Key words: Metabolic system, oxygen consumption, minute ventilation, carbon dioxide production, Bruce protocol.

Subjects
Fifteen healthy adults (10 males and five females), staff and students of a South East UK University gave their signed informed consent to participate. The research was carried out in accordance with the Declaration of Helsinki and was approved by the Faculty Ethics Committee. All subjects were familiar with graded exercise testing. Subjects were asked to refrain from eating for a minimum of 2 h prior to the test and from vigorous exercise 24 hours prior to testing.

The purpose of present study was to assess the agreement and consistency between gas exchange variables measured by the Innocor and the CardiO₂ metabolic systems during an incremental exercise test.

The Innocor yielded mean VO₂ and V_E that were not significantly different from those obtained by the CardiO₂ system.

The percentage VO₂ differences in the present study compare favourably with findings of Porszasz et al., 1994 who reported the difference of less than 5.9% between Medical Graphics system and the Douglas bag method. Engebretson, 1998 found that the Medical Graphics system produced values of VO₂ that were within 3.6% compared with the Douglas bag. Non-significant differences in mean VO₂ in the present study were not surprising, although there were non-significant differences in measured FeO₂ between the Innocor and CardiO₂ systems.

In the present study, the limits of agreement reported for VO₂ of -0.52 to 0.55 l.min^-1 are, however, wide and unacceptable in cardio-pulmonary exercise testing. Bassett et al., 2001 compared the ParvoMedics computerized system and Douglas bag method over a range of exercise intensities and reported that the limits of agreement of -0.08 to 0.11 l.min^-1 for VO₂ are acceptable. McLaughlin et al., 2001 compared the Cosmed portable metabolic system with the Douglas bag and reported wider limits of agreement (-0.33 to 0.15 l.min^-1). Surprisingly, the authors concluded that the portable metabolic system is acceptable for measuring oxygen consumption. This is in spite of these limits of agreement being ~10% of the reported mean peak VO₂ value of ~3.5 l.min^-1.

The Innocor measured V_E slightly higher than the CardiO₂ through all exercise stages except at peak exercise. Miles et al., 1994 showed that the Medical Graphics automated system produced the lowest V_E measurement among four different metabolic systems. On the other hand Engebretson, 1998 showed no significant differences between the Medical Graphics and the Douglas bag method in measured V_E, while La Mere et al., 1993 illustrated that the Medical Graphics system overestimated V_E by 3.1 l.min^-1 compared with the Douglas bag.

Despite non-significant differences in measured V_E, it should be noted that the Bland-Altman analysis indicated that the limits of agreement for V_E are wide (-8.7 to 10.7 l.min^-1). This is in contrast with finding of Bassett et al., 2001 who reported limits of agreement for V_E of -0.8 to 1.2 l.min^-1, even at higher maximum V_E values than those reported in the present study (~100 l.min^-1 vs. ~80 l.min^-1). Individual differences in V_E never exceeded 1.6 l.min^-1 (Bassett et al., 2001). On the other hand, McLaughlin et al., 2001 reported wider limits of agreement for V_E that were similar to those in the present study (~ -6 to 10 l.min^-1). Also maximum V_E was ~80 l.min^-1. However, we believe that limits of agreement for V_E reported in our study are wide and not acceptable in cardio-pulmonary exercise testing.

As suggested, the Cronbach's alpha coefficient should be used to indicate a degree of consistency between measurements (Cronbach, 1951). Bland and Altman, 1997 reported that the Cronbach's alpha should be a minimum of 0.90, and 0.95 would be desirable for clinical application. The results of the present study demonstrate that high consistency exists in measured VO₂ and V_E between the two systems. However, the results of the present study are an obvious example that reporting only the Cronbach's alpha without calculating the limits of agreement may lead researchers to make wrong assumptions and draw inappropriate conclusions.

Both systems were calibrated and checked for their accuracy for measurements of V_E, VO₂ and VCO₂ by an engineer before study was conducted. The report demonstrated that both systems met manufacturers' recommendations regarding the accuracy. From manufacturers' specifications it seems that the Innocor has capability to measure V_E more accurate than the CardiO₂ (± 1% vs. ± 3%). In contrast, the CardiO₂ has capacity to measure O₂ and CO₂ concentrations with accuracy of + 0.03% compared with ± 0.01% by the Innocor. However the results from present study clearly indicate that differences in measured gas exchange variables between the two systems are higher than those suggested.

When Beaver et al., 1973 compared metabolic measurements between an on-line breath-by-breath computerised system and a standard method, they suggested that differences in measured V_E and VO₂ may be due to temporal alignment of a gas flow or analyser dynamic response. Further potential source of error in V_E and VO₂ may be the method and equation used by the Innocor and the CardiO₂ to estimate the BTPS factor (Hodges et al., 2005). When a subject exhales during a cardiopulmonary exercise test, the air leaves the lungs and enters the spirometer at 33-35^oC (Cole, 1954). Most volume type spirometers assume instantaneous cooling of the air as it enters the spirometer, although errors can occur due to incorrect assumptions of instantaneous cooling of the air (Hodges et al., 2005). Depending on the environmental temperature, the BTPS correction factor could be as large as 10% (Crapo, 1994). As stated earlier, in order to obtain direct comparison, the two systems were placed in series. The distance of the subject to the metabolic analyser together with a bacterial filter through which air passed before reached the Innocor gas sensors may potentially affect the cooling of exhaled air. These possible differences in physical characteristics of the analysed air (e.g. temperature) may affect differences in V_E and VO₂ by the two systems. However, it is important to note that reversing the order of the systems, and subsequent measurement, was not possible due to equipment design.

Carbon dioxide production and consequently RER values reported by the Innocor were significantly lower than those of the CardiO₂. The limits of agreement for VCO₂ (-1.01 and 0.56 l.min^-1) were wider than those previously reported as acceptable, (e.g. -0.08 to 0.08 l.min^-1, as reported by Bassett et al., 2001). These differences in VCO₂ were due to lower measurements of FeCO₂ by the Innocor. Although the Innocor measured V_E slightly higher during all exercise stages, this was not enough to compensate for the significantly lower FeCO₂ when calculating VCO₂. RER values reported at peak exercise by the CardiO₂ appear to be more valid than those reported by the Innocor. This indicates that the Innocor underestimated FeCO₂ and VCO₂ compared with the CardiO₂.

Miodownik et al., 2000, when comparing a newly developed semi-automated metabolic system based on a Douglas bag design and the Medical Graphics system, reported a non significant difference of 1.5% in VCO₂. Engebretson, 1998 reported significantly lower VCO₂ by the Medical Graphics system compared to the Douglas bag method. By contrast, few studies have reported non significant differences in measured VCO₂ between the Medical Graphics systems and the Douglas bag method (Porszasz et al., 1994; Prieur et al., 1998), while Miles et al., 1994 reported that the Medical Graphics system measured a higher VCO₂ compared with three other metabolic systems.

It is accepted that an increased dead space and the distance between the Innocor sensors and the subject's mouth, may have contributed to the poor consistency and agreement in measured FeCO₂ and VCO₂. Analysing RER results at peak exercise it seems that this direct comparison study design may possibly affect more the Innocor CO₂ sensors than those in the CardiO₂. An additional problem is that of correcting water vapour pressure in the expired air, as this pressure may be quite different to that in the calibration gas (Davies et al., 1974). Although the gas analysers were adjusted automatically to ignore the contribution of water vapour (effectively measuring dry air), most CO₂ analysers are sensitive to the presence of water vapour (Macfarlane, 2001). As the dead space was increased in the present study, this could potentially enhance higher water condensation, and possibly effect CO₂ analysers and measurement of FeCO₂. Therefore the possible inability of infrared sensors to cope with the water vapour could have contributed to the discrepancies in measured FeCO₂. The possible measurement of the temperature of the sample near the flow detector followed by calculation and correction according to the absolute water vapour pressure could possible identify the source of error. However, the measurement of the temperature was not possible due to direct comparison study design and specific configuration of the Innocor respiratory valve unit where the sensors are located.

Lower FeCO₂ measured by the Innocor may be due to lower response time of the analyser. This particularly may be emphasized with higher breathing frequencies at higher intensity of exercise. Supporting this assumption Figure 3 demonstrates that the difference in VCO₂ was higher at the end of stage four than at the end of previous stages of the Bruce protocol. There is evidence to suggest the use of different algorithms for correcting response time (Ariely and Van Liew, 1981; Farmery and Hahn, 2000). Farmery and Hahn, using specific correcting methods, were able to reduce response times for measured gases almost fivefold. Therefore future investigations should evaluate the use of suggested algorithms for correcting the response time not only in the most commonly used metabolic analysers but also in those which have recently appeared.

Finally, the differences in measured CO₂ between the two systems may be explained by the technological factors. The Innocor uses a newly developed portable multigas analyser which uses the principle of photoacoustic spectroscopy with an infrared spectrum, while the CardiO₂ uses a standard non-dispersive infrared CO₂ sensor.