How the two sides of the heart adapt to graded impedance to venous return with head-up tilting.

OBJECTIVES

The study sought to probe whether the adaptation of the right ventricle to reduced preload may influence that of the left ventricle (interdependence) and whether and how this mechanism contributes to maintain an adequate pump function.

BACKGROUND

A study like this requires that subjects be normal, restraint of venous return be gradual, systolic function and diastolic filling and dimensions of either ventricle be monitored.

METHODS

Of 30 healthy men (mean [+/- SD] age 35 +/- 7 years) studied with Doppler echocardiography, 20 were studied in the supine position and after 20 degrees, 40 degrees and 60 degrees tilting for 10 min; the remaining 10 subjects were also studied at the same levels of tilting for 45 min.

RESULTS

At 20 degrees, heart rate, blood pressure and stroke volume were steady; the diastolic right ventricular area was reduced (p < 0.001); and the end-diastolic dimension of the left ventricle did not vary. Tilting at 40 degrees and 60 degrees increased heart rate and diastolic pressure, decreased systolic pressure and stroke volume and reduced the diastolic dimensions of both ventricles. At any tilting level, right and left peak early inflow velocities (E) were decreased, peak late velocities (A) were unchanged, and E/A ratios were reduced, suggesting that the atrial-ventricular pressure difference was diminished bilaterally and that the atrial contribution to ventricular filling was maintained. Tachycardia at 40 degrees and 60 degrees tilting was not associated with enhancement of left ventricular fiber fractional shortening or mean velocity of shortening for any corresponding end-systolic wall stress; changes in heart rate also did not correlate with those in fiber fractional shortening and velocity of shortening. The adaptive responses to the same degrees of tilting for a duration of 45 min were comparable to those at 10 min.

CONCLUSIONS

With moderate restraint of venous return, the left ventricle maintains filling and output in response to a reduction in right ventricular diastolic volume, which increases left ventricular compliance (interdependence), and to the pulmonary blood reservoir, which compensates for an immediate decrease in right ventricular stroke volume. The decreased lung blood volume would facilitate right ventricular ejection, resulting in a normal stroke output despite the reduced preload. Thus, mechanical adjustments fully compensate for moderate reduction of venous return. A more severe reduction requires chronotropic support for the maintenance of cardiac output. With prolongation of tilting time to 45 min, adaptive mechanisms do not become exhausted in normal persons.