Experimental and clinical tests of the oxyconsumeter: a new oxygen uptake monitor.

The prototype of a microprocessor controlled oxygen uptake monitor oxyconsumeter developed by Draegerwerk AG, Luebeck, FRG, has been tested. The measuring accuracy of this device was assessed with laboratory bench experiments utilizing both the nitrogen dilution technique and the hydrogen combustion technique to simulate oxygen uptake (VO2). The correlation coefficient between the simulated and the measured VO2 values was 0.9989 (p less than 0.05, n = 115). The average relative error of the VO2 values was -3.32% +/- 3.88% when breathing 21 vol% oxygen and -5.58% +/- 4.53% for 70 vol% oxygen (percent of reading). This was within the range given by the manufacturer (+/- 5% for 21 vol% to less than 40 vol%, +/- 10% for 40 vol% to less than 70 vol%) with few exceptions. Furthermore the oxyconsumeter was used in clinical experiments to determine oxygen uptake during general anaesthesia. Oxygen uptake was monitored using a non-rebreathing system with an externally triggered expiratory valve. The difference between preanaesthetic reference values and values determined during anaesthesia averaged -24.8 +/- 20.1 ml/min/m2 oxygen. This average relative change of -16.0 +/- 11.5% was statistically significant in 11 of 15 cases (p less than 0.05).

The efficiency of various humidifiers for respired gas.

Eight different breathing gas humidifiers were tested by humidity measurements using two independent methods under simulated clinical conditions. The comparison considering the influence of tidal volume and inspiratory/expiratory ratios revealed great differences between the individual devices depending on the kind of humidification and functional principles. Only four of the humidifiers tested: Ultrasonic Nebulizer NB 108, Hydrotrop 200, Bennett Cascade and Nicholas Humidifier, gave a constant performance of more than 30.75 g H2O/m3 absolute humidity or 70% relative humidity, related to 37 degrees C. However, humidification and heating of the inspired gas could be ensured only by the last three devices. The humidifiers Bird 500 cm3 Micronebulizer, Ultrasonic Nebulizer NB 100 and Draeger Compressor 660, were strongly dependent on the respiration parameters and exceeded the required minimum of 70% relative humidity only under certain conditions. The Engstroem ER-200 Vaporizer had a low performance and was not suitable as a breathing gas humidifier for longterm ventilation.

[Review of the development of air humidifiers working on the principle of evaporation with reference to their experimentally assured efficacy]. / Uberblick über die Entwicklung der nach dem Verdunsterprinzip arbeitenden Atemgasanfeuchter unter Berücksichtigung ihrer experimentell gesicherten Leistungsfähigkeit.

Basing on own measurements of the efficiency of customary water-bath humidifiers (humidifier of the ventilator RO-5 R, humidifier of the ventilator Engström ER 200, Bennett Cascade Humidifier, Hydrotrop 200 and Nicholas Humidifier) a critical review is given. As the results show only 3 of the humidifiers (Bennett Cascade Humidifier, Hydrotrop 200 and Nicholas Humidifer) make sure a really breathing air "conditioning" (moistening and rewarming). The humidifers of the ventilators RO-5 R and Engström ER 200 have only a very low efficiency, therefore they are not suitable for long term ventilation. Finally, the safety for the patient (the dangers of watercondensation, watertransport into the lungs, overheating, hypothermia and superhumidication) and the different expenditure of the clinical use of the humidifiers should be mentioned.