Brief review of physiology relevant to right heart and pulmonary artery catheterization

Let us begin by looking at one mechanical cardiac cycle. While the ventricles are in systole the atria are filling, as blood flows continually from the venous system, with the inferior and superior vena cava emptying into the RA and the pulmonary veins emptying into the left atrium (LA). Meanwhile, the tricuspid valve and the mitral valve remain closed as long as the pressure in the ventricles exceeds the pressure in the atria.

Blood flow in the heart occurs due to pressure gradients. As ventricular pressure decreases during isovolumetric ventricular relaxation (Figure 2.2a), it reaches a point where atrial pressure is higher than ventricular pressure and the mitral and tricuspid valves open due to hydrostatic pressure. The ventricles now rapidly fill (rapid ventricular filling; Figure 2.2b). This rapid filling characterizes the first phase of diastole. During the second phase of diastole, the pressures in the atria and the ventricles are the same. During this time only a small amount of blood normally flows into the ventricles (slow ventricular filling). This is primarily blood returning to the atria from the great veins or pulmonic veins, which then passes into the ventricle. Near the end of diastole, the atria contract (atrial systole), which generates an increase in pressure in the atrium and enhances blood flow into the ventricles (Figure 2.2c).

Systole begins when electrical activation of the ventricles (QRS complex) leads to mechanical activation of the ventricles. Very shortly after onset of ventricular contraction, the pressure in the ventricle increases to the point where it exceeds atrial pressure and the mitral and tricuspid valves close (all four valves are now closed). As the ventricles contract (isovolumetric contraction; Figure 2.2d), right and left ventricular pressures increase until RV pressure exceeds PA pressure and LV pressure exceeds aortic pressure. At these points, the pulmonic and aortic valves open and ejection occurs (ventricular ejection; Figure 2.2e). At the end of systole, isovolumetric ventricular relaxation begins and ventricular pressures fall.

A point to remember is that as heart rate increases, myocardial oxygen consumption increases while supply decreases. This is because time in diastole decreases and time in systole increases as heart rate increases, while most coronary artery filling occurs during diastole, but oxygen consumption of the heart is maximal during systole. About 70–90% of blood flow in the left coronary artery and one‐half of blood flow in the right coronary artery occur during diastole. During systole, intramural arteries and capillaries are compressed by the contracting heart muscle, which limits blood flow.

Vascular access

A PA catheter can be inserted via any vein that has uninterrupted access to the RA. The most commonly used insertion sites include subclavian veins, internal jugular veins, and femoral veins. The site is generally chosen based on patient preference and operator experience. All things being equal, the right internal jugular vein provides the shortest and straightest path to the heart. The use of ultrasound has been shown to increase success rates for venous access and to decrease complications. Complications associated with venous access include bleeding, hematoma, arterial puncture, infection, pneumothorax, and hemothorax (Table 2.2). Advantages and disadvantages of various access sites are listed in Table 2.3.

Right heart catheterization and placement of a PA catheter

A PA catheter can be positioned using either fluoroscopy or pressure monitoring. Fluoroscopy is preferred for femoral artery insertion and/or if there is significant hardware in the right heart (e.g., biventricular pacer). If continuous pressure monitoring is used, transduce the distal tip. Insert the PA catheter 20 cm and inflate the balloon with air. Central venous pressure (CVP) is initially obtained. As the catheter is being “floated” into the heart, it first enters the RA and then goes into the RV. Atrial waveforms consist of an A wave, a V wave, a C wave, and X and Y descents (Figure 2.3a). The A wave in the RA waveform coincides with atrial contraction and occurs simultaneously with the PR interval on the electrocardiogram (ECG), whereas the peak of the V wave in the RA waveform can be found near or at the end of the T wave. A further discussion of interpretation of atrial waveforms can be found in Chapter 5.

Entry of the catheter into the RV can be identified by the appearance of ventricular systole on the pressure tracing (Figure 2.3b). There will be a rapid upstroke reflective of ventricular contraction and a rapid decline reflective of ventricular relaxation. Diastole will be similar to the atrial tracing. A further discussion of interpretation of ventricular waveforms can be found in Chapter 3. Normal RV systolic pressure is approximately 25 mm Hg, with diastolic pressures ranging from 0–8 mm Hg. The only time you should see an RV tracing is when the PA catheter is being inserted or removed. An RV pressure tracing from the distal port of the catheter appearing on the monitor at any other time is an indication that the catheter is no longer correctly placed. With the tip of the catheter in the RV, the risk of ventricular tachycardia is markedly increased. Note that some continuous cardiac output catheters use a proximal port in the RV to monitor for correct positioning of the thermistor—the warning here is that if the pressure from the distal port of the catheter changes from PA to RV, that implies that the catheter tip is now residing in the RV.

As the catheter is advanced, it will leave the RV and enter the PA (Figure 2.3c). Remember that RV and PA systolic pressures are the same (both should coincide with the T wave on the ECG). Thus passage into the PA will be evident from diastolic pressures. If an RV waveform is still present approximately 20 cm after the initial RV pattern appears, the catheter may be coiling in the RV.

PCWP can be obtained by inflating the balloon on the catheter and advancing the catheter until it “wedges” (Figure 2.3d). When the balloon is “wedged,” blood flow is stopped in that portion of the PA. Pressure equalizes in a nonflowing segment and, since there are no valves between the pulmonary arteries and pulmonary veins or between the pulmonary veins and the LA, pressure at the tip of the balloon is reflective of LA pressure. The A and V waves of the PCWP tracing will be delayed relative to RA tracing and ECG, with the A wave starting in or at the end of the QRS complex, and the V wave starting later in the T–P interval. This is because LA pressure is reflected back to the PA catheter via the pulmonary veins and through a static column of blood. PCWP is an important measurement since it is an estimate of LV end diastolic pressure and thus LV preload.

Tempe et al. [11] measured the distance to various chambers in 300 adult patients who had PA catheters placed via the right internal jugular vein. The right ventricle was reached at 25 ± 3 cm, the PA at 36 ± 4 cm, and the wedge position at 43 ± 6 cm. The length of catheter required was directly related to the height of the patient and was greater in patients with valvular heart disease.

In general, indwelling PA catheters rest in the PA (Figure 2.4), continuously giving a readout of PA systolic and PA diastolic pressure. Mean RA pressure can be obtained if the CVP port is transduced (but will often appear damped because the lumen is small).

Always deflate the balloon before withdrawing the PA catheter.

Ensuring that accurate data is obtained from a PA catheter

Remember the old saying: “Trash in equals trash out.” The integrity of the data obtained from a PA catheter is dependent on the accuracy of the system. A few simple rules to start out with:

• The PA catheter needs to be “zeroed” at the level of the RA.
  
• Minimize the length of pressure tubing, the number of stopcocks, and the number of connections. Avoid narrow tubing, as the smaller the diameter of the tubing, the more pressure damping that occurs.
  
• Eliminate any air bubbles in the pressure line.
  
• The proper pressure scale needs to be used
  
• Pressure tracings need to be interpreted in conjunction with an ECG tracing.
  
• The effects of respiration have to be identified.

It is essential that the PA catheter be properly “zeroed” at the level of the RA, which in the supine patient is assumed to be in the fourth intercostal space at the midaxillary line. Proper “zeroing” removes the effect of gravity on pressures. To locate the level of the RA, two imaginary lines are drawn (Figure 2.5) and the place where they intersect is called the phlebostatic axis. To locate the phlebostatic axis, locate the fourth intercostal space on the edge of the sternum, draw an imaginary line along the fourth intercostal space laterally, along the chest wall, and draw a second line from the axilla downward, midway between the anterior and posterior chest wall (i.e., the midaxillary line).

For every centimeter that the transducer is not correctly leveled, pressures will change by 0.75 mm Hg. If the transducer is too high, pressures will be falsely low. Alternatively, if the transducer is too low, pressures will be falsely high.

Pressure is transmitted from the catheter to the transducer via tubing filled with fluid, usually saline. Small changes in pressure in the PA are transmitted through the tubing and cause deflection of the transducer membrane, which converts these changes into electrical signals. Anything that hampers transmission of these small pressure changes to the membrane will distort pressure measurements and cause pressure “damping.” Thus, it is important to use semirigid, noncompliant tubing, to minimize the length of tubing, and to make sure that there is no air in the pressure tubing. Other causes of pressure damping include long or compliant tubing, multiple stopcocks, transducer malfunction, loose connections, and partial occlusion of the port (e.g., with thrombus).

It is important that the proper scale be used when interpreting the pressure tracings. For example, when measuring LV pressure a scale of 0–200 mm Hg is useful. However, if this scale is used when examining RA pressure tracings, much of the information is lost about the “A” and “V” waves and “X” and “Y” descents that would be apparent if a scale of 0–25 mm Hg was used.

It is important that pressure tracings be interpreted in conjunction with an ECG recording. The most accurate method of obtaining hemodynamic data is to use a dual channel strip recorder, which will allow analysis of the waveform and ECG together.

When interpreting atrial pressure waveforms, emphasis should be placed on mean pressures or pressures obtained at end‐expiration. In many intensive care units, all waveforms are read at end‐expiration because this is when intrapleural pressures are negligible, which means that the waveforms are an accurate reflection of cardiac pressures.

In a spontaneously breathing patient, intrathoracic and thus intracardiac pressures will decline during inspiration and rise during expiration (Figure 2.6). The diaphragm pulls downward, creating a negative pleural pressure during inspiration, while the diaphragm relaxes and the elastic recoil of the lungs, chest wall, and abdominal structures compress the lung during expiration.

In patients who are mechanically ventilated, end‐expiration is still used to record pressures. The difference is that pressures increase with inspiration and decrease with expiration. If the patient is not breathing over the ventilator, then interpretation is straightforward. If the patient is ventilated and also breathing spontaneously, then waveform interpretation becomes more difficult, as inspiration can be associated with decreases in pressure (spontaneous respiration) and increases in pressure (ventilator). The spontaneous breath with pressure support will result in an initial decrease in pressure, which can be used to locate end‐expiration (just prior to the inspiratory effort marked by the dip in pressure). In patients receiving intermittent mandatory ventilation, identifying end‐expiration can be a challenge. There are newer ventilators that show graphic respiratory waveforms that provide a means for accurately identifying end‐expiration.

In the intensive care unit, manual interpretation of waveforms using a printed copy is necessary to ensure that pressures are interpreted at end‐expiration, unless a monitor is available that displays the ECG, respiratory recording, and PA pressure waveforms on a single screen.

Cardiac output

A brief overview of cardiac output is presented here. For a more detailed discussion, see Chapter 6.

There are two primary methods used to calculate cardiac output when utilizing a PA catheter: thermodilution and assumed Fick. The thermodilution method requires that a substance that is cooler than blood (usually saline) be injected through the CVP port of the PA catheter. This injection should be smooth and take less than 4 seconds. The temperature in the PA is measured and the change over time, as the cooler injectate passes through the PA, is used to calculate cardiac output. The computer in the cardiac output machine uses the following formula:

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