Wednesday, September 5, 2007
Hypertension and Heart Failure
I was sitting around the other day, just waiting to bite down into my deep-fried, salt-laden, double bacon cheeseburger sub sandwich, when a little medical school angel appeared on my shoulder and chirped, "wait Andy, what about your blood pressure? It's going to skyrocket!!"
Of course, at this point the little devil on my other shoulder retorted, "blood pressure? So what? Everyone has high blood pressure. What's the worst that this delicious bite of instant gratification could do?"
Luckily, it was cardiology section and the little medical school angel knew just how to answer such a health dilemma. We have all heard of the dangers of high blood pressure, and yet far too many of us carry the diagnosis; in 2003 there were more than 35 million doctor's visits for hypertension. I find that in order for me to want to change a behavior, such as the food that tastes so good but that I vaguely know is bad for me, I have to know why. Why is it so important that I keep my blood pressure normal?
First, let's look at circulation. I think that the circulation is most easily envisioned as a big loop, with a pump, the heart, propelling blood through progressively smaller tubes, arteries and arterioles. These eventually narrow into capillaries to distribute oxygen and nutrients and then expand again as veins to carry away waste and return to the lungs for more air. The tubes have a certain amount of resistance, especially as they get narrower, and hypertension occurs when the relationship between the output from the heart and the total peripheral resistance is altered. High blood pressure can injure many organs when the pressurized blood damages the vessels, including those of the retina (which may result in vision damage), the kidneys, and the brain (which may cause stroke). As we are in cardiology, however, I am worried now about the damage that hypertension inflicts on the heart.
The heart is a fairly simple pump. Blood flows into the right atrium, is contracted into the right ventricle, then sent into the pulmonary, or lung, circulation where it becomes oxygenated. It returns from there and enters the left atrium, is "kicked" into the left ventricle, and this, the strongest chamber of the heart, contracts to send fresh blood to the body. The principle behind this directional flow is that pressure must always decrease from one chamber to the next. Thus, pressure in the atrium is lower than in the veins, and pressure in the ventricle, when it is relaxed, is lower or equal to pressure of the atrium. When the ventricles contract, they increase the blood pressure so that it can perfuse the body, return to the heart, and the cycle begins again. When the heart contracts, this is called systole and the pressure produced is your systolic blood pressure. When it relaxes, this is diastole, and the pressure that remains in the vessels is diastolic blood pressure. This is higher in the vessel than in the heart because there is a valve that closes after the heart contracts. This means that the heart can relax and refill, while the vessels remain pressurized and able to go forward.
For such a simple pump, many things can go wrong. If the left ventricle is trying to pump against high blood pressure, as in hypertension, the ventricle will have to work harder to expel its blood. Recall that blood must go from high pressure to low; the ventricle has to work harder to overcome the high pressure in the aorta. Just like any other muscle, the heart will "get jacked" and you see hypertrophy, or increased size, of the ventricle. This bulking up means that less blood can get into the ventricle chamber, and so you might begin to experience heart failure. The problem is that since the circulation is a big loop, blocking one step results in backing up all the others. So, less blood pumped through the ventricle means blood, and pressure, builds up in the atrium, which then backs up in the lungs. This can result in congestive heart failure, where fluid can actually build up in the small alveoli of the lungs because of the pressure forcing it out of the small, weak capillaries. Pressure can continue back, so that you may get the right side of the heart involved, and even the venous return. So it is that the failure of your left heart to pump past the high blood pressure in your aorta can result in pulmonary edema (swelling with fluid) and right heart failure.
So, when my little medical school angel on my shoulder is confronted with a deep-fried, salt-laden, double bacon cheeseburger sub sandwich, it can fight back, knowing that since the circulation is one big loop, pumping up the pressure in one part will cause the rest of it to try and compensate. Heart failure (and maybe more) just isn't so appetizing.
P.S. Here is a very fun story of hypertension presenting as a medical mystery, from the New York Times.