Arterio-venous concentration difference of [51Cr]EDTA after a single injection in man. Significance of renal function and local blood flow

Summary. The present investigation was undertaken in order to study (1) the difference in arterial (C_a) and venous (C_v) concentration of [⁵¹Cr]EDTA (ethylenediaminetetra-acetate) after a single intravenous injection, (2) the impact of different physiological variables on this difference, and (3) the error introduced in the measurement of renal plasma clearance and total plasma clearance by using venous blood samples instead of arterial. In 13 patients with GFR ranging from 29 to 150 ml min^-1, C_a was higher than C_v immediately after the injection. After mean 38 min (range 12–82 min) the two curves crossed, and 180–300 min post-injection (p.i.) C_v was 5·9% higher than C_a (range 0·5–13·9%, P<0·001). The more reduced renal function, the smaller was the concentration difference. The areas under the arterial and the venous plasma concentration curves did not differ significantly at either 0–∞ or 0–300 min p.i., whereas the venous area 0–100 min p.i. underestimated the arterial area in the same period by 4·1% (P<0·05). In a computer simulation model, variation in the forearm capillary permeability–surface area product did not have any significant influence on the C_v–C_a difference, whereas the difference was very sensitive to even small changes in forearm blood flow within the physiological range. For measurement of renal plasma clearance it is recommended to use one long period: from the time of injection until 300 min p.i. or longer. If the clearance period is too short, the use of venous samples will overestimate the true renal clearance. Plasma clearance determined by venous and arterial blood samples does not differ significantly as long as the concentration is followed from the time of injection and a long period is applied. When simplified plasma clearance techniques are used, different results may be obtained from venous and arterial samples. The simplified techniques using venous blood samples—which usually include some empirical corrections—should be sufficiently reliable in daily clinical practice provided the forearm blood flow is reasonably high, e.g. exposure to cold should be avoided.