Effects of mechanical loading and hypercapnia on inspiratory muscle EMG

The electromyograms of the diaphragm and an external intercostal muscle were analyzed to see if the effects of hypercapnia on inspiratory muscle electrical activity could be distinguished from those of mechanical loading and to determine whether changes in inspiratory muscle electrical activity were a sueful measure of CO2 response during mechanical loading. Anesthetized dogs were studied: 1) during progressive hypercapnia without mechanical loading, 2) during flow-resistive and elastic loading at constant PCO2, and 3) during progressive hypercapnia and mechanical loading. Both mechanical loading and hypercapnia increased total inspiratory diaphragmatic and intercostal muscle electrical activity. However, inspiratory duration was increased by mechanical loads but reduced by hypercapnia. Because of these changes in inspiratory duration, the average rate of diaphragmatic electrical activity remained unaffected by mechanical loading before and after vagotomy but was increased by hypercapnia. In contrast, both hypercapnia and mechanical loading increased the average rate of intercostal muscle electrical activity. There was a greater increase in both total and average rate of intercostal muscle electrical activity during hypercapnia in the presence of mechanical loading than during unloaded breathing. However, the change in total and average rate of diaphragmatic electrical activity with PCO2 was unaffected by added mechanical loads. These results suggest that diaphragmatic but not intercostal muscle electrical activity can be used as an index of CO2 response even during mechanical loading.