Myocardial perfusion measured by dual-isotope acquisition of⁸¹rubidium/^81mkrypton: an experimental verification of the method

Summary

Purpose

The purpose of the study was to verify that the⁸¹Rubidium/^81mkrypton (⁸¹Rb/^81mKr) method allows the quantitative measurement of myocardial perfusion.

Principle

The potassium analogous cation⁸¹Rb is accumulated in the myocardium by the cell membrane-bound Na−K-ATPase. After accumulation an intracellular equilibrium establishes between⁸¹Rb 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 nuclide⁸¹Rb 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 pure⁸¹Rb by bolus injection into the aortic cannula. Spectroscopic measurements of⁸¹Rb 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 ratio⁸¹Rb/^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 between⁸¹Rb 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 ratio⁸¹Rb/^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.