INFLUENCE OF RECIRCULATION ON MYOCARDIAL CLEARANCE CURVES WITH XENON 133

Abstract. Myocardial clearance curves with inert radioactive gas as indicator regularly deviate from monoexponential curves. When Xenon is used as an indicator and injected into the left heart the major part is removed from the circulation by effective alveolar ventilation, while a minor fraction re‐enters the left heart and recirculates through all vascular beds including the coronary vascular system. The effect of this recirculation on the shape of the myocardial clearance curve has been investigated under conditions when deviation is especially pronounced, i.e. pacemaker‐induced tachycardia. Tachycardia causes increased coronary flow with steeper clearance curve, while the ventilation and hence the amount of recirculating Xenon is unchanged. The resulting marked deviation is significantly diminished by voluntary hyperventilation, which decreases the recirculation. This explains the decrease of deviation during physical exercise when both coronary flow and effective alveolar ventilation increase. The error imposed by recirculation on the calculation of coronary flow is on average less than 5% during pacemaker‐induced tachycardia. 1971 Association for the Publication of the Journal of Internal Medicine