Decreased Cardiac Output Systolic Stretch and Cardiac

Shock. When some of the cardiac muscle fibers are not functioning and others are too weak to contract with great force, the overall pumping ability of the affected ventricle is proportionately depressed. Indeed, the overall pumping strength of the infarcted heart is often decreased more than one might expect because of a phenomenon called systolic stretch, shown in Figure 21-7. That is, when the normal portions of the ventricular muscle contract, the ischemic portion of the muscle, whether this be dead or simply nonfunctional, instead of contracting is forced outward by the pressure that develops inside the ventricle. Therefore,

Systolic stretch in an area of ischemic cardiac muscle.

Cor Systolic Stretch

Systolic stretch in an area of ischemic cardiac muscle.

much of the pumping force of the ventricle is dissipated by bulging of the area of nonfunctional cardiac muscle.

When the heart becomes incapable of contracting with sufficient force to pump enough blood into the peripheral arterial tree, cardiac failure and death of peripheral tissues ensue as a result of peripheral ischemia. This condition is called coronary shock, car-diogenic shock, cardiac shock, or low cardiac output failure. It is discussed more fully in the next chapter. Cardiac shock almost always occurs when more than 40 per cent of the left ventricle is infarcted. And death occurs in about 85 per cent of patients once they develop cardiac shock.

Damming of Blood in the Body's Venous System. When the heart is not pumping blood forward, it must be damming blood in the atria and in the blood vessels of the lungs or in the systemic circulation. This leads to increased capillary pressures, particularly in the lungs.

This damming of blood in the veins often causes little difficulty during the first few hours after myocar-dial infarction. Instead, symptoms develop a few days later for the following reason: The acutely diminished cardiac output leads to diminished blood flow to the kidneys. Then, for reasons that are discussed in Chapter 22. the kidneys fail to excrete enough urine. This adds progressively to the total blood volume and, therefore, leads to congestive symptoms. Consequently, many patients who seemingly are getting along well during the first few days after onset of heart failure will suddenly develop acute pulmonary edema and often will die within a few hours after appearance of the initial pulmonary symptoms.

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Essentials of Human Physiology

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  • darren
    What is systolic stretch?
    4 years ago

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