How Digitalis Is Used
Digitalis is used to bolster the ailing heart in congestive heart failure. In this condition, the heart muscle has stretched while straining to pump blood against a back pressure. The back pressure may be caused by high blood pressure, or it may be the result of a leak caused by a faulty aortic valve or a hole in the wall (septum) dividing the right and left halves of the heart. When these conditions occur, the heart muscle, or myocardium, must exert greater and greater pressure to force blood through the body against the resistant force. Over time the strain will stretch the heart muscle, and the size of the heart increases. As the heart muscle changes in these ways, its pumping action becomes less and less effective. Congestive heart failure occurs when the myocardium has been stretched too far. At this juncture the patient must have a heart transplant or he will die.
The administration of digitalis, however, can forestall the critical stage of the disease. Digitalis has a direct and immediate effect on the myocardium. By a mechanism not well understood, digitalis increases the levels of intracellular calcium, which plays an important role in the contraction of the muscles. Almost as soon as the drug has been administered, the heart muscle begins to contract faster and with greater force. As a result, its pumping efficiency increases and the supply of blood to the body is enhanced. Digitalis also tends to bring about a decrease in the size of the ventricles of the failing heart as well as a reduction in wall tension.
In addition to its immediate effect on the heart muscle, the drug affects the autonomic nervous system, slowing the electrical signal that drives the heartbeat. As heart contractions become more efficient, the heart rate slows. For this reason, the drug is said to have a negative chronotropic effect (the prefix chrono- refers to time).
As digitalis stabilizes the myocardium, appropriate steps can also be taken to correct the original cause of the disease, if possible. The patient's blood pressure can be lowered with medications, or heart surgery can be performed to replace a faulty valve or patch a hole in the septum. When it is not possible to improve cardiac function by other means, the patient can be maintained on digitalis for many years.