Arterial-ventricular coupling evaluation by echocardiography

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Definition Arterial ventricular coupling summarizes the cardiovascular perfomance of a subject; in particular, it is the interaction between the ventricle, as pump, and the vascular system, as a load[1]. The concept of arterial-cardiac relationship is important in understanding the pathophysiology of cardiovascular diseases.


The term arterial-ventricular coupling indicates the interaction between left ventricle and arterial system[2]. This parameter is considered crucial in assessing the cardiovascular status and performance of a subject. The components that characterize arterial-ventricular coupling are [2] arterial elastance (Ea) and left ventricular elastance (Elv).

Contents

Arterial elastance

In the past arterial load was assessed in the frequency domain, but at the beginning of 80's[3] an estimation in the time domain was introduced in order to allow a cross-talk analysis of vascular and ventricular behaviour. Arterial load can be estimated by Ea, that is the change in volume for a change in pressure. This parameter includes arterial compliance, peripheral vascular resistance, characteristic impedance, and systolic-diastolic time intervals. Ea is obtained by pressure-volume loops[2] and it can be approximated by the ratio of end-systolic pressure (ESP) to stroke volume (SV). End-systolic pressure is computed as

ESP = 0.9 *( brachial systolic blood pressure)

whereas, SV is estimated as the difference between end-diastolic and end-systolic volumes, that are obtained by 2D ecocardiography. Cardiac volumes are measured from apical 4-chamber and 2-chamber views using the biplane method of disks (modified Simpson’s rule)[1]. We have to point out that the Ea estimation is drawn by the three-element Windkessel model and hence it does take into consideration the phenomenon of pressure wave reflection[2]. Moreover, to make this evaluation more accurate, instead of brachial pressure, an estimation of central blood pressure by tonometry might be adopted[4]. Finally, it is worth noting that Ea depends on many factors, including blood pressure, peripheral vascular resistance (PVR), arterial compliance (C) and heart rate (HR). In particular, in resting conditions variations in Ea are mainly due to variations in PVR/HR whose influence is three times higher than the impact of similar change in C[5]. During exercise the influence of vascular elasticity on Ea becomes dominant[2]. The unit of measurement of Ea is [mmHg/ml]. Ea in healthy subject at baseline is around 2.3±1 mmHg/ml[2].

Left ventricular elastance

The left ventricular contractility can be estimated by Elv, the left ventricular elastance[6]. This index can be computed as Elv = ESP /(ESV - V0) where ESP represents the end-systolic pressure, ESV the end-systolic volume and V0 is the volume intercept (x-axis) of the end-systolic P-V relationship[2]. Under phyisiological loading conditions V0 can be considered negligible[2]. Elv reflects left ventricular contractility, but it is also influenced by biochemical and morphological features of the cardiac tissues. In particular, changes during stress conditions (i.e. exercise or pharmacological) can give us information about left ventricle (LV) performance, whereas analysis at rest allows an integrated picture of LV contractility, function and geometry[2]. The unit of measurement of Elv is [mmHg/ml]. Elv in healthy subject at baseline is around 2.2±0.8 mmHg/ml[2]. Both Ea and Elv in order to obtain a more reliable evaluation can be normalized for body-surface-area (EaI and ElvI, respectively).

Arterial-ventricular coupling

The ratio between Ea and Elv can be used to estimate the interaction between the arterial system and the left-ventricle. Ea/Elv is an adimensional parameter. Ea/Elv in healthy subject at baseline is around 1±0.36 and this results in a balance between blood ejection from the heart to the periphery without abnormal pressure changes[2]. A suitable arterial-ventricular coupling ensures mechanical efficacy and energetic efficiency[7].

Arterial-ventricular coupling alterations

The interaction between the heart and the arterial system at rest can be altered in the presence of modifications that impact on the cardiovascular system, such as aging, hypertension and heart failure[1][2]. In particular, Ea and Elv increase with aging: their ratio remains balanced in men, and slighly decreased in women. As regards hypertensive patients they present higher Ea and Elv, and the arterial-ventricular coupling remains similar to normotensive subjects in men, and slightly lower in women. Finally, patients with systolic heart failure present higher Ea/Elv, but this ratio remains balanced in subjects with heart failure and preserved ejection fraction[1].

Arterial-ventricular coupling and exercise

The dynamic behaviour of cardiac-vascular interaction is intriguing, since this condition is exetremely important in patients' real life. In healthy young subjects both Ea and Elv increase during aerobic exercise, and their ratio decreases due to an higher change in cardiac contractility. This mechanism ensures the preservation of cardiac efficacy over energetic efficinecy[2]. The phenomenon is similar in hypertensive patients, whereas it is less evident in older subjects and in the presence of systolic heart failure[2].

References

  1. 1.0 1.1 1.2 1.3 Chi Young Shim. Arterial-Cardiac Interaction: The Concept and Implications. J Cardiovasc Ultrasound. 2011 June; 19(2): 62–66.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 P.D. Chantler, E. Lakatta, S.S. Najjar. Arterial-ventricuar coupling: mechanistic insights into cardiovascular performance at rest and during exrcise. J App Phisiol 105:1342-1351, 2008
  3. Sunagawa K, Maughan WL, Burkhoff D, Sagawa K. Left ventricular interaction with arterial load studied in isolated canine ventricle. Am. J. Physiol Heart Circ Physiol 245: H773-H780, 1983.
  4. Chen CH, Nevo E, Fetics B, Pak PH, Yin FC, Maughan WL, Kass DA. Estimation of central aortic pressure waveform by mathematical transformation of radial tonometry pressure. Validation of generalized transfer function. Circulation. 1997 Apr 1;95(7):1827-36.
  5. Segers P, Stergiopulos N, Westerhof N. Relation of effective arterial elastance to arterial system properties. Am J Physiol Heart Circ Physiol 282:1041-1046, 2002.
  6. Chantler PD, Lakatta EG. Arterial-ventricular coupling with aging and disease. Front Physiol. 2012;3:90.
  7. Starling MR. Left ventricular-arterial coupling relations in the normal human heart. Am Herat J 125:1659-1666, 1993.

Figures to be inserted (volumes example and central pressurre waveform)

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