Frequency, duration and predictors of bronchiolitis episodes of care among infants ≥32 weeks gestation in a large integrated healthcare system: a retrospective cohort study

Bronchiolitis is the most common lower respiratory condition in children younger than 2 years [1], and presents with a wide spectrum of clinical illness, from mild symptoms to severe, life-threatening respiratory distress. More severe bronchiolitis is the most frequent cause of infant hospitalization in the United States [2, 3] with costs estimated at $543 million per year [4]. Bronchiolitis also causes a substantial emergency department (ED) and outpatient burden [5–7]. Populations at increased risk for severe bronchiolitis include racial and ethnic minorities and those who had preterm birth, bronchopulmonary dysplasia (BPD) or congenital heart disease. [6, 8, 9]

Previous research on the epidemiology of bronchiolitis has focused on ascertaining rates of outpatient care, ED visits, and hospitalizations, with widely differing results that can be attributed to differences in study designs, methodologies, and populations. Estimates of the rate of outpatient visits for children <2 years range from 103 per 1000 children [2] to 238 visits per 1000 child years [5]. Estimates of the rate of hospitalization also vary. Koehoorn et al. [10] found a hospitalization rate of 17.1 per 1000 child years for children <1 year. Shay et al., estimated similar rates of hospitalization in the US for infants <1 year ranging from 12.9 per 1000 in 1980 to 31.2 per 1000 in 1996 [11], whereas Carroll et al. [5] estimated a higher rate of 71 per 1000 child years during 1995–2003. Carroll et al. [5] also estimated a rate of 77 ED visits per 1000 child years. Respiratory syncytial virus is an important cause of bronchiolits, and recent research has examined the prevalence of respiratory syncytial virus (RSV) among children presenting for medical care, and found that 15-20% of children presenting for care had RSV infection or co-infection [12].

These existing studies have tabulated the frequency of hospitalizations and individual outpatient visits for bronchiolitis, and have estimated the frequency of RSV infection among those receiving care, but have not provided a comprehensive description of the healthcare utilization for each infant over a specific time period. In order to provide such a comprehensive description, health care effectiveness researchers have promoted “episode of care” analysis. Hornbrook et al. [13] first defined an episode as a “series of temporally contiguous health services related to treatment of a given spell of illness or provided in response to a specific request by the patient.” Recently episode analysis has been used as an important tool for estimating costs, determining patterns of use, evaluating outcomes of care, and understanding medical practice variation [13–15], using the entire duration of care as the basis for measurement, with the assumption that this will be more effective than counting single outpatient visits or single hospitalizations [13, 14, 16]. To the best of our knowledge, no previous studies have defined episodes of care for bronchiolitis or have reported their frequency, duration or risk during the first 2 years of life.

In this study, we use an “episodes of care” (henceforth, “episode” or EOC) approach to determine the frequency, duration and risk factors for bronchiolitis in children <2 years among members of a large, integrated healthcare system in Northern California. Using this approach, we analyze our results by gestational age (GA) to examine the burden of disease among those born preterm, who are considered at high risk for bronchiolitis [17]. Below we describe the frequency, duration and characteristics of bronchiolitis episodes and the clinical and demographic predictors of bronchiolitis episodes longer than one day among preterm and term infants.

Our main analysis focused on bronchiolitis episodes, which included ≥1 medically attended visit for bronchiolitis. Among all infants (123,264), 19,968 infants developed ≥1 episode for bronchiolitis during their first two years of life. Overall, there were 23,748 episodes identified throughout the study period. Among infants who experienced an episode, the mean number of episodes per infant was 1.2 within the first two years of life, so that 16.2% of infants eventually developed bronchiolits. The overall mean (± SD) duration of episodes was 3.2 ± 4.7 days. Overall, 2,036 (8.6%) episodes included a hospitalization. Of note, 6,351 (26.7%) included a diagnosis of otitis media.

Among episodes lasting >1 day (40%), the mean duration was 7.0 ± 5.9 days; of all episodes, 13.4% lasted 2–3 days, 11.3% lasted 4–6 days, 10.0% lasted 7–13 days and 5.0% lasted ≥14 days. The mean number of outpatient visits among episodes lasting >1 day was 2.3, while the mean number of emergency department (ED) visits was 0.2. The majority (85.2%) of episodes with an ED visit for bronchiolitis had no other healthcare encounters during the episode. However, among children who had an ED visit for bronchiolitis, the mean episode length was 6.3 days and the proportion of infants admitted to the hospital was 35.6% (n = 626).

Preterm birth was associated with both frequency and duration of bronchiolitis episodes (Table 2). The episode rate and corresponding mean duration increased with lower GA. The rate during the first two years of life and mean duration of bronchiolitis EOC were 246 per 1000 infants and 4.4 ± 5.5 days, respectively, for infants born at 32–33 weeks and 204 per 1000 infants and 3.9 ± 5.0 days, respectively, for infants born at 34–36 weeks GA. In contrast, the rate of episodes among full term infants was lower, 149–178 per 1000 infants. Infants of 32–33 weeks gestational age were also twice as likely as term infants to have a bronchiolitis episode that included an ED visit and a hospitalization (14.2% vs. 6.4%). Figure 2 shows the increased trend in duration of bronchiolitis episodes in infants with lower GA. We also examined episodes by chronologic age. Overall, 10,380 (43.7%) episodes occurred in infants <6 months of age. For infants <6 months of age, the proportion of bronchiolitis episodes including a hospitalization was 11.5%; this proportion was 6% for those aged 6–24 months.

Gestational age, wk	Infants, n	Infants with at least one EOC, n	Rate of at least one episode per thousand children	EOCa, n	Mean episode duration, d (SD)	Episode consisting of ED visit and hospitalization and lasting >1 db, n (%)	Episode with any hospitalization, n (%)
32–33	1,649	406	246.2	523	4.4 (5.5)	37 (14.2)	101 (19.3)
34–36	7,969	1,622	203.5	2,003	3.9 (5.0)	72 (7.8)	254 (12.7)
37	7,855	1,400	178.2	1,681	3.4 (4.6)	44 (6.4)	151 (9.0)
38–40	86,708	13,701	158.0	16,172	3.3 (4.7)	396 (6.3)	1,288 (8.0)
≥41	19,083	2,839	148.8	3,369	3.2 (4.7)	70 (5.6)	242 (7.2)
Total	123,264	19,968	162.0	23,748	3.2 (4.7)	619 (6.6)	2,036 (8.6)

ED = emergency department. EOC = Episode of Care.

^aAcross episodes during the first two years of life, there were a total of 35,703 outpatient visits for a rate of 289 per 1000 children, a total of 1909 emergency department visits for a rate of 15 per 1000 children and 2169 hospitalizations for a rate of 17.5 per 1000 children.

^bDenominator is among episodes >1 day for each gestational age cohort.

Our study shows that bronchiolitis care episodes were frequent during the first two years of life in this study cohort and that, consistent with prior research, most occurred outside the hospital setting. In our large cohort, the duration of bronchiolitis episodes ranged from 1 to 27 days. Among episodes with duration >1 day, the mean duration was 7.0 ± 5.9 days, potentially indicating significant morbidity for this subset of infants. Preterm infants compared to full-term infants had higher overall bronchiolitis episode rates as well as higher rates of episodes with an ED and/or inpatient hospital admission during the first two years of life, which might reflect both an intrinsically higher risk of morbidity in this population and/or an increased level of parent and provider concern regarding their medical needs. In addition to prematurity, male gender, neonatal oxygen use, family history of asthma, and African-American or Hispanic race/ethnicity were associated with a higher frequency of at least one episode and/or a higher frequency of an episode >1 day.

Consistent with previous studies, the overall burden of disease in this population was relatively high, with 16.2% of children in this study having a documented episode of bronchiolitis before age two. An episode of care approach in characterizing the utilization patterns of infants and children with bronchiolitis may be useful for health services researchers, physicians, and parents in understanding a broader healthcare burden of disease. Analyzing episodes of care may be more informative than reporting the rates of discrete bronchiolitis-related medical encounters.

Our study has several important limitations. First, not all infants with bronchiolitis seek medical care. This study reports frequency and associated characteristics of bronchiolitis that led to a clinical encounter, but is not a prospective study of the entire burden of illness from bronchiolitis that includes patient reports of symptoms [21]. Nevertheless, the study design provided a good approximation of the total health care utilization associated with bronchiolitis. Second, we ascertained bronchiolitis diagnoses using ICD-9 codes. To be considered an episode, a subject needed to have ≥1 diagnostic code specific for bronchiolitis or viral pneumonia attributable to pathogens typically causing bronchiolitis (Category A). Because bronchiolitis is a clinical diagnosis, it is possible that some cases of bronchiolitis were instead coded as “fever,” “wheeze,” or “lower respiratory infection.”However, it is also possible that some pneumonia attributed to pathogens typically causing bronchiolitis was not in fact bronchiolitis, and that some episodes of asthma, viral pneumonia or upper respiratory infection were coded as “bronchiolitis.” Therefore, it is likely that the total error from miscoding of bronchiolitis is relatively small. Third, we estimated membership based on a combination of membership and use status in each cohort month. Because infants with a higher level of use might tend to have higher frequency of seeking medical care, it is possible that this might bias the results towards higher estimates of bronchiolitis episodes. Since the sensitivity analysis including only infants with 24 months of documented membership did not show much difference from the primary analysis, the effect of membership status is likely to be minimal. Fourth, there were no reliable data on smoking for this analysis. Because smoking in the home is a major predictor of bronchiolitis episodes and because maternal smoking is associated with preterm birth and with race/ethnicity, it is possible that smoking might confound the relationship between GA and incidence of bronchiolitis. However, in our previous work we have found that the overall rate of smoking among mothers delivering at KPMCP is 4%–8% [22]. Therefore, we do not believe this has had a large impact on our results. Fifth, because rates of episodes and episodes with hospitalization might be impacted by participation in an integrated healthcare delivery system, it is possible that our results may not be generalizable to other healthcare systems. However, because the KPMCP provides medical care to a large portion of infants born in Northern California, our data represent an important estimate of overall care episodes in this population. Sixth, our data on race/ethnicity include only the categories White, African-American, Asian, Hispanic and other, and we do not have access to more detailed data. Therefore, we cannot report how more specific subgroups of race/ethnicity might alter bronchiolitis risk.

Despite these limitations, our research design offers the unique opportunity to study an entire birth cohort using linked data with relatively little loss to follow-up and to report the health care utilization related to bronchiolitis in this prospective cohort. Thus, we are able to provide a comprehensive description of burden of disease in this population, including frequency and severity of illness. In addition, the large sample size of our cohort has allowed us to identify many predictors associated with the disease outcome, which include gestational age, race/ethnicity, maternal age, small for GA, presence of congenital anomaly, family history of asthma, at least one sibling <5 years of age in the home, and degree of oxygen exposure or having BPD.

In this cohort, 16.2% of infants had one or more episodes of bronchiolitis in the first two years after birth, with episodes lasting from 1 to 27 days. Preterm infants were more likely to have a bronchiolitis episode compared to term infants and were more likely than term infants to have longer periods of healthcare utilization, perhaps reflecting both the medical needs of this particular high-risk population group and the vulnerability perceived by providers and parents. Using an episode of care approach to study the epidemiology of bronchiolitis may provide additional information about burden of disease for both preterm and term infants.