Hemodynamic correlates of the fourth heart sound

¹ Division of Cardiology, The Chicago Medical School, Chicago, Illinois

Intracardiac phonocardiographic characteristics of the atrial sounds and the relation of these vibrations to mechanical and electrical events of the cardiac cycle were studied in 21 dogs. Simultaneous recordings of intracardiac (from RA, LA, RV, and LV) and external phonocardiograms and equisensitive intracardiac pressures confirmed that the atrial sound is commonly formed by two main sets of vibrations: the first coincident with the onset of the atrial "a" wave, the second with its highest pressure peak. It is suggested that the first group of vibrations is usually due to atrial contraction and the second to ventricular filling. A short reversal of presystolic AV pressures after atrial relaxation occurred in some of the cases, suggesting that here a valvular mechanism was involved in the production of the late component of the atrial sound. The rapid intravenous infusion of saline generally increased the amplitude of the atrial sound and made evident the presence of a second component. The administration of drugs inducing systemic or pulmonary hypertension failed to give consistent changes in the amplitude and timing of the atrial sound. An asynchronism between right and left atrial sounds was demonstrated, the right-sided presystolic vibrations preceding those of the left heart by 0.026 sec (AS₁) and 0.023 sec (AS₂), respectively, in accordance with the earlier electrical activation and contraction of the right atrium.