Hemodynamics is a 12-Letter Word!An intro to the basics.Part I: Basics with Wiggers
- Posted on: 6/19/08
- 0 Comments
- 19521 reads
There are other shortcomings in today’s advanced hemodynamic systems. Marking the correct points on the waveforms is one example, but the purpose of this article is not to point out all these issues, but to aid in educating cath lab staff in the basics of hemodynamics. In order to operate the hemodynamic systems, it is essential to understand the components of waveforms and expected values for the patient. Everyone needs to take advantage of the time savings these systems offer, but should educate ourselves enough to utilize them correctly, thus ensuring we don’t end up with false values, which can lead to unnecessary procedures, incorrect treatment or no treatment. There are many reports where staff did not pay attention to the markers on the waveforms and ended up reporting a pulmonary artery (PA) pressure of 75mmHg (system marked a spike) when the correct pressure should have been 50mmHg. There is a significant difference between the two and treatment options could change based on the values.
Why do cath labs today have so many staff members who do not understand hemodynamics? The answer is probably as simple as the fact that this knowledge is no longer expected of new staff. Fast-paced labs need staff to get up to functional speed as soon as possible. New staff are taught to scrub, circulate and run the hemodynamics banking, with the idea that they will learn the details of these processes later. Unfortunately, later never comes. It also may be that no veteran staff with a good understanding of hemodynamics is willing (or able) to take the time to teach newer members. There are also situations where some staff members want to be spoon-fed education and will never put forward the effort necessary to educate themselves. Ultimately, it is our responsibility as professionals to accept responsibility for our education. The managers, educators and senior staff can provide opportunities and teaching, but it is not until the student decides they wish to learn that education takes place. Perhaps most importantly, however, the most common situation may be that staff consider the subject of hemodynamics to be too complicated, intimidating and way over my head. In this article, we hope to dispel that myth. The goal is to communicate a relatively difficult topic in an easy-to-understand way. Part I will cover the phases of diastole (when the heart is relaxing) and systole (when the heart is contracting).
We will assume that the reader has a strong grasp on the anatomy of the heart and how the blood flows through the heart. In brief:
1. Unoxygenated blood moves from the inferior vena cava (IVC) and superior vena cava (SVC) to the right atrium (RA);
2. Through the tricuspid valve to the right ventricle (RV);
3. Through the pulmonic valve to the PA and through the pulmonary capillary wedge (PCW, not a structure, but important to remember) to the lungs.
4. Oxygenated blood moves from the lungs through the pulmonary veins to the left atrium (LA);
5. Through the mitral valve to the LV;
6. Then through the aortic valve to the aorta and systemic circulation.
Blood flow is absolutely essential to understanding hemodynamics. The field of hemodynamics is so large that it is nearly impossible to simply memorize the process of analysis and calculations. Each step of the way must make sense before you can move ahead. Try to visualize what is happening as each topic is discussed. If you do not have a good grasp on cardiac anatomy and blood flow, find a co-worker and a good diagram. Study until it makes sense to you.
Before beginning a review of the phases of systole and diastole, let’s try to understand what makes a waveform. What causes that line on the screen to go up and down? Without getting into transducers and fluid dynamics, think of it this way: wherever the catheter tip is, imagine a clear lens on the tip. You look through the catheter as you would a long, wiggly microscope. If the catheter tip is in the left ventricle (looking into the left ventricle), you are getting a left ventricular waveform. (This may sound almost too basic, but it will help you understand the wedge pressure and what it represents when this is discussed in the future.) Since we are looking in the left ventricle, what makes the line on the monitor go up and down? Pressure, obviously, but think of the whistle with the paper that unrolls like a frog’s tongue when you blow on it. This whistle is the catheter. Look straight up in the air and blow on it. The harder you blow, or the more pressure you create, the higher it goes. You can apply this same concept to looking at a waveform. The more pressure is exerted on the end of the catheter, the higher the line goes on the screen.
You may wonder why the article will review the phases of diastole and systole when all you want to know is how to identify if something is wrong with a pressure so the physician can be notified. Good question. It is important to understand how the valves work with the chambers to get blood through the body. This is probably the most difficult portion of understanding the basics, but if you can understand the phases of diastole and systole, when it comes time to analyze waveforms, everything will fall into place with greater ease. In reality, there will not be much to learn; rather, you will correlate and identify. When we first dove into hemodynamics, our mentor gave us a Wiggers diagram and said, Learn it, don’t study anything else until you know this inside and out and it makes sense to you. Boy, was he right! It made so much sense once we got it down pat.
As Part I, this article will focus only on when the valves are opening and closing, and when the chambers are contracting and relaxing. The important knowledge will be broken down into manageable chunks. Certainly there is much more to study, such as isometric contraction/relaxation, diastolic filling, passive filling, rapid filling, etc. The article will touch on these as they occur, but not in detail. At this time, it is important to understand how the valves and chambers work together, and what it produces on the computer screen or paper.
The Wiggers diagram shows three different waveforms, excluding the ECG at the bottom, that have been superimposed onto one another. These are three separate recordings that have been overlaid to match up the phases of the cardiac cycle. In the cath lab, the Wiggers diagram would be like having three catheters connected to the monitor at the same time: one in the left atrium, one in the left ventricle and one in the aorta, and all recording on the screen at the same time. Note that this diagram deals with the left side of the heart. To understand the right side, simply change the labels of the valves and chambers. It is the same process. Also, when referring to filling, contracting, or relaxing phases, the diagram is referring to ventricular filling, contracting and relaxing.
Please note that the alphabetical labels on the diagram do not correspond to actual waveform identification. The letters exist merely for the sake of discussion. However, some waveforms are labeled with letters (i.e., an atrial wave form has multiple waves identified as a wave, v wave, etc.). These waveforms will be addressed in future articles in this series.
Point a. Right now, the valve between the atrium and ventricle is open, which, in this instance, is the mitral valve. Immediately prior to point a, the atrium and ventricle are being filled passively. Passively means that blood coming from the lungs is spilling in. With the open mitral valve, blood passes easily into the ventricle. In a normal cardiac cycle, the atria will also contract, giving a final boost in volume to the ventricle before both ventricles contract, called the atrial kick. This atrial contraction supplies an additional boost of blood volume to the ventricle. The bump you see at point a is the atrium contracting (squeezing causes more pressure, so the line rises on the monitor). Blood is rushing into the ventricle until the ventricle gets the message to fire.
At point b, the ventricle has begun to squeeze, building pressure until it exceeds the pressure in the atrium and slams the mitral valve shut. Now the mitral valve has closed and the next path for the blood to travel is out to the aorta. Yet the aortic valve is still closed. What happens next for the blood to get through the aortic valve?
The area between points b and c is called isometric contraction. The term isometric comes from the Greek for having equal measurement and in this case, it refers to the volume of blood. Isometric contraction means the ventricle is contracting, but there is no change in volume. (Note that during isometric contraction and isometric relaxation, all valves are closed.) The isometric contraction period is necessary because the ventricle must generate enough pressure to open the aortic valve and deliver the blood to the body.
Point c. The ventricle builds pressure until it achieves a pressure greater than that of the aorta. Once it reaches this point, c, the aortic valve opens and blood rushes forcefully out into the aorta. Note how the aortic waveform is the same with the ventricle just after point c. Why? The valve is open, so the pressures equalize. After the aortic valve opens, the ventricle continues to squeeze and build pressure, forcing as much blood as it can out into circulation until the volume is depleted. The ventricle gets the message to relax, and the pressure starts to drop (point d).
The area between point c and e is called the systolic ejection period. That’s when heart muscle is really moving and contracting with great force. It is the period when blood is ejecting from the ventricle. The ventricle has forcefully ejected the blood, the blood is expelled into the aorta and now the pressure begins to fall. Once the pressure in the ventricle falls below the pressure of the aorta, the aortic valve slams shut, point e on the diagram. Often you will see a bump on the aortic waveform, referred to as the dicrotic notch. The dicrotic notch is from the rebound effect of the blood slamming against the aortic valve.
The ventricle has now expelled its blood and the aortic valve is closed (point e). Now the ventricle is going to relax and fill back up with blood. Between point e and point f is what is called isometric relaxation. Remember, isometric means the blood is not traveling anywhere, and anytime the heart is isometric, all the valves are closed. The ventricle is relaxing and decreasing in pressure. It continues to do so until the pressure in the ventricle becomes lower than the pressure in the atrium. At this point, the lower pressure in the ventricle causes the mitral valve between the atrium and ventricle to open (point f).
After the valve opens, the ventricle is still relaxing and decreasing in pressure, which causes a sucking effect, drawing blood out of the atrium very quickly. This is why you see a dip in pressure just after point f. The dip area is the rapid filling phase. The suction caused the dip in pressure, and the pressure in the ventricle and atrium are equalizing and rising until they nearly plateau or level off. The gradual climb (coming from the right edge of the diagram and starting over again at the left edge of the diagram) is called the slow filling period. We have reached our starting point, the slow filling, with the blood returning from the lungs and left atrium, and filling the ventricle.
It may take some review to truly cement the details of the diagram and its phases in your mind. Locate additional resources in order to increase your grasp of this topic. The Internet is a wonderful and obvious place to start. Search on Google for Wiggers diagram and you will see that many universities have relevant articles.
1. Todd JW. Todd’s Cardiovascular Review Book Volume I: CV Science, Patient Care, Anatomy, Physiology, and Pathology. 4th Ed. Spokane, WA; Cardiac Self-Assessment. Available on www.westodd.com
2. Kern MJ. The Cardiac Catheterization Handbook. St Louis: Mosby, 2003.