CPX/D Underestimates [latin capital V with dot above]O2 in Athletes Compared with an Automated Douglas Bag System

GORE, C. J., R. J. CLARK, N. J. SHIPP, G. E. VAN DER PLOEG, and R. T. WITHERS. CPX/D Underestimates V̇O₂ in Athletes Compared with an Automated Douglas Bag System. Med. Sci. Sports Exerc., Vol. 35, No. 8, pp. 1341-1347, 2003.

Purpose: Based on persistent reports of low oxygen consumption (V̇O₂) from Medical Graphics CPX/D metabolic carts, we compared the CPX/D against an automated Douglas bag system.

Methods: Twelve male athletes completed three, randomized 25-min bouts (5 min at 100, 150, 200, 250, and 300 W) on a cycle ergometer with intervening 30-min rests. One bout was measured on each of the CPX/D, the CPX/D with altered software (CPX/DΔ), and an automated Douglas bag system at Flinders University (FU). The CPX/DΔ software alteration was an apparent lag time correction factor of 60 ms.

Results: For the CPX/D, both V̇O₂ and V̇CO₂ were significantly lower than the FU system at 100-300 W, and the relative differences ranged -10.7 to -12.0% and -7.7 to -8.2%, respectively. Altering the software approximately halved the V̇O₂ discrepancy between the CPX/DΔ and FU systems. When data from all five workloads were pooled, V̇_E of the CPX/D (67.2 ± 26.4 L·min^-1) and CPX/DΔ (67.5 ± 26.9 L·min^-1) were significantly lower than for the FU system (70.5 ± 27.1 L·min^-1); and at 300 W, the relative differences were -4.0% and -3.4% for the CPX/D and CPX/DΔ, respectively. Altering the software changed the pooled %O₂ from 16.24 ± 0.40% for the CPX/D to 16.04 ± 0.39% for the CPX/DΔ, and these were significantly different than pooled data for the FU system (16.15 ± 0.39%).

Conclusions: During submaximal exercise, the CPX/D yields V̇O₂ values that are ∼11% lower than the criterion system, and the source of the discrepancy does not appear to be primarily related to volume measurement. A disturbing observation is that factory defaults for the lag time use different correction factors, which vary by 60 ms and this significantly alters V̇O₂ and V̇CO₂.

Measurement of oxygen consumption (V̇O₂) from the timed collection of expired gas fractions and ventilation is a fundamental procedure in any exercise physiology laboratory, although computerized metabolic carts have largely replaced the traditional Douglas bag (6) method. Prudent investigators continue to use Douglas bags to verify the accuracy of computerized, commercially available metabolic carts (3,16,20), but there is also a trend toward using computerized systems to verify other computerized systems (12,17,24,28).

Whereas the Douglas bag method collects a fully mixed sample of expirate in a single bag, newer systems often adopt the breath-by-breath approach that was pioneered by Beaver and colleagues (5). Critical to this approach is temporal alignment of the ventilation measured at the mouth with the associated gas fractions that are subsequently transported from the mouth to the gas analyzers (4). The latter interval is referred to as the lag time, which comprises the gas transport time plus the response time of the analyzers. A methodical investigation of this issue highlights that lag times in the range of 200-400 ms altered submaximal V̇O₂ from 1.95 to 2.83 L·min^-1, respectively (13). These investigators further concluded that a change in the lag time of greater than 30-50 ms results in statistically significant differences in V̇O₂. More recently, it has been demonstrated that respiratory frequency also affects the accuracy of breath-by-breath systems (19). Compared with a mean delay of 180 ms, an increase or decrease of 70 ms can yield V̇O₂ errors of ∼30% for a respiratory frequency of 70 breaths·min^-1 (19).

One breath-by-breath system in widespread use (2,7,15,21,23) is the Medical Graphics CPX/D (Medical Graphics Corporation, St. Paul, MN), but an extensive search of the published literature failed to locate a single paper that has validated this unit against the Douglas bag method at workloads routinely achieved by athletes. Although the CPX/D pneumotachograph has an accuracy within 1-2% of that of a piston pump (18), it has been reported that the CPX/D yields higher respiratory exchange ratios (RER) than other computerized systems during submaximal exercise (17). Each study has limitations because volume is only one component of a derived V̇O₂, and RER determined without a comparison with the Douglas bag method is inconclusive. On the other hand, a high RER could be indicative of an inappropriate lag time.

Despite obtaining excellent agreement between a metabolic calibrator and the CPX/D for V̇O₂ (9), multiple users in Australian and international laboratories have reported (personal correspondence) purportedly high RER and low V̇O₂ values compared with their previous systems when testing athletes. Unlike an athlete, the metabolic calibrator does not expire fully saturated gases, which may have confounded our earlier comparison. The anecdotal claims of discrepancy could reflect inaccuracy in one or both of the V̇O₂ systems at each institution or even that the gas drying on the CPX/D may be inappropriate. But with scientists from more than 20 different institutions making such claims (see Methods for details), we hypothesized that the CPX/D is inaccurate compared with a criterion Douglas bag system.