The heart and lungs are intimately coupled by their anatomical proximity within the thorax and, more importantly, by their responsibility to deliver the O2 requirements of individual cells and organs while excreting the CO2 by-product of metabolism. During critical illness, if these two organ systems fail, either alone or in combination, the end result is an inadequate O2 delivery to the body with inevitable tissue ischemia, progressive organ dysfunction, and if untreated, death. Thus, restoration and maintenance of normalized cardiopulmonary function is an essential and primary goal in the management of critically ill patients. Heart failure can impair gas exchange by inducing pulmonary edema and limiting blood flow to the respiratory muscles. Ventilation can alter cardiovascular function by altering lung volume, and intrathoracic pressure (ITP), and by increasing metabolic demands. These processes are discussed from the perspective of the impact that ventilation has on the cardiovascular system.
The ventilatory apparatus and the cardiovascular system have profound effects on each other.1,2 Acute hypoxia impairs cardiac contractility and vascular smooth muscle tone, promoting cardiovascular collapse. Hypercarbia causes vasodilation and increases pulmonary vascular resistance. Hyperinflation increases pulmonary vascular resistance, which impedes right-ventricular (RV) ejection and also compresses the heart inside the cardiac fossa in a fashion analogous to tamponade. Lung collapse also increases pulmonary vascular resistance, impeding RV ejection.3 Acute RV failure, or cor pulmonale, is not only difficult to treat, but it can induce immediate cardiovascular collapse and death.
Ventilator technologies and numerous vasoactive drugs have been developed as means to improve oxygenation of arterial blood. These advances are the subjects of other chapters in this volume. The complex interactions, however, between the heart, circulation, and lungs often leads to a paradoxical worsening of one organ system function while the function of the other is either maintained or even improved by the use of these technologies and drugs. To minimize these deleterious events, and in the hope of more efficiently and effectively treating critical ill patients with cardiorespiratory failure, a better knowledge and understanding of the integrated behavior of the cardiopulmonary system, during both health and critical illness is essential. Based on this perspective, the health care provider can more appropriately manage this complex and challenging group of patients.
Respiratory function alters cardiovascular function and cardiovascular function alters respiratory function. A useful way to consider the cardiovascular effects of ventilation is to group them by their impact on the determinants of cardiac performance. The determinants of cardiac function can be grouped into four interrelated processes: heart rate, preload, contractility, and afterload. Phasic changes in lung volume and ITP can simultaneously change all four of these hemodynamic determinants for both ventricles. Our current understanding of cardiovascular function also emphasizes both the independence and interdependence of RV and left-ventricular (LV) performance on each other and to external stresses. Complicating these matters further, the direction of interdependence, from right to left or left to right, can ...