompanied by a change of the fractional area of contraction, the increase of the right ventricular diameter might be explained by right ventricular compensation due to an imbalance between augmented right ventricular impedance and reduced venous return.

Haemodynamic effects of selective positive end-expiratory pressure after unilateral pulmonary hydrochloric acid-aspiration in dogs

We investigated 1) the effects of HCl-mediated acute left lung injury on regional juxtacardiac pressures and 2) the haemodynamic effects of different modes of ventilation before and after induction of left lung injury. The study was done in 7 mechanically ventilated, anaesthetized dogs. Juxtacardiac pressures and haemodynamic variables were recorded during 1) differential ventilation (DV) with zero positive end-expiratory pressure (PEEP = 0) and 2) DV with general (G) PEEP and selective right (R) and left (L) lung PEEP. Left lung injury increased left, but not right pleural pressure of pericardial pressure. Pulmonary vascular resistance (PVR) and pulmonary artery pressure (PAP) were increased moderately. Cardiac output (CO) did not change. GPEEP reduced LV filling and cardiac output markedly and by approximately the same degree before and after lung injury. The haemodynamic effects of LPEEP were minor before as well as after the induction of lung injury. RPEEP, which had only moderate haemodynamic effects during control, caused a marked reduction in cardiac function after the induction of left lung injury. The transmission of airway pressure to the pleura was reduced in the diseased lung. These results suggest that serious haemodynamic side effects may be avoided by applying PEEP selectively to the diseased lung.

Effects of positive end-expiratory pressure (PEEP) ventilation on the exocrine pancreas in minipigs

The effect of positive end-expiratory pressure (PEEP) ventilation on the pancreas was studied in 22 anesthetized minipigs. Five pigs were treated with intermittent positive pressure ventilation (IPPV) for 21 h (Group I) and six pigs were treated with a PEEP of 15 cm H2O for 21 h (Group II). We next explored the influence of PEEP ventilation while stimulating the pancreas with Ceruletide, a synthetic cholecystokinin (CCK) analog, at 2.3 micrograms/kg per h.i.v. for 3 h. Ventilation with IPPV for 4 h (n = 5, group III) was compared with PEEP of 15 cm H2O for 4 h (n = 6, group IV). Changes in serum-lipase were observed in all groups. The average lipase level rose from 12.6 U/l to 67 U/l group I and from 15.1 U/l to 129.2 U/l in group II (P = 0.025 for group I vs group II). In group IV (PEEP and Ceruletide), the mean lipase level rose about six times more than in group III (IPPV and Ceruletide). The difference was significant (P less than 0.00005). By three-factor ANOVA analysis, effects due to the drug (P less than 0.000006) and to PEEP (P less than 0.038) as well as a threefold interaction could be demonstrated, i.e., using Ceruletide a greater PEEP effect on lipase than without the drug (P less than 0.023) took place. There were no significant histological changes of the pancreas in groups I and III. In group II (21 h PEEP), vacuolization of acinar cells was evident; on the ultrastructural level, indication was given that these vacuoles derived both from the Golgi apparatus and from fusion of individual zymogen granules. In group IV (PEEP and Ceruletide), the focal appearance of fatty-tissue necrosis and acinar cells as well as hemorrhage of the gland was observed. We conclude that PEEP ventilation impairs the function of the pancreas and produces even more deleterious effects when the gland is stimulated. These effects should be considered when PEEP is used in clinical practice in the treatment of acute pancreatitis.

The effect of positive end-expiratory pressure ventilation on atrial filling

As a measure of atrial filling, left and right auricular diameter and free wall segment length were recorded by sonomicrometry during incremental positive end-expiratory pressure (PEEP) in eight acutely instrumented closed chest dogs. The effect of PEEP was assessed with the pericardium open (n = 6) and closed (n = 8). On both occasions, PEEP decreased left auricular diameter (P less than 0.05). PEEP also caused a reduction in right auricular diameter with the pericardium open (P less than 0.05), while the variable was unchanged with the pericardium closed. PEEP did not cause any changes in either left or right free wall segment lengths. Both left and right auricular pressure-diameter relationships were progressively shifted leftwards with incremental PEEP. These observations suggest that PEEP may reduce left ventricular output not only by interfering with passive ventricular filling, but also by reducing atrial dimensions.

Endocrine responses to positive end-expiratory pressure ventilation in patients who have recently undergone heart surgery

The effect of positive end-expiratory pressure ventilation (PEEP) on angiotensin II and atrial natriuretic factor (ANF) was studied postoperatively following heart surgery. In nine patients pressures were recorded in the radial artery, pulmonary artery and the right atrium. PEEP of 5 cmH2O (0.5 kPa) and 10 cmH2O (1 kPa) increased angiotensin II from 38.8 +/- 20.3 (mean +/- s.e.mean) to 56.7 +/- 29.6 (n.s.) and 66.7 +/- 28.7 (P less than 0.05) pmol/l, respectively. Plasma-ANF showed no significant changes during PEEP. Pulmonary artery wedge pressure increased from 12.9 +/- 2.0 to 14.1 +/- 2.0 (n.s.) and 18.5 +/- 2.1 (P less than 0.01) mmHg, and right atrial pressure from 8.3 +/- 1.7 to 9.8 +/- 1.7 (n.s.) and 12.9 +/- 1.7 (P less than 0.01) mmHg with 5 and 10 cmH2O (0.5 and 1.0 kPa) of PEEP, respectively. Systemic blood pressure tended to decrease (n.s.) with PEEP. In conclusion, PEEP markedly increased angiotensin II. This may represent an important compensatory mechanism, helping to prevent reduction in aortic pressure during PEEP. ANF, however, did not change with PEEP of 5 or 10 cmH2O (0.5 and 1.0 kPa).

Combined thermodilution and two-dimensional echocardiographic evaluation of right ventricular function during respiratory support with PEEP

In ten patients requiring respiratory support for an episode of acute respiratory failure (ARF), the best therapeutic level of PEEP was determined by measurement of changes in lung and chest wall compliance (CT) during a PEEP challenge from 0 to 20 cm H2O. During this challenge, hemodynamic monitoring combined with thermodilution measurement of right ventricular (RV) ejection fraction (EF) and two-dimensional echocardiographic measurement of RV size permitted assessment of the effects of increasing levels of PEEP on RV function. RV preload, as reflected by RV end-diastolic volume (EDV) and two-dimensional RV end-diastolic area (EDA), remained unchanged and RV diastolic compliance progressively decreased. On the other hand, RV systolic function, as assessed by RVEF and two-dimensional RV fractional area contraction (FAC), was progressively depressed. Substantial deleterious effects of PEEP were noted at high levels of PEEP including reduced CT and augmented pulmonary vascular resistance. Inadequate increase in RV preload to compensate for increased RV afterload resulted in depressed RV systolic function and contributed to the reduction in cardiac output. Finally, two-dimensional echocardiography proved to be more sensitive than fast-response thermodilution to evaluate change in RV function.