The proposed use of a 'screening test' to assess oxygenator performance

Clinical evaluation of nine hollow-fibre membrane oxygenators

In a comparative study we investigated the performance characteristics of nine hollow-fibre oxygenators. In a clinical setting, 10 units of each type of oxygenator were tested for oxygen exchange, transoxygenator pressure drop, heat exchanger performance and blood trauma. The oxygenators included are Maxima PRF Plus, Affinity, Forte, Affinity NT, Quantum, Optima, Capiox 1.8, Hilite and Quadrox. Ninety patients scheduled for elective coronary bypass surgery were enrolled in the study. Cardiotomy suction blood was not allowed to mix with the circulating blood. Samples were taken preoperatively (t1), immediately after cooling and crossclamping of the aorta (t2), during rewarming (t3), and postoperatively (t4). The results showed that under clinical conditions all of the tested oxygenators are well capable of meeting the normal and above-normal oxygen needs. The Quantum and Affinity NT are the best performing oxygenators with respect to oxygen transfer. The Quantum oxygenator with a small membrane surface area of 1.9 m2 has also the highest oxygen transfer per square metre. A direct relationship between shunt fraction and blood flow could not be established in all oxygenators. The lowest shunt fraction at 4 litres blood flow was seen in the Maxima, Quantum and Affinity NT groups (15%). At near physiological PaO2-values, we noticed a negative influence on the shunt fraction. Large differences exist in pressure drop between the oxygenators under standardized conditions with circulating priming, as well as during bypass. The lowest pressure drop was noticed in the Quadrox and Affinity NT, the highest in the Quantum, Capiox and Optima. However, we did not measure any differences in pressure drop after cooling and during re-warming. Differences exist between the heat exchanger performances. The Quadrox oxygenator is the most efficient, while the Maxima has the smallest performance factor. It was not possible to calculate heat exchanger performance for the Hilite due to incorrect temperature measurement. The high pressure gradients in some of the devices did not result in higher haemolysis. Plasma free haemoglobin values were low in each oxygenator. There are no differences in platelet drop postoperatively. The influence on blood trauma of the higher pressure drop in some of the tested devices, in combination with the higher centrifugal pump revolutions needed to overcome this gradient, has to be studied with longer perfusion times.

Use of the oxygen transfer slope and estimated membrane oxygen transfer to predict PaO2

The purpose of this investigation was threefold: (1) could the perfusionist accurately estimate oxygen transfer (VO2/minute) while on CPB; (2) could this estimate, and its position on the oxygen transfer slope (OTS), predict resultant PaO2 values within a specific range; and (3) could previously derived performance 'normals' be used during this study. Fifty-four sets of samples (both arterial and venous) from 27 oxygenators were used in this study. Each oxygenator provided one normothermic and one hypothermic set of samples. In 48 of the 54 samples (88.9%) the predicted VO2/minute was within +/- 10% of the actual VO2/minute. Thirty-nine of these 48 (81.25%) had resultant PaO2 values within our target range of 140 +/- 30 mmHg. The PaO2 for these 39 samples ranged from 110 to 168 mmHg with a mean of 133 mmHg. The percentage of predicted shunt (POPS) ranged from 59.0 to 192.4% with a mean of 109.3% (SD = 23.81%). With this degree of variability, we concluded that the perfusionist must assess VO2/minute as well as POPS in order to predict the resultant PaO2.