Effects of Pericardial Constraint on Left Ventricular Mechanoreceptor Activity in Cats

BACKGROUND: The purpose of this study was to assess the effect of pericardial constraint on the activity of left ventricular (LV) mechanoreceptors with nonmyelinated vagal afferents. METHODS AND RESULTS: Single-unit activity of cervical vagal afferents (conduction velocity, 1.6 +/- 0.5 m/s) was recorded in six cats anesthetized with alpha-chloralose. Discharge frequency during diastole (DFdiastole) and systole (DFsystole) was determined after correction for conduction delay of the nerve action potential. When the pericardium was closed and LV end-diastolic pressure (LVEDP) was approximately 5 mm Hg, DFdiastole and DFsystole were 1.3 +/- 1.0 and 0.3 +/- 0.1 impulses per second, respectively. Volume expansion increased LVEDP, LV transmural LVEDP, and segment length and was associated with a significant increase in DFdiastole. At a given LVEDP, DFdiastole was significantly greater in the absence of the pericardium than with the pericardium closed. Removal of the pericardium increased the slope of the relation between DFdiastole and intracavitary LVEDP but did not alter the slope of the relations between DFdiastole and transmural LVEDP and LV segment length. CONCLUSIONS: These results suggest that, rather than the absolute value of intracavitary LVEDP, transmural LVEDP and distension appear to be more important determinants of diastolic LV mechanoreceptor activity and that pericardial constraint may attenuate mechanoreceptor activity by limiting cardiac distension.