Describe the effects of positive-pressure ventilation on heart-lung interactions.
List indications for mechanical ventilation in patients with cardiac failure.
Discuss the role of continuous positive airway pressure (CPAP) in patients with cardiac failure.
Discuss the monitoring and weaning of patients with cardiac failure.
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 (CHF) or acute myocardial infarction (MI). Some of them benefit from noninvasive or invasive positive-pressure ventilation.
The normal changes in intrathoracic pressure during spontaneous breathing facilitate venous return and maintains adequate preload to the right heart. In addition, the negative intrathoracic pressure reduces left ventricular afterload. Left ventricular dysfunction with MI or severe CHF results in increased left ventricular preload, pulmonary edema, decreased cardiac output, hypoxemia, and increased 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. The result is decreased 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 (pleural pressure). 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).
Table 23-1Determinants of Cardiovascular Response to Positive-Pressure Ventilation
Positive End-Expiratory Pressure
Since positive end-expiratory pressure (PEEP) increases 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 pulmonary vascular resistance, thus increasing right ventricular afterload and decreasing ...