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BACKGROUND Both [13N]ammonia and [15O]water have been used to quantify myocardial blood flow with positron emission tomography using appropriate tracer kinetic models. A direct comparison of the two tracers with radioactive microspheres has not been performed in the same experimental preparation. METHODS AND RESULTS The two tracers have been tested for myocardial blood flow quantification in closed-chest dogs with circumflex coronary stenosis or permanent occlusion at rest and during adenosine-induced hyperemia. [13N]ammonia- and [15O]water-derived myocardial blood flow values have been compared with radiolabeled microspheres. Validation studies consisted of simultaneous measurements of blood flow with positron emission tomography and microspheres over a wide range of flow values. Blood pool and regional tissue activity curves were fitted with a three-compartment model for [13N]ammonia with and without arterial metabolite correction and with a single-tissue-compartment model for [15O]water. A correction for finite-resolution effect before the fit was also applied. In large regions of interest (5 cm3), a good correlation between the microsphere method and [13N]ammonia (with metabolite correction) was obtained (y = 3 + 0.78x, r = 0.94). The correlation with microspheres was slightly better with [15O]water (y = -3 + 0.89x, r = 0.97). Similar correlations were achieved in smaller regions of interest (1 cm3) as well as in akinetic segments and in central infarct regions. CONCLUSIONS Positron emission tomography with appropriate tracer kinetic models using [13N]ammonia and [15O]water provides an accurate quantitative method for measuring regional myocardial blood flow over a wide range of flow values in normally contracting or akinetic canine myocardium in the absence and in the presence of infarction.

Portopulmonary hypertension (POPH), a complication of chronic liver disease, may be a contraindication to liver transplantation (LT) because of the elevated risk of peritransplant and posttransplant morbidity and mortality. Because POPH is frequently asymptomatic, screening with echocardiography is recommended. The only reliable technique, however, for diagnosing POPH is right heart catheterization (RHC). The aims of this study were to evaluate the current estimated systolic pulmonary artery pressure (sPAP) cutoff value of 30 mm Hg and to determine a better cutoff value. One hundred fifty-two patients underwent pretransplant echocardiography between January 2005 and December 2010. These echocardiographic results were compared with pulmonary artery pressures measured during the pretransplant workup or at the beginning of the transplantation procedure (both by catheterization). With a cutoff value of 30 mm Hg, 74 of the 152 patients met the criteria for POPH on echocardiography, although the diagnosis was confirmed in only 7 patients during catheterization; this resulted in a specificity of 54%. It would have been more accurate to use a cutoff value of 38 mm Hg, which had a maximal specificity of 82% and, at the same time, guaranteed a sensitivity and negative predictive value of 100%. With the incorporation of the presence or absence of right ventricular dilatation, the specificity even increased to 93% for this new cutoff value. In conclusion, the prevalence of POPH was 4.6% among LT candidates in this study. We can recommend that LT candidates with an sPAP > 38 mm Hg should be referred for RHC. With the cutoff value increased from 30 to 38 mm Hg, the number of patients undergoing invasive RHC during their evaluation could be safely reduced. Liver Transpl 19:602-610, 2013. V C 2013 AASLD.Received November 11, 2012; accepted February 27, 2013. See Editorial on Page 573Portopulmonary hypertension (POPH), the presence of pulmonary hypertension in association with portal hypertension, is a known complication of chronic liver disease.1-9 Prospective studies and case-control studies have documented that POPH occurs in approximately 5% to 6% of patients with advanced liver disease. 5 In patients with portal hypertension, the association with pulmonary hypertension is seen in 2% to 6%.10,11 The incidence of POPH in patients referred for liver transplantation (LT) is 4% to 6%. 12Abbreviations: CO, cardiac output; LT, liver transplantation; mPAP, mean pulmonary artery pressure; mRAP, mean right atrial pressure; NA, not applicable; ND, not determined; NS, not significant; PCWP, pulmonary capillary wedge pressure; POPH, portopulmonary hypertension; PVR, pulmonary vascular resistance; RHC, right heart catheterization; RVEDD, right ventricular end-diastolic diameter; sPAP, systolic pulmonary artery pressure; SVR, systemic vascular resistance; TPG, transpulmonary gradient.