Effects of Lung Volume on Parasternal Pressure‐Generating Capacity in Dogs

Abstract

Previous studies have suggested that the optimum length for force generation of the parasternal intercostal (PS) muscles is well above functional residual capacity (FRC). We further explored this issue by examining the pressure‐generating capacity of the PS muscles as a function of lung volume in anaesthetized dogs. Upper thoracic spinal cord stimulation (SCS) was used to electrically activate the PS muscles. Changes in airway pressure and parasternal resting length (LR) during airway occlusion were monitored over a wide range of lung volumes during SCS. To assess the effects of parasternal contraction alone, SCS was performed following phrenicotomy and section of the external intercostal, levator costae and triangularis sterni muscles. With increasing lung volume, there were progressive decrements in the capacity of the PS muscles to produce changes in airway pressure. The relationship between PS pressure generation and lung volume was similar to a previous comparable assessment of the external intercostal muscles. The PS muscles shortened during passive inflation and also shortened further (by > 20% of LR) during SCS. Total shortening (passive plus active) increased progressively with increasing lung volume. Our results indicate that the capacity of the PS muscles to produce changes in airway pressure (a) falls progressively with increasing lung volume and (b) is similar to that of the external intercostal muscles. We speculate that the fall in PS pressure‐generating capacity is related, in part, to progressive reductions in end‐inspiratory length.

Effect of expiratory loaded breathing during moderate exercise on intercostal muscle oxygenation

“…Dyspnea can be exacerbated by DH when it occurs during exercise because when tidal breathing is shifted to a higher lung volume, inspiratory muscle efferent is changed due to increased work of breathing. Concomitantly, the length of the respiratory muscles is modified, which disrupts the tension-length relationship of the muscle fibers and the ability of the respiratory muscles to produce strength [35,36]. Consequently, the mismatch between the inspiratory muscle efferent and the respiratory mechanical/muscular response might exacerbate respiratory discomfort [37].…”