Table 2– Summary of component recommendations for inert gas washout system characteristics
Flow measurementInstantaneous flow accuracy within 5% across the range of flows encountered during clinical testing and volume accuracy within 3% using a precision calibration syringe.
Sample flowIdeally, all side-stream washout systems should correct for sample flow. If not performed or achievable, sample flow should be minimised: <20 mL·min1 for paediatric and <40 mL·min−1 for adult apparatus where gas sample point is proximal to flow metre.
Volume driftAccurate correction of volume drift is problematic due to difficulty separating technical and physiological components to observed drift. When an excessive drift, beyond the range usually observed, appears, attempts to identify physiological and/or technical causes (e.g. leaks) should be made as part of the routine quality control.
Gas analyser accuracyLinearity within 1% relative of full scale (e.g. 0–80% is ±0.8% at 80% N2) to ensure appropriate assessment of starting concentration, and within 5% relative of any lower value (e.g. 0.25% at 5% N2) down to 1/40 of the starting concentration. Initial assessment should incorporate both dry and humid conditions. Monitor gas analyser accuracy, stability and linearity annually using at least three reference points of gas concentration.
Gas analyser rise timeA 10–90% analyser rise time of <100 ms is recommended across all age groups.
Data sampling frequencyData sampling should ideally be≥100 Hz for both flow and inert gas concentration measurement.
Synchronisation of flow and gas signalsAlignment accuracy within 10 ms or one sample (whichever is longer).
Equipment-related dead spaceTotal equipment deadspace for young children should be <2 mL·kg−1 bodyweight, and ideally <1 mL·kg−1 in infants. Recommendations should be adhered to in older subjects, until further evidence is available. An upper limit of 70 mL should be adhered to for adults including hygiene filters if used.
Equipment-related resistanceShould be minimised for both inspiration and expiration to avoid effects on breathing pattern and FRC during test.
  • The table is expanded with further explanation in section E2.6 in the online supplementary data. N2: nitrogen gas; FRC: functional residual capacity.