The maternal and infant characteristics of the preterm and term infants included in this report are summarized in the . To assess the extent that infants included in the analyses differed from those not included, we also compared the included infants with (1) those excluded because of insufficient monitor use and (2) those meeting the monitor use criterion but not selected for inclusion. Consistent with previously published data, infants included in this study had mothers who were older, better educated, more likely to be of white race, more likely to be to be married, and less likely to smoke.2,3
Infants who met the monitor use criterion but were not selected for inclusion in this study did not differ significantly from those who were included in the analyses. As expected, compared with the mothers of term infants, mothers of preterm infants were significantly younger, had less education, and were more likely to be nonwhite, unmarried, and a smoker.
Demographic characteristics of preterm and term infants
The total of 3-minute epochs assessed in the preterm and term groups was 75 286 and 68 920, respectively. After exclusions for movement artifact and no Spo2 signal, 44 252 (59%), and 53 149 (77%) of epochs, respectively, remained for analysis. The median (extremes) number of hours assessed per infant was 17.9 (0.3; 66.2) and 26.4 (7.6; 55.8) in the preterm and term groups, respectively. The median (extremes) hours assessed per week for all infants combined was 102 (38; 121) and 126 (75; 152) in the preterm and term groups, respectively. Sleep position data were available in 44% and 61% of the preterm and term groups, respectively; the other infants had an indeterminate position or may not have had the sensor in place. Among those with sleep position recorded, 57% and 65% were supine (P = .043), and 28% and 25%, respectively, were prone (P = .941). Among all assessed epochs, only 0.06% and 0.08% in the preterm and term cohorts, respectively, included an event meeting the recording threshold for apnea (16 seconds).
Identification of a segment >10 seconds in duration meeting the criteria for measurement of baseline Spo2 was possible in 97% of epochs in the preterm and 98% of epochs in the term cohorts. For each infant, we calculated the 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentile for their baseline Spo2 measurements for each study week; average percentiles of baseline Spo2 by cohort and week are presented in . Because the symptomatic and asymptomatic preterm subgroups did not differ in respect to baseline Spo2, we combined the two subgroups for comparison of the aggregate PT cohort with the term cohort.
Baseline SpO2 percentiles. A, Preterm cohort of 103 infants at 36 to 59 weeks PMA. B, Term cohort of 99 infants at 43 to 65 weeks PMA.
For both preterm (weeks 36 to 59) and term infants (weeks 43 to 65), there was a significant quadratic trend in median Spo2 over time, reflecting an initial increase and then gradual decrease in median Spo2 (P < .001 for quadratic trend in each group). At 50 to 59 weeks PMA (common observation time), both the preterm and term cohorts showed a similar decreasing linear trend in median Spo2 (test for difference in slopes over time P = .478, test for significant common slope P < .001). Median Spo2 is 0.8% lower, on average, for preterm compared with term infants (P < .001, controlling for time).
Among all 3-minute epochs assessed, 1.4% and 0.7% (P < .001, aOR = 2.31 [1.49, 3.57] controlling for infant) in the preterm and term cohorts, respectively, had ≥1 episode of intermittent hypoxemia to <90%. Among all infants in the preterm and term cohorts, 79% and 65% (P = .064) of infants, respectively, had ≥1 intermittent hypoxemia episode. The median number of episodes of intermittent hypoxemia was four in the preterm and five in the term group, but the range was wide, 1-98 and 1-65, respectively.
The symptomatic and symptom-free preterm infants with any intermittent hypoxemia did not differ in their baseline comparisons or the frequency and severity of intermittent hypoxemia, and we therefore combined them for comparison with the term cohort. The percent of infants with ≥1 intermittent hypoxemia episode, by week, for the preterm and term cohorts is shown in . For both the preterm and term cohorts, the percent of infants showed an initial drop over time (from week 36 to week 44 for preterm, P < .001; from week 43 to 50 for term, P < .001). For preterm infants, there was no significant decrease in the percent with intermittent hypoxemia episodes after this initial drop (P = .275), and for term infants there was a slower but significant decline through week 65 (P < .001). Between weeks 50 and 59 (common observation time and after the initial drop in intermittent hypoxemia episodes), there was no significant difference between the term and preterm cohorts on the risk of having intermittent hypoxemia, controlling for week (P = .777). Examining the time spent with Spo2 <90% per week, those infants with ≥1 intermittent hypoxemia episodes showed a significant drop in time spent with Spo2 < 90% for both preterm (from weeks 36 to 45, P < .001) and term infants (from weeks 43-50, P < .001) (). After this initial drop, there was no significant change in the time spent with Spo2 < 90% for either the preterm (P = .732) or term (P = .332) groups. Between weeks 50 and 59 (common observation time, and after the initial drop in time with Spo2 < 90%) there was no significant difference in time with Spo2 < 90% between the preterm and term cohorts (P = .363).
Figure 4 Frequency of intermittent hypoxia (Spo2 <90%) in preterm and term cohorts. A, Percent of infants in preterm and term cohorts with ≥1 episode of intermittent hypoxemia. B, Number of seconds with Spo2 <90% during intermittent hypoxemia, (more ...)
In preterm infants, 1.4% of both prone and supine epochs had intermittent hypoxemia (aOR = 1.13 [0.60, 2.12]). In term infants, for epochs in the prone versus supine positions, 0.5% versus 0.9% had intermittent hypoxemia (aOR=0.73 [0.45, 1.18], adjusting for infant and PMA).
In the preterm and term cohorts, 60% and 57%, respectively, of epochs with intermittent hypoxemia (aOR = 0.98 [0.64, 1.52] adjusting for infant), included periodic breathing. Among all 3-minute epochs, 16% and 11% in the preterm and term cohorts, respectively, involved periodic breathing (aOR = 0.98 [0.64, 1.50] adjusting for infant and PMA). For preterm infants, the percent of epochs with periodic breathing was 12% versus 13% in the prone versus supine positions (aOR = 0.90 [0.77, 1.04] adjusting for infant and PMA). For term infants, the percent of epochs that included periodic breathing was 8.7% versus 10.9% for epochs in the prone versus supine positions (aOR = 0.99, [0.86, 1.15] adjusting for infant and PMA).
Comparing symptomatic (n = 30) and symptom-free (n = 73) preterm infants, fewer symptomatic infants had any intermittent hypoxemia (60% vs 79%, P = .041). However, there was no significant difference in the percent of symptomatic versus symptom-free patients with any intermittent hypoxemia episodes per week (aOR=1.16 [0.62,2.13] controlling for infant and week) or in the seconds/hour with Spo2 < 90. The time with Spo2 < 90 was 5.4 sec/h lower for symptomatic vs symptom-free infants (P = .302) controlling for week.