CHEST-WALL AND LUNG-MECHANICS DURING ACUTE HEMORRHAGE IN ANESTHETIZEDDOGS

Objectives: In trauma and in surgical patients, respiratory mechanics may change because of many factors, including the hypotension induced by hemorrhage. The effects of acute hemorrhage on elastic and resistiv e characteristics of the respiratory system were studied. Design: Pros pective study, Setting: Anesthesia research laboratory. Interventions: Acute hemorrhagic shock was induced in 24 supine anesthetized/paralyz ed, mechanically ventilated dogs by blood withdrawal over a 12-minute period to decrease systolic arterial pressure to 50 mmHg; additional b lood was subsequently withdrawn to maintain this pressure for 2 hours. Total respiratory system dynamic compliance and resistance and lung a nd chest wall compliances and resistances were measured. Measurements and Main Results: Total respiratory system dynamic compliance decrease d from control (0.03 +/- 0.002 L/cmH(2)O) by the first 10 minutes of s hock (p < 0.05) and was 9.8 +/- 2% lower than control 2 hours after th e induction of shock because of decreases in both lung (9.6 +/- 3%) an d chest wall (7.7 +/- 3%) compliances. Total respiratory resistance in creased 12.8 +/- 3% from control (3.08 +/- 0.19 cmH(2)O/L/s) after 2 h ours of shock (p < 0.05) because of an increase in chest wall resistan ce (21.6 +/- 8%, p < 0.05), Pulmonary resistance was not significantly increased (p > 0.05). In six control dogs, prepared similarly but not hemorrhaged, chest wall compliance and resistance did not change, but lung compliance gradually decreased by 17.8% during 150 minutes of an esthesia/paralysis. Lung resistance increased only after 100 minutes ( p < 0.05). Conclusions: (1) Hemorrhagic shock caused slight changes in the chest wall, but effects on lung mechanics were a consequence of p rolonged mechanical ventilation during anesthesia/paralysis, and (2) c hanges in respiratory mechanics caused by hemorrhagic shock are small and, unless other deleterious factors are present, would probably have little clinical significance. Copyright (C) 1997 by W.B. Saunders Com pany.