Limits of corrected flow time to monitor hemodynamic status in children

Objective. Doppler corrected flow time (i.e., corrected left ventricular ejection time) as a noninvasive tool for assessing hemodynamic changes has been previously reported for adult patients. Its use in paediatrics seems to be worthwhile but no data concerning its accuracy are presently available in this population. The purpose of this work was to study the relationships between corrected flow time (FT) and indices of systemic vascular resistance (SVR) and of myocardial contractility in healthy children. Methods. Twenty healthy children performed a graded maximal bicycle exercise in order to induce physiological hemodynamic alterations. Hemodynamic parameters were measured with an echocardiography-Doppler at rest and within a few minutes of post exercise. Cycle time (RR), mean aortic flow velocity, mean systolic velocity (MSV), FT, peak velocity (PV), and stroke distance were measured on the Doppler aortic velocity waveform. Cardiac index (CI) and SVR were calculated from the classical volumetric equation. Corrected FT was calculated by using Bazett's formula (FTb = FT/root) and a simplified formula FTc = FTmeasured + [1.29 . (HR - 60)]. Results. Post exercise, SVR, RR, FT, decreased, while CI, PV and MSV increased and stroke distance remained unchanged. After multiple regression analysis no significant correlation between SVR and FTb and SVR or FTc was noted. A significant correlation appeared between FTb and, respectively, PV (r = -0.83; p < 0.001), stroke distance (r = 0.78; p < 0.001) and RR (r = -0.52; p = 0.0016). A significant correlation was also shown between FTc and, respectively, PV (r = -0.71; p < 0.001) and stroke distance (r = 0.63; p < 0.001) but not with RR. Conclusions. These results show that the use of Bazett's formula correct FT could lead to hemodynamic misinterpretations, because it does not rule out all the heart rate effect. Moreover, in healthy children corrected FT appears as an inaccurate index to monitor physiological afterload alterations, because of the involvment of other hemodynamic factors such as contractility in its variation.