Chapter 23: Cardiac Failure

Introduction

Cardiovascular disease is the leading cause of death in the United States. As a result, many patients present to the emergency department or general patient care units with congestive heart failure or acute myocardial infarction. Many of these patients benefit from the application of positive pressure ventilation. Increasingly the respiratory support is applied noninvasively.

Overview

Heart-Lung Interactions

The normal changes in intrathoracic pressure during spontaneous breathing facilitate venous return and maintains adequate preload to the right heart. In addition, the negative mean intrathoracic pressure reduces left ventricular afterload. Left ventricular dysfunction with myocardial infarction (MI) or severe congestive heart failure results in increased left ventricular preload, pulmonary edema, decreased cardiac output, hypoxemia, and work-of-breathing. Of particular concern is the increase in blood flow required by the diaphragm and accessory muscles as a result of ventricular dysfunction. The respiratory muscles receive as much as 40% of the cardiac output during stress, which can result in a reduction of blood flow to other vital organs.

Effects of Mechanical Ventilation

With positive pressure ventilation, the mean intrathoracic pressure is positive. During inspiration, intrathoracic pressure increases, whereas it decreases with spontaneous breathing. This decreases left ventricular preload and afterload. In the patient with acute left ventricular dysfunction, this may enhance the performance of a compromised myocardium. In the hypovolemic patient, however, these effects may further decrease cardiac output.

The response of the cardiovascular system to positive pressure ventilation is dependent on cardiovascular and pulmonary factors. From a pulmonary perspective, the compliance of the lungs and chest wall affects the transmission of alveolar pressure into the intrathoracic space. The most deleterious effect on hemodynamics occurs with compliant lungs and a stiff chest wall, which results in greater pressure in the intrathoracic space. Cardiovascular volume and tone, pulmonary vascular resistance, and right and left ventricular function determine the effect of intrathoracic pressure on hemodynamics (Table 23-1).

Positive End-Expiratory Pressure

Since positive end-expiratory pressure (PEEP) elevates intrathoracic pressure, it reduces venous return and decreases preload. In the presence of left ventricular dysfunction with an elevated preload, PEEP generally improves left ventricular function. PEEP may increase ...