Parasternal intercostal, costal, and crural diaphragm neural activation during hypercapnia

The parasternal intercostal is an obligatory inspiratory muscle working in coordination with the diaphragm, apparently sharing a common pathway of neural response. This similarity has attracted clinical interest, promoting the parasternal as a noninvasive alternative to the diaphragm, to monitor central neural respiratory output. However, this role may be confounded by the distinct and different functions of the costal and crural diaphragm. Given the anatomic location, parasternal activation may significantly impact the chest wall via both mechanical shortening or as a "fixator" for the chest wall. Either mechanical function of the parasternal may also impact differential function of the costal and crural. The objectives of the present study were, during eupnea and hypercapnia, 1) to compare the intensity of neural activation of the parasternal with the costal and crural diaphragm and 2) to examine parasternal recruitment and changes in mechanical action during progressive hypercapnia, including muscle baseline length and shortening. In 30 spontaneously breathing canines, awake without confounding anesthetic, we directly measured the electrical activity of the parasternal, costal, and crural diaphragm, and the corresponding mechanical shortening of the parasternal, during eupnea and hypercapnia. During eupnea and hypercapnia, the parasternal and costal diaphragm share a similar intensity of neural activation, whereas both differ significantly from crural diaphragm activity. The shortening of the parasternal increases significantly with hypercapnia, without a change in baseline end-expiratory length. In conclusion, the parasternal shares an equivalent intensity of neural activation with the costal, but not crural, diaphragm. The parasternal maintains and increases its active inspiratory shortening during augmented ventilation, despite high levels of diaphragm recruitment. Throughout hypercapnic ventilation, the parasternal contributes mechanically; it is not relegated to chest wall fixation.NEW & NOTEWORTHY This investigation directly compares neural activation of the parasternal intercostal muscle with the two distinct segments of the diaphragm, costal and crural, during room air and hypercapnic ventilation. During eupnea and hypercapnia, the parasternal intercostal muscle and costal diaphragm share a similar neural activation, whereas they both differ significantly from the crural diaphragm. The parasternal intercostal muscle maintains and increases active inspiratory mechanical action with shortening during ventilation, even with high levels of diaphragm recruitment.