Use of negative extrathoracic pressure to improve hemodynamics after cardiac surgery

Little attention is given to the mode of mechanical ventilation after cardiac surgery. Positive pressure ventilation with positive end-expiratory pressure (PEEP) has been shown to reduce cardiac output. We hypothesized that positive pressure ventilation with continual negative pressure applied to the chest through a cuirass would increase cardiac output in coronary artery bypass graft patients immediately after surgery.

Twenty patients with a normal left ventricular ejection fraction were studied 2 hours after coronary artery bypass graft surgery. The patients were ventilated with synchronized intermittent mandatory ventilation (SIMV) and PEEP. Hemodynamic variables and blood gases were studied using four modes of ventilation after 15 minutes in each mode: A (baseline 1) = SIMV and 5 cmH(2)O of PEEP; B = SIMV without PEEP; C = SIMV without PEEP and with continuous negative pressure applied to the thorax at -20 cmH(2)O; D (baseline 2) = SIMV and 5 cmH(2)O of PEEP. The results of the two baselines were averaged.

All patients were hemodynamically stable during the trial. Heart rate, blood pressure, and gas exchange were not affected by the changes in ventilatory modes. With continual negative pressure, the stroke volume index and cardiac index were significantly increased relative to ventilation with SIMV and PEEP by 3.21 mL x min(-1) x m(-2) (9.0%) and 0.45 L x min(-1) x m(-2) (13.8%), respectively. Continual negative pressure also reduced venous and wedge pressure.

Continual negative pressure attenuates the negative effects of positive pressure ventilation on cardiac output. Although the improvement in this cohort with normal ventricular function is modest, this pilot study demonstrates that the mode of ventilation may have potentially important effects on cardiac output.