Epidemiology of apnea and bradycardia resolution in premature infants

Abstract

Background: There is little epidemiologic evidence to assess the maturation of respiratory control in premature infants.

Objective: To measure the success rate or the percentage of infants who have no additional events of various apnea- or bradycardia-free intervals after correcting for gestational age, postmenstrual age of the last apnea or bradycardia event, and the severity of the event.

Methods: This was a retrospective cohort study of infants born at 34 weeks' gestational age or earlier at 1 of 5 Kaiser Permanente Medical Care Program hospitals between 1998 and 2001. The success rates of various apnea- or bradycardia-free intervals were calculated after stratifying according to gestational age, postmenstrual age of the last event, or event severity.

Results: Among the 1403 infants identified in this study, 84.2% did not have an apnea event and 78.5% did not have a bradycardia event after they were otherwise ready for discharge. For the entire cohort, a 95% success rate was statistically reached, with a 7-day apnea- or bradycardia-free interval. Infants with a gestational age of 30 weeks or less had a 5% to 15% lower success rate than infants with a gestational age more than 30 weeks for any given apnea- or bradycardia-free interval. The success rate was reduced by an additional 5% to 10% if the last apnea or bradycardia event occurred at a postmenstrual age of more than 36 weeks. Including only the most severe events slightly improved the success rate of a given interval.

Conclusions: The risk of recurrence for apnea or bradycardia differs depending on the gestational age of the infant and the postmenstrual age of the last apnea or bradycardia event.

FIGURE 1

Inclusion criteria for the study project. The bar shows the course of a typical infant from birth (shown on the left-hand edge) through hospital discharge (shown on the right-hand edge). Arrows show important events for this project: the day of the last apnea or bradycardia event before the infant reached the ready-for-discharge date according to other criteria and the day when the infant was first ready for hospital discharge by other criteria. Three theoretical patients are shown above the solid line, and apnea or bradycardia events are shown to the right of the infant. Events shown as circles indicate events that occurred before the infant was ready for discharge according to other criteria and were not included in this study. Events shown as “X” indicate events that were included in the study because they occurred after the infant met other criteria for discharge. Patient 1 had the simplest course: the infant had no events after reaching the ready-for-discharge date. Patients 2 and 3 had events included in the study; the interval times are denoted by the “I2x” and “I3x” periods between events. The observation time began at the day of the last apnea or bradycardia event before the infant reached his or her individual ready-for-discharge date, denoted as the asterisk in the patient time lines.

FIGURE 2

Success rate of various observation times, stratified according to gestational age.

FIGURE 3

Success rate of various observation times, stratified according to the PMA of the last event and gestational age (GA).