Respiratory changes after open-heart surgery.

Breathing pattern was studied non-invasively in 20 coronary artery bypass surgery patients before the operation and post-operatively after weaning from mechanical ventilation. Post-operatively minute ventilation (VE), breathing frequency (Fr) and mean inspiratory flow (VT/TI) increased (28%, 42%, 27%; p less than 0.01, p less than 0.001, p less than 0.01, respectively), while tidal volume (VT) decreased (15%, p less than 0.025). CO2 production (VCO2) and oxygen consumption (VO2) increased postoperatively (p less than 0.001 for both), contributing to the increase in ventilatory demand. Reduced variation of VT and Fr (p less than 0.001, p less than 0.01, respectively) and number of sighs (p less than 0.001) were characteristic of the post-operative breathing pattern. Post-operatively an increase in the contribution of rib cage (%RC) to tidal volume in the supine position was observed suggesting reduced motion of the diaphragm. All patients had atelectasis, 17 had pleural fluid and only 6 normal vascularity post-operatively. The shallow breathing in combination with increased ventilatory demand, impaired gas exchange and the surgical trauma of the thorax predispose to postoperative respiratory complications.