Table 3.

Results of multilevel, multivariable regression analyses on sigh frequency, sigh morphology, tidal volume and variability in tidal volume

	Multivariable models			R 2
	Coefficient	CI 95%	P-value
Respiratory rate
Gestational age (w)	−0.5433	−0.8995, −0.18722	0.003	0.15
Birth weight z-score	−2.4207	−4.2435, −0.5980	0.010
Sigh frequency
Sighs/10 min
BPD	0.0047	−0.0002, 0.0096	0.059	0.03
Sigh morphology
VE,max/VT,pre
BPD	0.0036	0.0004, 0.0068	0.028	0.06
Maternal smoking	−0.4936	−0.8383, −0.1489	0.005
VE,max/VI,max
BPD	0.0016	0.0008, 0.0023	<0.001	0.09
Maternal smoking	−0.0791	−0.1606, 0.0023	0.057
Changes in VT
VT,pre
Gestational age (w)	0.0003	0.0001, 0.0004	0.001	0.40
Weight (kg)	0.0048	0.0037, 0.0061	<0.001
Sex, male	0.0016	−0.0001, 0.0034	0.058
VT,post
Weight (kg)	0.0034	0.0018, 0.0050	<0.001	0.12
Sex, male	0.0023	0.0002, 0.0045	0.036
VT,diff
BPD	−0.0000	−0.0001, −0.0000	0.024	0.04
VThigh after sigh (breaths)
BPD	−0.0297	−0.0515, −0.0079	0.008	0.03
VT,end
Gestational age (w)	0.0003	0.0017, 0.0005	<0.001	0.40
Weight (kg)	0.0045	0.0032, 0.0057	<0.001
Sex, male	0.0020	0.0002, 0.0037	0.027
Short-term variability in VT
CVpre
Gestational age (w)	0.0008	−0.0004, 0.0020	0.190	0.05
Sex, male	−0.0930	−0.1521, −0.0338	0.002
CVpost
Gestational age (w)	0.0073	0.0039, 0.0107	<0.001	0.16
Sex, male	−0.0394	−0.0786, −0.0002	0.043
Maternal smoking	−0.0692	−0.1350, −0.0035	0.041
CVdiff
Gestational age (w)	0.0055	0.0013, 0.0096	0.009	0.07
Maternal smoking	−0.0940	−0.1733, −0.0146	0.020
CVpost_slope
BPD	0.0188	0.0122, 0.0255	<0.001	0.15
CVend
Gestational age (w)	0.00102	−0.0010, 0.0031	0.025	0.16
Sex, male	−0.0323	−0.0528, −0.0118	0.003
Birth weight z-score	0.0172	0.0078, 0.0265	<0.001

Bronchopulmonary dysplasia (BPD) was expressed as the number of days on supplemental oxygen. Respiratory rate was averaged over the duration of the measurement. V_I,max/V_T,pre, maximal inspiratory volume during the sigh normalized to mean tidal volume at the beginning of the measurement; V_E,max/V_T,pre, maximal expiratory volume during the sigh normalized to mean tidal volume at the beginning of the measurement; V_E,max/V_I,max, ratio of maximal expiratory volume during the sigh/maximal inspiratory volume during the sigh; V_T,pre, mean tidal volume over the first 30 breaths of the measurement; V_T,post, mean tidal volume over the first five breaths after the sigh; V_T,diff, difference between V_T,pre and V_T,post; V_T,high, number of breaths after a sigh that exceeded 2 SD of V_T,pre; V_T,end, mean tidal volume over the last 30 breaths of the measurement; CV_pre, coefficient of variation in V_T,pre; CV_prior, coefficient of variation in V_T,prior; CV_post, coefficient of variation in V_T,post; CV_diff, difference between CV_pre and CV_post as an estimate of change in variability in tidal breathing upon a sigh; CV_{post_slope}, difference between CV_VT of first window after the sigh and second window after the sigh normalized to individual baseline CV_VT; CV_end, coefficient of variation in V_T.