PA Catheter Basics

PACs allow for assessment of Cardiac Output (CO), Pulmonary Artery Pressure (PAP), Mixed venous oxygenation (SvO2), Pulmonary Artery Wedge Pressure (PAWP), and Central Venous Pressure (CVP) through blood draws and waveform analysis. CO is the amount of blood pumped by the heart each minute and is simply heart rate multiplied by stroke volume (the amount of blood ejected with each beat). With a PAC, CO is determined by obtaining an SvO2 level, the patient’s body surface area, a hemoglobin level, and an SpO2 reading. All of these numbers are collected and inserted into a Fick calculator which gives us CO and cardiac index (CI). The only lab values needed to run a Fick is a mixed venous saturation (SvO2) and an arterial saturation. CI is merely CO with consideration of the patient’s body surface area (BSA), and is, therefore, more accurate for clinical decision making. 

PAP is a reflection of cardiac heart pressures and is read as a systolic and diastolic number. Pulmonary artery systolic pressure (PASP) is the pressure exerted by the right ventricle (RV) during systole on the PA. Whereas pulmonary artery diastolic pressure (PADP) is a measurement of the pressure in the PA during RV diastole. This is a continuous waveform measurement.

SvO2 is the percentage of oxygen attached to the hemoglobin in the blood leaving the right ventricle. Therefore, SvO2 can be used to determine if CO and oxygen delivery is sufficient for the patient. A Low SvO2 can indicate that either body tissues are taking more oxygen from the blood as compensation for increased oxygen needs or are not receiving enough oxygen to begin with (CO is not high enough to meet the body’s demand). An SvO2 is obtained through a blood draw from the yellow lumen of the PAC.

PAWP is a measurement of left ventricular end-diastolic pressure. Basically, this is a measurement of preload, or filling volume, to the left ventricle (LV). PAWP is obtained by utilizing the balloon port of the PAC. Between 0.5-1.5mL (a specific syringe exists that only allows 1.5mL, try not to lose it!) is injected into the red lumen, this causes the balloon to occlude antegrade flow from the distal PA to the pulmonary capillaries. Due to the occlusion created by wedging, it is important to have your monitor set up to “freeze” the screen when a wedge waveform is achieved. Since you do not want to be wedged for longer than 15 seconds, preparing to get a still of the wedged waveform, or “freezing” the screen, will allow you time to assess the waveform once the air is removed from the balloon. Only insert air until the waveform is “wedged”, freeze the screen, and deflate the balloon. Allow the balloon to deflate passively and ensure a PAP waveform is present following wedging. Not all PACs will wedge, often due to positioning. Do not wedge a PAC unless there is an order to do so from the clinician. 

Finally, CVP is a representation of RV preload and pressure. Using this measurement has many caveats and will require an entire post in itself! This is a continuous waveform measurement and can also be assessed with a CVC sans PAC.

So, what are the normal values for these measurements?

• CO 4-8 L/min
• CI 2.5-4 L/min
• PAS 15-25 mmHg
• PAD 2-12 mmHg
• PAP Mean 10-20 mmHg
• SvO2 65-70% (typical oxygen extraction is 25-30%)
• PAWP 8-12 mmHg
• CVP 2-6 mmHg

In order to get accurate readings, it is important to level and zero the lines prior to waveform analysis. Assess leveling anytime the patient changes position for accurate continuous monitoring.

Level first:

1. Find and mark the patient’s phlebostatic axis (the level of the right atrium; approximately 4cm posterior to the sternum at the fourth intercostal space), marking it will eliminate some variation between those leveling
2. Using an actual level, level the stopcocks (there will be one for each measurement: PAP and CVP) to the marked phlebostatic axis

Zero second:

1. Move the stopcocks off to the patient, open to atmospheric pressure
2. Select “Zero all” on the monitor, it is also possible to zero each line individually; just learn where the button is on your monitor
3. Turn the stopcocks back off to the patient

After leveling and zeroing the lines, waveform analysis can be performed. At this stage, it is crucial to measure the waveforms at the end of expiration. This is because pleural pressure is closest to atmospheric pressure at the end of expiration. To achieve this, it is helpful to monitor the respiratory waveform simultaneously. An accurate PAP is the peak and trough measurement of the waveform at end-expiration. CVP measurement is taken at the base of the c wave, this is just after the R wave on an ECG, and represents the final pressure in the RV before systole.