Phase equilibrium of oxygen in "nitrogen-filled vacutainers".

Nitrogen-filled “Vacutainers” (Becton-Dickinson, Rutherford, N. J. 07070) have been proposed for the anaerobic collection of specimens for blood gas determinations (1). We have indicated (2) that these devices, as currently supplied by the manufacturer, contain oxygen and may produce spuriously high P(02) and percentage oxyhemogobin (HbO) values. We had previously indicated preference for the use of common (air-filled) heparinized Vacutainers for percentage HbO determinations (3). Our preference was based on the conclusion of several authors (4-7) that data on blood gas for samples drawn in syringes and Vacutainers were comparable. The primary difference between these two blood drawing methods is contact of the blood sample with a gas phase in the Vacutainer. The fluids present in a Vacutainer will be subject to the fundamental gas laws; the behavior of the liquid and gas phases will be influenced by concentration gradients and solubiities. A liquid and a gas in a closed container tend to come to equilibrium. The P(02) of the gas and liquid phases in a nitrogen-filled (oxygen-free) Vacutainer tends to equalize, some oxygen going from the liquid to the gas phase. If the liquid phase is blood, this oxygen is derived from two sources, which are also in equilibrium: the physically dissolved and the hemoglobin-bound oxygen. The consequent transfer of oxygen can be estimated. To simplify this estimation we will consider two examples: (a) oxygen in physical solution in water, and (b) oxygen in both physical solution and hemoglobin-bound, as found in blood. In the former, changes in oxygen concentration will be reflected only in the P(02), while in the latter, both P(02) and percentage HbO will be affected. Example a: Consider the behavior of 4.5 ml of water drawn into a Vacutainer and equilibrated with a 1.1-ml bubble of nitrogen at 37 #{176}C. These volumes approximate those of commercially available nitrogen-filled Vacutainers. The amount of gas dissolved in a given volume of solvent is proportional to the pressure of that gas with which it is in equilibrium, at constant temperature (9). The absorption coefficient of oxygen in water at 37 #{176}C is 0.024 (10). Then 1 ml of water will dissolve v ml of oxygen at a P(02) of PTorr (1 Torr 133 Pa), such that