How to Think About Shock

1

Define shock

Shock is:

• Not just low blood pressure

• Not just decreased peripheral perfusion

• Not just limited systemic oxygen delivery

Ultimately, shock is decreased tissue respiration. Shock is suboptimal consumption of oxygen and excretion of carbon dioxide (CO ₂ ) at the cellular level.

2

Is shock related to cardiac output?

Yes. A healthy medical student can redistribute blood flow preferentially to vital organs. After a 3- to 4-unit bleed, your typical young gunslinger can still think and can tell you, “Four dudes jumped me.” From this history you have no idea what happened to him, but you do know that he is still perfusing his brain.

3

Is organ perfusion democratic?

No. Limited blood flow always is redirected toward the carotid and coronary arteries. Peripheral vasoconstriction steals blood initially from the mesentery, then skeletal muscle, and then kidneys and liver.

4

Is this vascular autoregulatory capacity uniform in all patients?

No. With age and atherosclerosis, patients lose their ability to redistribute limited blood flow. A 20% decrease in cardiac output (CO), or a fall in blood pressure to 90 mm Hg, can be life-threatening to a Supreme Court justice, whereas it may be undetectable in a triathlete.

5

For diagnostic and practical therapeutic purposes, can shock be classified?

Yes. See Table 4.1 .

• a.

  Hypovolemic shock mandates volume resuscitation.

• b.

  Cardiogenic shock mandates cardiac stimulation (pharmacologic and eventually mechanical).

• c.

  Peripheral vascular collapse shock mandates pharmacologic manipulation of the peripheral vascular tone (and direct attention to the cause of the vasodilation—typically sepsis).