Blood Pressure Is an Unreliable Indicator of the Shock State

In the preceding example the arterial pressure was forced to fall as the dog was bled. Similarly, the arterial pressure invariably falls in the decompensated phase of circulatory shock. Unfortunately, that observation has erroneously caused many to equate shock with hypotension. It must be stressed that an inadequate cardiac output rather than a low blood pressure is the primary lesion in this syndrome. Because the body has many mechanisms for defending the blood pressure (barore-flex, renin-angiotensin system, carotid bodies, anti-diuretic hormone, etc.), the body will meet a sudden drop in cardiac output with an intense peripheral vasoconstriction that may temporarily maintain the blood pressure. The signs of reduced cardiac output can still be seen, however, and will include pale, cold skin as well as low urine production (oliguria) due to reduced renal blood flow. The circulating catecholamines will cause sweating even though the skin is cold. The patient will often complain of thirst as the CNS tries to increase fluid intake in an attempt to restore blood volume. Not all shock states exhibit reduced cardiac output. A good example is septic shock (infection), where the oxygen requirements of the body are increased due to the fever and circulating toxins. In the case of sepsis, a cardiac output within normal limits may still be inadequate to meet the high nutritional needs of the periphery.

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