Infant Pulmonary Function Testing (iPFT)
All measurements were performed according to the ATS-ERS guidelines10–12. Infants were clinically stable for at least 4
weeks prior to testing. Prior to testing, parental written consent was
obtained. Infants were sedated using 100–150mg/kg triclofos sodium
syrup or the equivalent 50-75mg/kg chloral hydrate syrup.
Prior to 2019, flows and volumes were measured via a custom made iPFT
device using a heated pneumotachograph (series 3700A, Hans Rudolph Inc.,
USA) connected via a low-dead-space Rendell Baker face mask to a
differential pressure transducer and signal conditioner set to 100 Hz
(SCIREQ TD-05 & SC-24), as previously described13–16. Subsequent to 2019, flows and volumes were
measured using a commercial device MasterScreen BabyBody
(JAEGER/Vyaire). Differences in flow and volume measurements between the
two testing devices were normalized for all infants tested, regardless
of inclusion in current study.
iPFT measurements were uniform and included: Tidal breathing analysis
with measurements of respiratory rate (RR), tidal volume (Vt), and
Minute ventilation (MV); Baby-body plethysmograph for the measurement of
functional residual capacity (FRCpleth) and airway resistance (Raw);
Measurement of respiratory system compliance (Crs) with the multiple
breaths occlusion technique prior to 2019
(for the control group) and with
the single breath occlusion technique after 2019 (for the SARS-CoV-2
group); and Tidal and raised rapid thoraco-abdominal compression
techniques for the measurements of forced expiratory airway flows.
Forced expiratory flow at FRC (V̇maxFRC) was obtained from the tidal
forced expiratory flow-volume curve, and the forced vital capacity
(FVC), forced expiratory volume after 0.5 sec (FEV0.5)
and forced expiratory flows at 50%, 75% and 85% of vital capacity
(FEF50%, FEF75% and
FEF85%) from the raised-volume expiratory flow-volume
curve. Residual volume (RV) and total lung capacity (TLC) were
calculated from the results of the plethysmography measurements and of
maximal expiratory flow-volume curves.
Bronchodilator responsiveness (BDR) was defined positive if an
improvement of 20% in the V̇maxFRC was documented twenty minutes after
an inhalation of nebulized salbutamol as previously described17 or 4-5 puffs of Salbutamol inhaler (100mccg/puff)
via valve holding chamber.
Pulmonary function results of all infants included in the study were
categorized according to the physiological pattern diagnosed18: normal lung function pattern - V̇maxFRC
(%pred)> 80%, and FRC(%pred) 80% -110%, and FVC(%
pred)>80%; Obstructive pattern - VmaxFRC(% pred)
<80% and FRC(% pred)> 80%; Restrictive pattern
- VmaxFRC(%pred)<80% and FRC(% pred)<80%; Mixed
pattern (obstructive and restrictive) - infants who do not meet any of
the previous criteria.