11.2.6 Graphical representations

Two graphical systems are particularly well suited to illustrating the haemodynamic effects of valvular disease. These are the pressure-volume loop and the pressure-flow synchrony (Wiggers diagram).

Pressure-volume loop

The pressure-volume (PV) diagram is very useful for illustrating the working conditions of heart valve disease. It describes a cardiac cycle as a counterclockwise loop defined by four points: telediastole, protosystole, telesystole and protodiastole conditions (Figure 11.19). The lower part of the loop is defined by the ventricular compliance curve. When the preload is modified (hypovolaemia, nitrates, colloid filling), the PV loop is shifted to the left in hypovolaemia or to the right in hypervolaemia, but the telesystolic points remain aligned on a quasi-straight line called maximum elastance (Emax), the slope of which is proportional to ventricular contractility; its normal value is 4-5 mmHg/ml [1]. It is independent of preload, ventricular shape and size, but remains partially dependent on afterload [2].

Figure 11.19: LV pressure-volume (PV) loop. A: Normal PV loop. 1: end-diastolic point; 1→ 2: isovolumetric contraction; 2: start of ejection; 2→ 3: systolic ejection phase; 3: telesystolic point; 3→ 4: isovolumetric relaxation; 4: start of filling; 4→ 1: diastolic filling. SV: stroke volume (SV = Vtd - Vts). B: Construction of the maximum elastance slope Emax by aligning the telesystolic points of a family of curves obtained at different preloads in the same individual. Normal Emax value: 4-5 mmHg/ml [1]. C: PV loop in LV dysfunction; the slope of Emax is reduced (systolic dysfunction) and the compliance curve is straightened (diastolic dysfunction); the surface area of the loop is reduced and the ejected volume (SV systolic volume) is reduced. D: the product of volume x pressure is the hydrodynamic equivalent of mechanical work (mass x distance); it is also the definition of the surface area (S) of the PV loop; the SPV therefore represents the work of ejection (Téj). The work of pressure (Tpr) is defined by the area of the triangle between the compliance curve, the Emax slope and the PV loop.

The Emax slope is one of the most reliable indices of contractility. It increases with sympathetic stimulation and decreases with ventricular failure. The area of the P/V loop is the product of stroke volume and the pressure generated by the ventricle; it is equivalent to the work of ejection. The physical definition of work is mass x distance. In hydrodynamics, work is pressure x volume, i.e. (gm/cm²) · cm³ = gm · cm. The area of the triangle between Emax, the compliance curve and the PV loop corresponds to the work of pressure of the ventricle (tension of the arterial system).

Wiggers diagram

The synchronisation of events in a cardiac cycle is illustrated by the Wiggers diagram (Figure 11.20). This diagram is used to define a number of commonly used terms.

a: Pressure wave due to atrial contraction at the end of diastole.
x: diastolic pressure drop in the atrium that occurs during ventricular systole; this drop occurs in two stages, separated by the "c" drop. The first stage "x'" corresponds to atrial relaxation. The second "x''" is due to the descent of the mitral annulus during the longitudinal contraction of the left ventricle; this movement enlarges the atrium, whose pressure decreases.
c: two phenomena are involved in this small increase in PLA: 1) bulging of the atrioventricular valve during the protosystole; 2) continuous filling by the central veins when the atrial wall has finished relaxing.
v: as venous return continues during ventricular systole, the pressure in the atrium rises progressively until it is released into the ventricle at the start of diastole. The "v" wave therefore begins in telesystole and continues in protodiastole; its peak corresponds to the dicrotic of the arterial pressure and the peak velocity of the mitral flow E.
y: pressure drop in the atrium as it empties into the ventricle in diastole.
S: systolic component of the venous flow in the vena cava and the pulmonary vein; it is subdivided into phases S1 (atrial relaxation) and S2 (descent of the mitral annulus), corresponding to descent x' and x''. The flow S is a function of the pressure difference between the great veins and the atrium (mitral or tricuspid valve closed).
D: diastolic component of venous and pulmonary venous flow. As the atrioventricular valve is open at this point, this flow is a function of the pressure difference between the great veins and the ventricle.
A: Blood flow due to atrial contraction; this is anterograde through the tricuspid and mitral valves. As there are no valves in the vena cava or pulmonary veins, atrial contraction also produces a brief retrograde flow (Ar) in the large central veins.
E: passive atrioventricular flow at the beginning of diastole; this normally represents 80% of ventricular filling. Its velocity is related to the level of atrial pressure and intraventricular depression in protodiastole (suction effect); the peak velocity of mitral flow (Vmax 0.8 - 1.0 m/s) corresponds to the peak of intraventricular dP/dt and the peak of the atrial "v" wave.

The flow velocities recorded by Doppler echocardiography are controlled by the instantaneous pressure gradient between the upstream and downstream cavities. The peak flow velocity corresponds to the maximum gradient. By convention, systole is considered to extend from isovolumetric contraction to closure of the aortic valve (phases 1-2) and diastole from isovolumetric relaxation to closure of the mitral valve (phases 3-4-5-6-7). However, the O₂ consuming phase of the cardiac cycle, corresponding to myocardial systole, actually extends from isovolumetric contraction to the peak of protodiastolic intraventricular depression (peak Vmax of mitral flow E, maximum value of -dP/dt). Physiologically, therefore, systole extends to the peak of E-flow and diastole begins with the deceleration of this flow because protodiastolic relaxation is an active O₂consuming phenomenon corresponding to the persistence of contraction of the subepicardial oblique fibres (see Chapter 5, Diastolic Relaxation).

Fig11 20 en

Figure 11.20: Wiggers diagram extended with filling currents. Art: systemic arterial curve. LV: Left ventricular pressure. LV: Left atrial pressure. PV: pulmonary venous flow. MV: flow through the mitral valve. 1: Isovolumetric ventricular contraction. 2: Ventricular ejection phase. 3: isovolumetric relaxation. 4: Rapid filling. 5: Diastasis. 6: Atrial contraction. AO: Opening of the aortic valve. AC: Aortic valve closing. MO: Opening of the mitral valve. MC: closing of the mitral valve. DSp: duration of physiological systole corresponding to the O₂ consuming phases. DSm: duration of mechanical systole according to the conventional definition. The same synchronisation and variations in pressure and flow are found in the right cavities, but at lower pressures and velocities.

Graphical representations

The pressure-volume loop represents the evolution of a cardiac cycle; its area corresponds to the ejection work. Maximum elastance (Emax) is a quasi-straight line defined by the telesystolic points of the ventricle during variations in preload; its slope is an excellent index of contractility.

The synchronisation of flows and pressures shows that the peak flow corresponds to the peak of the upstream pressure and the trough of the downstream pressure.

References

ASANOI H, SASAYAMA S, KAMEYAMA T. Ventriculoarterial coupling in normal and failing heart in humans. Circ Res 1989; 65:483-93
SUNAGAWA K, SAGAWA K, MAUCHAN WL. Ventricular interaction with the vascular system in terms of pressure-volume relationships. In: YIN FCP, ed. Ventricular/vascular coupling. Clinical, physiological and engineering aspects. New York: Springer Verlag, 1987, 210-39.