The Pulmonary Artery Catheter
This chapter presents the spectrum of hemodynamic parameters that can be monitored with pulmonary artery catheters. The clinical applications of these parameters are described in future chapters.
I. Catheter Basics
A. The Principle
The pulmonary artery (PA) catheter is equipped with a small inflatable balloon at the distal end. When inflated, the balloon allows the flow of venous blood to carry the catheter through the right side of the heart and into one of the pulmonary arteries. This balloon flotation principle allows catheterization of the right heart and pulmonary arteries without fluoroscopic guidance.
B. The Catheter
The PA catheter is 110 cm in length (about 5–6 times longer than a central venous catheter), and has an outside diameter of 2.3 mm (about 7 French).
There are two internal channels: one channel emerges at the tip of the catheter, and the other channel emerges 30 cm proximal to the catheter tip (and should be situated in the right atrium when the catheter is properly positioned).
The tip of the catheter has an inflatable balloon (1.5 mL capacity) that helps to carry the catheter to its final destination.
A small thermistor (a temperature-sensing transducer) is placed near the tip of the catheter, and this allows measurement of the cardiac output by the thermodilution method (described later).
C. Placement
The PA catheter is inserted through a large-bore (8–9 French) introducer sheath that has been placed in the subclavian vein or internal jugular vein. The distal lumen of the catheter is attached to a pressure transducer to guide catheter placement. When the catheter emerges from the introducer sheath and enters the superior vena cava, a venous pressure waveform appears. The balloon is then inflated and the catheter is advanced, using pressure tracings to determine the location of the catheter tip, as shown in Figure 5.1.
The pressure in the superior vena cava shows small amplitude oscillations. This pressure remains un-changed when the catheter tip enters the right atrium.
When the catheter tip is advanced across the tricuspid valve and into the right ventricle, a pulsatile waveform appears. The peak (systolic) pressure is a function of the strength of right ventricular contraction, and the lowest (diastolic) pressure is equivalent to the pressure in the right atrium.
When the catheter moves across the pulmonic valve and enters a main pulmonary artery, the pressure waveform shows a sudden rise in diastolic pressure with no change in the systolic pressure. The rise in diastolic pressure is caused by resistance to flow in the pulmonary circulation.
FIGURE 5.1 The pressure waveforms encountered during placement of a pulmonary artery catheter. See text for explanation.
As the catheter is advanced along the pulmonary artery, the pulsatile waveform eventually disappears, leaving a nonpulsatile pressure (which is typically at the same level as the diastolic pressure of the pulsatile waveform). This is the pulmonary artery occlusion pressure, also called the wedge pressure, and it is a reflection of the filling pressure of the left side of the heart (see the next section).
When the wedge pressure tracing first appears, the catheter is held in place (not advanced further). The balloon is then deflated, and the pulsatile pressure waveform should reappear. The catheter is then secured in place.
In about 25% of cases, the pulsatile PA pressure never disappears despite advancing the PA catheter maximally (1). When this occurs, the PA diastolic pressure can be used as
a surrogate measure of the wedge pressure, except in the presence of pulmonary hypertension (when the wedge pressure is lower than the PA diastolic pressure).
D. The Balloon
The balloon should remain deflated while the PA catheter is left in place (sustained balloon inflation can result in pulmonary artery rupture or pulmonary infarction). Balloon inflation is permitted only when it is desirable to measure the wedge pressure.
When measuring the wedge pressure, DO NOT fully inflate the balloon with 1.5 mL air all at once (catheters often migrate into smaller pulmonary arteries, and a fully inflated balloon could result in vessel rupture). The balloon should be slowly inflated until a wedge pressure tracing is obtained.
Once the wedge pressure is recorded, the balloon should be fully deflated. Detaching the syringe from the balloon injection port will help prevent inadvertent balloon inflation while the catheter is in place.
II. The Wedge Pressure
A. The Principle
The principle of the wedge pressure measurement is illustrated in Figure 5.2.
Inflation of the balloon on the PA catheter obstructs blood flow (Q) in the pulmonary artery, and this creates a static column of blood between the tip of the catheter and the left atrium. In this situation, the “wedged” pressure at the tip of the catheter (PW) is the same as the pulmonary capillary pressure (PC) and the pressure in
the left atrium (PLA); i.e., if Q = 0, then PW = PC = PLA.
The wedge pressure will reflect left atrial pressure only if the pulmonary capillary pressure is greater than the alveolar pressure (PC >PA). This condition is not satisfied when the wedge pressure varies with the respiratory cycle (2) (see later).
If the mitral valve is behaving normally, the left atrial pressure (wedge pressure) is equivalent to the end-diastolic pressure (the filling pressure) of the left ventricle. Therefore, in the absence of mitral valve disease, the wedge pressure is a measure of left ventricular filling pressure.
B. Wedge vs. Pulmonary Capillary Pressure
The wedge pressure is often mistaken as a measure of the physiological pressure in the pulmonary capillaries,
but this is not the case (3,4) because the wedge pressure is measured in the absence of blood flow. When the balloon is deflated and flow resumes, the pressure in the pulmonary capillaries must be higher than the pressure in the left atrium (the wedge pressure); otherwise, there would be no pressure gradient for flow in the pulmonary veins.
FIGURE 5.2 The principle of the wedge pressure measurement. When flow ceases because of balloon inflation (Q = 0), the wedge pressure (PW) is the same as the pulmonary capillary pressure (PC) and the pressure in the left atrium (PLA), but this relationship occurs only when the pulmonary capillary pressure exceeds the alveolar pressure (PC>PA).
The difference between pulmonary capillary pressure (PC) and left atrial pressure (PLA) is determined by the rate of blood flow (Q) and the resistance to flow in the pulmonary veins (RV); i.e.,
Since the wedge pressure (PW) is equivalent to the left atrial pressure, Equation 5.1 can be restated as follows:
Therefore, in the presence of blood flow, the wedge pressure will always underestimate the pulmonary capillary pressure. The magnitude of the (PC–PW) difference is not possible to determine in individual patients because it is not possible to measure RV. However, this difference will be magnified by conditions that promote pulmonary venoconstriction, such as hypoxemia, endotoxemia, and the acute respiratory distress syndrome (ARDS) (5,6).
III. Thermodilution Cardiac Output
The PA catheter is equipped with a thermistor that allows the measurement of cardiac output by the thermodilution method. This is illustrated in Figure 5.3.
A. The Method
A dextrose or saline solution that is colder than blood is
injected through the proximal port of the PA catheter (usually located in the right atrium). This cools the blood in the right heart chambers, and the cooled blood then flows past the thermistor at the distal end of the PA catheter.Related posts:
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