INTRODUCTION

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CARBON DIOXIDE (CO₂) is a very important gas in human physiology and is extensively monitored in medical treatment, physiological studies, and various environmental situations. Inspired gas enriched with O₂ is used in many fields of human activity. For example, combinations of various levels of O₂ and CO₂ are common in diving and in medical treatment. At our institute, where we practice hyperbaric O₂ therapy and study the physiology of closed-circuit diving, combinations of various levels of O₂ and CO₂ are common. Infrared CO₂ analyzers are the most widely used. In different situations, we came up with conflicting results by using both our mass spectrometer and infrared CO₂ analyzer after calibration with certified commercial calibrating mixtures. Discrepancies in the readings of the CO₂ concentration in a gas mixture and unacceptable respiratory ratios are two examples. We therefore set out to study this conflict by using the Wösthoff precision pumps (H. Wösthoff, Bochum, Germany) to produce various precise concentrations of CO₂ with either N₂ or O₂ as the background gas. These precision pumps are known for their accuracy, and an analysis of a gas mixture prepared by the precision pumps that we performed by using the Micro-Scholander gas analyzer was proven accurate to within 0.01-0.02%. Infrared CO₂ analyzers' instruction manuals do not indicate any effect of N₂ or O₂ on CO₂ sensing.

	METHODS

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Three infrared CO₂ analyzers and a mass spectrometer were used.

Ametek CD-3A (Ametek, Thermox Instruments Division, Pittsburgh, PA). The instruction manual for the Ametek CD-3A (1) suggests zero calibration with either air or N₂ and span calibration with a CO₂-containing mixture (CO₂ >4% and up to 15%). There is no reference to interference by O₂ or N₂.

Servomex 1440 (Crowborough, East Sussex, UK). The instruction manual for the Servomex PA 404 CO₂ analyzer suggests "narrow-band interference filters and a solid-state detector provide a measurement which is inherently less affected by cross-sensitivity" (10). The manual does suggest some background effect of gases on the O₂ sensor, but there is no mention of the CO₂ sensor. It is suggested that "these effects can be compensated by either using the background gas as a zero or by offsetting the N₂ zero point by the amount of error induced by the background gas" (10). There is no mention of cross sensitivity of N₂ or O₂ on CO₂ sensing.

Dräger Multiwarn P CO₂ (Lübeck, Germany). The instruction manual for the Dräger Multiwarn P CO₂ (6) suggests zero calibration with pure N₂ and span calibration with a CO₂-containing mixture. They suggest calibrating with a "concentration corresponding to the typical values which are to be measured" (6). Nothing is mentioned regarding cross sensitivity of N₂ or O₂.

	RESULTS

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The CO₂ readings obtained from the CaSE mass spectrometer (calibrated with a certified commercial mixture, Scott Specialty Gases, Plumsteadville, PA), with either N₂ or O₂ as the background gas, are shown in Fig. 1. In Figs. 1 and 2, we present the difference between the analyzer readings of CO₂ and the precision pumps' input, on the y-axis, as a function of the percentage of CO₂ in the precision pumps' input. It can be seen that the CO₂ readings deviate only slightly from the input of the precision pumps and are similar for both background gases. When the Ametek CD-3A was calibrated by using N₂ as the background gas, other CO₂ readings showed small deviations from the expected concentrations (Fig. 2A). However, CO₂ readings deviated to a greater degree as a function of input CO₂ concentration when the background gas was O₂. The slope of the decrease was 0.075 (%error/%CO₂, r² = 0.99). There was no difference between new CO₂ readings and those from the initial set, when the background gas was changed back to N₂. Calibration of the Ametek CD-3A with O₂ as the background gas gave the opposite results (Fig. 2B). The different CO₂ readings with O₂ as the background gas yielded small deviations from the expected values, but replacing the O₂ with N₂ produced positive deviations that are linearly related to the input CO₂ concentration (slope = 0.11, r² = 0.98). Results similar to those obtained with the Ametek CD-3A were derived by using the Servomex 1440 (Fig. 2, C and D). The slope for the deviation of CO₂ concentrations in O₂ from the input when the calibration background gas was N₂ was 0.080 (r² = 0.99), and, in N₂ when the calibration background gas was O₂, was 0.10 (r² = 0.99). Similar results were obtained with the Dräger Multiwarn P analyzer (Fig. 2, E and F). When the background gas was the same as had been used for calibration, the CO₂ readings yielded a reclining s-shaped response that crossed the zero at the calibration concentration. However, the deviation of CO₂ concentrations for the other background gas (the gas that had not been used for calibration) was similar to that seen in the Ametek CD-3A readings: underestimation when the background gas was O₂ (Fig. 2E) and overestimation when the background gas was N₂ (Fig. 2F). The deviations were corrected by subtracting the deviation for the calibration background gas from the deviation of the noncalibration background gas ( in Fig. 2, E and F). The results gave a linear relationship for the corrected deviation as slope = 0.085, r² = 0.97, and slope = 0.09, r² = 0.78 in Fig. 2, E and F, respectively. The nonlinear concentration readings of the Dräger analyzer may be inferred from their instruction manual, which mentions a possible error of ±5% for the values 0 to the calibration value and ±10% from the calibration value to 1.5 times this value (6). In all the analyzers, the zero setting was not sufficient on recalibration with another background gas; sensitivity had to be calibrated as well.

View larger version (21K): [in this window] [in a new window]   Fig. 1.   CaSE mass spectrometer readings of CO2 concentrations. Difference between CO2 concentration readings and precision pumps' input is shown as a function of CO2 input from H. Wösthoff precision pumps. Both pure O2 and pure N2 were used as background gas for pure CO2 ( and , respectively).

View larger version (27K): [in this window] [in a new window]   Fig. 2.   Ametek CD-3A (A and B), Servomex 1440 (C and D), and Dräger Multiwarn P (E and F) readings of CO2 concentrations. Difference between CO2 concentration readings and precision pumps' input is shown as a function of CO2 input from H. Wösthoff precision pumps. Right: results from calibration with O2 as background gas; left: results from calibration with N2 as background gas. , CO2 readings when N2 was background gas; , readings when background gas was O2; , differences between errors for N2 and O2 with Dräger Multiwarn P.

	DISCUSSION

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A CO₂ infrared analyzer responds differently, depending on whether the background is N₂ or O₂. Because most common commercial calibrating tanks contain a high percentage of N₂, CO₂ readings in an O₂-enriched atmosphere are underestimated. For the three analyzers we tested (Ametek, Servomex, Dräger), this underestimation was 0.075, 0.085, and 0.080, respectively, for each 1% CO₂. Therefore, calibrating an infrared CO₂ analyzer with N₂ as the background gas would result in an 8% underestimation of CO₂ levels in O₂ as background. For example, when the analyzer reading is 4.6% CO₂, the true concentration is 5%. Conversely, calibrating the three analyzers with O₂ as the background gas gave an overestimation of 0.11, 0.09, and 0.10, respectively for each 1% CO₂ in N₂. Thus calibrating the infrared CO₂ analyzer with O₂ as the background gas will result in a 10% overestimation of CO₂ levels in a N₂-rich atmosphere.

There are a few fields in respiratory physiology where an error in the CO₂ reading could seriously affect the outcome. In hyperbaric physiology, a small change in CO₂ concentration can make a large difference, because PCO₂ is the product of the fraction of CO₂ and pressure. CO₂ is an important factor in O₂ toxicity (3), which will be enhanced by an inspired PCO₂ as small as 1 kPa (2). CO₂ is an important respiratory drive in hyperbaric conditions, and professional divers tend to hypoventilate and retain CO₂ in their tissues (7). Diving with breathing mixtures containing various concentrations of O₂ (Nitrox or closed-circuit diving) would affect the CO₂ monitored with infrared analyzers. The alveolar-arterial CO₂ difference is negligibly small in the healthy lung and is therefore used to calculate alveolar PCO₂ from arterial PCO₂ (5). An 8-10% error would represent a serious flaw in the computation of alveolar-arterial gas exchange. Various tests use the switch from an air- to an O₂-filled spirometer (4), and the CO₂ reading would be affected if an infrared analyzer were used. In clinical medicine, if the analyzer is calibrated with N₂ as the background gas, alveolar PCO₂ may be underestimated in patients breathing an enriched O₂ mixture. In artificial respiration, for example, when the ventilator is set to give 5% CO₂ in the alveolar gas, the true value may be close to 4.6%.

Lauber et al. (8) tested the accuracy of various infrared CO₂ analyzers as affected by an array of background gases and conditions. They concluded that "all tested analyzers were found to be safe for clinical use," allowing for a 12% error. However, the error established by the present study, which is probably related to the collision-broadening effect of O₂ (9), should be considered in scientific research. Presently, only a gas mixture with the same O₂/N₂ ratio as that used in the planned experiment can be used as a calibrating mixture in infrared CO₂ analyzers.