Effect of inspiratory resistive loaded breathing and hypoxemia on diaphragmatic function in the piglet.

The combined effects of inspiratory resistive loaded breathing (IRL) and hypoxemia on transdiaphragmatic pressure (Pdi) in nine 1-mo-old Yorkshire piglets were studied. IRL was adjusted to increase spontaneously generated Pdi five to six times above baseline but maintain arterial PCO2 < 70 Torr to prevent hypercapnic depression of diaphragmatic contractility. Measurements of ventilation, blood gases and pH, Pdi, diaphragmatic electromyogram, Pdi during phrenic nerve stimulation, diaphragmatic blood flow, and end-expiratory lung volume were obtained at baseline, after 2 h of IRL, and then after 1 h of hypoxemia (arterial PO2 approximately 40 Torr) combined with IRL. Diaphragmatic muscle samples were obtained after study completion and immediately frozen in liquid nitrogen for determination of tissue ATP, phosphocreatine, lactate, and glycogen levels. Ten 1-mo-old piglets were subjected to IRL alone and served as controls. IRL alone resulted in significant impairment of Pdi generation. The addition of hypoxemia for 1 h did not further compromise Pdi in comparison to control animals who were subjected to IRL alone. Blood flow to both the costal and crural segments of the diaphragm increased significantly during IRL; the addition of the hypoxemic stress resulted in further significant augmentation of blood flow to both segments of the diaphragm. No differences were noted in diaphragmatic muscle tissue ATP, phosphocreatine, or glycogen between control and IRL animals or between control and IRL plus hypoxemia animals. Muscle lactate levels increased significantly in the IRL plus hypoxemia animals only. The data from this study suggest that moderate hypoxemia during resistive-loaded breathing in the piglet does not accentuate diaphragmatic fatigue.