Ventilator support after coronary artery bypass surgery has been widely applied. Controlled ventilation for 12 to 24 hours following open heart surgery is believed to have reduced the incidence of postoperative pulmonary complications. Patients with obvious pulmonary insufficiency may require prolonged mechanical ventilator support.
Ventilator support can be provided with either pressure- or volume-controlled ventilators. Pressure-regulated respirators are used when “effective” compliance in high (ratio of tidal volume [ml]) to peak inspiratory pressure (cm H2O) is higher than 40, arterial PO2 in excess of 150 mm Hg (in F1 O2 = 0.5), and the chest film is clear. The volume-controlled ventilator is mandatory when lung compliance deteriorates and peak airway pressure in excess of 35 cm H2O is required. Furthermore, volume-controlled ventilator is indicated in patients with preexisting ventilatory abnormalities and intra-operative pulmonary insufficiency.
The oxygen-air mixture of the respirator is determined by serial blood gas analysis. The inspired oxygen concentration should be adjusted to produce an arterial oxygen tension between 120 and 150 mm Hg. The use of 100% O2 should be avoided because of the danger of producing damage to the lungs. Neurologic signs and symptoms may appear also. We generally administer oxygen concentration between 40% and 50%. A tidal volume of 15 ml/kg body weight is used and respiratory rate is set to maintain arterial PCO2 within normal limits, adding mechanical dead space as required (8 to 12 breaths/min). Hyperventilation should be avoided because the resulting hypocapnic alkalosis has been shown to interfere with myocardial oxygen supply . . .