Thomas L. Spray

Myocardial depression by anesthetic agents (halothane, enflurane and nitrous oxide): quantitation based on end-systolic pressure-dimension relations.

The end-systolic pressure-diameter relation of the left ventricle was used to examine the effect of halothane, enflurane and nitrous oxide on left ventricular (LV) contractility in 10 dogs chronically instrumented with dimension transducers to measure LV diameter and micromanometers to measure LV transmural pressure. Contractility was assessed by the slope (EES) of the end-systolic pressure-diameter relation. A new index that identifies the dose of anesthetic necessary to depress the inotropic state by 20% (ID20) was calculated to be 0.63% for halothane and 1.55% for enflurane, indicating a greater apparent myocardial depressant effect of halothane than enflurane. However, when these agents were compared at equi-anesthetic concentrations by normalizing the ID20 to the minimal alveolar concentration of each drug, they had comparable degrees of myocardial depressant effects. This measurement technique was used in 7 patients undergoing coronary artery bypass grafting conducted under narcotic anesthesia showing that halothane induced a similar depression of contractility. The use of ID20 should allow reclassification of anesthetic agents according to their myocardial depressant effects.

Transmural Gradient in High-Energy Phosphate Content in Patients with Coronary Artery Disease

In 16 patients undergoing elective coronary artery bypass, transmural biopsies were performed during bypass but before global ischemia. Subendocardial and subepicardial halves were separately assayed in each sampled tissue. Adenosine triphosphate (ATP) levels, total adenine nucleotide content (sigma Ad), and creatine phosphate (CP) content were significantly higher (p less than 0.005) in the subepicardium than the subendocardium in regions of the heart distal to major occlusions: 35.36 +/- 2.12 nmole/mg versus 28.7 +/- 1.7 (ATP), 42.24 +/- 2.04 versus 35.6 +/- 1.6 (sigma Ad), and 29.99 +/- 4.32 +/- versus 16.35 +/- 3.48 (CP). The opposite was true in two hearts with normal coronary arteries, in which high-energy phosphates tended to be higher in the subendocardium than the subepicardium. A transmural metabolic gradient therefore exists in regions of the myocardium distal to significant coronary occlusive disease. The subendocardiums relative depression in metabolic reserve cold determine its susceptibility to ischemic damage and influence techniques designed to preserve the heart during ischemia.

The Effect of PEEP on Left Ventricular Diastolic Dimensions and Systolic Performance Following Myocardial Revascularization

To quantitate the alterations in left ventricular (LV) dimensions and performance at successive levels of positive end-expiratory pressure (PEEP), 16 patients undergoing coronary artery bypass grafting (CABG) underwent instrumentation with ultrasonic dimension transducers to measure the minor-axis diameter of the left ventricle. Matched micromanometers were placed to measure intracavitary LV pressure and intrathoracic pressure. LV pressure and dimension data were recorded and computer analyzed during continuous positive-pressure ventilation at 0, 5, 10, and 15 cm H2O of PEEP 4 to 8 hours postoperatively. Preload was determined by the end-diastolic minor-axis diameter, cardiac output was measured by thermodilution, and indices of LV contractility assessed included the maximal velocity of minor-axis shortening and the slope of the end-systolic pressure-diameter relationship. PEEP produced a progressive increase in intrathoracic pressure associated with a fall in cardiac output; this was associated with a decrease in LV end-diastolic diameter and no significant change in the maximal velocity of minor-axis shortening or the slope of the end-systolic pressure-diameter relationship. Our results indicate that PEEP of 10 cm H2O or greater will produce a significant fall in cardiac output in patients following CABG, due to a decrease in preload rather than impaired LV contractility.

The Influence of Time on the Response to Dopamine after Coronary Artery Bypass Grafting: Assessment of Left Ventricular Performance and Contractility Using Pressure/Dimension Analyses

Pressure and dimension analyses were used to quantitate the changing cardiac response to dopamine over a 24-hour interval after coronary artery bypass grafting (CABG). Ultrasonic dimension transducers were utilized to measure the minor-axis diameter of the left ventricle, and matched micromanometers were inserted to measure intracavitary left ventricular pressure and intrathoracic pressure. Pressure and dimension data were recorded and analysed by computer during dopamine infusion at 0, 2.5, 5.0, and 10.0 micrograms per kilogram per minute, at periods designated as early (2 to 4 hours after CABG) and late (18 to 24 hours after CABG). Myocardial contractile responses to dopamine (peak velocity of minor-axis shortening, maximal excursion) were similar at each dose in the early and late studies. However, overall hydraulic performance, as reflected by cardiac outputs and the areas of the pressure/diameter work loops, had augmented late dose responses. This study suggests a major change in the relationship between the heart and peripheral control mechanisms that may partially explain diminishing inotropic requirements over time, in addition to the generally accepted occurrence of improvement in contractile state and functional reserve following cardiac operation.

Anesthesia-Induced Myocardial Depression: Quantitation of Effects on Systolic and Diastolic Function

The effect of general anesthesia induced with halothane and nitrous oxide on left ventricular (LV) contractility and diastolic mechanics was directly assessed in the chronically instrumented canine model. Seven dogs were instrumented with ultrasonic dimension transducers to measure LV diameter and micromanometers to measure LV transmural pressure. Contractility was assessed by the slope (EES) of the end-systolic pressure-diameter relationship PES = EES (LES - LD). Diastolic compliance was assessed by fitting end-diastolic pressure-dimension data to the exponential P = alpha (e beta epsilon L-1), where alpha and beta are nonlinear elastic coefficients. A new index (ID20) that identifies the dose of anesthetic necessary to depress the inotropic state by 20% was calculated to be 0.63% for halothane. Contractility was progressively decreased by halothane, with EES falling from 10.1 +/- 0.6 at control to 6.7 +/- 0.4 at 1% halothane and to 4.2 +/- 0.5 at 2% halothane (p less than 0.05 at each halothane level). However, similar levels of halothane did not significantly alter alpha and beta, nor did they significantly shift the exponential diastolic compliance curve from control. Seven patients who underwent coronary artery bypass grafting conducted under narcotic anesthesia showed a similar halothane-induced depression of contractility; 0.5% halothane decreased EES from 11.5 +/- 2.0 to 8.0 +/- 2.4 (p less than 0.01). Use of ID20 allows reclassification of anesthetic agents in accord with their myocardial depressant effects, which with halothane appears to be caused by decreased inotropism without alterations in diastolic chamber mechanics.

Myocardial Depression by Anesthetic Agents (Halothane, Enflurane and Nitrous Oxide): Quantitation Based on End-Systolic Pressure-Dimension Relations

The end-systolic pressure-diameter relation of the left ventricle was used to examine the effect of halothane, enflurane and nitrous oxide on left ventricular (LV) contractility in 10 dogs chronically instrumented with dimension transducers to measure LV diameter and micromanometers to measure LV transmural pressure. Contractility was assessed by the slope (EES) of the end-systolic pressure-diameter relation. A new index that identifies the dose of anesthetic necessary to depress the inotropic state by 20% (ID20) was calculated to be 0.63% for halothane and 1.55% for enflurane, indicating a greater apparent myocardial depressant effect of halothane than enflurane. However, when these agents were compared at equi-anesthetic concentrations by normalizing the ID20 to the minimal alveolar concentration of each drug, they had comparable degrees of myocardial depressant effects. This measurement technique was used in 7 patients undergoing coronary artery bypass grafting conducted under narcotic anesthesia showing that halothane induced a similar depression of contractility. The use of ID20 should allow reclassification of anesthetic agents according to their myocardial depressant effects.