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Factors Influencing Serial Measurements of Cardiac Volumes by Count-Based Methods: Effects of Elevated Catecholamines, Position, and Exercise on Technetium-99m-Blood Radioactivity Concentration

Division of Cardiology, Department of Medicine, Division of Nuclear Medicine, Department of Radiology, Department of Psychiatry and Behavioral Sciences, and Geriatric Research, Education, and Clinical Center, Seattle Veterans Affairs Medical Center, and the University of Washington, Seattle, Washington

Correspondence: For reprints contact: Wayne C. Levy, MD, Division of Cardiology (111C), Seattle VA Medical Center, 1660 S. Columbian Way, Seattle, WA 98108.

ABSTRACT

Most radionuclide methods for measuring cardiac volume require a determination of the blood radioactivity concentration. Thus, changes in blood radioactivity over time or during interventions might lead to spurious volume estimates unless blood radioactivity is serially measured. The effects of elevated epinephrine, posture and exercise on ^99mTc-labeled blood radioactivity concentration were studied in 15 young (mean age= 28 yr) and 14 older (mean age = 68 yr) healthy males. An epinephrine infusion of 50 ng/kg/min resulted in a 4.1% ± 1.0% increase in ^99mTc-blood radioactivity (p 0.001) compared to baseline. Sitting increased blood radioactivity concentration by 12.3% ± 3.0% (p < 0.0002) compared to the supine position and peak supine bicycle exercise caused an 11.0% ± 1.7% increase(p 0.0001) compared to supine rest. There was a significantly greater increase during peak supine exercise in the young compared to the older subjects (15.0% ± 2.3% versus 6.3% ± 2.0%, p 0.01). The mechanism of the increase in blood radioactivity concentration is uncertain, but presumably reflects the addition of hemoconcentrated red blood cells from the spleen and/or the loss of plasma volume. Failure to correct for the increased blood radioactivity concentration during exercise or pharmacological interventions will result in a significant error in serial measurements of cardiac volumes by methods requiring RBC radioactivity measurements.