HOW THE 2 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 ri ght ventricle to reduced preload may influence that of the left ventri cle (interdependence) and whether and how this mechanism contributes t o maintain an adequate pump function. Background. A study like this re quires that subjects be normal, restraint of venous return be gradual, systolic function and diastolic filling and dimensions of either vent ricle be monitored. Methods. Of 30 healthy men (mean [+/-SD] age 35 +/ - 7 years) studied with Doppler echocardiography, 20 were studied in t he supine position and after 20 degrees, 40 degrees and 60 degrees til ting for 10 min; the remaining 10 subjects were also studied at the sa me levels of tilting for 45 min. Results. At 20 degrees, heart rate, b lood pressure and stroke volume were steady; the diastolic right ventr icular area was reduced (p < 0.001); and the end-diastolic dimension o f the left ventricle did not vary. Tilting at 40 degrees and 60 degree s increased heart rate and diastolic pressure, decreased systolic pres sure and stroke volume and reduced the diastolic dimensions of both ve ntricles. At any tilting level, right and left peak early inflow veloc ities (E) were decreased, peak late velocities (A) were unchanged, and E/A ratios were reduced, suggesting that the atrial-ventricular press ure difference was diminished bilaterally and that the atrial contribu tion to ventricular filling was maintained. Tachycardia at 40 degrees and 60 degrees tilting was not associated with enhancement of left ven tricular fiber fractional shortening or mean velocity of shortening fo r any corresponding end-systolic wall stress; changes in heart rate al so did not correlate with those in fiber fractional shortening and vel ocity of shortening. The adaptive responses to the same degrees of til ting for a duration of 45 min were comparable to those at 10 min. Conc lusions. With moderate restraint of venous return, the left ventricle maintains filling and output in response to a reduction in right ventr icular diastolic volume, which increases left ventricular compliance ( interdependence), and to the pulmonary blood reservoir, which compensa tes for an immediate decrease in right ventricular stroke volume. The decreased lung blood volume would facilitate right ventricular ejectio n, resulting in a normal stroke output despite the reduced preload. Th us, mechanical adjustments fully compensate for moderate reduction of venous return. A more severe reduction requires chronotropic support f or the maintenance of cardiac output. With prolongation of tilting tim e to 45 min, adaptive mechanisms do not become exhausted in normal per sons.