Left ventricular adaptation to volume overload from large aortocaval fistula

The manner in which the left ventricle responds to an acute volume overload has not been well defined. Left ventricular performance was studied in six chronically instrumented resting awake dogs in the control state and serially after creation of a large abdominal aortocaval shunt. Ultrasonic transducers measured minor and major axis diameters and equatorial wall thickness. Left ventricular pressure was obtained with micromanometers. Cardiac contractility was evaluated by the load-independent contractility index, EMAX(sc) (slope of the normalized end-systolic equatorial midwall circumferential stress-equatorial midwall circumference relationship). By 1 week postshunt, the dog had clinical signs of congestive heart failure (ascites, dyspnea, limb edema); although systolic aortic pressure remained stable, heart rate, end-diastolic volume, pulse pressure (systolic minus diastolic pressure), cardiac output, minute work and dp/dt(max) were significantly increased. At 1 week the calculated left ventricular mass was increased by 10.1% ± 4.0% above control. EMAX(sc) was significantly increased immediately after shunting but returned to control at 1 day and was less than control at 1 week. Thus adaptation of the left ventricle to acute volume overload is characterized by use of inotropic, chronotropic, and Starling reserves. However, chronic volume overload is characterized by decreased inotropic state and an apparent increase in hemodynamic performance (dp/dt(max), cardiac output, minute work), which appears to be maintained by an increase in cardiac mass and by chronotropic and Starling reserves.