Hemodynamic Findings in Normal Children

Abstract

Extract: We have studied cardiac function and dynamics in 29 apparently normal children under 20 years of age.

Table I lists the values obtained for intracardiac and great vessel pressures. There was no correlation of these pressures with either age or increasing body size. Except for the possibility that our data is inadequate for patients under one week of age, these pressures as well as the ratio between pulmonary artery mean to systemic artery mean (fig. 1), appear to be constant throughout the pediatric age range. Arterio-venous differences ranged from 28 to 78 ml of O₂/l of blood. The mean was 44.3 ml/l with a S.D. of 15.0.

Left ventricular cavity diameters are shown for 11 normal children from our series and 9 patients from the literature (fig.4). The two measurements recorded in the youngest subjects are elevated, probably as a result of our inability to distinguish that area of left ventricular ‘wall’ occupied by the thymus. The ratio of the left ventricular cavity diameter to wall thickness was 7.0 (fig. 5).

Calculated systemic, total pulmonary and pulmonary arteriolar resistances appeared to be inversely related to age, height, and weight. Therefore, it seemed that non-linear regression formulae employing the inverse of age, height and weight would be more suitable (table III). Height alone gave the best correlation with all cardiac functions except output.

Calculated resistances for both systemic and pulmonary circulations fall with increasing age at approximately the same rate (figs. 6-8). As systemic blood flow increases to accommodate the needs of increasing body size, the denominator of the DC resistance term increases and ‘resistance’ decreases. Since in the absence of shunts, pulmonary blood flow must parallel the increase in systemic blood flow, pulmonary ‘resistance’ decreases pari passu.

Speculation: High speed digital computers now make the development of regression equations increasingly simple. These equations will be increasingly employed by clinical investigators in place of ‘surface area’.

It is also obvious that calculated ‘resistances’ tell us far less about the vascular bed than was formerly believed. Input impedance calculation will be employed in the future, particularly as better methods for measuring instantaneous flow become available.