Routine ventilatory management of cardiac surgical patients

Initial ventilatory settings are as follows: FiO 2, 0.6 to 0.8; tidal volume, 10 to 12 ml/kg; respiratory rate, 8 to 12 breaths/min; positive end-expiratory pressure (PEEP), 5 cmH2O; inspiratory-to-expiratory (I:E) ratio, 1:2; peak pressure limit, 30 cmH 2O. These are guidelines only and individual units may vary widely in approach. Flow- or pressure-generated tidal breaths are equally applicable, although mandatory pressure-generated breaths, such as those delivered by the pressure-control mode, are uncomfortable for conscious post-sternotomy patients (because of the high initial inspiratory flow rates and breath characteristics).

Typically, synchronized intermittent mandatory ventilation is established with the addition of pressure support up to 20 cmH 2O. This initiates a smooth weaning process in which respiratory rate is decreased as spontaneous activity increases.

A chest radiograph is obtained to check for pneumo- and hemothoraces, the position of intravascular catheters and the endotracheal tube, and the status of the lung fields. In most patients the aim of the ventilatory strategy is to achieve normal oxygen and CO 2 levels. Initial high inspired oxygen concentration is rapidly reduced once adequate oxygenation is ensured.

In patients with widespread atelectasis, prolongation of the I:E ratio and intermittent gentle hyperinflation may be necessary to recruit alveoli. High levels of PEEP (> 10 cmH2O) and high mean airway pressures (> 15 cmH2O) may impair cardiac performance, particularly in the presence of hypovolemia. Transmission of airway pressure to the pleural space is directly related to compliance; hence patients with emphysema are most at risk of ventilator-induced hemodynamic depression. Paradoxically, patients with postoperative pulmonary edema, despite poor cardiac function, may be less adversely affected by lung volume recruitment techniques.

Unstable cardiac patients may tolerate hypercarbia poorly which, in combination with other factors, may contribute to arrhythmia and low output syndromes. Unlike many other ventilated patients, permissive hypercapnia is rarely indicated. Adverse effects on pulmonary hemodynamics may be observed if CO 2 is allowed to rise, and thus the aim is to maintain normal values.

Individual minute volume requirements vary and relate to underlying lung disease, physiological dead-space, the degree of residual hypothermia, the presence of shivering, and the level of consciousness. Violent shivering increases CO 2 production and oxygen requirements. Thermogenic shivering can be treated with active warming blankets, filtered warm air generators, or overhead radiators. Non-thermogenic shivering may respond to small doses of meperidine (25 mg) or doxapram (10-20 mg).

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