Hypotension/Shock

Shock = state of hypoperfusion to tissues, resulting in decreased tissue oxygen delivery à end result is cell injury and death, organ failure, and death.
·         Tissue perfusion (Mean Arterial Pressure) is determined by Cardiac Output (= HR X Stroke Volume) and Systemic Vascular Resistance (SVR): 
              MAP = CO x SVR

• Main mechanisms underlying shock: Broadly speaking, the cardiovascular system is essentially a plumbing system, involving a pump (heart), the pipes and tank (vasculature), and fluid in the tank (blood).  (Sorry, future cardiologists, but it really is that simple).

Problems can occur at each of those levels:
1.  Hypovolemic Shock – not enough fluid in the tank

• Hemorrhage (i.e. trauma, GI bleed, etc)
• Severe dehydration
• Capillary leak (SIRS – e.g. severe pancreatitis, sepsis)

2.  Cardiogenic Shock – pump malfunction
a) Mechanical – Massive MI (e.g. affecting the LAD, causing LV dysfunction), Acute valvular dysfunction (e.g., chordae tendinae rupture from MI leading to acute MR, endocarditis causing valvular regurgitation), other mechanical problems post-MI (free wall rupture, VSD), also just progression of severe CH
b) Arrhythmogenic – Either tachy- or brady-arrhythmias

• Also consider Iatrogenic – overdose of negative inotropes (beta blockers, CCBs)

3.  Distributive Shock – vasodilation of the vasculature – the tank is expanded

• SIRS and Sepsis – most common cause
• Anaphylaxis
• Adrenal Shock (cortisol upregulates alpha receptors and catecholamine production and maintains vascular tone)
• Neurogenic Shock – loss of autonomic innervations of vasculature.  Often seen in spinal cord injuries.

4.  Obstructive shock – obstruction to the flow through the pump.  Important to recognize, since basically all of these require some sort of emergent physical intervention to relieve the obstruction.

• Massive PE
• Cardiac tamponade
• Constrictive Pericarditis
• Tension pneumothorax
• Abdominal compartment syndrome (compresses IVC and heart)

There are certainly clues on exam that can quickly point you towards one direction or another: looking at the BP, is there a wide pulse pressure (more indicative of distributive shock) or a narrow pulse pressure (more indicative of cardiogenic shock, although also hypovolemic and obstructive).  Is there fever (i.e. sepsis).  Is the JVP elevated?  Are the extremities warm, or cold/clammy? 

(Chanu Rhee MD, 3/24/11)