Shock Introduction

Hypotension is a sign of cardiovascular insufficiency. The cardiovascular system is made up of three key elements (figure 1):  a pump, tubing and a fluid. The pump propels the fluid, the tubing distributes and collects it. If a patient becomes hypotensive, either the pump is not working properly, there is not enough fluid in the system, or the distribution network is malfunctioning.

Under normal conditions, the various components of the cardiovascular system are interdependent, and if one element malfunctions, the others compensate to return the blood pressure to normal. An example of this is when a patient bleeds: the volume of fluid falls, and in response, the blood vessels tighten up (vasoconstrict), and the heart rate rises.

When there is evidence of end organ insufficiency (confusion, oliguria or lactic academia), then hypotension has caused “shock.” In shock – definitive evidence of hypotension (inadequate blood pressure to maintain tissue perfusion) – at least one element of the cardiovascular system has malfunctioned, and the others have failed to adequately compensate. The simplest way to describe shock is 1) Pump failure – “cardiogenic” shock, 2) Tubing malfunction- “distributive” shock, & 3) Fluid loss – “hypovolemic shock”.

The treatment of shock depends on the cause. The commonest type – hypovolemic – is treated by replacing the fluid that is lost, blood or plasma. Cardiogenic shock is managed by using therapies that improve overall cardiac function. Distributive shock is treated by either loosening up constricted blood vessels or tightening up dilated vessels. It is important to remember that shock is failure of circulation and failure of compensation, and two separate types overlap: e.g. in anaphylaxis, there is distributive shock due to mast cell degranulation and vasodilatation. In this situation the failure is the vasodilatation, but there is failure of compensation as the volume of blood in the vasculature is inadequate to maintain pressure, and the patient becomes “relatively hypovolemic”. The treatment therefore is fluid loading and vasoconstrictors.

Learning Objectives

1. To understand the physiological basis of cardiovascular function and why compensation fails in shock.
2. To use deductive reasoning to evaluate the hypotensive patient.
3. To develop a method for resuscitating and monitoring the shocked patient.

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