Measurement of cardiac output with and organ trapping of radioactive microspheres

Cardiac output in dogs and lambs was measured with radioactive microspheres. The radioactivity injected [I(i)] into left atrium and in an arterial reference sample [I(ar)] drawn at constant rate [Q(ar)] during injection was counted; cardiac output (CO) = Q(ar)[I(i)/I(ar)]. On comparison with known flows this method was accurate and precise (r = 0.94, s(y,x) = 0.11 liter/min). Comparing nonsimultaneous indocyanine green and microsphere cardiac outputs gave more variability (r = 0.90, s(y,x) = 0.41 liter/min). A method of determining organ extraction of microspheres after left atrial injection from reference samples from any artery and a representative vein draining the organ being studied is also presented. The percentages of radioactivity of microspheres 4-12 μ diameter not trapped by heart, kidney, and systemic circulation of lambs were 3.6 ± 2.9%, 2.8 ± 1.9%, and 5.5 ± 2.2% (mean and standard deviation). Under 0.1% of the radioactivity of 15 μ diameter spheres and 4-26% of the radioactivity of 4-12 μ diameter spheres were not trapped by dog kidneys. These results are based on radioactivity measurements and do not account for variations in sizes of trapped and nontrapped spheres. Diameters of nontrapped spheres compared to injected spheres suggest that nontrapping in the lamb was due to spheres under 7 μ in diameter.