1996 OPEN FORUM Abstracts
NITRIC OXIDE DELIVERY - EVALUATION OF CONSTANT GAS FLOW MIXING IN THE INSPIRATORY LIMB OF AN ADULT VENTILATOR CIRCUIT
Hill W. BSc. RRT, Miller C.C. BA. RRT,, Dyer D. BSc. RRT. King Faisal Specialist Hospital and Research Center, Riyadh, Saudi Arabia
Background: With intermittent positive pressure ventilation (IPPV), constant flow NO titration into the inspiratory limb of the breathing circuit might theoretically expose the patient to undetected boluses of NO and NO2, especially if longer exhalation times are used, because constant flow gas continues to fill the circuit during the ventilator's exhalation time. Method: We evaluated circuit gas mixing by measuring the peak: baseline ratio (P:B ratio) on a capnograph, of a tracer gas (5% CO_2) titrated into an adult ventilator circuit (Siemens, 900C, Sweden). 5% CO_2 gas was used as a surrogate (tracer) test gas in place of NO for three reasons: 1) 5% CO_2 has the same density as NO gas (1.04g/m^3). 2) Currently available electrochemical nitric oxide analyzers are too slow to detect bolus effects, (0-90% response in 20 seconds or less) in the pulmonox II Pulmonox Medical Corporation, Alberta, Canada 3) Capnographers are designed to detect breath to breath CO_2 fluctuations, (0-90% response in 75 ms) in the Novametrix CO_2SMO, Wallingford, USA. METHOD: During IPPV at 7.5 L of minute ventilation, 800ppm NO gas was titrated into the inspiratory limb, at the ventilator's inspiratory limb, at the ventilator's inspiratory outlet to give concentrations of 5-45 ppm. NO and NO_2 were measured in the inspiratory limb, adjacent to the patient wye. 5% CO_2 was then delivered at the exact same flowrates as the NO gas had been, and the P:B ratios of CO_2 were then measured in the inspiratory circuit adjacent to the patient wye. Inspiratory: Expiratory (I:E) ratios were set at 1:3, 1:2, 1:1, 2:1, and 3:1. In a second experiment, P:B ratios were measured at 5 points in the inspiratory limb, with and without a 950 mL circuit mixing chamber.
Results: Measured NO & NO_2 & NO & CO_2 flows during IPPV @ 7,5 L/min
NO ppm 5 10 15 20 25 30 35 40 45
NO_2 ppm 0 0 0 0.1 0.3 0.5 0.7 0.8 0.9
NO or CO_2 (L/min) 0.2 0.2 0.35 0.40 0.46 0.52 0.58 0.62 0.68
Over all I:E ratios, the measured P:B ratios of CO_2 were 1.0 indicating complete gas mixing at patient wye. In the second experiment (Figure 1) the ventilator gas and the constant flow gas (5% CO_2) were completely mixed (A) before they reached the humidifier, (P:B ratio=1) when the mixing chamber is used. Mixing occurred much nearer the patient (B) when a normal circuit is used. Conclusion: We found complete mixing of the surrogate (CO_2) gas at the patient wye under all conditions, simulating NO delivery up to 45 ppm. Conditions that favour complete gas mixing are: lower doses of required gas mixture, introduction of the constant flow gas before the humidifier, normal or inversed I:E ratios (shorter exhalation time) and the addition of a mixing chamber between the gas inlet.
(See original for figure)
Reference #: OF-96-080