Myocardial perfusion measured by dual-isotope acquisition of 81rubidium/81mkrypton: an experimental verification of the method.

PURPOSE: The purpose of the study was to verify that the 81Rubidium/81mkrypton (81Rb/81mKr) method allows the quantitative measurement of myocardial perfusion. PRINCIPLE: The potassium analogous cation 81Rb is accumulated in the myocardium by the cell membrane-bound Na-K-ATPase. After accumulation an intracellular equilibrium establishes between 81Rb and its radioactive daughter nuclide 81mKr. The flow-sensitive method evaluates the disturbance of this equilibrium by perfusion which washes out the short-lived diffusable daughter nuclide 81mKr while the mother nuclide 81Rb remains cell-bound.
METHODS: Isolated rabbit hearts were prepared in a modified Langendorff technique which allowed quantitative collection of the coronary sinus efflux. The myocardium was labeled with pure 81Rb by bolus injection into the aortic cannula. Spectroscopic measurements of 81Rb and 81mKr gamma radiation were performed using a germanium detector. Perfusion was varied in the range between 0 and 4 ml/min/g. The activity ratio 81Rb/81mKr was determined and compared with coronary sinus effusion.
RESULTS: 81Rb/81mKr activity ratio was closely related (r = 0.98) to perfusion as described by the predicted equation Rb/Kr = (F/2.96 P) + 1. (Rb/Kr = activity ratio between 81Rb and 81mKr in the myocardium; F = myocardial perfusion (ml/min/g); P = partition coefficient (myocardium/perfusate) for Krypton gas). The constant P was found to be 1.14 +/- 0.06 (mean +/- SEM). No saturation of this ratio at high flow rates was observed.
CONCLUSION: The activity ratio 81Rb/81mKr is a valid quantitative measure for myocardial perfusion in isolated hearts. The main limitation of the method before application in man is the correction of the different gamma ray tissue absorption of both nuclides.