Ventricular function curves and the Frank Starling law of the heart

Clinical evaluation of the overall pump function of the heart is usually based on the ventricular function curve, which relates a measure of ventricular performance (cardiac output, stroke volume, or stroke work) as a dependent variable to a measure of ventricular preload, such as ventricular filling pressure, end diastolic volume, end diastolic diameter, cross sectional area, or end diastolic wall stress.9 10 78 The ventricular function curve describes the fundamental dependence of ventricular performance on ventricular preload and represents the expression of the Frank-Starling law of the heart: ''the energy of contraction is a function of the length of the muscle fibre".79 (Fig 2.11). This law is based on the length-tension relationship of myocardial sarcomeres, which at rest operate below the maximum length (Lmax) on the ascending limb of their length-tension relationship.

Increasing stretch on the sarcomeres towards Lmax increases the number of activated cross bridges and the developed tension.

The dependence of ventricular performance on afterload and contractile state is reflected by displacement of the ventricular function curve with changing contractility and/or afterload. For example, with decreasing afterload and/or increasing contractility, the curve is shifted upwards and to the left; an opposite shift is observed with increasing afterload and/or deteriorating contractility.78 Hence, the function of the ventricle cannot be described by one function curve; instead, a family of ventricular function curves, reflecting the changing load and inotropic states, gives a better characterisation of the ventricular function.78 79 Incremental pacing also shifts the cardiac output curve upwards; the magnitude of the shift is, however, less than during exercise at a comparable heart rate, as a result of an additional positive inotropic effect of increased sympathetic tone during exercise.80 At higher preloads the ventricular function curve tends to plateau and further augmentation in ventricular filling no longer enhances ventricular performance. The so called "descending limb of the Starling curve", which has given rise to considerable controversy in the past, is now

Frank Starling Law

Fig 2.11 Ventricular function curves. The curves describe the relationship between resting length of contractile fibres (preload) and ventricular performance (cardiac output, stroke volume, stroke work), known as the Frank-Starling law. An enhanced inotropic state shifts the relationship leftward and upward; depressed inotropy has an opposite effect.

Fig 2.11 Ventricular function curves. The curves describe the relationship between resting length of contractile fibres (preload) and ventricular performance (cardiac output, stroke volume, stroke work), known as the Frank-Starling law. An enhanced inotropic state shifts the relationship leftward and upward; depressed inotropy has an opposite effect.

considered to be an artefact caused by non-physiological experimental conditions.79

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