Once venous access is obtained, the catheter is advanced with the balloon inflated to the right atrium, then the right ventricle, the pulmonary artery, and finally into a pulmonary capillary wedge position. Advancing the catheter can be done under fluoroscopy, or by simply watching the pressure tracings as they transition from each chamber of the heart until an appropriate wedge tracing is identified. Pulmonary artery catheters are frequently placed in ICU settings where fluoroscopy equipment is not available, but fluoroscopy can be especially helpful in situations where the cardiac anatomy or valvular pathology make passage of the catheter into the pulmonary artery difficult, such as is frequently the case with severe tricuspid regurgitation. The careful use of a guidewire (0.018–0.025 in.) under fluoroscopy is sometimes needed to augment stiffness and achieve correct placement of the catheter in the pulmonary artery.

As the catheter passes first into the right atrium, a normal pressure waveform should be identified (Fig. 6.2). The a-wave represents atrial contraction, followed by the x-descent as the atrium relaxes. The c-wave represents the period of brief, isovolumetric contraction of the right ventricle where the tricuspid valve briefly bows upward under RV systolic pressures, prior to the opening of the pulmonic valve, at which point the RV empties and the tricuspid pulls away from the RA allowing the x-decent to continue. The v-wave represents venous return to the RA as the pressure gradually rises. Finally, the y-decent represents passive filling of the RV due to opening of the tricuspid valve, just prior to atrial contraction. The easiest way to correctly identify the a-wave is to note that it occurs slightly after the p-wave on the EKG.

As the catheter passes into the RV, an obvious change in the pressure tracing will be noted as systolic pressures rise (Fig. 6.3). RV end-diastolic pressure should closely approximate the mean RA pressure, and should be measured at the end of diastole, just prior to isovolumetric contraction.

Just like the pressure waveform that is measured in the systemic arterial circulation, the pulmonary artery waveform will have a rise in pressure during systole, followed by a gradual decrease in pressure during diastole (Fig. 6.4). Systolic pressure should closely approximate RV systolic pressure. A dicrotic notch during diastole occurs at the time of closure of the pulmonic valve, and interrupts the otherwise smooth descent of the pressure tracing. This is analogous to the dicrotic notch seen in an arterial line tracing due to closure of the aortic valve.

Once the balloon passes distally enough in the pulmonary artery to completely occlude the vessel, the pressure transducer will no longer be able to measure the normal pulmonary artery pressures waveforms behind it. At this point, a continuous column of blood exists between the tip of the catheter, through the pulmonary capillaries, and all the way through the pulmonary veins. Thus, once in correct wedge position, the hemodynamic tracing will approximately transduce the pressure waveform of the left atrium. Understanding this phenomenon will help one to troubleshoot when the waveform does not appear as would be expected. One common mistake is made when the catheter is incompletely in a fixed position, and the tracing represents partial wedge and partial pulmonary artery pressures. When this happens, it can be helpful to wedge the catheter under fluoroscopy, as the normal systolic and diastolic movement of the balloon will terminate, and the balloon will hold its position when correctly wedged. Another way to confirm that the catheter is in a correct position (although it is very rarely required) is by the very slow and careful drawing back of blood from a wedged catheter. Checking the oxygen saturation of this blood should reveal a normal arterial concentration (as it is drawn back across the capillaries from the pulmonary veins). Pressure tracings of the left atrium will appear similar to that of the right atrium, other than being somewhat higher. A PCWP tracing will differ somewhat from a directly measured left atrial pressure due to the column of fluid that separates the two, and this often leads to dampening of the waveform and a larger delay between events seen on the EKG and events measured on the pressure waveform (i.e. the delay between the p-wave and a-wave will be shorter on the RA tracing and longer on the PCWP tracing).

Given that all of the measurements made during a right and left heart catheterization are done within the thoracic cavity, it is important to remember the normal relationship between intra-thoracic pressures and the respiratory cycle. Frequently, the pressure tracings will rise and fall by a substantial amount during the respiratory cycle, and it is often confusing to decide the correct point to make measurements. The key is to understand that pressures in the thoracic cavity always return to “normal” at the end of expiration. In a person breathing independently, inspiration causes a small amount of negative pressure within the thoracic cavity, and this returns to normal during expiration. In a patient on a positive pressure ventilator though, the opposite will occur, and the pressure will rise as air is forced into the lungs during inspiration, and return to normal during expiration. Thus, measurements should always be taken at the end of expiration, which is the point at which the pressure in the lungs is equal to the pressure in the atmosphere, and is also referred to as the Functional Reserve Capacity (FRC) of the lungs. In patients who are on ventilators with substantial positive end expiratory pressure (PEEP), the measured pressure at end-expiration may slightly over-estimate the patient’s normal physiologic pressures.