Augmentation Index, Left Ventricular Contractility, and Wave Reflection

Abstract

Abstract-Augmentation index (AIx), a correlate of mortality, is thought to be influenced by left ventricular contractility and wave reflections. However, the relationship of AIx with left ventricular contractility changes has never been assessed, and the wave reflection theory has recently been questioned. This study sought to examine arterial waveform changes in response to reduced "wave reflection" and increased left ventricular contractility induced by dobutamine. Key Words: left ventricle Ⅲ heart contractility Ⅲ cardiac inotropism Ⅲ blood pressure Ⅲ hemodynamic Ⅲ cardiovascular physiology Ⅲ augmentation index A ugmentation index (AIx) is a surrogate indicator of left ventricular (LV) systolic loading that may be recorded noninvasively from superficial arteries (eg, radial or carotid artery) or estimated at the ascending aorta from the radial artery using a generalized transfer function. 1 Several studies suggest that AIx may be a useful therapeutic target in selected patient populations. For example, the change in AIx in response to antihypertensive therapy was shown recently to be a robust predictor of the change in LV mass in hypertensive patients treated over 1 year. 2 The AIx has also been shown to independently correlate with the extent of coronary artery disease, 3 LV hypertrophy, 4 urinary albumin excretion, 5 diabetic retinopathy, 6 maximal aortic intima-media thickness, 7 cardiovascular events, 8,9 and all-cause mortality. 10 There is a widely held belief that AIx is principally determined by the magnitude of arterial wave reflection 11 where, in instances of increased heart rate (eg, that induced by drugs, 12 pacing, 13 or exercise 14 ), ejection duration is shortened and the reflected pressure wave is shifted from systole to diastole, thereby decreasing both AIx and LV systolic stress. However, recent findings with new methodology have suggested that wave reflection only plays a minor role in arterial pressure waveform morphology. [15][16][17] Discrepancy between the 2 explanations may be resolved by analyzing the relative changes in discrete components of arterial pressure waveforms (ie, systolic and diastolic pressure-time integrals, as well as arterial reservoir pressure and wave pressure 18 ) in response to changes in heart rate. The first aim of this study was to assess these waveform parameters in response to increased heart rate induced by dobutamine. We hypothesized that an increase in heart rate would be accompanied by an increase in the magnitude of systolic, rather than diastolic, components of the waveform.Furthermore, AIx is known to be influenced by numerous factors, 12-14,19 -22 including alterations in either cardiac or vascular function, because the shape of the arterial pressure waveform is a result of the interaction between the heart and the arterial system. 23 As such, the pattern of LV contractility is commonly cited as playing an influential role on the magnitude of AIx. 24 These conclusions are based on pacing studies in humans that show a strong negative correlation

Central Hemodynamics and Target Organ Damage in Hypertension

“…Additionally, recent studies have emphasized the importance of the aortic reservoir function in determining the systolic pressure waveform (Sharman et al 2009;Davies et al 2010). The relative contribution of this reservoir pressure wave (in comparison with the reflected pressure wave) to late-systolic augmentation is currently a matter of debate (Mynard et al 2012;Segers et al 2012), while it has long been well-recognized that the diastolic pressure waveform is determined by the Windkessel (reservoir and recoil) function.…”

Arterial stiffness, central blood pressure and body size in health and disease

“…18,28 This 'high-output, low-resistance' haemodynamic milieu will reasonably be expected to result in significantly lower central SBP compared with peripheral SBP (increased SBP amplification), and thereby potentially help explain the differential relationships between body size and central versus peripheral BP. 23,29 Further studies are needed to understand the relation between body size and central BP.…”

Aortic Reservoir Pressure Corresponds to Cyclic Changes in Aortic Volume

“…3,4 Indeed, augmentation of central BP may be largely attributable to forward wave propagation (as a result of left ventricular [LV] ejection) and proximal aortic reservoir function. [5][6][7][8][9] Importantly, wave separation theory obscures the pressure buffering role of the highly elastic proximal aorta (ie, the aortic reservoir), and a failure to consider this function may lead to incorrect interpretations of the physiology underlying central BP waveform morphology.The reservoir-excess pressure concept is an alternate method proposed to explain the underlying physiology of the aortic BP waveform. This method has been used invasively to study changes in aortic BP associated with both aging and exercise.…”