On-line systems, based on mass spectrometers for determination of gas fractions, directly measure all inspired (FIN2, FIO2, FICO2) and expired (FEN2, FEO2, FECO2) gas fractions, thereby reducing the number of necessary assumptions for these systems. It is of relevance to both on-line and Douglas bag methods that although the assumption that N2is inert has been challenged in the past (Dudka et al., 1971; Cissik et al., 1972), the assumption is generally considered appropriate, particularly during exercise when minute ventilation is elevated (Wilmore and Costill, 1973).
to correct the volume meter reading to the actual volume. Sandals (2003) has determined the precision of such a calibration approach to be 0.057 l.
When this level of precision is considered for SO2determination, a value for SO2precision of 0.86% is calculated.
Ambient pressure is normally determined with a mercury barometer via a Vernier scale. Barometers should be regularly checked for their accuracy. This is possible by applying the following equation (WMO, 1996):
PBLABPBSL/(H/29.27TLAB)
where PBSLis the barometric pressure (PB) at sea-level, PBLABis the PBfor the laboratory (and both pressures are in hPa where 1 mm Hg 1.33 hPa), H is the laboratory elevation in metres (from ordinance survey map), and TLABis the laboratory temperature in Kelvin.
A good quality barometer will normally have a resolution of 0.05 mmHg, and it can be assumed that measurement can be accurately made to within
0.2 mmHg. When this level of precision is considered for SO2determination, a value for SO2precision of 0.03% is calculated (Sandals, 2003).
Expired gas temperature is normally determined at the inlet port of the dry gas meter with the use of a thermistor probe. Commonly available probes have a resolution of 0.1C, and are normally factory calibrated with maximum accuracy in the region of 0.2C. Errors in the measurement of expired gas temperature may have a cumulative effect on SE translation from ATPS to STPD (and therefore SO2determination) via both the determination of gas temperature itself, and the use of gas temperature in the determination of the partial pressure of water vapour in the gas (PH2O). Taking a maximum possible error of 0.2C in the determination of gas temperature, this translates into a precision of 0.07% in the conversion of SEfrom ATPS to STPD. This is further compounded by a 0.2 mmHg error in PH2Odetermination, resulting in an accumulated precision of 0.1% in the conversion of SE from ATPS to STPD and hence SO2determination (Sandals, 2003).
The sample volume removed for determination of gas fractions should be accurately determined. This is normally done by removing gas from the Douglas bag at a known flow rate. The accuracy of the flow rate may be checked by filling a bag with a known gas volume, and timing the emptying of the bag. Sandals (2003) has noted significant discrepancies in the actual flow rate and that at which flow controllers are set. Once flow rate is known, the precision of determination of sample volume has been shown to be
0.007 l·min1. When this level of precision is considered for SO2determination, a value for SO2precision of 0.11% is calculated (Sandals, 2003).
Having considered factors that might influence the accuracy and precision of the determined volume of expirate, factors that influence the accuracy and precision of the gas fractions are now considered. Due to the influence of gas fraction determination on SO2andSCO2, the importance of accurate and pre-cise determination of expired fractions of oxygen (FEO2) and carbon dioxide (FECO2) cannot be overemphasised. Table 11.1, produced by Sandals (2003), demonstrates just how influential errors in FEO2and FECO2can be in the deter-mination of SO2 and SCO2. For example, in the heavy exercise intensity
domain, a 1% overestimation for FEO2converts to a 4.61% underestimation of SO2. This is because the FEO2variable is used twice in the calculation of SO2
and the error incurred at the first stage of the calculation is in the same direction as that which is introduced at the second stage. In the calculation of SCO2 no variable is used twice so this amplification effect does not occur.
However, since similar factors will influence the determination of SO2 and SCO2, errors in both calculations should be minimised.
A potential source of error that is often ignored is the contamination of expirate with any residual gas in the bag after evacuation. Whilst vacuum pumps are commonly employed to thoroughly evacuate Douglas bags, residual gas remains mainly in the non-compressible part of the bag (the neck) between the bag and the two-way valve. Minimising the volume of the ‘neck’ of the bag is an important part of minimising this potential error. However, it is also possible to quantify the volume of the ‘neck’ of the bags, and correct for the contamination effect of residual gas (Prieur et al., 1998). A correction may be performed by knowing the volume and the concentration of the gas contained in the ‘neck’ of the bag following evacuation. Flushing the bags with room air of known con-centration prior to evacuation allows for such a correction. If this procedure is followed, one may have increased confidence in measurement of the oxygen fraction (FEO2), and carbon dioxide fraction (FECO2) in the expirate. Sandals (2003) has calculated that with such a correction the precision is 0.031 l for the residual volume, which translates to 0.05% for SO2determination.
When using partial pressure analysers, water vapour partial pressure presents a gas fraction diluting effect, which may lead to a further source of error (Beaver, 1973; Norton and Wilmore, 1975). The water vapour is evident (as water droplets) as the expirate cools to room temperature in the bag. Partial pressure analysers are commonly used in off-line PGE systems, whereas on-line systems are increasingly incorporating mass spectrometry to determine gas frac-tions. Mass spectrometers are not influenced by water vapour partial pressure.
When using partial pressure gas analysers, water vapour should be dealt with consistently when calibrating the analysers (when using dry bottled gases and moist air) and when analysing the expirate (when using saturated air). A possible approach is to first saturate all gas presented to the analysers using Nafian tubing (e.g. MH Series Humidier; Perma Pure Inc, New Jersey, USA) immersed in water, and then cool and dry the gas using a condenser (e.g. Buhler PKE3; Paterson Instruments, Leighton Buzzard, UK) to a consistent 106 DAVID V.B. JAMES ET AL.
Table 11.1 Effect of a 1% increase in FEO2and FECO2on the error incurred in the calculation of V.
O2and V.
CO2at three levels of exercise intensity
Exercise 1% increase in FEO2 1% increase in FECO2
intensity
% error % error % error % error
in V·
O2 in V·
CO2 in V·
O2 in V·
CO2
Moderate 3.07 0.00 0.21 1.01
Heavy 4.61 0.00 0.24 1.01
Severe 7.94 0.00 0.30 1.01
saturated water vapour pressure (e.g. 6.47 mmHg at 5.0C; see Draper et al., 2003). When adopting this approach, the calibration of the analysers and the resulting accuracy of the expired gas fraction measurement are improved.
A two-point calibration (zero and span) is commonly used for the O2and the CO2analyser. In each case, adjusting the zero setting is equivalent to alter-ing the intercept of a linear function relatalter-ing the analyser readalter-ing to the output from the sample cell, while adjusting the span is equivalent to altering the slope of this relationship. For both analysers, the zero setting is adjusted to ensure that the reading on the analyser is zero when gas from a cylinder of N2is passed through the analyser (the zero gas). For the O2 analyser the span setting is adjusted to ensure that the reading on the analyser is 0.2095 when outside air is passed through the analyser. For the CO2analyser the span setting is adjusted to ensure that the reading on the analyser is the same as the gas fraction of a gravimetrically prepared cylinder (normally 0.0400 CO2). Precise measure-ments of the atmospheric O2fraction since 1915 have been in the range of 0.20945–0.20952 (Machta and Hughes, 1970) and recent data suggest that a realistic current value for the CO2fraction would be ~0.00036 (Keeling et al., 1995). It is not clear why the 0.2093 and 0.0003 values for FIO2and FICO2, respectively, have been so widely adopted in the physiological literature.
However, it is plausible that they arose from Haldane’s investigations of mine air at the start of the twentieth century and have been assumed to be constant over time (Haldane, 1912). The data from the meteorological literature show that the O2fraction in fresh outside (atmospheric) air is relatively constant, varying by ~0.00002 over a year (Keeling and Shertz, 1992). The precision of the gravimetrically prepared gas mixtures is reported to be within 0.0001 of the actual nominal gas fraction (BOC Gases, New Jersey, USA). Taking the worst-case scenario, a precision of 0.0001 for the measured expired gas fractions, translates into a precision of 0.34% for SO2.
Table 11.2 presents an overview showing that after careful consideration of potential sources of error, and taking action to minimise each potential error, the degree of precision may be quantified. Sandals (2003) has calculated that when each source of uncertainty is combined in the calculation of SO2, an overall value for precision may be derived. Whilst we have presented preci-sion for each potential source of error based on certain assumptions (e.g. heavy intensity exercise and 45 s collection of expirate), Sandals (2003)
Table 11.2 Effect of measurement precision on the determined V· O2for heavy intensity exercise with 45 s expirate collections
Measurement Precision in V·
O2(%)
Volume (with dry gas meter) 0.86
Ambient pressure (with barometer) 0.03 Expired gas temperature (with thermistor probe) 0.10 Sample volume (with flow controller) 0.11
Residual volume 0.05
Expired gas fractions (gas analysers) 0.34
has determined overall precision for a range of assumptions (see Table 11.3).
For the assumptions made earlier in this chapter, the overall precision of SO2
determination is calculated to be 1.4%. Of particular note is the finding that the degree of precision is increased as exercise intensity increases (for a given collection period) or the expirate collection duration increases (for a given exercise intensity).