Accuracy of Blood Volume Determination via Central Venous Catheters William M. Nadel, MD, Phoenix, Arizona, Edwln C. Sneider, BS, NM (ASCP), Phoenix, Arizona James D. Colt, MD, FACS, Phoenix, Arizona
Blood volume determinations using radioiodinated (iodine 131) serum albumin (RISA) are frequently performed via central venous catheters. This technic has the advantage of potentially greater accuracy due to less venous pooling in the extremity. This is especially true with repeated or serial determinations when the blood volume determination is most valuable. Use of an indwelling central venous catheter also affords the advantage of ease of administration and sampling and prevents paravenous injection. The question of accuracy using a central venous catheter for injection and sampling has been raised before. Ladegaard-Pedersen and Engell [I] calculated the volume of a steel basin using RISA injected through a catheter and injected directly into the basin and found no significant difference between the two calculations. Hoye [2] measured directly the isotope absorbed by 1 foot of plastic tubing and calculated that 3 per cent of the isotope was lost. None, however, measured directly the amount of isotope absorbed by tubing used for intravenous use. This study measures the isotope absorbed by some commonly used central venous catheters and the errors thereby introduced. We are the first to directly measure isotope absorption by catheters rather than indirectly calculate absorption. Method
A glass flask was used as the container for the in vitro experiment. The background count was determined and found to be low each time. Between each determination the flask was washed thoroughly. The volume of fluid in the flask was first determined by gravimetric technic. It was found to be consistent. The top of the flask was open so that only barometric pressure could have altered the volume and this amount is negligible. The temperature in the Fran the Veterans A&ninisbation Hospiil. 7th %r& 8 Indian schoal Road, Phoenix, Arizona. Reprint re-qtests should be addressed to James D. Colt, MD, Chief, Vascular Surgery, Veterans Administration Hospital, 7th Street & lndii School Road, Phoenix, Arizona 85012.
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laboratory varied less than 5°C. Normal saline was used as the fluid. The volume of fluid used totally in each test was within the volumes of the average blood volume in humans, 3,800 to 5,800 ml. A measured volume of normal saline was placed into the flask and the catheter to be tested was inserted into the fluid to the depth it would be inserted into the tissues of a patient. A magnetic stirrer (also checked for background count) was used to constantly agitate the fluid. Commercially prepared normal saline was then allowed to flow into the flask at a rate of 100 ml/hour. The RISA was then injected through a stopcock (checked for background and absorption) while the normal saline was flowing-as is done in vivo. After 15 minutes a sample was withdrawn from the stopcock after 5 ml had been withdrawn to clear the catheter. The usual method of counting, using a Nuclear of Chicago Well Scintillation Counter, was followed. Calculations were performed exactly as is prescribed for in vivo tests. The catheter and the stopcock were counted separately and the amount of radioactive material absorbed in the collector was calculated as was the equivalent volume of fluid that the error represented. The catheters used were a 14 gauge, 12 inch long Abbott catheter made of polyvinylchloride, a 14 gauge, 12 inch Deserette catheter, and a 14 gauge, 24 inch Deserette polyurethene catheter. The procedure was repeated twenty times for each type of catheter. The volume determination was performed using the same procedures as for blood volume determinations in vivo in our hospital. The same personnel, materials, and methods were employed. Results
The root mean square error in the determination using the 1.2inch Deserette catheter was 85.43 ml or 2.1 per cent. The root mean square error introduced by the RISA absorption by the catheter was 82.97 ml or 0.12 per cent. The root mean square error in the determination using the 24 inch Deserette catheter was 82.97 ml or 2.0 per cent. The root mean square error introduced by the RISA absorption by the catheter was 0.21 per cent. The root mean square error in the determination using the 12 inch Abbott
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Nadel, Sneider, and Colt
catheter was 69.42 ml or 1.7 per cent. The root mean square error introduced by the RISA absorption by the catheter was 0.11 per cent. The 24 inch Deserette catheter absorbed more RISA than did the 12 inch Deserette catheter (p < 0.01). There was also a reliable difference between catheters of the same length manufactured by different companies. The 12 inch Deserette catheter absorbed more RISA than did the 12 inch Abbott catheter (p < 0.01). The coefficient of correlation between the amount injected and the amount absorbed differed between catheters manufactured by the two companies. The 12 inch Abbott catheter demonstrated a coefficient of correlation of gamma = 0.66, indicative of variance of absorption from sample to sample, as compared with gamma = 0.92 (24 inch), and gamma = 0.93 (12 inch) for the two lengths of Deserette catheters. These coefficients of correlation differed reliably between manufacturers (p < 0.01). Comments
All of the results obtained were well within the accepted accuracy of RISA blood volume determinations of plus or minus 5 per cent. The errors introduced by isotope absorption by the catheter averaged 0.15 per cent. Although there was a statistically significant difference between the amount of RISA absorbed by the different lengths and cathe-
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ters, it was not of clinical significance. Although Abbott catheters absorbed the least amount of RISA, the Deserette catheters were more reliable in the variance of amount absorbed from sample to sample of the catheter. Conclusion
An attempt was made to reproduce as closely as possible in the laboratory the clinical technic of blood volume determination in an effort to control as many variables as possible. We find that in directly counting the radioactivity absorbed by commercially available central venous catheters, that amount of absorption is not statistically significant and is not of any clinical significance. We conclude that the use of the central venous catheter as a port for blood volume determination is safe, accurate, and better than peripheral vein use. Acknowledgment: Our results were subjected statistical analysis by Doctor Robert Haygood Arizona State University.
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References 1. Ladegaardhdersen HJ, Engell Hc: Blocd volume determination using one catheter located in a central vein. Acfa Chir Stand 135: 105, 1969. 2. Hoye RC: Simultaneous measurement of red cell, plasma, and extracellular fluid volume in the surgical patient. J Lab C/in M&69: 663,1967.
The Am&can Journal 04 Surgery