Positron emission tomography (PET) with
rubidium-82 (82Rb) has been developed to measure regional myocardial perfusion and to detect transient
ischemia both in the experimental laboratory and in humans. There are known and separate contaminating effects of the 82Rb signal by disturbances in wall motion, wall thinning, and the partial volume effect that occur during transient
ischemia. In nine anesthetized greyhounds, PET with 82Rb (T1/2 = 78 sec) was used to determine the regional myocardial uptake of this
cation during a control period that consisted of a mild
stenosis of the left anterior descending coronary artery in the absence of
ischemia (to limit
reactive hyperemia), during 10 min of total occlusion and, finally, at 30 and 60 min of recovery with release of the occlusion but not of the
stenosis. Separately,
rubidium-81 (81Rb); T1/2 = 4.58 hr) was given as a peripheral
intravenous injection 2 hr before the study to allow this long-lived tracer to distribute in the
potassium space of the myocardium. Observations during control and
ischemia revealed marked decreases in 82Rb uptake (0.84 +/- 0.12 to 0.28 +/- 0.12, p = 0.001) in affected regions and were paralleled by similar decreases in
microsphere blood flow (0.88 +/- 0.08 to 0.12 +/- 0.10 ml/min/g, p = 0.003), which gradually recovered by 60 min postischemia. Lesser decreases in 81Rb activity (0.84 +/- 0.11 to 0.76 +/- 0.17, p = 0.83) were observed in the same regions during
ischemia, but these were immediately reversible. Separate in vitro postmortem experiments in eight rabbits confirmed a linear relationship between plasma and myocardial activities of stable
potassium and 81Rb although there was a greater concentration of 81Rb in the myocardium that in the plasma relative to
potassium (y = -3.29 +/- 0.79 x, s.e.e. 1.91, r = 0.95). These studies demonstrate that if 81Rb is given intravenously to distribute into the
potassium pool, tomograms of the heart may be recorded to measure the
potassium-rich mass of myocardium providing information about the acute effects of wall thinning during
ischemia.
Rubidium-81 used in this way may be helpful in assessing the effects of wall thinning and/or
scar when other tracers are being used to assess perfusion or metabolism.