Comparative cardiac dynamic effects of dobutamine and isoproterenol in conscious instrumented dogs.

The cardiac dynamic consequences were evaluated of constant infusions of dobutamine and isoproterenol at graded dose levels into conscious, healthy instrumented dogs. Measurements were made of simultaneous changes in left ventricular internal diameter, pressure, aortic pressure and rate of rise of left ventricular pressure(dP/dt), as well as the left ventricular electrogram. From these primary variables, derived variables were computed using programs in a minicomputer system. The data showed that, with increasing doses of dobutamine there were significant linear increases in all measured indexes of myocardial contractility, such as the rate of rise of left ventricular pressure at a developed isovolumic pressure of 40 mm Hg (dP/dt/P40), mean velocity of left ventricular fiber shortening, ejection fraction and stroke work. These changes in myocardial contractility occurred without changes in end-diastolic volume, mean aortic pressure or heart rate when dobutamine was infused in doses of 5 to 20 mug/kg per min. Isoproterenol also produced linear changes in indexes of myocardial contractility but in doses of 0.02 to 0.10 mug/kg per min, it produced a significantly higher heart rate at any given level of contractility than that produced by dobutamine. Cardiac minute work (heart rate X stroke work) was increased by both drugs. However, with infusions of isoproterenol the amount of cardiac minute work was significantly limited because of the changes in heart rate, whereas with dobutamine cardiac minute work could be increased to a higher level as a function of changes in myocardial contractility alone without changes in heart rate. These data suggest that dobutamine selectively increases myocardial contractility.

Regional coronary blood flow in ponies.

Regional coronary blood flow was measured by injecting radioactive microspheres (15 mum +/- 5 in diameter) into the left atrium of anesthetized ponies with surgically prepared open thorax before and during occlusion of the coronary arteries. The normal blood flow to the myocardium of the interventricular septum and the left ventricular wall were highest, followed in decreasing order by the right ventricular wall, the interatrial septum, the atrial walls, and the valves. Measurement of transmural blood flow in the normal left ventricle yielded a mean endocardial/epicardial flow ratio of 1.36 in the free wall. The left ventricular flow ratio was 1.33 in the septal wall. The percentage of the left ventricular myocardium made ischemic during occlusion of the right coronary artery or of the left coronary artery (cranial descending and circumflex arteries) was approximately equal. Blood flow to the ischemic areas of the left ventricle after occlusion of coronary arteries ranged from 3.8 to 20.6% of the normal flow. A disproportionate decrease in flow to the endocardial regions of the left ventricle was also observed in ischemic areas (mean inner/outer left ventricular wall flow ratio was 68.89% of the normal flow ratio).

Estimation of left ventricular mass in conscious dogs.

The short-axis area-length method for the estimation of left ventricular mass (LVM) and volume was validated in 24 formaldehyde-fixed canine hearts, using the truncated ellipsoid model. Estimates of the short-axis muscle area were highly correlated with directly measured values (R = 0.92; P less than 0.001). Similarly, LVM calculated using the area-length method showed good correlation with the actual weight of the left ventricle (LVW) (R = 0.85; P less than 0.001). When the regression equation was used, retrospectively, to correct the LVM estimates, the correlation between the actual LVW and the calculated LVM was markedly improved (R = 0.96; P less than 0.001). In awake dogs instrumented for measuring transverse and long axis ventricular dimensions using sonomicrometry, short-axis two-dimensional echocardiograms were used to convert the sonar external transverse dimensions to true diameters. The combined use of the corrected diameters and the regression equation to calculate LVM in the intact dog resulted in values that were highly correlated with the actual LVW (R = 0.95; P less than 0.001). Application of this technique for monitoring LVM in awake instrumented dogs demonstrated the method to be reproducible from day to day and to be sensitive enough to detect serial changes in mass such as during the development of left ventricular hypertrophy.