21 Jan A framework for flow time measured by Doppler ultrasound
21 January, 2025
A framework for flow time measured by Doppler ultrasound
Jon-Emile S. Kenny
Abstract
Background
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.
Heart rate correction
Thus far, FT was discussed without any heart rate (HR) correction, which is commonly performed clinically. There are numerous equations used to correct for heart rate (e.g., Wodey, Bazett, Weissler) [24], but why might this be physiologically necessary?
If the truncation of systole with increased HR is due only to reduced LV filling (i.e., EDV or preload), then the fall in absolute FT would directly reflect decreased ε (or SV, globally).
However, the chronotropic response also increases myocyte shortening velocity—the so-called ‘Bowditch effect [18]’—which diminishes systolic time for any given ε. Accordingly, there is a mild-to-moderate correlation between HR and ε′ [25, 26]; correcting for HR, in theory, accounts for this phenomenon.
Beyond accounting for chronotropy, there are no known equations that adjust for inotropic or afterload state when correcting systolic time.
Conclusions
Systolic duration measured by Doppler ultrasound is directly proportional to the distance traversed by a single cardiac myocyte and indirectly proportional to the velocity of its shortening.
Globally, this translates to a direct relationship between time and the LVOT VTI (or SV) and an indirect relationship with mean ejection velocity. Studies of myocardial strain and strain rate clarify this relationship. Increased contractility, chronotropy and decreased afterload all increase ε′ which reduces FT and vice versa.
Changes in the systolic time domain can be used to infer SV∆, so long as other cardiac parameters are considered.
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