APPENDIX. working blood volume was also rather large; Evans, Grande, and. equilibrated to the new mixture is partially dependent upon the rate

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1 APPENDIX A SIMPLIFIED HEART OXYGENATOR CIRCUIT FOR BLOOD- FED HEARTS. By J. YULE BOG-UE and R. A. GREGORY.' SINCE 1934 studies on the carbohydrate metabolism of the blood-fed heart without lungs have been carried out in this laboratory, using the heart oxygenator preparation of Evans, Grande, and Hsu [1934]. The advantages and uses of this preparation have been clearly shown by those authors. The apparatus, though perfectly satisfactory for many types of experiments, is complicated, and requires a trained assistant to maintain it. We found that it was not possible to do more than one experiment every other day, since a whole day was required to dismantle, clean, sterilise, dry, and erect the apparatus. Further, two thermostatically controlled water-baths were necessary. The working blood volume was also rather large; Evans, Grande, and Hsu recommend that 1 litre of blood should be left in the circuit. "Bleeders" were required to give a minimum of 800 c.c. of blood in addition to that drawn off from the experimental animal at the changeover in the heart-lung stage. Blood volumes of this order as compared with the heart weight (circa 100 g.) have the additional disadvantage that chemical changes in the blood as the result of cardiac metabolism are relatively small for shorter periods of time, especially when the work done by the heart is light. Because of this, a slight error in the chemical estimations will at times, upon calculation, appear significant. Reduction of the dead space in the circuit would greatly diminish these effects. Finally, the oxygenator is supplied by the right ventricle, so that the flow through it is dependent upon the output of the latter and therefore upon the condition of the heart. While satisfactory in normal conditions, this is not the case when the output of the right ventricle is small. With small outputs the surface of the oxygenator is incompletely covered, the blood running down in streaks which results in poor oxygenation. When the gas mixtures in the oxygenator are changed, the time taken for the total blood in the circuit to become equilibrated to the new mixture is partially dependent upon the rate of flow through the oxygenator, and therefore on the right ventricular output, which may have been reduced as the result of the experimental Sharpey scholar. 105

2 u Bogue and Gregory conditions. This is particularly noticeable when a change is made from an asphyxial period with N2/CO2 to an O2/CO2 mixture. Our modified circuit retains the advantages of the Evans, Grande, and Hsu constant level pump circuit, and at the same time overcomes the disadvantages mentioned above. Fig. 1 shows the apparatus as set out in our circuit. It consists essentially of two reservoirs, VR which supplies the heart, and SR, a stock reservoir joined together by means of a wide bore rubber tube F FIG. 1.-Diagram of circuit (not to scale). Dimensions given at end of paper. OT. Defibrinated blood which has been filtered once through coarse muslin and tightly packed glass wool, and then four times through fine linen, is poured into the reservoir SR. Twro Dale-Schuster pumps suffice to operate the two blood pumps P, placed in parallel (only one pump is shown in the diagram). The pumps are started, the blood pumped from the reservoir SR is passed through the fine muslin filter F and led on to the spinning-plate of the Drinker pattern oxygenator DO, and from there into the reservoir VR by means of the wide bore, curved glass tube, C. In order to avoid frothing it is essential that the blood should flow in an unbroken stream down the lower side of this tube. This is assured by cutting the upper end of the tube diagonally and so fixing it in the oxygenator outlet that the lower part of the diagonal cut is flush with the upper surface of the rubber bung, and by having the tube of very wide bore. The entry of this tube to the reservoir VR should be about 1 cm. below the level of the overflow tube OT. Blood is added to SR until VR is full and an excess over-

3 A Simplified Heart Oxygenator Circuit for Blood-fed Hearts 107 flows via OT into SR. In order to avoid frothing the tube OT must also be of wide bore so that the blood flows only down its lower side. When large volumes of blood are used in the circuit, OT can be arranged to enter SR at a higher level than that shown in the diagram; this ensures thorough mixing of the blood. An additional 100 c.c. of blood over that required to fill VR to overflowing with the pumps operating is added. The capacity of the pumps, reservoirs, oxygenator, tubes, and cannule when working is 350 c.c., as compared with 650 c.c. in the circuit of Evans, Grande, and Hsu. The minimal working volume including sufficient blood to allow for 25 c.c. samples at halfhourly intervals over a period of two hours is about 450 c.c. The reservoir SR can be filled to a maximum of 1350 c.c. Air bubbles must be removed from the valves and tubes and from the filter F, by opening the spring clip at its top. The output of the pumps is adjusted so that it exceeds that which can be delivered by the right ventricle; we usually set it at about 1 litre/min. Further adjustment is unnecessary. Under these conditions a constant and adequate rate of flow through the oxygenator is maintained, and the blood-level in VR is kept constant by the overflow of the excess to SR. The two reservoirs and the blood pumps are immersed into a thermostatically controlled water-bath situated at the end of the operating table. The oxygenator and the tubes leading to it are enclosed in a coat of thick felt. The two sides of the heart are supplied with blood from the reservoir VR via the cannulae 1 and 2 which are inserted into the superior vena cava and the left auricular appendix respectively. The rate of inflow can be independently controlled by the regulators R and L. After the apparatus has been filled with blood as described above, the stages by which the preparation is set up are the same as those described by Evans, Grande, and Hsu. The left pulmonary artery is isolated and ligatured and the left lung removed. The usual heartlung preparation is then made. The variable pneumatic resistance and air cushion AR is set to give a left ventricular pressure of about 50 mm. Hg. As in all heart oxygenator preparations, all the blood in the circuit and any additional blood which may be used in subsequent stages of the experiment must be allowed to circulate through the heart-lung preparation. This required about 30 minutes in the Evans, Grande, and Hsu circuit. We find, however, that this time can be halved if the output from the left ventricle is passed temporarily to the reservoir SR-this prevents any short-circuiting and ensures thorough mixing of the blood. During this time the thorax is widely opened by cutting the ribs. Cannula No. 2 is inserted into the left auricular appendix, great care being taken to remove all air. The inflow into the left auricle is started and adjusted by the regulator L. This is checked

4 108 Bogue and Gregory by measuring the left ventricular output at the graduated vessel, AO. The left ventricular output is now returned to VR, and the right inflow R is clamped. The left pulmonary artery is then cannulated and the right lung tied off at the root and removed. The right inflow is restarted, the heart removed and placed in a warmed dish. The preparation is now complete. The output of the right ventricle which includes that from the cannula 1 plus that returning from the cardiac veins is returned via cannula 3 to the reservoir SR to be pumped up to the oxygenator. The rate of flow can be measured by the graduated vessel PO. The coronary flow is measured by clamping off R and taking the right ventricular output at PO. Samples of coronary blood for analysis can be drawn off at S2, and compared with arterial samples taken simultaneously at S, or at AO. Ventricular pressures are recorded by means of mercury manometers connected to the side tubes shown in cannulse 3 and 4, and the auricular pressure by water manometers, connected to the inferior vena cava on the right side and to a side tube in cannula 2 on the left side. A thermometer is inserted at T in the superior vena caval cannula. If necessary it is possible to make the preparation using the same dog as bleeder and experimental animal by adopting the following modifications:-about 400 c.c. of blood is drawn off from a dog of not less than 15 and not more than 20 kg. The blood is defibrinated and filtered as before, but instead of being placed in the reservoir SR is poured into the reservoir VR up to the level of the overflow (110 c.c.). The operative procedure is as above, except that during the changeover when making the heart-lung preparation, the reservoir VR is topped up with the remaining blood as the level falls. The blood collected from the animal, together with that used for washing out the heart, is defibrinated and filtered and then added to the stock reservoir, and the pumps started. After allowing the blood to circulate through the heart-lung preparation the procedure is the same as above. Dimensions of components: Oxygenator DO, 30 x 25 cm.; outlet tube C, 1-6 cm. diameter. Reservoir VR.-Overall length, 43 cm.; overflow level, 33 cm. Diameter of wide part, 3-5 cm.; constricted part, 1V5 x 26 cm. Overflow tube OT, internal diameter, 2-2 cm. Stock Reservoir SR.-Overall length, 30 cm.; greater diameter, 7-2 cm.; constricted part 4 x 11 cm. Other tubes-usually 1 cm. diameter. Warm bath-28 x 45 x 29 cm. The lettering in the diagram corresponds with that on p. 59 of Evans' Recent Advances in Physiology, Newton, Churchill, London, 1936.

5 A Simplified Heart Oxygenator Circuit for Blood-fed Hearts 109 SUMMARY. A simplified form of the Evans, Grande, and Hsu constant level pump heart oxygenator preparation is described in which the dead space has been considerably reduced. The blood-flow through the oxygenator is kept constant by means of a pump, and is not dependent upon the cardiac output. It is possible to make the preparation using the same dog as bleeder and experimental animal. We wish to express our thanks to Professor C. Lovatt Evans for his kindness and help and the provision of every facility in this work. We are indebted to the Government Grants Committee of the Royal Society for a grant to one of us (J. Y. B.), and to the Thomas Smythe Hughes Fund for a grant to R. A. G., from which grants the cost of animals was met. REFERENCE. EVANS, C. L., 24, 283. GRANDE, F., and Hsu, F. Y. (1934). Quart. J. exp. Physiol.