The myocardial perfusion reserve, defined as the ratio of hyperemic and basal myocardial blood flow, is a useful indicator of the functional significance of a coronary artery lesion. Rapid magnetic resonance (MR) imaging for the noninvasive detection of a bolus-injected contrast agent as a MR tracer is applied to the measurement of regional tissue perfusion during rest and hyperemia, in patients with microvascular dysfunction. A Fermi function model for the distribution of tracer residence times in the myocardium is used to fit the MR signal curves. The myocardial perfusion reserve is calculated from the impulse response amplitudes for rest and hyperemia. The assumptions of the model are tested with Monte Carlo simulations, using a multiple path, axially distributed mathematical model of blood tissue exchange, which allows for systematic variation of blood flow, vascular volume, and capillary permeability. For a contrast-to-noise ratio of 6:1, and over a range of flows from 0.5 to 4.0 ml/min per g of tissue, the ratio of the impulse response amplitudes for hyperemic and basal flows is linearly proportional to the ratio of model blood flows, if the mean transit time of the input function is shorter than approximately 9 s. The uncertainty in the blood flow reserve estimates grows both at low (< 1.0 ml/min/g) and high (> 3-4 ml/min/g) flows. The predictions of the Monte Carlo simulations agree with the results of MR first pass studies in patients without significant coronary artery lesions and microvascular dysfunction, where the perfusion reserve in the territory of the left anterior descending coronary artery (LAD) correlates linearly with the intracoronary Doppler ultrasound flow reserve in the LAD (r = 0.84), in agreement with previous PET studies.

Adenosine is a potent vasodilator used extensively to study the coronary circulation of animals. Its use in humans, however, has been hampered by lack of knowledge about its effects on the human coronary circulation and by concern about its safety. We investigated in humans the effects of adenosine, administered by intracoronary bolus (2-16 jig), intracoronary infusion (10-240 ,ug/min), or intravenous infusion (35-140 pg/kg/min) on coronary and systemic hemodynamics and the electrocardiogram. Coronary blood flow velocity (CBFV) was measured with a 3F coronary Doppler catheter. The maximal CBFV was determined with intracoronary papaverine (4.5±0.2. resting CBFV). In normal left coronary arteries (n=20), 16-pg boluses of adenosine caused coronary hyperemia similar to that caused by papaverine (4.6±0.7 * resting CBFV). In the right coronary artery (n=5), 12-,ug boluses caused maximal hyperemia (4.4± 1.0 * resting CBFV). Intracoronary boluses caused a small, brief decrease in arterial pressure (similar to that caused by papaverine) and no changes in heart rate or in the electrocardiogram. The duration of hyperemia was much shorter after adenosine than after papaverine administration. Intracoronary infusions of 80 ,g/min or more into the left coronary artery (n=6) also caused maximal hyperemia (4.4±0.1. resting CBFV), and doses up to 240 pg/min caused a minimal decrease in arterial pressure (-6±2 mm Hg) and no significant change in heart rate or in electrocardiographic variables. Intravenous infusions in normal patients (n=25) at 140 ,ug/kg/min caused coronary vasodilation similar to that caused by papaverine in 84% of patients (4.4±0.9 -resting CBFV). At submaximal infusion rates, however, CBFV often fluctuated widely. During the 140-,ug/kg/min infusion, arterial pressure decreased 6±7 mm Hg, and heart rate increased 24± 14 beats/min. One patient developed 1 cycle of 2:1 atrioventricular block, but otherwise, the electrocardiogram did not change. In eight patients with microvascular vasodilator dysfunction (ACBFV, <3.5 peak/resting velocity after a maximally vasodilating dose of intracoronary papaverine), the dose-response characteristics to intracoronary boluses and intravenous infusions of adenosine were similar to those found in normal patients. These studies suggest that maximal coronary vasodilation can be achieved safely with intracoronary adenosine administration and that intravenous infusions at a rate of 140 ,ug/kg/min cause near-maximal coronary hyperemia in most patients. (Circulation 1990;82:1595-1606 M aany studies of the coronary circulation require the use of drugs that can safely and reliably produce maximal coronary hyperemia of brief duration (for example, for measurement of coronary flow reserve, thallium-201 scintigraphy, and echocardiographic imaging).1-6 An ideal agent for these studies would cause maximal

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The myocardial perfusion reserve can be quantified with first-pass MR imaging. In patients with microvascular dysfunction, the myocardial perfusion reserve matches the reduced coronary flow reserve.