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The aim of our study was to develop and validate a simple surgical model in the sheep which allows control of the gas composition of the blood supplying the carotid and central chemosensitive area independently of the rest of the body. This approach was made possible due to the specific features of the cranial circulation in the sheep. An extracorporeal circuit, consisting of a pump and a gas exchanger, was placed at the level of the two common carotid arteries to create a pressure gradient between the carotid and the systemic systems and to reverse blood flow in the vertebral vessels via the occipital arteries. When a pressure gradient of about 40 Torr was created between the systemic and carotid circulation, we found that no blood could reach the carotid bodies and the medulla without passing though the extracorporeal circulation. This was established (1) by measuring vertebral blood flow; and (2) by injecting either a coloured suspension or particles labelled with 99m*Tc into the systemic or the carotid circulation. The slope of the relationship between minute ventilation (V̇E) and systemic arterial PCO2 (Pa,CO2) during high CO2 inhalation in seven hyperoxic vagotomised and anaesthetised sheep was dramatically reduced, but remained above zero, when Pa,CO2 was maintained constant in the cephalic circuit (0.11 ± 0.15 vs. 0.70 ± 0.35 l min−1 Torr−1 for the control tests). This residual V̇E response to CO2 inhalation remains to be explained since it could not be accounted for by any of the chemical or circulatory changes occurring in the cephalic circulation. Nevertheless, this preparation provides an easy method of maintaining chemical and circulatory homeostasis at the chemoreceptor level.

New methods of respiratory support are needed to reduce the high mortality rate of acute respiratory failure. To simplify the procedures of extracorporeal CO2 elimination under apneic oxygenation, one approach is to replace the membrane lung by a hemodialyzer and to administer an alkali, since hemodialysis requires a lower blood flow rate than blood-gas exchange. This study compared the effectiveness of trishydroxymethyl aminomethane (THAM) and NaOH in this procedure. Twelve male Anglo-Poitevin dogs (25 to 33 kg) were anesthetized, curarized and mechanically hypoventilated (&Vdot;E = 41% of the control value). After not less than 15 min, a venovenous shunt was used for dialysis with blood flow of 7-10 ml. min.-1kg-1 for at least 8 hours. The dialysate contained no acetate, bicarbonate or lactate, but was alkalinized to a pH of 8-9 by the addition of NaOH. A solution of THAM (0.5 N) was infused into the right heart at the rate of 0.30 ml.min.-1kg-1 in six animals, and NaOH (0.15 N) was infused in the other six at the rate of 0.80 ml.min.-1 kg-1. The injected volumes were compensated for by an equivalent amount of ultrafiltration. Elimination of CO2 (mean †CO2 = 2.3 ml.min.-1kg-1) was the same with both methods and the difference for the electrolytes and acid-base equilibrium was only very small. However, hemolysis was six times greater with NaOH than with THAM. Despite ultrafiltration, a similar marked weight gain was observed from the second hour of the experiment in the NaOH series, but only after 7 hours with THAM. It thus appears that hemodialysis combined with alkalinisation is still too complex a procedure to be safely applied in acute or chronic pulmonary failure

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