Discussion
This study describes respiratory healthcare utilization in the year following NICU discharge for infants with BPD discharged with home oxygen, in the setting of a standardized home oxygen weaning guideline. We found that pre-discharge pCO2 was not associated with higher risk of readmission or other hospital respiratory encounters but was associated with prescription of systemic steroids and inhaled corticosteroids. Higher pCO2 at 36 weeks correlated with longer duration of home oxygen or later corrected gestational age at which oxygen was discontinued.
Infants with BPD are at increased risk for hospital readmission and other healthcare utilization. Kovesi et al. previously showed that an elevated capillary pCO2 prior to discharge was associated with an increased risk of readmission or a severe adverse event, defined as late pulmonary hypertension, reintubation or death after discharge3. Since that publication, we have talked to colleagues at many institutions who, like our institution, recommend discharge after a pCO2 is less than 60; this is the first study of which we are aware that evaluates outcomes in the setting of such a recommendation. In our study, we did not find that capillary pCO2 at discharge or at 36 weeks was associated with more readmissions or reintubations; no infants in our cohort died after discharge and we had few infants with late pulmonary hypertension. The lack of association between pCO2 and readmission could be secondary to our recommendation to discharge infants after the pCO2 is <60 mm Hg. We saw the same non-association between pCO2 and readmission in the referral group, but similar to our NICU group, few referral infants had high pCO2s prior to discharge. Considering our readmission rate for infants discharged with home oxygen therapy was not high, it is possible that close outpatient follow up was successful in controlling symptoms that otherwise may have led to readmissions 14,15. Infants were seen in pulmonary clinic 4-6 weeks following discharge from the NICU, and then every 4-6 weeks to 2-3 months until they were off supplemental oxygen. It has been noted that outpatient management explains some variation in readmission rates for premature infants15,16. For infants with BPD discharged with home oxygen, one example of close outpatient management may be the use of inhaled and systemic steroids, which we noted to be more common in infants with a higher pre-discharge pCO2. Ryan and colleagues similarly found in the Prematurity and Respiratory Outcomes Program that inhaled steroids and bronchodilators increased over the first year 8. It possible that outpatient management such as early use of inhaled or systemic steroids helps prevent our highest-risk patients from being admitted or presenting to the emergency room. This raises the issue of whether to continue the recommendation to delay discharge for infants with higher pCO2. Our institution’s time to discharge for infants with BPD is still similar or shorter than other children’s hospital institutions, so the impact on NICU length of stay would likely be modest14. But if close outpatient follow-up can mitigate some potential risks, there may be benefits to individual patients by facilitating earlier discharge. Future implementation and quality improvement work will be needed to determine best practices moving forward.
Clinical practice guidelines of the American Thoracic Society recommend home oxygen therapy for infants with BPD who have chronic hypoxemia; home oxygen therapy is used in at least half of infants with BPD discharged from U.S. NICUs 17-20. After NICU discharge, however, there are few published guidelines for the close monitoring required to wean home oxygen therapy in the outpatient setting 19,21,22. We developed an oxygen weaning protocol in 2012 to standardize the care of infants being discharged with home oxygen in our clinic. Using this guideline, we were able to effectively wean home oxygen using home oximetry studies. Only 3 patients received sleep studies due to concerns for the quality of the home oximetry study. Our use of this clinical guideline enabled us to assess duration of home oxygen use as a secondary outcome in this study. We found that a higher pCO2 at 36 weeks corrected age, but not pre-discharge, was associated with a longer duration of home oxygen or later corrected gestational age at which oxygen was discontinued. We noted that pCO2 at 36 weeks was correlated with other measures of NICU illness severity, such as gestational age, days of mechanical ventilation, and BPD severity. Kaempf and colleagues have previously shown that a 36-week capillary blood gas pCO2 correlated with NICU illness severity; our findings suggest that in the setting of a home oxygen weaning guideline, NICU illness measures such as 36-week pCO2 may be used to predict duration of outpatient home oxygen therapy 23. Rhein and colleagues recently reported results of a randomized clinical trial of two different strategies to wean home oxygen in the outpatient setting; they noted that infants whose parents reported more home weaning attempts were safely weaned from oxygen faster 24. In this context, our findings present an opportunity to use NICU illness measures to tailor outpatient management and counsel families in a more individualized fashion. If parents have a better idea of the expected duration of home oxygen therapy for their infant, it may influence them to participate more actively in the home oxygen weaning process.
Strengths of this study include the level of clinical detail available in a single center, the use of an outpatient clinical guideline and high degree of follow-up. There are several limitations. The biggest limitation was that this was a single-center study with a general recommendation not to discharge infants until pCO2 < 60 mmHg; we tried to examine associations between pre-discharge pCO2 and outcomes from referral NICUs, but in a medium sized referral area those NICUs may already follow a practice similar to our own. We cannot determine from chart review what led to decisions regarding NICU discharge. Even in the cohort from our own NICU, although outpatient oxygen weaning follows a guideline, NICU weaning of respiratory support is not protocolized; we were unable to determine how much of infants’ NICU length of stay was attributable to our guidelines regarding pCO2. Although our clinic follows a protocol for follow-up of infants with home oxygen, we were not able to control the exact timing of when appointments were made for each family that may affect the exact duration of home oxygen. We tried to mitigate this somewhat by also evaluating proxy measures of ability to wean home oxygen such as room air trials in clinic. For future studies of oxygen discontinuation, we will delineate between the providers approval to discontinue home oxygen and when the oxygen is removed from the home.
In conclusion, we found that pCO2 prior to NICU discharge or at 36 weeks corrected age was not associated with differences in one-year respiratory readmissions. Higher pCO2 at 36 weeks was associated with later corrected gestational age at which oxygen was discontinued, as well as longer duration of home oxygen therapy. In the setting of an outpatient oxygen weaning protocol, measures of neonatal illness severity at 36 weeks such as pCO2 may be useful in communicating expectations for home oxygen therapy to families. The association between pre-discharge pCO2 and subsequent healthcare utilization, including duration of home oxygen therapy requires further study, especially as hospitals look toward earlier discharge of preterm infants.