OBTAINING A REPRESENTATIVE BLOOD-SAMPLE IN LACTATE TRACER STUDIES

Reasons why venous tracer infusion with arterial sampling [(v-a) mode] has advantages compared to arterial infusion and venous sampling [(a-v) mode] for studies of blood lactate kinetics are presented. Arterial tracer infusion can result in biased tracer input due to streaming and unequal blood flow distribution. The procedure is impractical for human studies. Venous sampling from the jugular, or any other peripheral or great vein, provides a sample wich may, or may not represent mixed venous systemic blood, which exists oly in the pulmonary artery. Venous sampling will not represent cardiac lactate matabolism because the coronary arteries drain into the coronary sinus. Venous sampling, as well as pulmonary artery sampling, will also ignore lactate metabolism in the lungs which drain into the left atrium from bronchial and pulmonary circulations. Turnover rates calculated from either venous or arterial specific activities underestimate true tissue turnover. Correction for either measurement depends on good estimates of blood flows to lactate exchanging and non-exchanging tissue. Equilibration between lactate and pyruvate pools does not invalidate the use of tracers to measure lactate turnover. The (v-a) mode with venous infusion and arterial sampling has advantages for lactate tracer studies.