Inter-Neonatal Intensive Care Unit Variation in Discharge Timing: Influence of Apnea and Feeding Management

DISCUSSION

Length of hospital stay in premature infants is most strongly influenced by GA at birth and birth weight.^1–3 Despite studies that document a predictable pattern of attainment of maturational milestones in premature infants necessary for safe discharge to home,^3,,4 little is known about how variation in management or discharge practices among practitioners or NICUs affects length of stay. We hypothesized that inter-NICU variation in the recognition and documentation of physiologic maturity and the required margin of safety before discharge would affect length of stay in individual NICUs.

When analyzed by GA groups, we observed the expected progression of more mature behavior with older GA at birth (Table 2). Although the occurrence of apnea and the need for PG feeds usually are well documented, we did not record reasons for incubator use because this is often difficult to assess from the medical record. It is possible that incubator use, especially in the gestationally older infants, was not related to immature temperature regulation but rather to other nursing issues (eg, phototherapy). Although the 34-week infants attained physiologic maturity at a slightly older PMA, the margin of safety was slightly longer in the younger infants. Thus, the PMA at discharge was nearly identical across the GA groups.

In contrast to the GA group analysis, length of hospital stay varied significantly between the 15 NICUs. The interval between the earliest and latest discharging NICU was approximately 1.5 weeks. Differences in the PMA at discharge across sites remained significant when corrected for GA at birth and other clinical factors that affected discharge timing, including birth weight z score and outborn status. Other clinical parameters examined, including hospital level of care, feeding method at discharge (breast or bottle), and sex did not affect the distribution of PMA at discharge.

Because our study sample was a homogeneous population of premature infants free of significant medical or surgical complications, we expected that the distribution of PMA at attainment of physiologic maturity would be similar across sites. However, NICUs varied widely in the PMA at resolution of apnea of prematurity, oral feeding (Figs 3 and4), and independent thermoregulation. This inter-NICU variation in recorded attainment of maturational milestones was the most significant influence on length of stay. In contrast, the margin of safety, although slightly different between NICUs, did not systematically alter discharge timing (Fig 5).

It is possible that despite our restrictive inclusion criteria, subtle differences between hospitals in severity of illness may have influenced the development of physiologic maturity and hence the PMA at discharge. However, infants born at the youngest GAs included in this study, who might be expected to be sicker at birth, attained documented physiologic maturity at a slightly earlier PMA than the 34-week infants (Table 2). In addition, GA at birth had no effect on PMA at discharge. We included both inborn and outborn infants in the study population, which should have obscured any differences in initial severity of illness between hospitals. Indeed, no effect of the level of care provided at the hospitals on PMA at discharge was observed. We are aware of no data that demonstrate that other biological or demographic factors that might differ between hospitals, such as ethnicity, sex, or socioeconomic status, might alter the development of physiologic maturity in premature infants. Our results strongly suggest that variation in care practices, rather than differences in clinical characteristics, contributed to differences in the recognition of maturity, and hence discharge timing, between hospitals.

In the absence of a GA or other biological effect, the inter-NICU differences we observed in the timing of apnea resolution could be influenced by several factors, including monitoring methods used to detect apnea or bradycardia and documentation practices. We observed significant differences between sites in the duration of use of pulse oximetry, and longer duration of use was associated with later documentation of apnea resolution. In a study in which oximetry was used in addition to standard cardiorespiratory monitoring of seemingly well-convalescent preterm infants, 10% of the infants studied experienced prolonged episodes of hypoxemia without associated bradycardia or prolonged apnea.¹¹ Although their use was not documented in our study, pneumogram recordings of cardiorespiratory activity of premature infants otherwise considered ready for discharge often reveal abnormalities undetected by standard monitoring techniques.^12,,13 It is possible that different approaches to monitoring of convalescent preterm infants might have altered the recording of apneic events and thereby affected discharge timing.

Inter-NICU differences in policies regarding monitor alarm settings and documentation of events might also affect recording of the last apneic or bradycardic episode. No consensus exists about the clinically significant lower limit for bradycardia or the significance of bradycardia without associated apnea in otherwise healthy premature infants approaching discharge to home.^3,,14,15 Hospitals are also likely to differ in nursing response and documentation of these events. We included all apneic or bradycardic episodes documented by the nursing staff regardless of their severity in part to account for this potential variability. It is likely that such differences in monitoring and documentation practices between NICUs included in the study sample contributed to the observed variation in timing of apnea resolution.

We did not differentiate between apneic spells that occurred on or off methylxanthine therapy. It is likely that the majority of the last apneic or bradycardic episodes recorded occurred while off methylxanthines because few infants were discharged on this therapy (Table 2). However, differences between NICUs in the PMA when methylxanthines were stopped might have obscured the true timing of the development of mature cardiorespiratory control in some infants and influenced our results.

In the later-discharging NICUs, infants took a longer time to reach full oral feedings and independent thermoregulation in addition to a longer duration of documented apnea or bradycardia episodes (Figs 3 and4). Although little is known about developmental aspects of feeding behavior, mature cardiorespiratory and feeding behavior often develop in parallel.^3,,4 It is not clear whether specific feeding practices can shorten the time course to transition from gavage to full oral feedings.^16,,17 However, just as differences in monitoring and documentation might prolong the apparent time to resolution of apnea, some feeding practices might delay the recognition of mature feeding behavior. For example, unit policies that limit oral feedings until infants have reached a specific postnatal weight or PMA might lengthen hospital stays in infants who are otherwise ready for discharge. We speculate that in the absence of a biological explanation, individual NICUs or practitioners with a more conservative approach to feeding, apnea management, or incubator use may prolong hospital stays even in an otherwise healthy population of premature infants.

Because of the observed inter-NICU variability in attainment of mature behavior, we chose to define the margin of safety as the interval between complete physiologic maturity (adequate temperature control, cessation of apnea and bradycardia, and oral feeding) and discharge to home rather than the more standard definition using only the date of last apnea.⁴ This definition takes into account variation in NICU practices that might delay or accelerate the recognition of mature feeding or cardiorespiratory behavior in individual hospitals. As expected, we observed inter-NICU differences in the measured margin of safety from the attainment of physiologic maturity to discharge to home (Fig 5). The small variability in the duration of the margin of safety did not have a systematic effect on length of hospital stay. Results were similar when we examined the interval between the last apneic or bradycardic event and discharge. In a 1996 survey of neonatologists regarding apnea management, Darnall et al reported a range of the number of apnea-free days required before discharge to home.⁴ Differences were small, however; >70% of the those surveyed required an interval of 5 to 7 days, consistent with the results in our study (Fig 5, Table 2). Our results suggest that the discretionary interval between mature behavior and discharge to home, though slightly different between NICUs, is not a major contributor to differences in discharge timing in the healthy premature population included in this study.

Our results have important implications for discharge guidelines for premature infants. Early discharge programs for premature infants recently have been described, evaluated,^14,18–21 and included in proposed national guidelines for hospital discharge of the high-risk neonate.²² These programs generally have combined more intensive parental teaching with stronger community-based supports for parents of premature infants who have demonstrated physiologic maturity, regardless of weight or PMA. Several studies have demonstrated the effectiveness of these programs, with significant reductions in length of stay and hospital costs within single institutions.^18,,19 Discharge timing of premature infants is a complex process influenced by both medical and nonmedical factors. However, the disparity in the timing of documented physiologic maturity between the NICUs included in our study calls into question whether early discharge programs will be equally effective in different institutions without including guidelines for monitoring and documentation practices.