The objectives are to measure diaphragm contractility directly in animals with respiratory alterations and to describe how pressure support ventilation affects diaphragm contractility. Breathing is a type of muscular exercise that may be assessed by measuring the mechanical work performed by respiratory muscles or by estimating the total energy cost of breathing by measuring oxygen consumption. The diaphragm is responsible for approximately 80% of the work of breathing and in certain situations, when the diaphragm becomes fatigued, patients experience respiratory difficulty. Nurses at the beside can monitor diaphragm fatigue only through indirect measures such as dyspnea, tachycardia, asychronized rib cage and abdominal movement. The overall research objective of the proposal is to utilize a miniaturized ultrasound device to directly measure diaphragm contractile movements in an animal model as a index of the work of breathing. The specific aims of this proposal are: effectiveness studies related to diaphragm contractility. investigate diaphragm to conduct fatigue during respiratory acidosis (normoxic/hypercapnic states) to explore diaphragm fatigue during hypoxia (hypoxia/normocapnic, hypoxia/hypocapnic & hypoxia/hypercapnic) states, and to examine the effectiveness of pressure support ventilation in rats to reduce diaphragm fatigue. In all studies, diaphragm fatigue will be defined as a 50 % reduction in the diaphragm contractility waveform produced by an ultrasound device. During the experiments diaphragm fatigue will be induced in rats that will develop diaphragm fatigue as assessed by the miniaturized ultrasound device by altering the inspired oxygen and carbon dioxide concentrations. These rats will be exposed to various concentrations of oxygen and carbon dioxide to produce different respiratory alterations (normoxic, hypoxic, normocapnic, hypercapnic & hypocapnic states). These experiments will mimic ventilation/perfusion alterations commonly observed in nursing practice by changes in arterial blood gas concentrations. The experiment will include mechanically ventilated rats with induced diaphragm fatigue as assessed by the ultrasound device. These experimental rats will be exposed to four levels of pressure support ventilation (PSV) to determine the effect of PSV on diaphragm fatigue. All baseline data from all rats will serve as control measures.