The study finds significant beneficial effects of hospital volume increases from low (≤ 25 infants annually) to medium or medium-high (72 infants annually) or high levels (144 infants annually) on reducing VLBW and VPB in-hospital mortality in the study countries. The largest incremental survival gains occur with volume increases from very low to medium levels (from ≤ 25 to 72 infants annually). There are significantly smaller survival gains with volume increases beyond these threshold and potential survival losses with volume increases beyond 144 infants. To our knowledge, there has been no specific evaluation of non-linearity in volume effects at very high thresholds in previous studies.
The study is the first to assess the effects of VLBW volume on in-hospital mortality in South American populations using a unique sample that has significant demographic, socioeconomic and geographic diversity, suggesting that it is well representative of large proportions of the study birth populations. The results are particularly relevant for less developed countries given the high VLBW mortality rates in hospitals with low volumes, which are more than twice as high as in developed countries. For example, the mortality rate of infants born in the study hospitals with less than 25 VLBW infants annually is 41%, compared to about 17.1% in California hospitals in 1997–2002 (Bartels et al. 2006
; Chung et al. 2010
; Phibbs et al. 2007
; Phibbs et al. 1996
). The high mortality risks at very low volume hospitals in the study countries emphasize the importance of developing policies that reduce the rates of delivering VLBW infants at these hospitals.
While the study has several strengths, some study limitations warrant discussion. One limitation is that we are unable to completely evaluate the extent to which the ECLAMC sample represents the whole infant populations in the study countries due to the lack of readily accessible population-level data. However, as described above, there is significant geographic diversity and variation in demographic and socioeconomic characteristics in the study sample, which enhance its representativeness. A related issue is that there are no readily available data to compare the neonatal intensive care levels between the ECLAMC hospitals and other hospitals in the study countries, in part due to the lack of a standardized system and data on neonatal care level (to our knowledge). Nonetheless, the extensive variation in volume and other observed characteristics between the study hospitals suggests that they are representative of a large proportion of the hospitals in the study countries.
In developed countries, higher risk pregnancies are generally referred into higher hospital volumes, which may attenuate the volume effects when ignored. The study models include several theoretically relevant variables and confounders which may in-part account for any potential self-selection into hospital volume based on fetal and maternal health risks and pregnancy progress. Furthermore, there is no consistent pattern of selection of higher risk pregnancies into higher volumes based on observed confounders in the study sample (Tables A7 and A8
in the Appendix list the distributions of the study variables by hospital volume). However, it is possible that there are additional unobservable risk factors that affect self-selection and that may attenuate the volume effects especially for very high volumes which show negative incremental survival gains compared to lower volumes and become insignificant when accounting for continuous hospitalization status.
One limitation is the censoring of the in-hospital mortality outcome for the group of infants who were still hospitalized at the last observation. We evaluate this issue using multiple approaches and find overall that this has no effects on the main results and study implications. Also, we have no data on admission to and length of stay in intensive neonatal care units. While this does not limit the estimation of the total hospital volume effects on mortality, this information would be useful to evaluate the process through which higher volume reduces mortality. We leave this question to future studies when such data become available.
The positive volume effects are generally consistent with findings from other populations (Bartels et al. 2006
; Chung et al. 2010
; Phibbs et al. 2007
; Phibbs et al. 1996
). However, we cannot directly compare all the estimated volume effects to previous studies due to differences in volume distributions and the non-linear volume effects. Furthermore, there are no previous studies of volume effects on VLBW mortality in South American populations to compare to. The study sample is smaller than those of other population studies. However, it provides adequate variation to evaluate the volume effects. The sample includes a small number of very high volume hospitals (see Appendix Tables A7 and A8
). However, to our knowledge, these hospitals have no unobserved special characteristics that are confounding their volume effects. A related issue is that ECLAMC provides mortality and prenatal data only on a subset of births in the ECLAMC-affiliated hospitals. However, this limitation mainly relates to sample size and less to representativeness, as ECLAMC imposes no selection criteria that bias sample representativeness. VLBW infants form a small percentage of the birth population (about 1–2%), making it extremely challenging to identify a larger sample without the existence of national-level birth-registry and hospital-discharge datasets. On practicality grounds, the ECLAMC sample is, to our knowledge, one of few data resources that allow studying volume effects on VLBW and VPB mortality for the study populations.
The study findings support regionalized care for at-risk pregnancies. However, in less developed countries where large populations have limited access to high volume providers typically located in highly urban areas, care regionalization may leave those non-urban residing mothers with high patient travel/time costs. Further, high volume providers in highly urban settings may already be operating on the flat-of-the-curve with minimal survival gains and even potential losses with increasing volume. The study finds that large survival returns in less urban areas may be achieved by increasing the access of at-risk pregnancies to hospitals with annual volumes of 49–72 VLBW infants. Additional smaller survival gains may be achieved at high levels up to 144 infants annually.