USE OF O-15 TO MEASURE OXYGEN-CARRYING CAPACITY OF BLOOD SUBSTITUTES IN-VIVO

A method for determining oxygen-carrying capacity of blood substitutes has been developed using the short-lived cyclotron-produced positron- emitting isotope O-15. This method measures the oxygen-carrying capaci ty of the blood substitutes in vivo in the presence of red blood cells and allows determination of changes in the oxygen-carrying capacity o ver time after exchange transfusion. This method is applied to the blo od substitutes of liposome-encapsulated hemoglobin (LEH) and cell-free hemoglobin (Hb). We have used O-15 (half-life of 2 min) to quantitate the lung uptake and tissue delivery of [O-15(2)]LEH. Lung uptake stud ies were performed in intu bated, catheterized rats after a 40% exchan ge transfusion of bovine LEH (LEBH; 0.68 g Hb/kg body wt), human hemol ysate LEH (LEHH; 1.0 g Hb/kg body wt), or free bovine hemoglobin (SFHS ; 0.56 g Hb/kg body wt). A bolus inhalation of O-15(2) (3-5 mCi) was g iven at 15 min, 3 h, and 24 h posttransfusion. Arterial blood samples were collected, spun, and separated into LEH, red blood cell, and plas ma fractions. O-15 activity and hemoglobin content were determined for each fraction. Oxygen-carrying capacity was calculated as a percentag e of the original red blood cell fraction removed. For LEBH, the carry ing capacity was 15% at 15 min, 13% at 3 h, and 1% at 24 h. For LEHH, the carrying capacity was 30% at 15 min, 26% at 3 h, and 19% at 24 h. The marked decrease in carrying capacity at 24 h for LEBH compared wit h LEHH was attributable to the increased formation of methemoglobin in the circulating LEBH rather than increased removal from circulation, because total hemoglobin concentrations measured for both LEH samples decreased at a similar rate during the 24 h. The presence of methemogl obin reductase and other naturally occurring antioxidants in the LEHH may be responsible for maintaining the higher levels of oxyhemoglobin. Oxygen-carrying capacity for SFHS also decreased over time but at a m uch sharper rate compared with both LEH formulations. The carrying cap acity for SFHS of 8% measured at 15 min decreased to 0.3% at 3 h and u ndetectable levels at 24 h. This sharper decrease in carrying capacity for SFHS is attributable to the rapid removal of the hemoglobin from circulation.