EFFECT OF UPRIGHT TILT ON VENTRICULAR VASCULAR COUPLING IN CHRONICALLY INSTRUMENTED PRIMATES

Studies of the hydraulic loading conditions on the heart in humans, es pecially pulsatile load, have primarily been limited to the supine sta te. Therefore, we have chosen a nonhuman primate model, the baboon, to assess left ventricular/vascular coupling in both supine and upright positions. Primate subjects were studied by catheterization under seda tion and then after surgical implantation of transducers. This allowed the evaluation of postural stress in the chronically instrumented con scious baboon and then after light dissociative doses of ketamine. Bas ic hemodynamic variables were evaluated for baboons in supine and upri ght positions. Fourier analysis was applied to aortic pressure and flo w to obtain input and characteristic impedance and the ratio of pulsat ile (W(p)) to total (W(t)) left ventricular power (W(p)/W(t)). The aor tic reflected, or backward, pressure was also calculated. Peripheral r esistance increased (P = 0.01) and reflected pressure decreased (17.74 +/- 1.50 vs. 15.21 +/- 2 mmHg; P < 0.01) in upright subjects. Charact eristic impedance and W(p)/W(t) were unchanged. Postoperatively, perip heral resistance increased (2,651 +/- 311 vs. 3,667 +/- 276; P < 0.05) and mean power and W(t) decreased (P < 0.01) without changes in refle cted pressure. All variables were unchanged after light dissociative d oses of ketamine. Thus there is no significant change in efficiency of left ventricular/vascular coupling formulated in terms of W(p)/W(t) o r input impedance with postural stress.