Summary Definition Of Contractility

• Physiological definition—Systolic myocardial work done with given pre- and afterload

• Physiological index—Ventricular stroke work index maximum slope (dp/dt) of ventricular isovolumetric contraction curve

• Practical concept—Ejection fraction for given CVP and MAP

• Practical index—Ejection fraction

Figure HE.29.

Contractility can be increased by various factors including:

• Increased serum calcium levels

• Sympathetic stimulation

• Parasympathetic inhibition

• Positive inotropic drugs

Contractility can be decreased by various factors including:

• Decreased serum calcium levels

• Parasympathetic stimulation

• Sympathetic blockade

• Myocardial ischaemia or infarction

• Hypoxia and acidosis

• Mismatched ventriculo-arterial coupling

Measurement of Contractility

Contractility is an index of work performance at a given pre- and afterload. Ideally these parameters should be controlled during measurement, which is often impractical. Accordingly, pre- and afterload should be recorded during assessment of contractility. The interpretation of contractility measurements is then made at the given pre- and afterload.

The calculation of ventricular work done during systole requires an integral of the ventricular PV loop area. This is not a practical measurement, but an index loosely reflecting systolic work is stroke work, obtained from the following product:

SW - (SV) x (mean arterial pressure - filling pressure).

Figure HE.30 illustrates how this product approximates to the PV loop area for the left ventricle.

SV work index (SVWI) is a more useful indicator of contractility and is derived in the following way. SV work index is calculated by normalizing stroke work for body surface area by using stroke index in its calculation. Thus, for the left ventricular stroke work index (LVSWI):

Normal values for LVSWi are 45-60 g.m/m2.

While for the right ventricle (right ventricular stroke work index, RVSWi):

Venfricjlor pressure

S'ncjiue volume

Ventricu'or volume

S'ncjiue volume

Ventricu'or volume

Figure HE.30 Left ventricular stroke work estimate for ventricular pressure-volume loop area

Normal values for right ventricle SVWi are 5-10 g.m/m2.

Other measures of contractility include ejection fraction and ventricular function curves.

Ejection fraction is measured by radionuclide ventriculography or transthoracic echocardiography. it is often derived from the fractional area change measurement. EF and FAC are both sensitive to pre-and afterload. These latter parameters should also be evaluated with EF or FAC. There is a clear association between EF and prognosis in cardiac patients.

Ventricular function curves can be plotted between an index of ventricular filling (e.g. CVP or PCWP) and an index of ventricular performance (e.g. CO or SV). Factors increasing contractility will shift the curve upwards and to the left while those decreasing contractility will shift it downwards and to the right.

Heart Rate

The heart rate is normally determined by the spontaneous depolarization rate of the SA node pacemaker cells. The normal heart rate of 60-80 bpm is much slower than the intrinsic rate of the denervated heart (110 bpm). This is because of the dominant parasympathetic tone in the intact cardiovascular system (Figure HE.31).

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