Unchanging cardiac activity while increasing respiratory activity at the start of exercise in man: a beat-by-beat analysis by means of the impedance cardiography method.

Both respiratory and cardiodynamic parameters were analyzed (the latter non invasively by means of a computerized impedance cardiograph) in 6 subjects at the start of voluntary dorsal ankle flexions. Increased mean inspiratory flow with a slow reduction in end tidal PCO2 was shown in the breath following the start of movements, and these findings indicate the nervous origin of this hyperventilation. Within the same breath, left cardiac output, stroke volume and ventricular ejection time remained unchanged, and the same is true of heart rate and ventricular performance indexes depending on both the first derivative of the base thoracic electrical bioimpedance and on the mean left ventricle ejection flow, as well as in the systemic venous return index depending on the basal value of the thoracic impedance. These results, indicating that no cardiodynamic intervention in the neurogenic hyperventilation was seen, show that nervous stimuli elicited at the start of voluntary exercise may exert a direct action on respiratory control structures.