Blood Pressure and Shock

Shock is what happens when the energy supply to cells is inadequate for their demands.  Unfortunately, shock is often equated with a reduction in blood pressure, but there are several mechanisms for reduction in blood pressure, and they don’t all result in decreases in the energy supply to cells.

A low blood pressure can result from one or more of these physiologic alterations:

• Low circulating volume
• Low pump performance which can be due to
  • poor cardiac filling (i.e., low circulating volume, but also can be due to obstructive problems such as tension pneumothorax or cardiac tamponade)
  • poor contractility (often due to ischemia from coronary artery disease)
  • low metabolic demand
• Vasodilation

The above phenomena relate to the hydraulic properties of the circulation, characterized by pressures and flows.  While in many cases, these conditions could be associated with a shock condition, but not necessarily so.  For example, many individuals normally have a low blood pressure resulting from a low cardiac output and/or a vasodilated vascular bed.  However, this circulation matches their metabolic demands. 

If metabolic demands are not met by the circulatory hydraulics, then shock is likely present or soon will be present.  On some occasions, this is due to circulatory insufficiency, such as with hypovolemic or cardiogenic shock.  But in other states, it may be due to excessive metabolic demands, such as during septic shock.

The possible mechanisms by which the energy supply for cells can deteriorate

• inadequate circulatory volume (hypovolemic shock)
• poor pump performance (cardiogenic shock)
• inadequate fuel content (acute anemia, severe hypoxemia)
• poor uptake of fuel (septic [or distributive] shock, anaphylactic shock)
• excessive metabolic demands (septic shock)

Neurogenic “shock” is not on the list above because there is no problem with fuel supply that is impaired in neurogenic shock. Unless the individual tries to stand up, in which case the brain gets inadequate oxygen delivery because the heart doesn’t have the vascular tone in the lower extremities to prevent blood pooling.  Of course, this is a self-correcting problem, because the decreased oxygen delivery to the brain causes them to pass out, fall down, and once again be level so that the brain’s blood flow is restored.

It would be a true paradigm shift in medicine if these physiological principles were understood in managing critically ill patients.