Estimation of coronary flow reserve with the instantaneous coronary flow velocity versus pressure relation: a new index of coronary flow reserve independent of perfusion pressure

Evaluation of Microvascular Disease and Clinical Outcomes

Although coronary microcirculatory dysfunction occurs in numerous cardiac conditions and influences prognosis, it has been largely ignored in clinical practice due to the lack of adequate methods for its assessment. Microcirculatory dysfuntion may result from a variety of causes, including structural remodelling (arterioles or capillaries), dysregulation (paradoxical arteriolar vasoconstriction), hypersensitivity to vasoactive factors or adrenergic drive, and extravascular compression of collapsable elements. Thus, the selection of a method to interrogate coronary microcirculation should be based on the suspected cause of dysfunction. This article reviews such assessment tools and their prognostic information.

Determining exercise-induced blood flow reserve in lower extremities using phase contrast MRI

PURPOSE

To study the changes in limb blood flow during lower extremity exercise using phase contrast (PC) MRI in normal volunteers.

MATERIALS AND METHODS

Healthy volunteers performed plantar flexion exercise (<1 W) for four minutes. Flow velocity was measured using cardiac-gated, cine PC-MRI sequences (fast gradient recalled echo [GRE]; multishot echo planar imaging [EPI]) on a 3T scanner at the level of the superficial femoral artery (SFA): 1) preexercise; 2) immediately postexercise; 3) during three minutes recovery; and 4) postrecovery.

RESULTS

At rest there was a triphasic flow waveform in the SFA. During exercise it changed to a monophasic pattern with an increase in total flow; there were variable changes in vessel size and flow velocity. The waveform regained the triphasic pattern during recovery. The exercise-induced flow reserve (FR) was 167 +/- 90%.

CONCLUSION

PC-MRI demonstrates that the resting triphasic flow waveform transforms into a monophasic pattern with submaximal exercise and returns to baseline with recovery. This increase in the regional blood flow allows for measurement of exercise-induced FR in the SFA.

The diastolic flow-pressure gradient relation in coronary stenoses in humans

OBJECTIVES

We assessed the feasibility and reproducibility of the instantaneous diastolic coronary flow velocity-pressure gradient relation to characterize different degrees of coronary stenoses.

BACKGROUND

Assessment of the hemodynamic significance of coronary stenoses can be difficult. Using sensor-tipped guidewires, various physiologic indexes can be determined in the catheterization laboratory. Each of the current methods, however, has limitations.

METHODS

After positioning a Doppler flow wire and a pressure wire distal of a coronary stenosis, the flow velocity signals and the proximal and distal pressure were sampled simultaneously, at baseline and after intracoronary administration of adenosine. The instantaneous diastolic flow velocity and pressure gradient of single cardiac cycles at baseline, at maximal and intermediate hyperemia were plotted. Data were fitted with a regression line using the equation: Delta P = 0 +kv+Sv(2). Measurements were performed in 11 normal coronary arteries, 20 intermediate stenoses and in 7 severe stenoses before and after percutaneous transluminal coronary angioplasty plus stenting.

RESULTS

We found significant differences between normal coronary arteries, intermediate and severe stenoses. Percutaneous transluminal coronary angioplasty nearly normalized the highly abnormal flow-pressure gradient relation in the severe stenoses. A high degree of reproducibility was observed. In 3% of the measurements, analysis was not possible due to the occurrence of pressure drift or bad flow velocity signals.

CONCLUSIONS

It is feasible to assess the diastolic flow velocity-pressure gradient relation over a wide range of stenoses. It characterizes the hemodynamics of epicardial coronary stenoses and allows discrimination between normal coronary arteries, intermediate and severe stenoses.

Effects of norepinephrine on the renal vasculature in normal and endotoxemic dogs

Septic shock is often complicated by systemic hypotension despite normal or increased cardiac output. Restoration of arterial pressure usually requires the administration of systemic vasopressor agents, such as norepinephrine. However, because norepinephrine induces vasoconstriction in other vascular beds, it may decrease visceral blood flow, impairing visceral organ function. Because sepsis is often associated with impaired peripheral vascular responsiveness, we hypothesized that, unlike in normal circulatory conditions, norepinephrine would improve visceral organ blood flow in sepsis by selectively increasing organ perfusion pressure. Thus, in nine pentobarbital-anesthetized, mechanically ventilated dogs, we measured the effect of norepinephrine infusion (0.3 microgram/kg/min) on renal, hepatic, and portal steady-state pressure-flow relations (P/Q) and the dynamic vascular P/Q, created by transient inferior vena caval occlusion, under basal and endotoxic conditions. Norepinephrine increased organ perfusion pressures during both control and endotoxemic conditions. However, even after controlling for the pressure effect using a general linear model, NE was associated with an increase in renal blood flow both before and after endotoxin administration. We conclude that, unlike the effects of administering norepinephrine under baseline conditions, norepinephrine infusion during endotoxic shock actually increases renal blood flow and that this effect is not the result of an increase in perfusion pressure alone.