| 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 |
| Interface |
| 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) [101–103] |
| 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.