• Our team
• Services
• Equipment suppliers
• Contacts
• Members

Indirect calorimetry - Background

Dr Christopher Gore, under the auspices of the National Sport Science Quality Assurance program (NSSQA), has developed two 'first principles' calibrators for indirect calorimetry systems.

Traditionally, the approach to system calibration has involved the calibration of individual components. The accuracy and linearity of O₂ and CO₂ gas analysers are checked with at least three precision gases, and the accuracy of the volume device is verified at either constant or pulsatile flows spanning the physiological range up to 220 L.min ^-1 at body temperature and pressure saturated (BTPS).

An integrated approach to system calibration is biological calibration, which requires the simultaneous or sequential measurement of subjects during sub-maximal exercise against both a computerised system and a reference system based on a Tissot tank or Douglas Bag. Such tests are time consuming and it is even more difficult to check the accuracy of a computerised system when subjects are at maximal aerobic power (VO₂) because the biological variability in VO_2max is approximately 5% (VL Katch et al., Medical Science Sports Exercise 14: 21-25, 1982). While sub-maximal steady-state exercise checks are useful, verification of a computerised system at maximal flow rates of up to 220 L.min^-1  (BTPS) are important, because errors in volume measurement are likely to be the greatest source of imprecision in the calculated VO_2max. Therefore, the computerised system should be verified throughout the complete physiological range. Huszczuk et al. (1990) described a gas exchange calibrator with a simulated oxygen consumption ( O₂) of 0.2-5.0 L.min^-1  that is based on dilution of a calibration gas (21% CO₂ , balance N₂ ) with room air. However, several features of this calibrator limit its application. Firstly, it can only be used with systems that measure expired ventilation (VE), thereby precluding commonly used computerised systems that have a volume device on the inspiratory side of the circuit. Secondly, data are only provided up to a maximum ventilation of approximately 125 L.min^-1 , which is much less than the maximum for athletes. Thirdly, the calibrator dead space will alter the composition of expired gas in a breathing frequency and tidal volume dependent manner. Despite this potential for dead space to influence precision, it is unclear how Huszczuk et al. (1990) dealt with this complex issue.

Given the identified limitations of previous calibrators and their inability to match the ventilatory flows produced by athletes during high intensity exercise, a calibrator able to simulate high aerobic power (VO_2max calibrator) was constructed.

Information from: CJ Gore, PG Catcheside, SN French, JM Bennett, J Laforgia. Automated VO_2max calibrator for open-circuit indirect calorimetry systems. Medical Science Sports Exercise 1997; 29(8):1095-1103.

MaxII metabolic calibrator

The MaxII metabolic calibrator is a versatile device that can be used for routine calibration of most indirect calorimetry systems that assess the ventilation and aerobic power of athletes. The MaxII metabolic calibrator has the capacity to test indirect calorimetry systems that measure either inspired or expired volume. It can achieve ventilations in excess of 240 L.min^-1  (BTPS) and correspondingly high VO₂ and VCO₂ of greater than 7.0 L.min^-1  . Accordingly, the MaxII calibrator is able to assess the accuracy of an indirect calorimetry system throughout the physiological range for elite athlete populations.

As a guide, the accuracy of predicted F_EO₂ and F_ECO₂ is approximately ± 0.10% absolute, and ventilation is accurate to within ± 1%. Consequently, predictions of VO₂ and VCO₂ are accurate to within ± 3-5%. If all errors are minimal, a 0.10% absolute error in F_EO₂ will change VO₂ by 3%, but if errors in gas fractions, ventilation and environmental conditions are cumulative then the total predicted error might be as large as 5%. The accuracy of the above predictions is also limited, in part, by the accuracy of the temperature, barometric pressure and relative humidity measurements provided at the test laboratory.

MaxII test overview

Typically, the test VO₂ system is assessed against the MaxII metabolic calibrator in two phases.

Phase 1: dry mode

The MaxII calibrator is run in a dry mode. The gas fractions, VE (BTPS), VO₂ and VCO₂, measured by the VO₂ system are compared with those values predicted from MaxII. The predicted gas fractions from MaxII depend upon mixing known volumes of alpha standard gas (21.00 ± 0.02 % CO₂ balance N₂) with known volumes of room air (20.93% O₂ , 0.03% CO₂ , balance N₂). MaxII uses two pistons; with a small piston to deliver precise volumes of 21% CO₂ into the big piston that contains precise volumes of air. By using a variety of 'tidal volumes' for the small and big pistons, a range of expired gas fractions can be generated to span the physiological range. Similarly, by varying the piston frequency the 'minute ventilation' of MaxII can vary between 10 and 400 L.min^-1 . The calibrator is set to run in a steady-state condition for 4-5 min at 6-10 different small/big piston configurations to span the range of F_EO₂ and F_ECO₂ and ventilation of athletes.

Phase 2: waterbath mode

The MaxII metabolic calibrator is used in conjunction with a customised, airtight waterbath and run in both DRY and WET modes. The deadspace of the waterbath and connecting tubing is matched on both occasions to assess the effectiveness of the VO₂ drying system. In dry waterbath mode, the waterbath uses an aluminium plate that matches the water level when the waterbath is used in wet waterbath mode. In wet waterbath mode, the waterbath is filled with hot water (~60°C). In this arrangement, the inspired air is ambient room air, while expired air passes across the waterbath and exits fully saturated with water vapour at about 37°C. The waterbath is thus designed to provide the machine equivalent of a human mouth; inspired air is ambient and expired air is hot and saturated with water vapour. For this phase of testing, the measured F_EO₂ with and without hot (37°C) and humid (100% RH) expirate are assessed to test the efficiency of the gas drying system utilised by the VO₂ system.

Analysis of MaxII results

Phase 1: dry mode

Predicted values from the MaxII calibrator are regressed against mean values obtained from the test VO2 system for F_EO₂, F_ECO₂, VE (BTPS), VO₂ and VCO₂. Correlation and standard error of estimate are also calculated for measured versus predicted values. The optimum result is to have a correlation near to 1.00 and a small standard error of estimate (for example, 0.03% for F_EO₂). In addition, but not quite as critical if measurements are conducted over a small range such as 16-18% O₂, the regression intercept should be near zero and the slope near 1.00. Plots are made of these regressions and also of the difference between the measured and predicted values.

The figures below represent some typical results for Phase 1 testing from two example VO₂ systems.

Phase 2: waterbath mode

The modelling equations that predict the gas fraction and ventilation of MaxII in the dry mode are not used when the waterbath is incorporated because the extensive deadspace compromises the accuracy of these calculations. Rather, measured values in DRY waterbath mode versus WET waterbath conditions are regressed against each other for F_EO₂. 

MaxII test costs

NSSQA laboratories

The MaxII metabolic calibrator is available to laboratories seeking NSSQA accreditation in the oxygen consumption stream for the detailed assessment of VO₂ testing systems. For more information about accessing MaxII, contact:

Email: NSSQA@ausport.gov.au

Non-NSSQA laboratories

Laboratories outside the NSSQA accreditation program can access the MaxII metabolic calibrator on a fee-for-service basis. The table below shows the approximate costs for the various components of MaxII assessment of a VO₂ system. In addition to the costs noted below, there are costs for transport or freight of MaxII to the destination laboratory. Travel and accommodation expenses for NSSQA staff are also payable by the client laboratory.

The above prices are subject to variation without notice and are GST exclusive. Prices depend on rates provided by external suppliers. Individual quotes will be given on request.

Based on the above, the following approximate costs for a full MaxII assessment of VO₂ testing system(s), not including costs for transport/freight of MaxII to the destination laboratory, NSSQA staff travel and accommodation, would be:

• 1 system = ~ A$4,900
• 2 systems = ~ A$7,300 or ~ A$3,650 per system
• 3 systems = ~ A$12,200 or ~ A$4,065 per system (requires a second bottle of alpha grade gas)
• 4 systems = ~ A$14,600 or ~ A$3,650 per system