Cardiovascular Coupling

In clinical practice assessment of cardiac function often relies on the available measurements of filling pressures (CVP and PCWP), arterial pressures (pulmonary and aortic) and CO. These measurements are dependent on two sets of characteristics that reflect:

• Contractile performance of the heart

• Elastance (PV relationship) of the vascular system

Assessing the relative effects of cardiac contractility and vascular system elastance on pressure measurements is important clinically, since it can influence therapeutic decisions.

Interpretation of pressure and CO measurements is aided by a consideration of the interaction or coupling between the heart and the vascular system. This can be performed at the arterial side (ventriculo-arterial coupling) for arterial pressures, or on the venous side of the heart (ventriculovenous coupling) for CVP.

Ventriculo-Arterial Coupling

Coupling between the ventricle and arterial system is illustrated by plotting the ventricular elastance and arterial elastance on the same diagram. The ESP (P) then lies at junction of the curves that are approximately at right angles to each other. Changes in arterial pressure are reflected by a shift in the position of P. If pre load (EDP) is maintained constant, then the direction of displacement of P identifies the degree to which ventricular contractility or arterial elastance is responsible (Figure HE.33).

Figure HE.33 Ventriculo-arterial coupling diagram

The left ventricle and arterial system can be seen as two elastic chambers with opposing elastances, Ees and Ea respectively. The distribution of blood is determined by the individual elastances of the two chambers. Theoretical analysis suggests that the LV will deliver maximal work when Ees = Ea but will work with maximal mechanical efficiency when Ees = 2 x Ea. The implication is that there is an optimum ventriculoarterial coupling ratio for these elastances. If these elastances are mismatched the ventricle may fail, e.g. should the ventricle eject for a prolonged period against a very low afterload.

Ventriculovenous Coupling

Description of cardiovascular coupling at the venous side of the heart requires the use of a vascular function curve. This curve describes the changes occurring in CVP as venous return is removed at different rates by the heart. It is obtained by varying the CO and recording the resulting CVP, at constant intravascular volume. The vascular function curve is a pressure-flow relationship at the venous side of the vascular system, dependent on the balance between vascular tone and intravascular volume. Changes in the gradient reflect alterations in vascular tone. Thus, vasodilatation causes a decreased gradient, while an increased gradient is associate with vasoconstriction. Changes in the intercept reflect altered intravascular volume.

A ventriculovenous coupling diagram can be drawn by plotting a vascular function curve and a ventricular function curve on the same axes. The ventricular function curve is described earlier and in Figure HE.23. These curves intersect at an operating point (Q). When Q becomes displaced by cardiovascular changes, the direction of displacement indicates the relative contributions of vascular tone and ventricular contractility effects to the changes (Figure HE.34).

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Figure HE.34 Ventriculovenous coupling diagram

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