PATTERNS OF INSPIRATORY MUSCLE SHORTENING DURING HYPOXIA AND HYPERCAPNIA IN DOGS

The shortening of parasternal intercostal muscles (Para) and crural (C ru) and costal diaphragms (Cos) are not precisely understood. We there fore examined shortening patterns of these inspiratory muscles by usin g chronically implanted sonomicrometers in dogs. To avoid acute effect s of surgery, measurements were performed 3 weeks after implanting the sonomicrometers. Patterns of length changes of Para, Cru, and Cos wer e measured during hypoxia and hypercapnia under two levels of anaesthe sia. Respiratory length change (Delta L) was assessed as a percentage change relative to the resting length at functional residual capacity (LFRC). Peak tidal shortening was defined as the maximal change from L FRC (Delta L/LFRC). Under light anesthesia, the Delta L/LFRC was the s ame among the three muscle groups at all tidal volumes (VT). Under dee p anaesthesia, the Delta L/LFRC both of Cru and Cos exceeded that of P ara. Under light anaesthesia, the maximal shortening velocity (Delta L /LFRC)/Delta t) of Cru was greater than that of Para. Under deep anaes thesia, the (Delta L/LFRC)/Delta t of Para was exceeded by that both o f Cru and Cos, Furthermore, the (Delta L/LFRC)/Delta t of each inspira tory muscle was greater during hypoxia than during hypercapnia at equa l volume. We conclude that: 1) the contribution of the diaphragm to ve ntilation increases during deep anaesthesia; 2) the muscle shortening velocity during hypoxia of hypercapnia is lower in parasternal interco stal muscles than in the diaphragm; and 3) there is no difference in t he shortening pattern between crural and costal diaphragms.