Stages of Shock
Classically there are 4 stages of shock:
1. Initial: Hypoperfusion causes hypoxia. This leads to mitochondrial dysfunction. The cells become leaky and switch to anaerobic metabolism. This switch produces lactic acid and a resultant metabolic acidosis.
2. Compensatory: During this phase the body employs several mechanisms in an attempt to correct the metabolic upset of shock:
- Hyperventilation: This will create a respiratory alkalosis in an attempt to neutralize the metabolic acidosis of shock returning the body to a normal pH or hydrogen ion concentration. Clearly if lung function or respiratory drive mechanisms are impaired this compensation will be less effective.
- Catecholamine response: In response to hypotension, noradrenaline and adrenaline will be released in an attempt to maintain both SVR and CO (see formula A and B).
- Renin-angiotensin response: Vasopressin excretion is increased leading to fluid retention (to maintain circulating volume) and vasoconstriction.
3. Progressive: The mechanisms listed above can only compensate for worsening shock for so long. Eventually they will begin to fail. At this point cellular dysfunction begins to spiral out of control, metabolic acidosis worsens and the arteriolar and precapillary sphincters constrict such that blood remains in the capillaries. The pressure within the capillaries will increase. This, combined with membrane dysfunction, will lead to fluid loss into the extravascular interstitial spaces. This will further worsen the intravascular volume status.
4. Refractory: At this stage organs fail and the shock can no longer be reversed. Death normally occurs rapidly.