Influence of body position on diaphragmatic and scalene activation during hypoxic rebreathing.

We measured electromyographic activity of the diaphragm (EMGdi) and scalene (EMGsc) during isocapnic progressive hypoxic ventilatory responses in five normal males in the supine and upright seated positions. The slope of the regression line relating EMGdi expressed as a percentage of maximum to percent fall in arterial oxyhemoglobin saturation was 93% steeper upright than supine (P < 0.005), whereas the slope of EMGdi activity to minute volume of ventilation was 73% higher upright than supine (P < 0.05). In addition, the slope of EMGsc activity relative to percent fall in arterial oxyhemoglobin saturation and minute ventilation was greater upright than supine (151%, P < 0.001 and 61%, P = 0.056, respectively). Greater EMGsc activity upright than supine was similar to findings during hypercapnic rebreathing. However, the greater EMGdi activity upright than supine stands in contrast to hypercapnic rebreathing where it was previously shown that EMGdi activity was not affected by a change in body position. We conclude that during hypoxic ventilatory responses both EMGdi and EMGsc activities are more pronounced upright than supine. Diaphragmatic activation during progressive hypoxia in response to a change in body position is different from that seen during progressive hypercapnia.