Question

What factor(s) can create positive feedback, making shock following hemorrhage get worse instead of better? Choose all that apply.

Answer 1

There are two types of circulatory shock

One that is caused by decreased cardiac output.

It's of two types-

1. Cardiac abnormalities that decrease the ability of the heart to pump blood. These include especially myocardial infarction but also toxic states of the heart, severe heart valve dysfunction, heart arrhythmias, and other conditions. The circulatory shock that results from diminished cardiac pumping ability is called cardiogenic shock.

2. Factors that decrease venous return also decrease cardiac output because the heart cannot pump blood that does not flow into it. The most common cause of decreased venous return is diminished blood volume, but venous return can also be reduced as a result of decreased vascular tone, especially of the venous blood reservoirs, or obstruction to blood flow at some point in the circulation, especially in the venous return pathway to the heart.

Second that occurs without diminished cardiac output.

Like excessive metabolic rate, so even a normal cardiac output is inadequate, or abnormal tissue perfusion patterns, so most of the cardiac output is passing through blood vessels besides those that supply the local tissues with nutrition.

IN THE ABOVE QUESTION THE STATED CONDITION IS OF DECREASED VENOUS RETURN. LETS DESCRIBE IT IN DETAIL.

Hypovolemic-haemorrhagic shocks are of two types.

Non Progressive -If shock is not severe enough to cause its own progression, the person eventually recovers. Therefore, shock of this lesser degree is called nonprogressive shock, or compensated shock, meaning that the sympathetic reflexes and other factors compensate enough to prevent further deterioration of the circulation.

and progressive shocks - when the arterial pressure falls low enough, coronary blood flow decreases below that required for adequate nutrition of the myocardium. This weakens the heart muscle and thereby decreases the cardiac output more. Thus, a positive feedback cycle has developed, whereby the shock becomes more and more severe. (PROGRESSIVE SHOCKS make shock following hemorrhage get worse instead of better)

FACTORS AFFECTING PROGRESSIVE SHOCK ARE;

Vasomotor Failure. In the early stages of shock, various circulatory reflexes cause intense activity of the sympathetic nervous system. This, as discussed earlier, helps delay depression of the cardiac output and especially helps prevent decreased arterial pressure. However, there comes a point when diminished blood flow to the brain’s vasomotor center depresses the center so much that it, too, becomes progressively less active and finally totally inactive.

Blockage of Very Small Vessels—“Sludged Blood.” In time, blockage occurs in many of the very small blood vessels in the circulatory system and this also causes the shock to progress. The initiating cause of this blockage is sluggish blood flow in the microvessels. Because tissue metabolism continues despite the low flow, large amounts of acid, both carbonic acid and lactic acid, continue to empty into the local blood vessels and greatly increase the local acidity of the blood. This acid, plus other deterioration products from the ischemic tissues, causes local blood agglutination, resulting in minute blood clots, leading to very small plugs in the small vessels.

Increased Capillary Permeability. After many hours of capillary hypoxia and lack of other nutrients, the permeability of the capillaries gradually increases, and large quantities of fluid begin to transude into the tissues. This decreases the blood volume even more.

Cardiac Depression Caused by Endotoxin. Endotoxin is released from the bodies of dead gram-negative bacteria in the intestines. Diminished blood flow to the intestines often causes enhanced formation and absorption of this toxic substance. The circulating toxin then causes increased cellular metabolism despite inadequate nutrition of the cells; this has a specific effect on the heart muscle, causing cardiac depression.

Generalized Cellular Deterioration. As shock becomes severe, many signs of generalized cellular deterioration occur throughout the body. One organ especially affected is the liver, as illustrated in Figure 24-5. This occurs mainly because of lack of enough nutrients to support the normally high rate of metabolism in liver cells, but also partly because of the exposure of the liver cells to any vascular toxin or other abnormal metabolic factor occurring in shock.

All the factors just discussed that can lead to further progression of shock are types of positive feedback. That is, each increase in the degree of shock causes a further increase in the shock.

(REMEMBER The Frank Starling law For heart- it represents the relationship between stroke volume and end diastolic volume. The law states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles, before contraction (the end diastolic volume), when all other factors remain constant.[] As a larger volume of blood flows into the ventricle, the blood stretches the cardiac muscle fibers, leading to an increase in the force of contraction. The Frank-Starling mechanism allows the cardiac output to be synchronized with the venous return, arterial blood supply and humoral length, without depending upon external regulation to make alterations.)

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Below is the diagram explaining the positive feedback effect which deteriorates gradually