Safety of single-site adenosine thallium-201 scintigraphy

Fluctuation of adenosine concentration by modes of intravenous infusion based on mathematical simulation and experiments

OBJECTIVE

Adenosine, which has been used for a myocardial perfusion scan, shows rapid clearance from blood because of its short half-life of <10 seconds. This simulation study evaluates influences of modes of radionuclide injection on ventricular adenosine concentration when one intravenous injection line is used.

METHODS

Assuming that radionuclide injection is a unit impulse, time-activity curves were measured in the left ventricle (LV) and fitted by a gamma function. Typical patterns of concentration fluctuation when adenosine infusion was temporarily modified were calculated by the convolution integral of input function and unit impulse response. Variation of concentration was measured by experiments using continuous 99mTc injection and co-infusion of water via a three-way stopcock. Modes of co-infusion with various infusion speeds and volumes were examined.

RESULTS

Intermission of adenosine infusion and rapid displacement by radionuclide co-injection significantly influenced the adenosine concentration in LV. Intermission of adenosine infusion for 2 seconds caused a 15% decrease in the adenosine concentration in the left ventricle. When a square-shaped input was assumed, a three-fold higher concentration of adenosine for 3 seconds created by radionuclide injection resulted in a +42% increase in the LV concentration. Based on a measured input function, radionuclide injection using three-way stopcock through one route caused a two- to three-fold increase in the steady concentration in the vein just after injection. When 0.5 ml of radionuclide was slowly co-injected, with three ways opened, it caused a relatively low fluctuation, creating a +34% to -47% change in concentration of LV. A flush of radionuclide with physiological saline significantly increased the adenosine concentration in LV, when short half-lives were assumed.

CONCLUSION

An intravenous adenosine and radiopharmaceutical injection in the same line is feasible. However, the fluctuation of concentration depends significantly on the mode of injection. To minimize the fluctuation, a slow injection of a small volume of a myocardial imaging agent via a co-injection route, with three ways opened, is recommended.

Myocardial infarction related atrial fibrillation: role of endogenous adenosine

Exogenous administration of adenosine induces atrial fibrillation in up to 7.0% of patients. Animal studies affirm endogenous adenosine released in response to tissue hypoxia may play a mechanistic role in arrhythmias associated with myocardial ischaemia or hypoxia. Therefore, atrial fibrillation occurring early after the acute phase of myocardial infarction involving atrial tissue may be secondary to an excessive accumulation of adenosine that leads to a shortening of atrial refractory period. Early in the course of acute inferior myocardial infarction, two patients (males aged 45 and 68) suffered new onset sustained atrial fibrillation that was abrupt in onset and complicated their clinical management. They were administered 250 mg theophylline as a slow intravenous injection at a rate of 100 mg/min or until conversion to normal sinus rhythm occurred. Both patients converted to normal sinus rhythm within five minutes of the administration of theophylline. In up to 52 hours of continuous ECG monitoring after the theophylline administration the atrial fibrillation did not recur. Neither patient experienced any adverse outcome from theophylline administration. These observations are the first reported in humans or laboratory animals to suggest that atrial fibrillation, presumably due to elevated interstitial atrial concentration of adenosine caused by myocardial ischaemia, can be terminated with an adenosine receptor antagonist. However, the hypothesis that excessive accumulation of endogenous adenosine in atrial tissue may induce atrial fibrillation is well substantiated by other investigators. Thus, A1 adenosine receptor antagonists may prove to be valuable in the management of ischaemia related atrial fibrillation.