A framework for flow time measured by Doppler ultrasound

Abstract

The duration of mechanical systole-also termed the flow time (FT) or left ventricular ejection time (LVET)-is measured by Doppler ultrasound and increasingly used as a stroke volume (SV) surrogate to guide patient care. Nevertheless, confusion exists as to the determinants of FT and a critical evaluation of this measure is needed. Using Doppler ultrasound of the left ventricular outflow tract velocity time integral (LVOT VTI) as well as strain and strain rate echocardiography as grounding principles, this brief commentary offers a model for the independent influences of FT. This framework establishes that systolic duration is directly proportional to the distance traversed by a single cardiac myocyte and indirectly proportional to its shortening velocity. Grossly, this translates to a direct relationship between FT and the LVOT VTI (i.e., SV) and an indirect relationship with mean ejection velocity. Thus, changes in the systolic time can infer SV change, so long as other cardiac parameters are considered.

Fig. 1

The left ventricular ejection and flow times. A) 4 cardiac cycles obtained via trans-esophageal echocardiography. Velocity increases in the downwards y-axis and x-axis is time. LVET is the duration the aortic valve is open and ejecting blood, the left ventricular ejection time. VTI is velocity time integral. B) Flow time from the common carotid artery. Velocity increases upwards on the y-axis and the x-axis is time

Fig. 2

Relating left ventricular outflow tract Doppler ultrasound and strain echocardiography to flow time. A) cartoon of 3 cardiac cycles with equal left ventricular outflow tract velocity time integral (LVOT VTI); see text for details. B) Analogy using single cardiac myocyte to understand the relationship between time, distance and velocity. C) Framework relating flow time to LVOT VTI (i.e., distance), mean ejection velocity ($\overline{v}$), strain (ℇ) and strain rate (ℇ'). EDV is end-diastolic volume, ESV is end-systolic volume. Note that increasing preload (i.e., EDV) will also increase flow time, but only if EDV rises relative to ESV (i.e., increased stroke volume) and with constant mean ejection velocity