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Neurogenic Shock-Increased Vascular Capacity
Shock occasionally results without any loss of blood volume. Instead, the vascular capacity increases so much that even the normal amount of blood becomes incapable of filling the circulatory system adequately. One of the major causes of this is sudden loss of vaso-motor tone throughout the body, resulting especially inmassive dilation of the veins. The resulting condition is known as neurogenic shock.
The role of vascular capacity in helping to regulate circulatory function was discussed, where it was pointed out that either an increase in vas-cular capacity or a decrease in blood volume reducesthe mean systemic filling pressure, which reducesvenous return to the heart. Diminished venous return caused by vascular dilation is called venous pooling of blood.
Causes of Neurogenic Shock. Some neurogenic factorsthat can cause loss of vasomotor tone include the following:
1. Deep general anesthesia often depresses thevasomotor center enough to cause vasomotor paralysis, with resulting neurogenic shock.
2. Spinal anesthesia, especially when this extends allthe way up the spinal cord, blocks the sympathetic nervous outflow from the nervous system and can be a potent cause of neurogenic shock.
3. Brain damage is often a cause of vasomotorparalysis. Many patients who have had brain concussion or contusion of the basal regions of the brain develop profound neurogenic shock. Also, even though brain ischemia for a few minutes almost always causes extreme vasomotor stimulation, prolonged ischemia (lasting longer than 5 to 10 minutes) can cause the opposite effect—total inactivation of the vasomotor neurons in the brain stem, with consequent development of severe neurogenic shock.
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