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<0.01,p<0.001,p<0.01, respectively), while tidal volume (VT) decreased (15%,p<0.025). CO₂ production (VCO₂) and oxygen consumption (VO₂) increased postoperatively (p<0.001 for both), contributing to the increase in ventilatory demand. Reduced variation of VT and Fr (p<0.001,p<0.01, respectively) and number of sighs (p<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.