The forces underlying left ventricular ejection were investigated by applying a wavefront analysis to blood pressure (P) and velocity (U) waveforms measured in the ascending aorta of anesthetized dogs (n = 13). Wavefronts travel forward (to the periphery) and/or backward (to the heart) after peripheral reflection. They are characterized by the rate of pressure change they cause, i.e., the time derivative of pressure (dP/dt): compression wavefronts have dP/dt > 0; expansion wavefronts have dP/dt < 0. Wave intensity is defined as (dP/dt)(dU/dt), where dU/dt is the time derivative of U. Forward wavefronts contribute positively to wave intensity and backward wavefronts contribute negatively. Therefore, wave intensity indicates whether the effects of forward wavefronts are predominant or whether those of backward wavefronts predominate in the formation of pressure and velocity waveforms. Under control conditions, wave intensity was positive in early and late systole, indicating that forward compression and expansion wavefronts dominate aortic acceleration and deceleration, respectively. Compression wave intensity was increased during inotropic stimulation by dobutamine (10-15 mug/kg per min i.v. infusion +161% +/- 31% mean change in peak value +/- SEM (%), P < 0.05), and was reduced during P-blockade by propranolol (1 mg/kg i.v. injection, -58% +/- 7%, P < 0.05). Expansion wave intensity was unchanged by dobutamine and propranolol (17 = 6). In a separate group of animals (n = 7), expansion wave intensity was reduced during vasodilatation by nitroglycerin (0.5 mg i.v. injection and 0.02 mug/kg per min infusion; -32 % +/- 12 %, P < 0.05), but was unchanged during vasoconstriction by methoxamine (2 mg i.v. injection). However, methoxamine reduced compression wave intensity (-46% +/- 14%, P < 0.05). These results indicate that (1) compression and expansion wavefronts generated by the left ventricle dominate acceleration and deceleration in the ascending aorta, (2) compression wave intensity is related to the inotropic state of the left ventricle, but is reduced during vasoconstriction, and (3) expansion wave intensity is reduced during vasodilatation. This time domain analysis of traveling wavefronts readily provides information concerning the dynamics of the ventriculoarterial interaction.
