We talked yesterday about big, flabby, weak, stretched out of shape hearts. These hearts can’t pump blood very efficiently. The body demands that a certain amount of blood get pumped around every minute on a regular basis — that’s Cardiac Output. If the heart can’t pump strongly enough to get that done with a normal number of beats per minute, the body may compensate by just having more beats per minute. That only works up to a point, though. It’s possible for the heart to beat so fast that it doesn’t have time to fill with blood between heart-beats. Then you have a pulse deficit, meaning that when you feel the pulse inside the thigh, there aren’t as many pulses as there are heart-beats. This is bad. When you can still compensate for the heart’s weakness by increasing heart rate, and tightening down blood vessels to keep pressure up to vital organs, you feel okay. When this isn’t enough to maintain cardiac output, you are de-compensated. Now you don’t feel good and you can’t tolerate exercise. Eventually you don’t even have enough capacity to just sit there and breathe, much less eat, sleep comfortably, or exercise.
While there are many different medications available, there just a few basic approaches to helping a patient with a weak, flabby heart. The analogy I like to use is that of an old horse pulling a loaded wagon up a hill. When he is having trouble getting the wagon up and over, what can you do?
Step one: find a hill that’s not so steep, or go around. For our heart patient, that means don’t overdo the exercise. This is not like someone who has had a heart attack and is exercising to build up the remaining muscle. With congestive heart failure, overdoing the exercise can put you in a crisis situation. Another thing that makes the hill steep is being overweight. Pounds of fat are miles of capillaries (microscopic blood vessels) and more work to do. Lose the excess weight.
Step two: unload the wagon a little. Here we’re talking about reducing the circulating volume of fluid. The reason that you restrict salt intake is that sodium is a very active little molecule in retaining water in the body. Excess sodium in the diet means excess water retained, which is more load in the wagon. Diuretic drugs allow the kidneys to let more water escape the body in the urine, again dropping the circulating fluid volume. If you’ve accumulated fluid in the lungs or abdomen, it helps pull some of that out, too. Furosemide (a.k.a. Lasix, a.k.a. Salix) is one of the most commonly used diuretic drugs for patients with congestive heart failure.
Step three: quit putting the brakes on the wagon wheel. When a weak heart allows blood pressure to fall, the body puts an inappropriate defense mechanism into play. In the wild, animals just don’t live long enough to get this disease. In the wild, if you’ve got low blood pressure, you’re either bleeding or in shock, with blood vessels dilated and not functioning properly. The body’s response to this situation is to tighten those blood vessels to stop the leak. This is a great idea if you’re bleeding. On the other hand, if your blood pressure is low because your heart is weak, tightening those blood vessels just makes it harder for the weak heart to get its job done. It’s a lot harder to pump fluid through a soda straw than through an open pipe. Enalapril (a.k.a. Vasotec, a.k.a. Enacard) is a popular drug used to block this inappropriate body defense response.
Step four: whip the horse. Make that weak, flabby heart work harder. Digoxin (derived from the older digitalis) has been a standby for many years. This drug makes the weak muscle contract more forcefully, giving more output per beat, and a slower heart-rate. Unfortunately, it has a narrow margin of safety. The right dose makes your heart work more efficiently, but just a little too much makes it worse. It must be dosed carefully, and tends to be used when the other approaches are not working.
There are certainly other ramifications to this disease, and many other drugs beside those mentioned. There are also other types of heart disease to which this discussion does not apply.
I’m no cardiologist. When I have a patient with severe heart disease, we do our homework. In addition to listening to his chest, we take X-rays to evaluate heart size and shape, and the condition of the lungs. An echocardiogram (using the ultrasound) lets us see the heart pumping and valves functioning in real time, and lets us measure how well it is contracting. Blood tests evaluate the extent of damage to other organ systems. We also use the Idexx Telemedicine service (formerly Cardiopet). The clips go on your pet, the telephone goes on the box, and the ECG (electrocardiogram) prints out in New Jersey. There a specialist inteprets the ECG and the other data we have collected and consults with us on the best way to manage the patient. Many pets with heart disease can have added years of a good quality of life.