Diaphragm shortening and intrathoracic pressure during hypercapnia in rats

Abstract The purpose of this study was to determine the relationship between intrathoracic pressure (ΔITP) and diaphragm shortening (DS) during the development of diaphragm fatigue. Fatigue of the diaphragm was produced by having rats breath 15% CO 2 in O 2 . Diaphragm shortening increased significantly to 178% of control during the first 5 min of hypercapnia and then decreased to 86% of control at approximately 80 min. Twenty minutes after terminating hypercapnia, DS increased to 115% of the prehypercapnic value. ΔITP increased to 199% of control following 5 min of hypercapnia and continued to increase, reaching 267% of control at the end of the hypercapnic period. Twenty minutes later, ΔITP was 147% of control. These results illustrate that during increased respiratory work, DS can decrease while intrathoracic pressure remains increased. These findings suggest that intrathoracic pressure may not always reflect the contractile status of the diaphragm. These findings are consistent with other studies indicating that as the diaphragm fatigues, accessory respiratory muscle activity increases to maintain ΔITP.