Measurement of exercise cardiac output by thoracic impedance in healthy children

The purpose of this study was to track changes in stroke volume during exercise by impedance cardiography in order to validate the method, and to obtain such data in a large number of healthy children for reference purposes. One hundred and fifteen healthy children (aged 7-19 years) performed progressive exercise to voluntary exhaustion with work increments every minute on a cycle ergometer. Oxygen uptake (̇ VO₂) was measured on a breath-by-breath system. Cardiac output was measured with an ICG-M501 impedance cardiograph. Stroke volume was normalized for body surface area and expressed as stroke volume index. Cardiac output was regressed against ̇ VO₂, and differences between stroke volume index at rest and exercise were assessed by repeated measures analysis of variance. Cardiac output increased linearly with ̇ VO₂ in all subjects: individual slopes and intercepts averaged 5.16 (1.56) 1·min^-1 per 1·min^-1̇ VO₂, and 4.25 (1.92) 1·min^-1, respectively [mean(SD)]. Stroke volume index rose by an average of 29% from rest to exercise, reaching a maximum of 52 m1·m^-2 in boys and girls. Most subjects demonstrated a continuous, gentle rise in stroke volume index with increasing work rate, though a minority demonstrated a falling index as work increased above the anaerobic threshold, despite rising cardiac output. Impedance cardiography accurately tracks cardiac output and can be a useful clinical and research tool in pediatric cardiology and exercise physiology.