Stimulation of ventilation by normobaric hyperoxia in exercising dogs.

In order to describe the factors which, during hyperoxic exercise, can counteract the chemoreceptor-mediated inhibition of ventilation by O2, minute ventilation (VE) and the pulmonary gas exchange were studied breath-by-breath in four dogs running on a treadmill (5 km x h(-1)) for 10 min during and following exposure to O2 of different durations. We found that a brief inhalation of O2 applied during the steady state of the VE response provoked a reduction in VE by 6.5 +/- 0.9 l x min(-1) whereas hyperoxia applied 2 min before the onset of exercise and maintained for 2.5 min during the running tests had a significantly weaker effect on VE (-1.8 +/- 0.2 l x min(-1), P < 0.05). The rise in pulmonary CO2 output (VCO2) during the prolonged O2 exposure was less than in normoxic exercise leading to a deficit of CO2 eliminated by the lungs of 181 ml. The return to air breathing provoked a rise in VE, which reached within 73 s a much higher level than the control tests (22.9 +/- 3.6 vs. 19.5 +/- 2.2 l x min(-1), P < 0.05); VE then subsided to control levels with a long exponential decline. The CO2 deficit during O2 breathing, was fully compensated after recovery in air within 6 min. No stimulatory effect on ventilation was observed at rest at the cessation of a similar exposure to O2 despite a higher end-tidal PCO2 (+4 +/- 1 mmHg) than in exercise. In conclusion, the stimulatory effect of O2 during exercise can be clearly revealed after recovery in air and seems to operate through a more complex mechanism than that thought to be involved at rest. We propose that the changes in CO2 stores in the exercising muscles could contribute to O2-induced stimulation during exercise, possibly through stimulation of muscle afferents responding to local circulatory changes. Finally, the observation that during continuous dopamine (DA) infusion (5 microg x kg(-1) x min(-1)) the VE response to recovery in air was only a slow decrease, suggests that the arterial chemoreceptors potentiate O2-induced hyperventilation, or that the vascular actions of DA counteract part of the effects provoked by CO2 accumulation in the exercising muscles.

EC CO2R: oxygenator or hemodialyzer? An in vitro study.

In respiratory support of patients with acute respiratory distress syndrome (ARDS), the extracorporeal CO2 removal (EC CO2R) technique should be the earliest and easiest procedure so as to have the lowest blood flow rate. Extracorporeal circulation (ECC) can be achieved using an oxygenator for CO2 removal under the dry form (dissolved CO2) or a hemodialyser for CO2 removal under the wet form (bicarbonates). This study investigated different methods allowing an increase in CO2 transfer, using liquid flow rates up to 0.330 l/min. The experimental set-up employed heated (38 degrees C) aqueous polyelectrolytic solutions mimicking the venous blood (pH 7.20, PCO2 53 mmHg). Four in vitro methods were tested: Series I: a DIDECO D702 oxygenator without blood (= liquid) acidification, Series II: D702 oxygenator with inlet HCl acidification, Series III: a HOSPAL H10-10 hemodialyzer without dialysate alkalinisation, Series IV: H10-10 hemodialyzer with NaOH dialysate alkalinisation. Maximum gas flow in the oxygenator and dialysate rate in hemodialyzer were 5 and 0.55 l/min respectively. For the four series the CO2 transfer (TCO2) (mean +/- S.E. ml/min) and pH out were: [table: see text] The difference between the four series was statistically significant (t-test). Acidification using the oxygenator increased CO2 transfer by 80%, but CO2 elimination was better with hemodialysis.

[CO2 elimination via extracorporeal circulation in respiratory assistance: artificial lung or kidney? Experimental study]. / Circulation extracorporelle (CEC) d'élimination de CO2 en assistance respiratoire: poumon ou rein artificiel? Etude expérimentale.

In the context of a respiratory assistance protocol dissociating oxygenation of the blood from elimination of carbon dioxide, it is possible to rest the lungs which are used for oxygen exchange by diffusion via functional zones. CO2 is eliminated via extracorporeal circulation. In order to simplify this method, the authors investigated the optimal conditions for this elimination at flow rates similar to those used for haemodialysis (0.33 l/min). This study was designed to evaluate the in vitro elimination of CO2 obtained by a membrane artificial lung with and without "doping" by acidification (10 ml/min of 0.01 N HCl) of the blood inlet compared with that obtained with a haemodialysis machine with and without alkalinisation (0.5 ml/min of 0.5 N NaOH) of the dialysate inlet. The results obtained showed that haemodialysis with an alkaline bath ensured CO2 elimination 36% superior to that of the artificial lung associated with acidification.

Metabolic CO2 removal by dialysis: THAM vs NaOH infusion.

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 (VE = 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-1kg-1. The injected volumes were compensated for by an equivalent amount of ultrafiltration. Elimination of CO2 (mean TCO2 = 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.

CO2 removal with hemodialysis and control of plasma oncotic pressure.

CO2 removal by hemodialysis, associated with systemic alkalinization, is the simplest method of metabolic CO2 elimination. In this experimental work, the authors investigated the efficacy of this protocol in modifying an alkaline perfusate by addition of dextran 40. The results, unlike those of preceding experimental series without dextran, disclosed no significant change in weight, hemodynamic variables, electrolyte concentrations or osmotic or oncotic pressures after 12 hours. Sixty-five percent of metabolic CO2 was eliminated with a bypassed blood flow rate of only 8% of cardiac output.