Adaptations of the left ventricle to chronic volume overload induced by mitral regurgitation in conscious dogs.

To assess the time-course of adaptive responses of the left ventricle to chronic volume overload, dogs were instrumented with a left ventricular (LV) micromanometer and pairs of ultrasonic crystals for the measurement of LV wall thickness (WTh), LV chamber diameter (D), and longitudinal segment length (L). Following a control study, mitral regurgitation (MR) was created by a transventricular section of the chordae tendineae. Heart rate was controlled during each study by atrial pacing. Plasma norepinephrine levels at rest were determined by high-performance liquid chromatography. Eight days (mean) after the onset of MR, enddiastolic (ED) D had increased by 9% from 34.2 +/- 2.4 mm (SEM) (P less than 0.001), with significant thinning of the wall thickness (from 8.2 to 7.7 mm, P less than 0.001). Consequently the calculated cross-sectional area (CSA) of the left ventricular wall remained the same. Peak wall stress (WSt) and EDWSt increased by 20% and 152%, respectively. During the subsequent 4 weeks, EDD progressively increased, averaging 11% above the control at 4 weeks, while EDWTh returned to the control level. Thus, the development of hypertrophy was clearly evidenced by an increase in CSA (by 8% over the control, P less than 0.001). These changes were accompanied by a consistent reduction in both peak WSt and EDWSt. Mean velocity of circumferential fiber shortening (meanVcf) and percentage shortening were significantly augmented following the onset of MR and remained at the same level thereafter, indicating no further use of the Frank-Starling mechanisms during chronic ventricular dilation.(ABSTRACT TRUNCATED AT 250 WORDS)