SUMMARY We have characterized N-13 ammonia as a myocardial blood flow imaging agent suitable for positron-emission computed tomography. However, the mechanisms of uptake and retention of this agent in myocardium are not known, and effects of altered metabolism were not considered. Therefore, we studied the uptake and retention of N-13 ammonia in myocardium under various hemodynamic and metabolic conditions in open-chest dogs. N-13 ammonia was extracted nearly 100% during its initial capillary transit, followed by metabolic trapping that competed with flow-dependent back diffusion. At control flows, the first capillary transit extraction fraction (E) of N-13 ammonia averaged 0.82 ± 0.06. It fell with higher flows by E = 1 -0.607 exp -125/F. Myocardial N-13 tissue clearance half-times were similarly inversely related to blood flow, and ranged from 110-642 minutes. Cardiac work and changes in the myocardial inotropic state induced by isoproterenol and propranolol did not affect E or the tissue clearance half-times. Low plasma pH reduced E by an average of 20%, while elevated plasma pH had no effect. Decreases in flow below control also were associated with a fall in E. Inhibition of glutamine synthetase with L-methionine sulfoximine impaired metabolic trapping of N-13 ammonia and implicates the glutamic acid-glutamine reaction as the primary mechanism for ammonia fixation. The product of E times flow predicts the myocardial N-13 tissue concentrations, which increased by 70% when flow was doubled. Thus, blood flow and metabolic trapping are the primary determinants of myocardial uptake and retention of N-13 ammonia. The relative constancy of metabolic trapping over a wide range of hemodynamic and metabolic conditions demonstrates the value of N-13 ammonia as a myocardial blood flow imaging agent. N-13 AMMONIA* has been characterized as an indicator for the noninvasive visualization of regional myocardial perfusion by positron computed tomography (PCT).' Use of N-13 ammonia has also permitted noninvasive detection of mild, 47% diameter coronary stenosis in the intact dog.2Because fixation of N-13 ammonia in myocardium occurs through metabolic pathways, alterations in both the hemodynamic and metabolic state of the heart could modify the uptake of N-13 ammonia, and hence, limit its value as a flow indicator. In blood, N-13 (NH3) ammonia exists primarily in its ionic species, NH4+, the ammonium ion, which apparently can substitute for K+ on the sodium-potassium transmembraneous exchange system in red blood cells.3 It thus may be actively transported into myocardium. On the other hand, NH3 can diffuse across cell membranes because of its lipid solubility and is rapidly replenished by conversion of NH4+ to NH3 as it leaves the vascular space.4" 5Transmembrane exchange therefore may occur through an active transport mechanism or *The term ammonia is used to refer to the chemical equilibrium of NH3 NH4+ in which the prominent form is NH4+.

Positron emission tomography (PET) was applied to the measurement of myocardial perfusion using the perfusion tracer 13N-labeled ammonia. 13N ammonia was delivered intravenously to 13 healthy volunteers both at rest and during supine bicycle exercise. Dynamic PET imaging was obtained in three cross-sectional planes for 10 minutes commencing with each injection. The left ventricle was divided into eight sectors, and a small region of interest was assigned to the left ventricular blood pool to obtain the arterial input function. The net extraction of 13N ammonia was obtained for each sector by dividing the tissue 13N concentration at 10 minutes by the integral of the input function from the time of injection to 10 minutes. With this approach for calculating net extractions, rest and exercise net extractions were not significantly different from each other. To obviate possible overestimation of the true 13N ammonia input function by contamination by '3N-labeled compounds other than 13N ammonia or by spillover from myocardium into blood pool, the net extractions were calculated using only the first 90 seconds of the blood and tissue time-activity curves. This approach for calculating net extractions yielded significant differences between rest and exercise, with an average ratio of exercise to rest of 1.38+0.34. Nonetheless, the increase was less than predicted from the average 2.7-2.8-fold increase in double product at peak exercise or the 1.7-fold increase in double product at 1 minute after exercise. However, when the first 90 seconds of dynamic data were fit with a two compartment tracer kinetic model, average perfusion rates of 0.75+±0.43 ml/min/g at rest and 1.50± 0.74 ml/min/g with exercise were obtained. This average increase in perfussion of 2.2-fold corresponded to similar average increases in double product. Thus, the noninvasive technique of PET imaging with 13N ammonia shows promise for future applications in determining absolute flows in patients with coronary artery disease. (Circulation 1989;80:1328-1337 T he reference standard for diagnosing coronary artery disease has long been considered to be coronary angiography. However, the limitations of this shadow technique, the variability of subjective readings of angiograms, and the variable relations of percent stenosis and coronary perfusion have emphasized the need for improved means to assess the functional severity of coronary

Extraction and clearance kinetics of

The usefulness of [1l-tC]acetate as a tracerof overall myocardial oxidative metabolism for use with positron emission tomography has been investigated in 12 closed-chest dogs. Myocardial ltC activity clearance kinetics after intravenous administration of [1-"C]acetate in dogs have been determined noninvasively by positron emission tomography. Biexponential fitting of regional myocardial`C time-activity curves was performed to give clearance half-times and fractional distribution. The rate constant kl for the early rapid phase of "C activity clearance was found to correlate linearly with myocardial oxygen consumption (y=0.0156x+0.039; SEE=0.023; r=0.95). kl was approximately 7% lower in septal sectors compared with the left ventricular free wall, suggesting that regional oxygen consumption in the septum was lower; a concomitant regional attenuation of blood flow in the septum relative to the left ventricular free wall was also observed. In dogs using carbohydrates as the predominant fuel, kl oxygen consumption was somewhat more than in dogs using predominantly free fatty acids (0.021±0.002 compared with 0.018+0.002, p <0.01), indicating that increased carbohydrate consumption is associated with a small increase in kl at constant oxygen consumption. It is concluded that measurement of myocardial [1-"Cjacetate kinetics allows noninvasive determination of cardiac oxygen consumption by positron emission tomography and that the technique is relatively insensitive to myocardial fuel selection. (Circulation 1989;79:134-142) P ositron emission tomography (PET) provides a unique opportunity for the noninvasive study of regional metabolism in vivo.1 While metabolic tracers currently in use for PET are

Experimental studies of animals have previously demonstrated the validity of [1-ttClacetate as a tracer of oxidative metabolism for use with positron emission tomography. The present study was undertaken to define in normal human volunteers the relation between myocardial clearance kinetics of [1-"C]acetate, and the rate-pressure product as an index of myocardial oxygen consumption. Twenty-two studies were performed of 12 volunteers. The rate-pressure product was increased with continuous supine bicycle exercise in six studies. Of the 16 resting studies, seven were performed in the fasted state and nine following an oral glucose load, to define possible effects of substrate availability on the tracer-tissue kinetics. Myocardial tissue time-activity curves were biexponential. Clearance of activity was homogeneous throughout the myocardium. The rate constants ki, obtained from biexponential fitting, and kn..n, obtained by monoexponential fitting of the initial linear portion of the time-activity curves, correlated well with the rate-pressure product. Although the correlation coefficient was higher for kl than for km.,, (0.95 vs. 0.91), analysis on a sectorial basis showed less regional variability in kn..O. This suggests that korno,0 which is more practical than kl because it requires shorter acquisition times, may be more clinically and experimentally useful for detection of myocardial segments with abnormal oxygen consumption. Overall, changes in myocardial substrate supply were without significant effect on the relation between the rate constants (kl and k...0) and the rate-pressure product, although a small decrease in km0,,0/rate-pressure product was observed following oral glucose by paired analysis in four subjects. It is concluded that [1-"C]acetate can be used for the noninvasive measurement of myocardial oxygen consumption in humans with positron emission tomography, and, thus, has clinical and experimental potential as a tool for the understanding and diagnosis of myocardial disease. (Circulation 1989;80:863-872) acid kinetics in myocardium.3-12 Although characteristic changes in uptake and clearance of labeled fatty acids have been observed in myocardial ischemia, these changes reflect the net effect of diminished oxygen supply on each of the steps in fatty acid utilization and are not specific for impairment of 3-oxidation.2 Uptake and clearance of labeled fatty acids are markedly affected by substrate availability,13-15 in addition to myocardial workload.16 To characterize more precisely the metabolic state of the myocardium, a selective tracer of mitochondrial oxidative function is required.In the early 1980s, [1-1"C]acetate was examined both in animal studies17 and in patients with coro-

The kinetics of [1-14C]acetate oxidation in isolated perfused rat hearts have been determined over a range of perfusion conditions. Effluent measurements demonstrated that 14CO2 cleared biexponentially over 50 minutes after bolus injection of [1-14C]acetate into normoxic hearts perfused with 5 mM glucose and 10 mU/ml insulin. The clearance half-time (t1/2) for the predominant initial clearance phase was 3.1 +/- 0.5 minutes (n = 4). MVO2 was varied over a fourfold range by hypoxia and phenylephrine stimulation (t1/2, 7.2 +/- 1.2 and 2.2 +/- 0.2 minutes, respectively) and in the presence of alternate substrates (lactate, 2 mM; DL-3-hydroxybutyrate, 20 mM; and palmitate, 0.1 mM), which did not modify either tricarboxylic acid (TCA) cycle flux or acetate kinetics. A good correlation (r = 0.93) was observed between k, the rate constant for the initial phase of 14CO2 clearance, and TCA cycle flux, estimated from oxygen consumption. In contrast to results with [1-14C]acetate, lactate (2 mM) increased t1/2 for 14CO2 clearance from a bolus injection of [1-14C]palmitate from 3.0 +/- 0.4 minutes (n = 3) at control to 4.3 +/- 0.2 minutes (n = 3, p less than 0.01). Addition of acetate in nontracer amounts (0.5 or 5 mM) caused significant underestimation of TCA cycle flux when estimated with [1-14C]acetate. 14CO2 clearance accounted for 88-98% of total effluent 14C between 10 and 20 minutes after [1-14C]acetate bolus injection; rate constants for clearance of 14CO2 and total 14C clearance were very similar during this period, and these two rate constants did not differ significantly from each other under any conditions tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies at necropsy have shown that the cardiomyopathy of Duchenne muscular dystrophy selects the posterobasal and contiguous lateral left ventricular (LV) walls as initial and primary sites of myocardial dystrophy in the absence of small-vessel coronary artery disease in these areas. The present investigation was designed chiefly to determine whether a myocardial metabolic abnormality could be identified in these same areas during a patient's life. Positron emission computed tomography was used to study regional LV metabolism with '8F 2-fluorodeoxyglucose, and metabolism and/or perfusion was studied with '3NH3. In addition, all subjects had the following performed: thallium-201 scans, technetium-99m multiple-gated equilibrium blood pool imaging, electrocardiograms, vectorcardiograms, and M mode and two-dimensional echocardiograms. '8F 2-fluorodeoxyglucose activity was selectively increased in the posterobasal and posterolateral walls of the left ventricle in 1 1 of 12 patients with technically adequate images, indicating accelerated regional exogenous glucose utilization.13NH3 activity was selectively decreased in the same areas in 13 of 15 patients, indicating either a regional metabolic alteration in uptake and trapping, a reduction in regional blood flow, or both. These data identify a myocardial metabolic abnormality concentrated in specific segments of the LV free wall in living patients with Duchenne dystrophy. Circulation 69, No. 1, [33][34][35][36][37][38][39][40][41][42] 1984 grams (ECGs) are found even in early childhood.56 Tall right precordial R waves and increased R/S amplitude ratios together with deep Q waves in leads 1, aVL, and V5-6 are characteristic of the classic rapidly progressive pseudohypertrophic X-linked dystrophy of Duchenne (figure 1).3-12 A reduction in or loss of electromotive forces caused by myocardial dystrophy in the posterobasal left ventricle (anterior shift of the QRS) and contiguous lateral wall (deep Q waves in leads I, aVL, and V5 6) is believed to be the mechanism responsible for the characteristic ECG.7-9, II Studies at necropsy showed that the above regions are the initial and most extensive sites of myocardial fibrosis,8 10. 11 which is preceded by ultrastructural (subcellular) abnormalities." The primary posterobasal abnormality spreads to the epicardial third of the contiguous lateral left ventricular (LV) free wall with progressive transmural fibrous replacement.", 1 ' There is relative sparing of the ventricular septum and comparatively minimal involvement of right ventricular and atrial

To assess myocardial glucose metabolism and perfusion in 142 myocardial segments with defects seen at thallium-201 single photon emission computed tomography (SPECT), 27 studies with positron emission tomography (PET) utilizing nitrogen-13 ammonia and fluorine-18 deoxyglucose were performed in 26 patients. Myocardial infarction was defined on the basis of concordant reductions in segmental perfusion and glucose utilization; myocardial ischemia, on the basis of preservation of glucose utilization (metabolic viability) in segments with hypoperfusion at rest. Of the 142 segments analyzed, 101 had fixed defects, 31 had partially reversible defects, and ten had completely reversible defects. Preserved glucose utilization was identified in 47 (46.5%) of the segments with fixed defects and 20 (64.5%) of the segments with partially reversible defects. Of the ten segments with completely reversible defects, five (50%) were normal, and five (50%) exhibited ischemia at PET. Visual improvement in a persistent thallium defect at delayed imaging was not associated with residual glucose metabolic activity. Thus, PET can be used to detect glucose metabolic activity in a significant proportion of myocardial segments with fixed or partially redistributing defects seen at thallium SPECT, which suggests that the extent of tissue viability in patients with ischemic heart disease is underestimated at thallium scintigraphy.