Left ventricular stroke volume and output in healthy term infants.

Left ventricular output, left ventricular stroke volume, and systemic vascular resistance were measured noninvasively in 16 healthy term infants at 6 predefined time intervals from less than 15 minutes to 72 hours after birth. The blood flow velocity in the ascending aorta was measured by range-gated Doppler technique and multiplied by the cross-sectional area, measured by 2-dimensional and M-mode echocardiography to yield left ventricular output. Stroke volume was calculated by dividing left ventricular output by heart rate. Mean arterial blood pressure was measured by oscillometric technique and used for calculation of systemic vascular resistance. A poor association between heart rate and left ventricular output was found, whereas there was a very close relationship between stroke volume and left ventricular output. There was also a reciprocal relationship between systemic vascular resistance and stroke volume. This suggests that stroke volume and not heart rate is the main determinant of neonatal left ventricular output and that the low postnatal afterload might strengthen this relationship.