Critical care: cardiovascular physiology and support

Summary

Cardiovascular pathophysiology

The function of the circulation is to transport oxygen and nutrients to the tissues and to remove metabolic waste products. For this to happen, there must be:

1. ▪ Enough oxygen in the blood

2. ▪ Enough blood flowing (cardiac output)

3. ▪ Enough blood pressure to let tissues regulate their own perfusion.

The oxygen content of the blood is determined by haemoglobin concentration and saturation.

Blood pressure

This is determined by the equation BP = CO × TPR.

(BP = blood pressure, CO = cardiac output, TPR = total peripheral resistance.)

Thus hypotension can be due either to low cardiac output or to inappropriate vasodilation. Treatment usually requires correction of theabnormal variable.

Cardiac output

Cardiac output is determined by the following:

1. Rate: too high a heart rate prevents adequate filling of the ventricle and reduces preload and cardiac output. Bradycardia reduces cardiac output as ejection simply does not happen often enough.

2. Rhythm: loss of atrial contraction in junctional rhythms or atrial fibrillation also reduces preload and hence cardiac output by up to 30%.

3. Preload: Starling's law states that the force of contraction of a cardiac muscle fibre is proportional to its initial length. The fibre length is determined by the ventricular volume. However volumes are difficult to measure clinically, and the simplest substitute is the central venous pressure (CVP). […]