Table 7– Multiple-breath washout (MBW) measurement acceptability criteria
Testing position
 Infants: supine position with head in midline and in sniffing position to optimise upper airway patency
 Preschool and above: seated position, with head in midline
 Infants and preschoolers: suitable facemask with adequate therapeutic putty volume to ensure adequate facemask seal and reduction of the pre-gas sampling point VD without obstructing the airway opening
  Nasal mask measurements are feasible during periods of preferential nasal breathing [100] but require further study
  Box shaped flow–volume loops may indicate an upper airway obstruction or external obstruction of the airway opening by therapeutic putty
 Older subjects: nose clip and maintain tight mouthpiece seal
Three technically acceptable MBW runs should be performed, with acceptability defined by the following criteria
 Wash-in phase (or pre-washout phase for N2 MBW)
  Stable VT and end-expiratory lung volume over the preceding 30 s
   Deviation in end-expiratory lung volume at start of test within 10% of mean VT of preceding five breaths
   An irregular small volume breath immediately prior to starting the washout may also lead to error in end tidal estimate of starting alveolar concentration
  Equilibration of exogenous wash-in gas within the breath cycle (i.e. inspiratory versus expiratory end tidal concentration)
   Variability <1% relative to mean inspired concentration (i.e. <0.04% if the inspired concentration is 4%)
  Adequate starting end-tidal inert gas concentration, stable over 30 s (i.e. equal to inspired gas concentration)
 Washout phase
  Regular breathing pattern
   Sufficient breath size for adequate phase III slope identification (if SnIII analysis being performed)
   Breathing protocols of 1 L VT are recommended in older adolescents (e.g. >16 yrs) and adults but may not be feasible in all age groups (e.g. VT 1.0–1.3 L) [101103]
  No evidence of significant trapped gas release with larger breaths; release of trapped gas
   Invalidates SnIII analysis and increases measured LCI
   May be difficult to avoid in advanced CF lung disease
  No coughing
  Specific to infants during critical periods of the wash-in/washout
   No evidence of apnoeas (may significantly decrease FRC)
   No evidence of sighs (may significantly elevate FRC)
   Critical period defined as the 10 breaths prior to achieving equilibration or during the first 10 breaths of the washout
 Criteria for test termination
  At least three consecutive breaths with end tidal inert gas concentration values below 1/40 of starting inert gas concentration
  If SnIII analysis alone, then at least 6 TO must be included
  If moment analysis is being performed then at least 6 TO should be included, as data collected at 8 TO in normal subjects are likely to be compromised by poor gas signal quality
 No evidence of leak occurring during the test
  Resident inert gas (e.g. N2) - leak indicated by the following during the washout phase
   Sudden spike in N2 concentration during inspiration (consistent with post-gas sampling point inspiratory air leak)
   Premature rise in N2 signal early in expirogram of following breath, where N2 concentrations should be zero in the initial absolute dead space portion (consistent with pre-gas sampling point inspiratory air leak)
   VD,aw decrease
   Sudden step changes of the volume trace
   Step-up of N2 concentration plotted versus TO
  Exogenous inert gas: leak indicated by
   Failure of equilibration between inspiratory and expiratory inert gas concentrations during wash-in (consistent with pre- or post-gas sampling point leak)
   Sudden drop in inspiratory inert gas concentration during wash-in (consistent with post-gas sampling point leak)
  VD,aw increase during washout
 Sufficient interval between runs when using resident inert gases to allow inert gas concentration to return to baseline values
  Twice the washout time is a conservative recommendation. If a shorter interval is used, then the operator must demonstrate that alveolar concentrations has been resituated [104]
  This period may be lengthy in advanced obstructive disease
  Inadequate duration may significantly decrease measured FRC
The following should trigger further investigation for artefact but are not a reason to exclude tests alone
  Marked volume drift during testing or sudden changes in volume (without other evidence of leak)
  FRC or LCI variability >10%, measured as the difference between maximum and minimum values
Tests where FRC differs by >25% from the median FRC value across the three tests should be automatically rejected
Test equipment and performance must adhere to infection control guidelines
 Use of bacterial filters may significantly increase VD and preclude the use of certain systems in younger age groups
  • N2: nitrogen gas; VD: deadspace volume; VT: tidal volume; SnIII: normalised phase III slope; LCI: lung clearance index; CF: cystic fibrosis; FRC: functional residual capacity; TO: lung turnovers, calculated as cumulative expired volume/FRC; VD,aw: deadspace volume of the conducting airways.