Systolic Time Intervals at Rest and During Exercise

Abstract

The systolic time intervals (STI) determined by high fidelity techniques from simultaneous fast speed recordings of the electrocardiogram, phonocardiogram, and external carotic arterial pulsation offer precise measurements of the sequential phases of cardiac systole in man. The intervals which offer the most accurate measurement of the isovolumic and ejection phases of systole are the pre-ejection period (PEP) and the left ventricular ejection time (LVET), respectively (Fig. 1). Electromechanical systole (QS₂) is the sum of these sequential phases. Among normal individuals, each of the STI varies inversely with heart rate with the greatest regression slope for QS₂ and LVET, and the smallest slope for PEP. Among patients with left ventricular dysfunction due to arteriosclerotic, hypertensive and primary myocardial disease, PEP lengthens and LVET shortens, while QS₂ remains within normal limits (Fig. 2). A convenient expression of the changes in STI with LV disease is the ratio PEP/LVET (P/L). In patients with chronic LV disease, the P/L varies inversely with the angiographically measured ejection fraction as well as indices of left ventricular performance derived from the first derivative of isovolumic left ventricular pressure. On interpreting the P/L, it is important to recognize the influence of several clinically pertinent variables which affect LV performance and simultaneously the P/L.