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

Funding Acknowledgements Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): Swedish Society of Pulmonary Hypertension Background In adult patients with pulmonary arterial hypertension (PAH), right ventricular (RV) failure may worsen rapidly, constituting a negative prognostic factor.  In this population, non-invasive assessment of RV function is challenging. RV stroke work index (RVSWI) reflects right ventricular function and has been proposed to predict outcome in PAH. However, RVSWI assessed by echocardiography (ECHO) has not been thoroughly compared to measures with right heart catheterization (RHC) in adults. The aim of the present study was to therefore evaluate RVSWI derived by echocardiography (RVSWIECHO) vs. RHC (RVSWIRHC). Methods  Fifty-four consecutive treatment naïve adult patients with PAH, were retrospectively analysed. All patients performed echocardiography and RHC with a median time of 1 day [IQR 0-1 days]. RVSWIRHC was calculated as: (mean pulmonary arterial pressure (mPAP) – mean right atrial pressure (mRAP)) x stroke volume index (SVI)RHC. Four methods for RVSWIECHO were evaluated: RVSWIECHO-1 = Tricuspid regurgitant maximum pressure gradient (TRmaxPG) x SVIECHO, RVSWIECHO-2=(TRmaxPG-mRAPECHO) x SVIECHO, RVSWIECHO-3 = TR mean gradient (TRmPG) x SVIECHO and RVSWIECHO-4=(TRmPG–mRAPECHO) x SVIECHO. Vena cava inferior diameter and its collapsibility were used for estimation of mRAPECHO.. Pearson’s correlation coefficients were used and data was expressed as mean ± standard deviation. Results Mean RVSWIRHC was 1132 ± 352 mmHg*mL*m-2.  Mean RVSWIECHO-1-4 was: 1904 ± 568, 1732 ± 531, 1090 ± 366 and 918 ± 336 mmHg*mL*m-2, respectively. There was no significant difference between RVSWIRHC and RVSWIECHO-3 in mean values, although they exhibited the lowest correlation, but moderate (r = 0.66). The strongest correlation was demonstrated for  RVSWIECHO-2 (r = 0.78, p < 0.001), followed by a moderate correlation for  RVSWIECHO-1 and RVSWIECHO-4 (r = 0.75 and r = 0.69, p < 0.001). The absolute (relative) bias for RVSWIECHO-1 was  -772 ± 385 (-50 ± 20%) mmHg*mL*m-2, RVSWIECHO-2 -600 ± 339 (-41 ± 20%) mmHg*mL*m-2, RVSWIECHO-3 42 ± 286 (5 ± 25%) mmHg*mL*m-2 and for RVSWIECHO-4 214 ± 273 (23 ± 27%) mmHg*mL*m-2. Conclusion RVSWIECHO-3, using the mean tricuspid gradient and SVIECHO, showed no significant difference to RVSWIRHC, albeit a moderate correlation between the methods. RVSWIECHO-1, RVSWIECHO- 2 and RVSWIECHO-4 exhibited moderate to strong correlations to RVSWIRHC, but poor concordance between absolute values. The clinical utility of RVSWIECHO and RVSWIRHC in assessing RV function in relation to PAH prognosis, risk stratification and treatment response remains to be evaluated in a larger clinical context.