Positive end expiratory pressure produced a 12 percent decrease in left ventricular end diastolic volume index and a 9 percent decrease in cardiac index. Volume loading studies revealed no alterations in left ventricular performance (the relation between cardiac index and left ventricular end diastolic volume index) or left ventricular systolic function (the relation between systolic blood pressure and left ventricular end systolic volume index).
PEEP produced a 5 percent decrease in right ventricular end diastolic volume index (not significant), and volume loading studies found no change in right ventricular performance (the relation between cardiac index and right ventricular end diastolic volume index). Despite a decrease in oxygen demand (cardiac index), neither coronary sinus blood flow nor myocardial oxygen consumption were altered by positive end expiratory pressure. Myocardial lactate utilization was variable while receiving PEEP because half of the patients continued to extract lactate while the other half produced lactate, suggesting ischemic anaerobic metabolism. Those patients who released lactate while receiving PEEP had no depression in myocardial performance or left ventricular systolic function.
This study was designed to evaluate the hemodynamic and myocardial metabolic response to increasing increments of positive end expiratory pressure between 4 and 8 hours following elective coronary bypass surgery. Previous studies found no significant change in ventricular function or myocardial metabolism between 4 and 6 hours after elective, uneventful coronary bypass surgery. Therefore, this period was employed to evaluate the response to PEEP. Visit to Canadian Health Care
The assessment of the hemodynamic response to PEEP requires a reproducible method of measuring ventricular volumes and ejection fractions. Nuclear ventriculographic studies provide an accurate assessment of left ventricular ejection fraction with a maximum error of ±5 percent at our institution. The calculation of left ventricular volumes combines the error of the thermodilution stroke index measurement (±10 percent) and the left ventricular ejection fraction measurement. The reproducibility of the left ventricular end diastolic volume measurement was found to be within 5 percent for volume indices between 50 and 70 ml/m2. The thermodilution-derived left ventricular volumes also correlated well with scintigraphic volume measurements and contrast ventriculographic volumes. The change in left ventricular volumes induced with increasing levels of PEEP has less variability than the volume measurements. The assessment of right ventricular function is difficult by all currently available methods. Nuclear ventriculographic measurement provides a reproducible estimate of right ventricular ejection fraction, which avoids assumptions about right ventricular geometry.-* Slutsky and colleagues found that the reproducibility and interobserver variability for right ventricular ejection fraction measurements were less than 4 percent. However, right atrial and pulmonary arterial overlap can result in underestimation of right ventricular ejection fraction and overestimation of right ventricular volumes. Despite its limitations, nuclear ventriculographic measurement offers significant advantages over other techniques to assess ventricular function. Both echocardiographic study and sonomicrometric examination provide estimates of ventricular volumes which are critically dependent on ventricular geometry. Postoperative septal wall motion abnormalities also complicate the assessment of ventricular function when using these techniques.
The calculation of ventricular volumes by nuclear ventriculography permits an assessment of right and left ventricular performance (the relation between cardiac index or stroke index and right or left end diastolic volume index). However, pressure-volume relations provide more sensitive indices of systolic and diastolic function.’ Unfortunately, the assessment of transmural pressures is difficult in patients who are ventilated with PEEP. Both pleural and esophageal pressure measurements may be misleading because of pressure variations at different levels in the thorax and in different positions near the pericardium. Neither pleural nor esophageal pressures were measured in this study, and we did not report intrathoracic pressure measurements.
We found no decrease in right or left ventricular performance or left ventricular systolic function with the application of 15 cm H20 PEEP. Calvin and colleagues, employing nuclear ventriculographic measurement, also found no depression in left ventricular function with the application of PEEP. Jardin et al and Scharf et al found that volume loading at 20 to 25 cm HaO PEEP shifted the interventricular septum to the left. They postulated that ventricular interaction may have compromised left ventricular function. We were unable to detect a shift of the interventricular septum to the left with the application of 15 cm H20 PEEP and volume loading.
Positive end expiratory pressure did not change coronary sinus blood flow or myocardial oxygen consumption significantly. Tucker and Murray suggested that coronary blood flow decreased with the application of PEEP. The thermodilution technique does not permit an assessment of regional perfusion. A decrease in subendocardial blood flow could have occurred in our study, but might not have been detected by our technique. Myocardial lactate utilization provides the best evidence for heterogeneous coronary perfusion. Gertz and colleagues found that a decrease in myocardial lactate extraction to levels below 0.2 mmol/L represented lactate extraction from normally-perfused regions and lactate production from inadequately-perfused regions in patients with coronary artery disease. The decrease in myocardial lactate utilization with the application of 15 cm HsO PEEP may have represented underperfusion, anaerobic metabolism and lactate release from poorly-perfused regions of the heart. The 17 patients who had evidence of ischemic anaerobic metabolism during the application of 15 cm H20 PEEP may have had subtle alterations in ventricular function which were not detected by our techniques. Decreased diastolic compliance (the relation between diastolic pressure and volume) is the most sensitive index of myocardial ischemia, but could not be employed in this study because in-trathoracic transmural pressures were not calculated. Our patients were recovering uneventfully following coronary bypass surgery and had normal ventricular function. Patients with myocardial ischemia may be at greater risk of ventricular decompensation with the application of PEEP. Ischemic myocardial metabolism may be the mechanism for the depressed cardiac function following the application of PEEP, as reported by Hechtman and colleagues.’ Some patients with acute respiratory distress due to sepsis have abnormal cardiac function. In this setting, the application of PEEP may induce ischemic myocardial metabolism and depress ventricular function.
The application of 15 cm H20 of PEEP was found to decrease left ventricular preload and cardiac index. Right and left ventricular performance were unchanged. Myocardial metabolic studies suggested that some patients developed cardiac ischemia with PEEP without hemodynamic alterations. Therefore, PEEP should be employed cautiously in patients who are at risk for myocardial ischemic injury.