Contractility is the amount of mechanical work that is done for a given preload and afterload. Factors affecting contractility include are given in Box 6.1.

Box 6.1 Factors affecting contractility Increased contractility

• Sympathetic stimulation

• Positive inotropic drugs, for example, dobutamine, adrenaline Reduced contractility

• Parasympathetic stimulation

• Negatively inotropic drugs, for example, p-blockers

• Ischaemia/infarction

• Hypoxaemia

• Low serum calcium

Figure 6.1 The Frank-Starling curve. In isolated myocytes, the tension developed during contraction is dependent on the initial length of the muscle fibre. Laplace's law relates wall tension (T) to internal pressure (P) in an elastic sphere where R is the radius: P= 2T/R. Thus, the Frank-Starling relationship for an isolated muscle fibre translates into a relationship between stroke volume and left ventricular end-diastolic pressure (LVEDP)

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