Clinical applications of cardiopulmonary interactions

The hemodynamic consequences of both spontaneous and positive-pressure ventilation may be profound and may have opposite effects on cardiovascular stability in differing patient populations. Thus, no firm rules apply as to the specific response that will be seen in all patients and under all conditions. Some generalities, however, are probably reasonable. In patients with markedly increased work of breathing, hypervolemia, or impaired LV pump function, the institution of mechanical ventilatory support can be lifesaving because of its ability to support the cardiovascular system, 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 institution of mechanical ventilatory support may induce cardiovascular instability, which, if not corrected, can lead to total cardiovascular collapse. Similarly, withdrawal of ventilatory support invariably increases intrathoracic blood volume and LV afterload and can be thought of as a type of cardiovascular stress test. Patients who pass this test easily can usually be successfully weaned from mechanical ventilatory support, whereas those who fail often are not ready to be weaned. Some patients who fail weaning trials do so because of the cardiovascular effects of spontaneous ventilation, not because the work of breathing is too great. Identification of such patients early on may improve their treatment by directing supportive therapies toward cardiovascular rather than ventilatory endpoints. However, in many situations, it will be difficult to single out a primary process determining cardiovascular instability, because multiple factors are compounded to create the observed situation and the patient's response to initiation of ventilatory support or weaning. Thus, the clinician is left with a series of therapeutic options, which if depending on the patient's response, suggest specific origins of the ventilatory and cardiovascular dysfunction. In that regard, the initiation and withdrawal of ventilatory support can be seen as a ventilatory probe into the determinants of cardiovascular homeostasis in the ventilatory-dependent patient.