Preload and afterload

Preload can be defined as the initial length of the muscle fibre before contraction (described by the Frank-Starling curve in Figure 6.1). In a normal heart, we take this to be equivalent

Hypoxaemia, acidosis

Hypoxaemia, acidosis

Failing heart

LVEDP

Table 6.2 Receptors in the circulation

Receptor

Action

Where

a receptors

Vasoconstrict

Peripheral, renal,

coronary circulation

P1 receptors

Increase contractility,

Heart

HR, and CO

P2 receptors

Vasodilate

Peripheral and renal

circulation

Dopamine (DA)

Range of actions

Renal, mesenteric,

receptors

(see later)

coronary circulation

CO, cardiac output; HR, heart rate.

CO, cardiac output; HR, heart rate.

to the end-diastolic volume. This is not easily measured. It may be thought of as the filling pressure of the ventricles. In clinical practice the central venous pressure (right ventricle) and pulmonary arterial occlusion pressure (left ventricle) are used to estimate filling pressures - but these have several limitations.

Afterload is defined as the tension developed in the ventricular wall during systole. It is affected by preload, systemic vascular resistance, and external pressure on the ventricles (for example, from positive pressure ventilation). It may be thought of as a measure of how forcefully the ventricles have to contract to eject blood.

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