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Chapter: Medical Physiology: Cardiac Failure

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Low-Output Cardiac Failure- Cardiogenic Shock

In many instances after acute heart attacks and often after prolonged periods of slow progressive cardiac deterioration, the heart becomes incapable of pumping even the minimal amount of blood flow required to keep the body alive.

Low-Output Cardiac Failure— Cardiogenic Shock

In many instances after acute heart attacks and often after prolonged periods of slow progressive cardiac deterioration, the heart becomes incapable of pumping even the minimal amount of blood flow required to keep the body alive. Consequently, all the body tissues begin to suffer and even to deteriorate, often leading to death within a few hours to a few days. The picture then is one of circulatory shock. Even the cardiovascular system suffers from lack of nutrition, and it, too (along with the remainder of the body), deteriorates, thus has-tening death. This circulatory shock syndrome caused by inadequate cardiac pumping is called cardiogenicshock or simply cardiac shock. Once a person devel-ops cardiogenic shock, the survival rate is often less than 15 per cent.

Vicious Circle of Cardiac Deterioration in Cardiogenic Shock.

The discussion of circulatory shock emphasizes the tendency for the heart to become pro-gressively more damaged when its coronary blood supply is reduced during the course of the shock. That is, the low arterial pressure that occurs during shock reduces the coronary blood supply even more. This makes the heart still weaker, which makes the arterial pressure fall still more, which makes the shock still worse, the process eventually becoming a vicious circle of cardiac deterioration. In cardiogenic shock caused by myocardial infarction, this problem is greatly com-pounded by already existing coronary vessel blockage. For instance, in a healthy heart, the arterial pres-sure usually must be reduced below about 45 mm Hg before cardiac deterioration sets in. However, in a heart that already has a blocked major coronary vessel, deterioration sets in when the coronary arterial pressure falls below 80 to 90 mm Hg. In other words, even a small decrease in arterial pressure can now set off a vicious circle of cardiac deterioration. For this reason, in treating myocardial infarction, it is extremely important to prevent even short periods of hypotension.

Physiology of Treatment. Often a patient dies of cardio-genic shock before the various compensatory processes can return the cardiac output (and arterial pressure) to a life-sustaining level. Therefore, treat-ment of this condition is one of the most important problems in the management of acute heart attacks.

Immediate administration of digitalis is often used for strengthening the heart if the ventricular muscle shows signs of deterioration. Also, infusion of whole blood, plasma, or a blood pressure–raising drug is used to sustain the arterial pressure. If the arterial pressure can be elevated high enough, the coronary blood flow often will increase enough to prevent the vicious circle of deterioration. And this allows enough time for appropriate compensatory mechanisms in circulatory system to correct the shock.

Some success has also been achieved in saving the lives of patients in cardiogenic shock by using one of the following procedures: (1) surgically removing the clot in the coronary artery, often in combination with coronary bypass graft, or (2) catheterizing the blocked coronary artery and infusing either streptokinase or tissue-type plasminogen activator enzymes that causedissolution of the clot. The results occasionally are astounding when one of these procedures is instituted within the first hour of cardiogenic shock but of little, if any, benefit after 3 hours.


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