The hemodynamic consequences of weaning from positive-pressure ventilation may be profound. In patients with markedly increased work of breathing, hypervolemia, or impaired left ventricular pump function, spontaneous ventilation may worsen cardiovascular function by adding an additional stress. Spontaneous ventilation will promote further cardiogenic pulmonary edema and hypoxemia. The re-institution of mechanical ventilatory support, by decreasing the work cost of breathing and abolishing the negative swings in intrathoracic pressure, can be lifesaving in this setting because of its ability to support the cardiovascular system while decreasing global O2 consumption, independent of any beneficial effects that mechanical ventilation may have on gas exchange.
In patients with decreased pulmonary elastic recoil, increased pulmonary vascular resistance, hypovolemia, or airflow obstruction, the withdrawal of ventilatory support invariably increases intrathoracic blood volume and left ventricular afterload and can be used as a cardiovascular stress test to ascertain whether the patient's cardiovascular status is primarily determined by venous return of right ventricular ejection pressure. Thus the initiation and withdrawal of ventilatory support can be seen as a ventilatory probe into the determinants of cardiovascular homeostasis in the ventilator-dependent patient. Patients with stable but limited cardiac reserve may often not be weaned from mechanical ventilatory support unless supplemented by exogenous positive inotropes.
Perhaps the best documentation of cardiopulmonary interaction during weaning is the ventilator-dependent subject with chronic obstructive pulmonary disease. Weaning of such patients will tax the cardiovascular system. Many patients with severe chronic obstructive pulmonary disease and apparently adequate respiratory parameters for weaning may go into severe cardiogenic pulmonary edema during the weaning trial (Lem.alre.et.a/: 1988). Similarly, left ventricular ejection fraction may decrease in such patients during weaning trials but not during positive-pressure ventilation, suggesting that occult left ventricular failure is a common occurrence during the weaning of patients with chronic obstructive pulmonary disease from mechanical ventilation.
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