Drawbacks of Gas Exchange Measurement

The obtained data are influenced by age, anthropometrical measures (height, weight, lean body mass) and gender. The aerobic capacity of women is about 15 to 30% below that of men, mainly due to a difference in mass of contracting muscle. Starting at age 12 there is a difference between boys and girls in accumulation of body fat. There is a greater accumulation of body fat (which does not contribute to an increase in aerobic capacity) in females. Maximal aerobic capacity is strongly related to lean body mass.

There is a decrease of resting oxygen rate as age increases (about 1-2 ml/kg), but the submaximal walking oxygen rate at self selected speed does not significantly decrease. Since there is a increase in self selected walking speed with age, there is decrease of oxygen cost with age.

Although one is aware of the influence of the parameters discussed above, there are no specific recommendations available with respect to the optimal method to normalise oxygen measurements. In most instances (eg. Cosmed K4B²) mass is used as a starting point for normalisation. Age is not recommended as a method for normalisation. The inverse body surface area was used by Bowen et al (1998) to develop a regression equation to determine a predicted oxygen cost for children regardless of disability.

Data interpretation requires knowledge of the variability of measurements made. Minimal information exists with respect to intra-day variability for either able bodied subjects or patients with neuromuscular disorders. We found oxygen cost to be the most repeatable from day to day with a variability of 9.5 %, followed by oxygen consumption 12.2 % and PCI 26 %. Bowen and Boyd reported comparable figures. Based on the results for repeatability, a significant clinical change requires oxygen cost changes ranging from 0.2-0.3 ml/kg/meter walked. There is however a lack of strong recommendations with this respect. Many of the improvements made by orthopaedic treatment and physiotherapy are not detectable by the degree of inter-day variability.

References

Bowen TR, Lennon N, Castagno P, Miller F, Richards J. Variability of energy-consumption measures in children with cerebral palsy. J Pediatr Orthop 1998;18(6):738-42.

Rose J, Haskell WL, Gamble JG, Hamilton RL, Brown DA, Rinsky L. Muscle pathology and clinical measures of disability in children with cerebral palsy. J Orthop Res 1994;12(6):758-68.