Hypertension & Diving

By Dr. Sawatzky

Hypertension is an extremely common medical problem in Canada (over 6 million adults affected) and I would like to thank the reader who suggested I write a column on this topic. I am always looking for ideas so if you have something you would like me to research and write about, please contact DIVER Magazine.

Hypertension is simply defined as high blood pressure. For this definition to make sense we have to know what blood pressure is and what controls it. Blood pressure is defined in Stedmans Medical Dictionary as, "the pressure or tension of the blood within the arteries, maintained by the contraction of the left ventricle, the resistance of the arterioles and capillaries, the elasticity of the arterial walls, as well as the viscosity and volume of the blood; expressed as relative to the ambient atmospheric pressure". As usual, this definition makes very little sense unless you already know all about blood pressure!

First, a simple review of how blood moves around the body. The heart is just a pump (much to the disgust of the romantics I'm sure who think the heart has something to do with love or emotions) and its job is to push the blood through the circulatory system. You should remember from your basic dive course that the heart is actually two pumps, each with two chambers. The right side of the heart collects blood returning from the body. When the heart contracts, the right atrium contracts first, complet ing filling of the right ventricle. The right ventricle contracts a fraction of a second later, forcing blood out through the pulmo nary valve and into the pulmonary artery. The pulmonary artery divides, sending blood to the right and left lungs. The blood returns from the lungs in the pulmonary veins to the left side of the heart. The left atrium connects to the left ventricle. When the left ventricle contracts, it forces the blood out through the aortic valve and into the aorta. The aorta distributes blood to the rest of the body by dividing into large arteries which divide into smaller arteries until the smallest arteries which are called arterioles. The arterioles divide into the capillaries in the tissues where gases and nutrients are exchanged with the cells. The capillaries join together to form venules which join to form small veins which join to eventually form the inferior and superior vena cava which return the blood to the right side of the heart to complete the circle.

The pulmonary artery of course has a blood pressure, but when we talk about a person's blood pressure we are usually referring to the pressure of the blood in the other arteries. Blood pressure is constantly changing. The left ventricle contracts and forces a bolus of blood into the aorta and the aorta stretches to receive the blood. The left ventricle then relaxes and the blood tries to flow back into the left ventricle but the aortic valve closes and prevents this. The blood then has no choice but to flow in the other direction, down the aorta away from the heart and into the large arteries of the body. As the blood is leaving the aorta, the aorta is relaxing from the elastic stretch and the pressure in the aorta is falling. After a second or so, the heart contracts again and the left ventricle forces another bolus of blood into the aorta, stretching it and forcing the pressure up again. When we measure blood pressure, we measure the peak arterial pressure which is the pressure when the artery is maximally stretched when the bolus of blood first arrives and we call this the systolic pressure. We also measure the minimum arterial pressure which is the pressure when the artery is minimally stretched just before the next bolus of blood arrives and we call this the diastolic pressure. For example, in the normal healthy resting adult, the blood pressure would be around 120/80. This means that the highest pressure would be 120 millimeters of mercury (mm Hg) and the lowest pressure would be 80 mm Hg. The mean arterial pressure is the average pressure in the arteries and in the above person it would be about 96 mm Hg. The next time someone says

their blood pressure is something over something, you will know what they mean.

So what? The blood pressure has to be high enough to force the blood through the circulation and back to the heart. The problem is that the higher the blood pressure, the more the poor arteries are stretched.

The walls of the arteries are living tissue and if you force them to work harder than they were designed for, they break down and wear out faster than they should. This is called degeneration and it is an amazingly complex process. The end result however is that the lumen or opening inside the artery becomes smaller. This allows less blood to pass, eventually not allowing enough blood to get through to feed the tissues supplied by the artery.

These damaged arteries are also likely to suddenly block, stopping all blood flow through them. The tissues supplied by this artery now get no blood and die. If this artery supplies tissue in the heart, we call this process a heart attack, if they supply tissues in the brain, we call it a stroke. As can be seen, keeping the blood pressure within the normal range is very important.

Lets now look at the major factors that determine our blood pressure. In the definition of blood pressure and ensuing discus sion, we saw that the contraction of the left ventricle generated the blood pressure. If the heart is failing to contract forcefully enough our blood pressure will be too low (heart failure) and the blood will not circulate through the body. If the heart beats faster and stronger (exercise) you would expect the blood pressure to go up but it does not. What happens is that the smallest arteries relax and dilate. This allows the blood to flow away faster and keeps the pressure in the normal range. The body controls where the blood goes by controlling which arterioles relax or contract. When we exercise, the arterioles in the working muscles relax and the amount of blood flowing through the muscles can increase 100 times compared to the blood flow in a resting muscle. Another determinant of blood pressure is the elasticity of the arterial walls. As we get older, the arterial walls tend to become less elastic. Therefore, when they receive the bolus of blood from the heart, they can not dilate as much and the systolic pressure tends to be higher. That is why in an older person a systolic pressure of 130 mm Hg or even 140 mm Hg is quite common. The final determinant of blood pressure is the viscosity and volume of the blood. The viscosity is the thickness of the blood. Thick blood flows poorly and this results in higher blood pressure. As the volume of blood in the body increases, the circulation can the thought of as filling up and therefore the blood pressure will again be higher.

We know that the higher your blood pressure (BP), the greater the likelihood you will suffer a heart attack or stroke but at what levels do we treat it? First, it is only the average pressure in the arteries that matters. Our blood pressure changes all the time and therefore, before your doctor will say you have high blood pressure, he will want you to have it measured on at least three visits. Some people only have high blood pressure when they are in the doctors office (white coat hypertension) and this can be checked by having the person wear a machine that takes their BP every three minutes for 24 hours. The true average blood pressure can be determined this way. If the average diastolic (resting) pressure is below 90 mm Hg, this is considered normal. If it is above 100 mm Hg, your blood pressure should be treated as these pressures increase your risk of a heart attack or stroke a great deal. If your average diastolic BP is between 90 mm Hg and 100 mm Hg however the choice is less clear. What I do in my own practice with these individuals is to strongly encourage them to alter their lifestyles to lower their BP back into the normal range. I then follow their BP on a regular basis to see how they are doing. If after six months or so they have not succeeded in getting their BP normalized, I start medication.

Recent large controlled trials have shown that in middle aged men, having a systolic BP of 130-139 mm Hg or a diastolic BP of 85-89 mm Hg doubled the risk of dying from a heart attack or developing end stage kidney disease. Having a systolic BP over 120 mm Hg doubled the risk of stroke. Another study showed that both men and women with mildly elevated BP have twice the risk of developing congestive heart failure. Many studies have shown that reducing the BP reduces the risk of developing the above problems. Therefore, preventing your BP from rising and getting it back to normal if it does go up is very important.

Other medical conditions are known to increase the likelihood of developing problems if your BP is elevated. Therefore, if you have diabetes, bad cholesterol levels, are older than age 60, smoke, or have a strong family history of cardiovascular disease, your BP must be aggressively controlled.

High blood pressure can be controlled through changes in lifestyle and with a wide variety of drugs. It can also be prevented in most people by living healthy lifestyles in the first place! I have already run out of room in this column but let me quickly run through the things we will discuss in greater detail next time.

The single most important determinant of hypertension is smoking, so stop, immediately! Other factors which we can change are obesity, high salt intake, high alcohol intake, lack of physical exercise (especially aerobic exercise), and inadequate intake of potassium and calcium. There are a very large numbers of drugs used to treat hypertension and we will look at the possible concerns with diving while taking them. In short, uncontrolled hypertensives are at risk of heart attacks and strokes. Heart attacks are often triggered by exercise. Diving is quite strenuous exercise. The most common cause of death while diving is a heart attack. Might there be a connection?