Ventriculoarterial coupling

The coupling of the left ventricle with the peripheral vascular system has been intensely studied in terms of the end systolic P/V relationship.69 In this model, the left ventricle and the arterial system are regarded as two coupled elastic chambers. The distribution of blood between these chambers is determined by their relative elastances. The elastance E is a measure of respective chamber stiffness and is represented by the slope of the P/V relationship (see Fig 2.7).The instantaneous elastance increases from a low value during diastole to its maximum value (Emax) close to end systole. The pressure varies inversely and linearly with the stroke volume according the following equation:

where Ees = end systolic elastance related to contractile properties of the ventricle, Pes = end systolic ventricular pressure, Ved = end diastolic volume, SV = stroke volume and V0 = volume-axis intercept of the P/V relationship.

The arterial system is characterised in this model by the relationship between end systolic pressure and stroke volume. The slope of this relationship, the effective arterial elastance (Ea=Pes/SV), serves as an index of total external load opposing ejection. Ea comprises resistance, compliance, and characteristic impedance of the arterial vascular bed. During the ejection phase, elastance varies with time; throughout ejection, ventricular elastance progressively increases from the onset to the end of ejection whereas arterial elastance progressively decreases (time varying elastance). The effective stroke volume resulting from the ventriculoarterial coupling is determined by the intersection of the ventricular end systolic P/V and arterial end systolic P/V relationships.48 69 Graphic analysis of the ventriculoarterial coupling provides expeditious information on left ventricular function and its determinants during acute changes in loading conditions.38 45 The left ventricle delivers maximal external work (stroke work) when the Ea/Ees ratio approximates 1. The mechanical efficiency of the ventricle, relating work to amount of energy consumed, is maximal when Ea is approximately half of Ees.70 In normal subjects, Ees and Ea values of 3-5-7-0 and 1-6-4 0 mm Hg/ml, respectively, were found.46 71 The Ea/Ees ratio was 0-4-0-6 and increased significantly with increasing afterload as a result of increased Ea An afterload reduction caused Ea and the ratio E/Ees to decrease. Positive a a a es inotropic stimulation also decreased EJE^ as a result of an increase in Eoc,.71

a es es

The pressure/volume area (PVA) encompassed by the end systolic and diastolic P/V relationship and the systolic part of the P/V loop provides an interesting insight into

myocardial energetics.72-74 PVA is composed of the PE area (PE being the potential energy of an isovolumic contraction) and the EW area (EW being the external work of ejecting contraction) (Fig 2.10). PVA (= PE + EW) is a measure of the total mechanical energy of contraction and was shown to correlate linearly with left ventricular myocardial oxygen consumption The slope of this relationship represents the oxygen cost of the PVA

and its reciprocal represents the mechanical efficiency. This linear t-^^/PVA relationship remains constant during changes in preload and afterload, but is shifted upwards in parallel manner by positive inotropic stimulation. In contrast, negative inotropic

Ventriculoarterial Coupling

Fig 2.10 Left ventricular pressure-volume relationship and pressure-volume area (PVA): ESPVR (end systolic pressure-volume relationship), EDPVR (end diastolic pressure-volume relationship), Ees (end systolic elastance), Ea (effective arterial elastance), EW (left ventricular external work (stroke work)), PE (left ventricular potential energy). The PVA is the sum of EW and PE areas.

Fig 2.10 Left ventricular pressure-volume relationship and pressure-volume area (PVA): ESPVR (end systolic pressure-volume relationship), EDPVR (end diastolic pressure-volume relationship), Ees (end systolic elastance), Ea (effective arterial elastance), EW (left ventricular external work (stroke work)), PE (left ventricular potential energy). The PVA is the sum of EW and PE areas.

interventions shift the relationship downwards, again without changing its slope.75 In patients with normal left ventricular function, a contractile efficiency of 40% was found.76 A decrease in efficiency from 46% to 35% was demonstrated in healthy volunteers during positive inotropic stimulation with dobutamine.77 The ratio of external work (EW or stroke work) to PVA can be used to express the mechanical efficiency of the whole left ventricle. In patients with normal left ventricular function, this ratio was 0 6. It decreased with an increase in afterload and increased when afterload was reduced.71

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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