Mechanical, neural and vascular determinants of diaphragm function

The respiratory muscles play a role in respiratory failure when the efficient performance of the work of ventilation and/or their supply of metabolic substrates is disrupted. In this report a model of inspiratory muscle action is presented. The inflationary pressure applied to the lungs and the lung apposed rib cage is partitioned into two parts. One component is attributable to the action of rib cage muscles and the other is due to the interaction between upper and lower rib cage compartments. These contributions were found to be equal.The role of afferent impulses travelling in the phrenic nerve in the control of respiratory muscle activity was investigated by electrical stimulation of its central cut end. Activation of these fibres exerts a non-uniform effect on the activities of the upper airway, rib cage and abdominal muscles and may influence respiratory muscle recruitment.The roles of blood flow and oxygen delivery in determining diaphragm function was investigated. The rate at which diaphragmatic fatigue develops is diminished at high rates of blood flow and this effect is not related to the associated increase in oxygen delivery. The critical oxygen delivery at which oxygen consumption becomes supply dependent is the same for the resting diaphragm as for the rest of the body tissues. Activation of the diaphragm results in a higher critical oxygen delivery, however, this effect is mitigated by an increase in the critical oxygen extraction ratio.The role of nitric oxide in regulating diaphragmatic blood flow and oxygen uptake was investigated by infusion of N$ sp{G}$-nitro-L-arginine. This treatment increased diaphragmatic vascular resistance, reduced the duration and magnitude of reactive vasodilation and increased the oxygen consumption and critical extraction ratio in the contracting diaphragm