Table 1.1 Factors affecting preload, afterload, contractility and heart rate

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Factors affecting preload

Blood volume
Heart rate
Body position
Phase of respiration (affects intrathoracic pressure)
Respiratory disease (affects intrathoracic pressure)
Muscular pumping of venous blood
Locomotion (affects intrathoracic pressure and venous return)
Atrial contraction
Valvular regurgitation

Factors affecting afterload

Heart rate
Vascular tone (and therefore blood pressure)
Aortic stiffness
Myocardial tension (affected by hypoxia, volume overload)
Metabolic rate
Preload
Valvular regurgitation

Factors affecting contractility

Noradrenaline release from sympathetic nerves (most important)

Increased circulating catecholamines	)
Increased heart rate (Bowditch effect)	> increase contractility
Increased afterload (Anrep effect)	)

Drugs

Metabolic factors:

hypoxia	) increase or
acidosis	> decrease
hype rcapnia	) contractility

Myocardial disease

Factors affecting heart rate

Heart disease	Fever
Exercise	Pain
Excitement	Anaemia
Afterload	Systemic disease
Preload