The same as central venous pressure (because there are no valves between the superior vena cava and the right atrium). RAP is a venous pressure, expressed in a mean, and its waveform is a low-pressure, venous waveform. The RAP is the first recorded pressure when a PA catheter is being inserted and is continuously monitored by the proximal port. This port recognizes pressure changes only in the right atrium and the increase in pressures are represented by upright waves in the waveform. This pressure reflects preload to the right side of the heart. It is not as accurate as pulmonary artery wedge pressure (PAWP).

The right ventricle is a higher-pressure chamber, and its pressure has a systolic and diastolic reading. Its waveform is more arterial-looking in nature than the RA waveform. Right ventricular pressure is not continuously monitored, but will be noted and recorded as a PA catheter is inserted. While the catheter is in the RV, ventricular ectopy may occur. (If sustained ventricular tachycardia occurs, the catheter is withdrawn into the RA.) If a patient's PA waveform develops morphology or values suggestive of RV pressure, particularly in the presence of ventricular ectopy, the catheter should be evaluated to see if it has drifted from the pulmonary artery to the right ventricle.

Picture of PA catheter

The PAP is constantly monitored by the distal port of the PA catheter. The pulmonary artery systolic pressure (PAS) approximates the systolic pressure in the RV, but the pulmonary diastolic pressure (PAD) is higher, and the waveform less steep. The PA waveform includes a diastolic notch which represents closure of the pulmonic valve. In the absence of lung or mitral valve disease, the PAD approximates the pulmonary artery wedge pressure (PAWP) and can, therefore, aid in evaluation of preload without wedging the catheter.

When the balloon on the PA catheter is inflated, the catheter floats further into the pulmonary catheter until it wedges against the vessel, giving us a view of what's beyond. Because there are no valves between the pulmonary artery and the mitral valve, the PAWP allows us to look at the pressure in the left atrium (LAP). PAWP is the most accurate reflection of left atrial pressure, therefore of left ventricular end-diastolic pressure (LVEDP), or preload. Anything that affects preload, from total blood volume, to venous return, to compliance of the left ventricle and its ability to receive that volume, will affect PAWP. Like the RA waveform, this reflection of LAP consists of a waves and v waves (c waves are not usually present).

We have already mentioned SVR as the best measure of afterload. SVR is a derived parameter, calculated by the bedside monitor, which takes into account the venous and arterial pressures as well as the cardiac output.

Any resistance to ventricular emptying will be reflected as an increase in SVR. Hypertension, vasoconstriction (hypothermia, hypovolemia), and aortic stenosis all increase SVR. Likewise, vasodilation from hypothermia or sepsis will cause a
decrease in SVR.

The amount of work the right ventricle performs with each heartbeat. This is an excellent measure of contractility of the RV.

The amount of work the left ventricle does with each heartbeat. This is an excellent measure of the contractility of the LV.

Measurement of the mixed venous oxygen saturation, or the saturation of blood in the pulmonary artery, can be continuous with special PA catheters. This blood is normally unoxygenated, having not yet traveled through the lungs, with a saturation of 60-80%. SvO2 is clinically important as a function of supply and demand; how much oxygen is being extracted from the blood by the organs, before this blood is returned to the right heart. SvO2, therefore, serves us in evaluating the supply and demand of oxygen to the tissues. It is influenced by oxygen delivery (hemoglobin, SaO2, cardiac output) as well as oxygen consumption.