Now You Tell Me I Have Pulmonary Hypertension!-July-2011

 

First you tell me I have lung disease

Now You Tell Me I Have Pulmonary Hypertension!

by John R. Goodman, BS RRT

This 2-part article was written to give you a better understanding of Pulmonary Hypertension and how it develops in patients with chronic lung disease.

Part 1: Pulmonary Hypertension (PH) Defined

ulmonary Hypertension (PH) may be defined as high blood pressure in the arteries that go from the heart to the lungs. It is interesting to note that the definition of an artery is a “blood vessel that carries blood away from the heart.” Generally arteries are high in oxygen content and the blood therein is bright red. Blood in the pulmonary “artery” however, is very low in oxygen, as it has just been pumped into the pulmonary artery by the right ventricle. The right ventricle receives all the venous blood from the lower extremities as well as venous blood draining in from the head neck, and shoul- ders. Venous blood returning from all parts of the body is a darker red color as the blood has released its oxygen to the billions of cells of the body. This is perfectly normal and is reflected in the normal saturation values associated with arterial and venous blood. If we consider a normal oxygen saturation (SaO2) at sea level as right around 97%, most people would be shocked to learn that the “normal” saturation of venous blood (PvO2) averages about 75%.

Unlike the left ventricle which must generate much higher pressures to pump oxygenated blood throughout the whole body, the right ventricle normally does not need to squeeze very hard to move the venous blood over to the lungs to pick up that all important oxygen. As an exam- ple, we can use just the upper pressure measurements to give an idea of the difference between the right side of the heart (pulmonic) and the left side of the heart (systemic).

When a physician, or nurse takes your blood pressure with a blood pressure cuff, the result is normally reported out as (for example) 120/80. The top number is called the systolic pressure and in the normal adult it does average about 120 mm of pressure. (We will not worry about the bottom number at this time.) This is how much pressure or force is necessary to keep the blood circulat- ing through the millions and millions of blood vessels (including capillaries) throughout the body. Since this is a reflection of the function of the left ventricle, we simplify things to say this represents left heart function.

Going over to the right ventricle, and remembering the right ventricle only has to pump blood over to, and though both lungs, it takes much less pressure. Normal right ventricular pressure is reported as 25/15. So simple math shows us that normally pressures on the left side of the heart are almost five times higher than on the right (120 vs. 25)! Unfortunately, there is no test as simple or as easy as taking your blood pressure that can measure your right- sided heart pressures. Most commonly the information is obtained non-invasively though the use of an echo cardiogram, standard chest xray or CAT scan. Positive confirmation is made by doing a catheterization of your right heart and measuring pressures directly. This is obviously an invasive procedure, and is normally performed in a cardiac catheterization lab specifically designed for this procedure.

There are many, many non-pulmonary causes of Pulmonary Hypertension. If you have a specific disease or condition that you think may be related to your PH, I strongly encourage you to begin with a simple Google search. But, due to space limitations, I must limit our discussion to the known relationship between pulmonary disease and the development of PH. While I normally don’t like to generalize the term “lung disease,” it is possible for me to do just that due to the one common denominator of all chronic lung disease. That common denominator is chronically low levels of oxygen in the blood, also known as hypoxemia. But, more on this later. As a quick note, you may see the terms Pulmonary Hypertension (PH) and Pulmonary Arterial Hypertension (PAH) used inter- changeably. To slightly add to the confusion, both PH and PAH, have been known in the past as “secondary pulmonary hypertension.” That is, heart disease that is secondary to lung disease. For all intents and purposes this is just nomenclature unless you see the use of PPH. PPH has been used for many years as an abbreviation for Primary Pulmonary Hypertension. This is a rare disorder (perhaps 4–6 cases per million) where the patient is born with the disease which may show up shortly after birth, or lay lurking in the shadows to pop up later in the patient’s life. Since no one really knows what causes PPH, it is better known today as Idiopathic (unknown cause) Pulmonary Arterial Hypertension (IPAH). Again, I must keep this discussion limited to PH that develops second- ary to chronic lung disease.

Now that we have been introduced to the disease or condition known as PH, we can discuss how it develops. There is an old slogan in medicine that happens to be very, very true. That slogan is “the body in its infinite wisdom.” What this means is that the human body has the marvelous ability to compensate for physiologic alterations that may be happening internally. A corollary to that statement might be, “the body never overcompensates.” As I stated earlier, PH develops as a result of chronically low blood oxygen levels. Over the past 20 years other factors have been found that hasten the development of PH or even make it worse.

Here is where the body in its infinite wisdom comes into play. If blood oxygen levels stay low enough, long enough, the body attempts to “compensate” for this deficiency. It does so by constricting blood flow though the pulmonary arterial system. Here is what the body is thinking. Okay, I’ve got less oxygen in the blood flow- ing through both the whole body and the lungs themselves. For whatever reason (pulmonary disease), there are less oxygen molecules being made available to the capillaries responsible for transporting that oxygen

throughout the body. So the body compensates by con- stricting (actually shrinking) these tiny blood vessels so that there is a better matching of oxygen breathed in to the blood still circulating through the lungs. The body in effect says, “Well, there is less oxygen available to pick up, so let’s do a better job at matching up those sections of the lungs that are still working pretty well, by re- routing blood flow preferentially to those units.” Truly, this is an example of the “body in its infinite wisdom” at work.

From the body’s perspective, this will have the effect of making sure the highest percentage of lung units will be functioning at their optimal best. Makes sense, doesn’t it? However, as we all know there are no free lunches in life. In this case making things better for the lungs can make things worse for the heart. We start the explanation by asking an obvious question – What is the difference between a drinking straw and a garden hose?

The answer to all of us is both simple and obvious. It is, of course, the diameter. You don’t have to be a hydraulic engineer to figure out that it is a heckuva lot easier to pump a fluid (like blood) through a series of garden hoses than it is through a bunch of drinking straws. This can be fur- ther complicated by the fact that chronic hypoxemia can also cause the blood to become thicker than normal. In another example of the body compensating for a chronic condition, more red blood cells are released into the bloodstream in order to “deliver” more oxygen to the cells of the body. If this condition goes on long enough, the viscosity of the blood goes from something like tomato juice to ketchup! This is reflected in your blood work as elevated Hemoglobin and Hematocrit levels.

Now is a good time to remember that the right side of the heart is the low pressure side. Since less pressure has to be generated, the muscular wall of the right ventricle is thinner and does not pump with as much force as the left ventricle. This means the right ventricle is much more subject to resistance downstream. Well, where is down- stream from the right ventricle? That’s right, the lungs. So first we have the constriction of the pulmonary arteries due to chronic hypoxemia, and then there may be the double whammy of having to pump thicker blood.

The Body Can Only Compensate So Much

Even the body in its infinite wisdom has limits to how much it can compensate for a chronic condition. At first the muscular wall of the right ventricle tries to keep up with the rising pulmonary pressures. Like any muscle that is exercised, it actually gets larger and for a period of time pumps with more force. The right side of the heart does make a valiant effort to keep up with the rising pressure. To quickly review, the normal pressure in the main pulmonary artery is most commonly given as 25/15. (There may be some quibbling as to the exact numbers,

but overall we can use this figure.)

If you look closely at the two illustrations (upper right), you can see what the end result will predictably be as the narrowing of the pulmonary artery(s) continues, the right ventricle enlarges to a point where it starts to become dysfunctional. If this “back pressure” continues unabated, it can cause fluid to back up throughout the entire circulatory system. This puts further strains on both the left and right sides of the heart, and you can see how a vicious cycle is created … and this cycle can lead to some very serious consequences including heart failure.

Like many diseases, PH is classified according to the Pulmonary Artery Pressure (PAP) measurement as follows:

Mild PH = a PAP of 26–34 mm of pressure. Moderate PH = a PAP of 35–44 mm of pressure. Severe PH = a PAP of 45 mm or greater.

Common Signs and Symptoms of PH Include:

• Dyspnea (shortness of breath), both at rest and seen especially with This usually starts slowly and gets worse over time.
• Dizziness, perhaps suddenly passing out.
• Lethargy or fatigue.

• 

  Enlarged Right Ventricle

  Chest pain; cough; swelling of the ankles or With this checklist of signs and symptoms, it is easy to see why your doctor must order a number of tests to either rule in PH, or rule PH out and some other condition in as the cause of the symptoms. Even then, your doctor must have a pretty high suspicion of PH in order to both make the correct diagnosis and not miss some other “sneaky” conditions such as pulmonary emboli (blood clots) in the lung, interstitial lung disease, certain forms of heart or heart valve disease, connective tissue disease and even sleep disordered breathing.

Once all of the blood work, echo cardiograms, radi- ologic testing, ECGs, and exercise testing is performed, your doctor will have a pretty good idea if you indeed have PH. These are all non-invasive tests that get a patient into the “ballpark.” Your doctor may well want to be positively sure of the diagnosis by having you undergo a cardiac catheterization. This is an invasive procedure where a small catheter is inserted into one of your larger veins and then advanced into the right side of your heart. Measurements made here are exact and you can be quickly classified as mild, moderate or severe PH.

Part 2 in the September/October issue will discuss treatments for PH.

John Goodman RRT is Executive Vice President of Technical/ Professional Services at Transtracheal Services, Denver, CO.