Reflected pressure waves in the ascending aorta: effect of glyceryl trinitrate

Reflected pressure waves returning to the ascending aorta are an important contribution to aortic systolic pressure and thus the load on the left ventricle. The effect of glyceryl trinitrate on pressure wave reflections in the ascending aorta was studied using the transmission of arterial pressure between two high fidelity pressure transducers. Glyceryl trinitrate 0.3 mg sublingually reduced systolic arterial pressure by 11 mmHg owing to a reduction of the late systolic pressure peak. Mean arterial pressure fell 2 mmHg, but heart rate and aortic flow did not change. The aortic input impedance was reduced at the first harmonic (control 18.4(4.1); glyceryl trinitrate 10.8(2.4) kPa.s.litre-1; p less than 0.005) but characteristic impedance was not changed (control 12.7(3.8); glyceryl trinitrate 14.2(3.3) kPa.s.litre-1). The first two harmonics of apparent phase velocity were reduced by glyceryl trinitrate (1.05 Hz: control 3314(798); glyceryl trinitrate 1772(495) cm.s-1; p less than 0.01; 2.1 Hz: control 1246(269); glyceryl trinitrate 754(127) cm.s-1; p less than 0.05), yet the foot to foot wave velocity was unchanged (control 688(112); glyceryl trinitrate 726(112) cm.s-1). There was a significant reduction in the amplitude of the global reflection coefficient at 1.05 Hz (control 0.70(0.09); glyceryl trinitrate 0.48(0.08); p less than 0.001) and at 2.1 Hz (control 0.48(0.07); glyceryl trinitrate 0.23(0.06); p less than 0.005) with no significant change in phase. Glyceryl trinitrate reduces cardiac pulsatile load by diminishing the amplitude of wave reflections arriving back in the aorta during systole yet has no effect on aortic compliance or arteriolar resistance. This study demonstrates a method of evaluating the effect of vasoactive drugs on cardiac pulsatile load.