Stroke volume index and flow rate in the healthy population. What is normal?

Stroke volume index (SVI) ≤35 ml/m2 is traditionally considered as low flow, even though it is actually a volume. Flow rate (FR) (stroke volume/ejection time) is a parameter that truly expresses transvalvular flow. Its "normal" value ≥200 ml/s is a cut-off point that emerged from in vitro studies in the 90s. Flow rate in healthy patients and its relationship with SVI is unknown.

The aim of this study was to assess FR in healthy patients, its causal variables and its relationship with SVI.

This was an observational, prospective study, including 313 healthy patients, who underwent a comprehensive transthoracic echocardiogram. Left ventricular ejection parameters (FR and SVI) were calculated and defined as normal or abnormal according to international recommended thresholds. Linear regression between FR and BSA was conducted. A multivariable analysis was performed to evaluate the determinants of FR

A total of 31.7% of patients presented abnormal FR and/or SVI, 7% had both parameters below normal values and 23.6% had discordant values: 10.8% normal SVI with low FR and 12.7% low SVI with normal FR. In a multivariable analysis including those factors affecting FR, the only variable with significant association was body surface area (BSA). The 5th percentile of FR normalized to BSA was 100 ml/s/m2.

A high proportion of healthy patients presents SVI and/or FR below reference values and discordance between FR and SVI. Flow rate is highly influenced by BSA and should thus be adjusted by it. Since in our cohort of patients 95% of the population presented an indexed FR ≥100 ml/s/m2, this could be postulated as cutoff point.