Vasodilator reserve in collateral-dependent myocardium as measured by positron emission tomography

Myocardial blood flow can be accurately quantitated in patientsusing positron emission tomography and oxygen-15 labelled water.The purpose of this study was to determine the vasodilator reservein myocardium completely perfused by intramyocardial collateralblood flow. We hypothesized that altered relative flow reservein such regions would correlate with the degree of ischaemiaobserved in these patients during exercise. The technique involves the inhalation of the positron emittingtracer C¹⁵O₂ which is converted to freely diffusible H₂¹⁵O bythe lung. With rapid dynamic scanning, arterial and regionalmyocardial tissue concentrations can be obtained and time activitycurves generated. With a two-compartment kinetic model, myocardialblood flow can be accurately quantitated over a wide range ofblood flows. Five patients with stable exertional angina andnormal ventricular function studies and who had an occludedmajor epicardial artery which completely opacified via intramyocardialcollateral blood flow were studied. Myocardial blood flow (MBF)was measured both at rest and following an infusion of intravenousdipyridamole (0.56 mg. kg^–1) and the results were comparedwith measurements obtained from a group of eight normal volunteers.During resting conditions, MBF in the control group was 0.86±0.10ml.g^–1. min^–1 and in the patient group was 0.99±0.10ml. g^–1. min^–1 in normally perfused myocardium (ns)and 0.86±0.14 ml. g^–1. min^–1 in collateral-dependentmyocardium (ns). Following dipyridamole, MBF increased to 3.58±0.89ml. g^–1. min^–1 in the control group and to 2.97±0.94ml. g^–1. min^–1 in the normal regions of the patients(ns). In the collateralized regions of the patients, the increasewas less than that observed in the control group (1.66±1.02,P <0.005). Absolute coronary flow reserve (ACFR) (dipyridamoleMBF/resting MBF) in the control group was 4.1±0.8 andin the patient group was 3.1±1.1 (ns) in normal regionsand 1.9± 1.0 (P <0.001) in collateralized regions.Relative coronary flow reserve, the ratio of ACFR in collateralizedvs that of normally perfused myocardium was determined in eachpatient and correlated well with total exercise time (r = 0.98;P <0.01) and peak double product (r = 0.85; P = 0.06) observedduring a symptom-limited modified Bruce treadmill test. These studies support the hypothesis that vasodilator reservein the distribution of non-infarcted collateral-dependent myocardiumis abnormal compared with normally perfused myocardium. Thedegree of altered flow reserve correlates well with the degreeof ischaemia during symptom-limited exercise, and may explainwhy these patients experience angina at high work loads.