Whole body potassium and total exchangeable potassium in elderly patients with cardiac-failure.

Body potassium status of patients with cardiac failure may be estimated by a number of methods, but increasing reliance is being placed upon radioisotope dilution with 42K which measures the total exchangeable potassium. Total exchangeable potassium comprises between 86 per cent and 97 per cent of whole body potassium in healthy subjects. We have measured total exchangeable potassium in 22 oedema-free elderly patients with stable cardiac failure and compared the results with simultaneously determined measurements of whole body potassium obtained by whole body counting. The mean whole body potassium was 2360 +/- 640 mmol. The mean value of total exchangeable potassium measured at 24 hours was 1820 +/- 610 mmol (77% of whole body potassium) and increased further to 2000 +/- 600 mmol (84%) when measured after 48 hours. In patients with cardiac failure and, perhaps, also other patients with a history of fluid retention, the mixing of a tracer dose may be significantly delayed, which if not appreciated may lead to an overestimate of potassium depletion.