USING THE RESERVOIR WAVE APPROACH TO STUDY THE HORIZON EFFECT

INTRODUCTION Differing theories and models have been explored in the field of arterial hemodynamics in an effort to better understand how the blood flows in the body. The existence of discrete reflection sites remains a source of disagreement. As a wave proceeds along an artery, any local change in impedance will result in partial reflection. Thus, because of the great complexity of the arterial system, it has been suggested that no distinct reflector sites should exist. On the other hand, there is recent experimental evidence using the reservoir-wave approach (RWA) that implies discrete positive and negative reflection sites. The pattern of wave propagation and reflection is plausible, as are the modifications produced by pharmacologic interventions [1]. The classical, frequency-domain, “impedance analysis” approach uses Fourier analysis to break down pressure and flow waveforms into summations of sinusoids, resulting in an impedance spectrum [2]. The RWA is a novel alternative, positing that measured pressures and flows are the instantaneous sums of “excess” (wave-related) and reservoir (volume-related) components [2]. The “Horizon Effect” (HE) [3] implies that a reflector site can never be reached no matter how far into the periphery one measures (Figure 1). As such, it supports the idea that there are no distinct reflector sites. Measuring peripheral pressure and flow and using the RWA, the purpose of this study was to evaluate the HE and the question of fixed reflector sites. METHODS Eight anesthetized pigs were catheterized and pressure and flow were measured simultaneously at 4 locations: the aortic root and the brachial, carotid and renal arteries. Pharmacologic interventions were used to manipulate propagation and reflection patterns. MatLab (The MathWorks Inc., Natick, MA) was used to calculate the reservoir pressure, the local wave speed and to carry out wave intensity analysis (WIA) to obtain the forwards and backwards components of pressure. RESULTS Analysis has been completed on 4 of the 8 animals studied. Each showed minimal backward wave activity at the aortic root and in the carotid artery. However, in the brachial and renal arteries, no backward waves could be detected. DISCUSSION AND CONCLUSIONS Backward waves were scarcely detected in the periphery using either the RWA or classical analysis. This may suggest that the pig model was inappropriate to study the HE. Porcine anatomy does not allow measurement at remote peripheral sites and reflection sites cannot be studied if no reflections are to be found. Further analysis of the data is needed before more definitive conclusions can be made

On the Mechanics Underlying the Reservoir-Excess Separation in Systemic Arteries and their Implications for Pulse Wave Analysis

Several works have separated the pressure waveform p in systemic arteries into reservoir pr and excess pexc components, p = pr + pexc, to improve pulse wave analysis, using windkessel models to calculate the reservoir pressure. However, the mechanics underlying this separation and the physical meaning of pr and pexc have not yet been established. They are studied here using the time-domain, inviscid and linear one-dimensional (1-D) equations of blood flow in elastic vessels. Solution of these equations in a distributed model of the 55 larger human arteries shows that pr calculated using a two-element windkessel model is space-independent and well approximated by the compliance-weighted space-average pressure of the arterial network. When arterial junctions are well-matched for the propagation of forward-travelling waves, pr calculated using a three-element windkessel model is space-dependent in systole and early diastole and is made of all the reflected waves originated at the terminal (peripheral) reflection sites, whereas pexc is the sum of the rest of the waves, which are obtained by propagating the left ventricular flow ejection without any peripheral reflection. In addition, new definitions of the reservoir and excess pressures from simultaneous pressure and flow measurements at an arbitrary location are proposed here. They provide valuable information for pulse wave analysis and overcome the limitations of the current two- and three-element windkessel models to calculate pr

Oxygenation-sensitive CMR for assessing vasodilator-induced changes of myocardial oxygenation

<p>Abstract</p> <p>Background</p> <p>As myocardial oxygenation may serve as a marker for ischemia and microvascular dysfunction, it could be clinically useful to have a non-invasive measure of changes in myocardial oxygenation. However, the impact of induced blood flow changes on oxygenation is not well understood. We used oxygenation-sensitive CMR to assess the relations between myocardial oxygenation and coronary sinus blood oxygen saturation (SvO₂) and coronary blood flow in a dog model in which hyperemia was induced by intracoronary administration of vasodilators.</p> <p>Results</p> <p>During administration of acetylcholine and adenosine, CMR signal intensity correlated linearly with simultaneously measured SvO₂(<it>r</it>²= 0.74, <it>P </it>< 0.001). Both SvO₂and CMR signal intensity were exponentially related to coronary blood flow, with SvO2 approaching 87%.</p> <p>Conclusions</p> <p>Myocardial oxygenation as assessed with oxygenation-sensitive CMR imaging is linearly related to SvO₂and is exponentially related to vasodilator-induced increases of blood flow. Oxygenation-sensitive CMR may be useful to assess ischemia and microvascular function in patients. Its clinical utility should be evaluated.</p

Mechano-energetics of the asynchronous and resynchronized heart

Abnormal electrical activation of the ventricles creates major abnormalities in cardiac mechanics. Local contraction patterns, as reflected by measurements of local strain, are not only out of phase, but often also show opposing length changes in early and late activated regions. As a consequence, the efficiency of cardiac pump function (the amount of stroke work generated by a unit of oxygen consumed) is approximately 30% lower in asynchronous than in synchronous hearts. Moreover, the amount of work performed in myocardial segments becomes considerably larger in late than in early activated regions. Cardiac Resynchronization Therapy (CRT) improves mechano-energetics of the previously asynchronous heart in various ways: it alleviates impediment of the abnormal contraction on blood flow, it increases myocardial efficiency, it recruits contraction in the previously early activated septum and it creates a more uniform distribution of myocardial blood flow. These factors act together to increase the range of cardiac work that can be delivered by the patients’ heart, an effect that can explain the increased exercise tolerance and quality of life reported in several CRT trials