Hemodynamics is a 12-Letter Word!An intro to the basics
- Posted on: 6/19/08
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Part I (March 2007) discussed the need for cath lab staff to have an understanding of hemodynamics. We reviewed the importance of building a foundation of knowledge, starting with a basic understanding of how blood flows through the heart, and what the chambers and valves do to get the job done. We hope readers felt encouraged in their study of the Wiggers diagram and found answers to questions they may have had.
Part II focuses on the need to assist physicians and other staff in providing better patient care. The intent is to help professionals new to the cath lab gain a basic understanding of hemodynamics. The more capable one becomes in the understanding and interpretation of hemodynamics, the more the physician will trust in the staff and be better able to focus attention on catheter manipulation and treatment options. In this article, we will review the normal pressures of the heart and their associated waveforms, then look at a few situations where the physician needs immediate notification.
Many of us recall that when entering the cath lab and beginning to study normal values, every book seemed to have a different set of numbers for what was normal. It can become quite confusing, so we will share a simple way to remember the normal ranges. First, let's look at how pressures are reported.
Sometimes in the cath lab, the physician will ask, What is the mean wedge pressure (or "EDP" or "PA diastolic")? How to report a pressure can be confusing. When a physician asks, What is the PA pressure?, do you report the mean, the systolic over diastolic, or something else? What about the right atrial pressure? When you look at the monitor, both pressures have three numbers in a similar format: X/X/X. Which number or numbers do you report?
These are all valid questions, and sorting it out is not as complicated as it might appear. All the numbers associated with each pressure and waveform are significant, but are only needed when looking at specific pathologies. For example, if we are looking for mitral regurgitation, the values of the a-wave and v-wave of the pulmonary capillary wedge pressure (PCWP) are compared, but if we are looking for diastolic dysfunction of the left ventricle, the mean pressure is important. Our focus in this article, however, will be on how pressures are commonly reported and what their expected values should be.
The majority of the patients we see in the cath lab are there because something is suspected to be wrong. There may also be prior pathologies (systemic hypertension, pulmonary hypertension, valvular disease, etc.) contributing to the reason the patient is in the lab. To be honest, the values we study today most likely will not be the values you see in the lab, because if someone is normal, chances are you will not find them in the cath lab. However, to paraphrase C.S. Lewis, you cannot recognize a crooked line unless you first have some idea of a straight line. The same principle applies to learning hemodynamics. You must learn the normal before being able to recognize the abnormal.
Let's review what values are reported for each chamber, what those expected values should be and what the waveforms will look like.
Reported & Expected Heart Chamber Values
The right atrium (RA), left atrium (LA) and the pulmonary capillary wedge pressure (PCWP) are all typically reported as a mean pressure. When I say mean pressure, don't think angry, think average. The mean pressure is obtained through a simple formula that the hemodynamic system will calculate for you. It is the last number in the series X/X/X. The PCWP might be 12/16/8, (or some systems show it as 12/16), so the mean pressure is 8. An easy way to remember this is that just those waveforms that are squiggly are reported as mean pressures. Aortic, pulmonary and ventricular pressures all have a triangular appearance, while the rest are bumpy lines. You can also simply memorize that the atriums and wedge are reported as mean.
As a side note (to be discussed in greater detail later in this series), the wedge pressure is what we use to measure the left atrial pressure, since we have no easy access to the left atrium and the wedge is a direct reflection of that pressure. Look at a good heart diagram with lungs. If we remember that the catheter is like a telescope measuring what we are looking at and that there are no valves between the wedge and the left atrium, then it is going to be the same pressure, considering there are no severe lung restrictions. We block the blood from the pulmonary artery with the balloon, preventing input from the pulmonary artery. What is left to measure is the pressure on the distal side of the balloon the left atrium.
Everything remaining is reported as systolic over diastolic. The left ventricle (LV), right ventricle (RV), pulmonary artery (PA) and the aortic (AO) pressure are all reported similarly. The only difference is that the ventricles' diastolic pressure is referred to and measured as end-diastolic pressure. If we remember the Wiggers diagram from Part I, this is a point just before the rapid upslope of the ventricular waveform, at the end of the diastolic phase. Table 1 will help clarify, but it does get a confusing and it just takes some time in the lab and memorization to know which numbers correlate. As an example, the LV pressure will show up as 120/3/7 on most systems. What these numbers represent is systolic/diastolic/end-diastolic. When the physician asks, What is the EDP?, it is typically the last number in the series.
The PA and AO pressures are reported as systolic over diastolic and the monitoring systems will almost always display in a systolic/diastolic (mean) format, or, for example, 120/80 (70) (Table 1).