critical care medicine
Monitoring and Testing the Critical Care Patient
Pulmonary Artery Catheter (PAC) Monitoring
Use of a pulmonary artery catheter (PAC, or Swan-Ganz catheter) is becoming less common in ICU patients. This balloon-tipped, flow-directed catheter is inserted via central veins through the right side of the heart into the pulmonary artery. The catheter typically contains several ports that can monitor pressure or inject fluids. Some PACs also include a sensor to measure central (mixed) venous oxygen saturation. Data from PACs are used mainly to determine cardiac output and preload. Preload is most commonly estimated by the Pulmonary artery occlusion pressure (pulmonary artery wedge pressure) . However, preload may be more accurately determined by right ventricular end-diastolic volume, which is measured using fast-response thermistors gated to heart rate. Despite longstanding use, PACs have not been shown to reduce morbidity and mortality. Rather, PAC use has been associated with excess mortality. This finding may be explained by complications of PAC use and misinterpretation of the data obtained. Nevertheless, some physicians believe PACs, when combined with other objective and clinical data, aid in the management of certain critically ill patients. As with many physiologic measurements, a changing trend is typically more significant than a single abnormal value. Possible indications for PACs are listed in the table Potential Indications for Pulmonary Artery Catheterization . Cardiac disorders Acute valvular regurgitation Cardiac Tamponade None Complicated Heart Failure (HF) None Complicated Acute Myocardial Infarction (MI) None Ventricular septal rupture Hemodynamic instability* Assessment of volume status Shock None Hemodynamic monitoring Cardiac surgery Postoperative care in critically ill patients Surgery and postoperative care in patients with significant heart disease Pulmonary disorders Complicated Pulmonary Embolism (PE) None Pulmonary Hypertension None * Particularly if inotropic drugs are required. The pulmonary artery catheter (PAC) is inserted through a special catheter in the subclavian (usually left), internal jugular (usually right), or, less often, a femoral vein with the balloon (at the tip of the catheter) deflated. Once the catheter tip reaches the superior vena cava, inflation of the balloon permits blood flow to guide the catheter. The position of the catheter tip is usually determined by pressure monitoring (see table Normal Pressures in the Heart and Great Vessels for intracardiac and great vessel pressures) or occasionally by fluoroscopy when available. Entry into the right ventricle is indicated by a sudden increase in systolic pressure to about 30 mm Hg; diastolic pressure remains unchanged from right atrial or vena caval pressure. When the catheter enters the pulmonary artery, systolic pressure does not change, but diastolic pressure rises above right ventricular end-diastolic pressure or central venous pressure (CVP); ie, the pulse pressure (the difference between the systolic and diastolic pressures) narrows. Further movement of the catheter wedges the balloon in a distal pulmonary artery. Once in place in the pulmonary artery, the balloon should be deflated. A chest x-ray confirms proper placement. The systolic pressure (normal, 15 to 30 mm Hg) and diastolic pressure (normal, 5 to 13 mm Hg) are recorded with the catheter balloon deflated. The diastolic pressure corresponds well to the occlusion pressure, although diastolic pressure can exceed occlusion pressure when pulmonary vascular resistance is elevated secondary to primary pulmonary disease (eg, Idiopathic Pulmonary Fibrosis , 30 seconds to prevent pulmonary infarction. Normally, pulmonary artery occlusion pressure (PAOP) approximates mean left atrial pressure, which in turn approximates left ventricular end-diastolic pressure (LVEDP). LVEDP reflects left ventricular end-diastolic volume (LVEDV). The LVEDV represents preload, which is the actual target parameter. Many factors cause PAOP to reflect LVEDV inaccurately. These factors include Mitral Stenosis , mitral regurgitation, high levels of positive end-expiratory pressure ( Elevated PAOP occurs in left-sided Heart Failure (HF) . Decreased PAOP occurs in hypovolemia or decreased preload. Mixed venous blood comprises blood from the superior and inferior vena cava that has passed through the right heart to the pulmonary artery. The blood may be sampled from the distal port of the PAC, but some catheters have embedded fiberoptic sensors that directly measure oxygen saturation. Causes of low mixed venous oxygen content (SmvO2) include anemia, pulmonary disease, carboxyhemoglobin, low cardiac output, and increased tissue metabolic needs. The ratio of arterial oxygen saturation (SaO2) to (SaO2 − SmvO2) determines the adequacy of oxygen delivery. The ideal ratio is 4:1, whereas 2:1 is the minimum acceptable ratio to maintain aerobic metabolic needs. Cardiac output is measured by intermittent bolus injection of ice water or, in newer catheters, continuous warm thermodilution (see Measurement of cardiac output and flow ). The cardiac index divides the cardiac output by body surface area to correct for patient size (see table Other variables can be calculated from cardiac output. They include systemic and pulmonary vascular resistance and right ventricular stroke work (RVSW) and left ventricular stroke work (LVSW). Pulmonary artery catheters (PACs) may be difficult to insert. Overview of Arrhythmias , particularly ventricular arrhythmias, are the most common complication. Pulmonary infarction secondary to overinflated or permanently wedged balloons, pulmonary artery perforation, intracardiac perforation, valvular injury, and Pulmonary artery rupture occurs in<0.1% of PAC insertions. This catastrophic complication is often fatal and occurs immediately on wedging the catheter either initially or during a subsequent occlusion pressure check. Thus, many physicians prefer to monitor pulmonary artery diastolic pressures rather than occlusion pressures. 