Relationship between heart rate, left ventricular output, and stroke volume in preterm infants during fluctuations in heart rate.

The relationship between changes in heart rate, left ventricular output, and left ventricular stroke volume was studied in 18 preterm infants, with mean gestational age 29 wk (range 26-33 wk) and mean postnatal age 10 d (range 1-21 d). To yield left ventricular output, the blood flow velocity in the ascending aorta was measured by range-gated Doppler technique and multiplied by the aortic cross-sectional area measured by cross-sectional and M-mode echocardiography. Stroke volume was calculated by dividing left ventricular output by heart rate. The individual mean left ventricular output correlated poorly with heart rate (r2 = 0.17), and, accordingly, there was a closer relationship between stroke volume and left ventricular output. In only four of the 18 infants was a significant correlation between heart rate changes and left ventricular output found. Substantial changes in stroke volume were seen in most infants, and in 13 of the 18 infants the changes exceeded 25%. These variations in stroke volume were closely related to left ventricular output. In 15 of the 18 infants, the maximum heart rate change was associated with a stroke volume change in the opposite direction. The group average of the maximum heart rate increase in each individual, 24 bpm (18%), corresponded to a decrease in stroke volume of 0.15 mL.kg-1 (9%) (p less than 0.05) and an increase in left ventricular output of 22.7 mL.min-1.kg-1 (9%) (p less than 0.05). A reciprocal relationship was seen between afterload and left ventricular output.(ABSTRACT TRUNCATED AT 250 WORDS)