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).
Do you notice what we have done with the normal values in Table 1? They are all multiples of 5. If you read authoritative books on hemodynamics, you will find each differs slightly, giving normal ranges on the values of the chambers. Rather than memorize that the normal mean for PCWP is 6-12, it is much easier to remember that the normal mean for PCWP is about 10. If you are just learning the expected values for heart pressures, look at Table 1, see what is typically reported and then memorize that value. Table 2 shows the simplified values to memorize.
A little on down the line, you will find it is easy to remember the atrial mean should always equal the associated ventricle's EDP. For instance, the PCWP or LA mean (remember we use the PCWP to represent the LA) should be equal to the LVEDP. Why? Because the valve is open between the two chambers at the point it is measured and the pressures are equal. Likewise, the RA mean should equal the RVEDP.
As you can see, learning the expected values for each chamber of the heart is simply memorizing a few numbers. Spend five minutes a day for a couple of days and you will have it down pat.
The absolute first thing to do when looking at a waveform is to note the scale on which it is recorded, which will allow you to almost immediately identify the chamber. Typically, the RA, RV, PA and PCWP are recorded on a 0-50mmhg scale, while the LV and AO are on a 0-200mmHg scale. In circumstances involving a form of hypertension or dysfunction, you may have an alternate scale. Most often, however, you will always see a 0-50mmHg or 0-200mmHg scale. If it is 0-200mmHg scale, ask yourself whether the waveform is tall and peaked, going nearly to the bottom of the scale during diastole. If so, it is a LV pressure. If it is triangular, then it is an AO waveform. If the scale is 0-50mmHg and if the waveform is triangular, it is the PA; a tall and peaked waveform is RV; about a mean of 5mmHg, RA; a mean of about 10mmHg, PCWP.
For ease of discussion, we will not look at each individual chamber's associated waveform. Instead, let's break them up into three types of waveforms: atrial, ventricular and triangular (artery). The atrial waveform (Figure 1), in very basic terms, represents both the RA and the PCWP (LA) waveforms. We commonly report the mean RA and PCW pressure, so the focus should be on where the overall waveform is in relation to the pressure scale. When glancing at the waveform, is it in the 0-5, 5-10, 10-15, or 15-20mmHG range? Learn to identify where the average of the pressure is located. An atrial pressure is going to be primarily a double-humped pressure; think of a double-humped camel. In our initial discussion, think of these humps representing the a-wave and v-wave. When looking with a more critical eye, you will find there are differences between the RA and PCWP, but the first step is to learn that if you see a double-humped, squiggly-looking waveform, it is an RA or PCWP. The PCWP will be higher than the RA in a normal patient. Think about the PCWP representing the LA. The LA is arterial; ergo, higher pressure. If the waveform is in the 0-5mmHg range, it is the RA; if in the 5-15mmHg range, it is the PCWP. Remember, however, that in patients with certain pathologies, these ranges will be abnormal. The PCWP could be 30mmHg, for example.
Let's move on to the ventricles (Figure 2). Notice that ventricular waveforms are tall and peaked, with the lower part of the waveform going to the bottom of the scale and sometimes even off the bottom of the scale. Our primary focus on ventricular waveforms, unless looking for specific pathologies, is the end-diastolic pressure (EDP). It is measured just before the rapid upstroke in the waveform. These pressure waveforms are easy to identify, because they usually take up the whole scale from top to bottom. To tell the difference between the LV and the RV waveform, look at the scale. The LV is arterial, so the pressures will be higher. Expect RVEDP to be about 5mmHg and LVEDP to be about 10mmHg.
At this point, only the PA and AO pressure waveforms remain. These two also look very similar, and in order to differentiate between the two, look at the scale and pressure recorded. Figure 3 shows how the PA and AO pressure waveform is triangular in shape, typically with a distinctive notch on the downslope of the waveform. Reporting of the PA and AO pressures is usually systolic over diastolic. Another differentiating note is that the pressure is peaked like the ventricular waveforms, but does not go all the way to the bottom of the scale, and will look wider in appearance. Expected values for AO are 120/80 and PA, 25/10.
Since we are usually taking care of and diagnosing patients with something not normal, the reality of frequently seeing an AO pressure of 120/80 or a PCWP mean of 10mmHg is rare. Future articles in this series will discuss some of the common causes of these pressure variances.
Three Things Crucial to Identify
Finally, and probably most importantly, there are three things essential for cath lab staff to identify. These three things, if not identified, can potentially have serious consequences for the patient. They are dampening, ventricularization and critically elevated LVEDP. If any of these occur during a catheterization, the physician needs to be notified immediately in order to correct or prevent further deterioration in the patient's condition.