We examined the association of PNRs on outcomes of moderately preterm infants. The main result from this study is that PNRs alone were not reliably associated with outcomes of care in this population. Despite exhaustive efforts, we were unable to reliably discriminate quality of care delivery among participating NICUs. Our finding of more generous nurse-staffing in small NICUs should, therefore, not be understood to mean that lower staffing ratios do not affect quality of care. Our interpretation is rather, that other measures need to be developed to more reliably discriminate quality of care in this relatively healthy population.
The greater nursing resource use in small NICUs does not necessarily reflect excess staffing capacity but rather the need for flexibility in staffing, because patient-census fluctuations per staff are proportionally larger in small NICUs. In addition, nurses in small NICUs may be performing a variety of duties that are the responsibility of dedicated ancillary staff in large NICUs. However, despite the higher salaries that nurses typically command when compared with ancillary staff, and despite the similar outcomes between small and large NICUs, the greater nursing resource use in small NICUs may not necessarily constitute an efficiency problem from the perspective of society. Although large NICUs may achieve efficiencies of scale with regard to the use of resources, they may be less able to offer other benefits parents value, such as reduced travel time and improved opportunities to spend time with their children or return to work.
The relation between nurse-staffing and adverse outcomes might follow a curve in which outcomes do not change perceptibly across a range of PNRs until they reach an inflection point at which nurses are unable to account for increasing workload. For the measures analyzed in this study, it seems that nurse-staffing in the participating NICUs operates on the flat part of the curve. It is important to emphasize that this study is hypothesis generating and cannot determine causality. Thus, whether lowering the PNRs would lead to fewer or more adverse outcomes is unknown but clearly warrants empiric examination in the context of the chronic nursing shortage. Differences in nurses’ job profile between small and large NICUs, the extent to which responsibilities are adjusted to patient acuity, and nursing quality need to be examined, among other questions.
In addition to the relative health of our study population, several reasons might have prevented us from finding a significant relation between PNRs and clinical outcomes. Our measurement unit for this study (daily PNRs) may not be sensitive enough to detect the kind of staffing adjustments NICUs are able (or unable) to make in response to a sudden rise in patient severity, census, or both. For example, in the Kaiser Permanente Medical Care Program, it is extremely uncommon to have more than a 2-hour lag in obtaining an additional nurse when one is needed. Thus, changes in staffing levels are occurring in a time frame that is much narrower than that measured in our study.
Traditional measures of care outcomes, such as chronic lung disease or nosocomial infections, occur infrequently in moderately preterm infants. In contrast, average daily weight gain is attributable to all patients and may, therefore, be a more sensitive change indicator. Indeed, we found that an additional patient per nurse resulted in a 24% decrease in average daily weight gain. Explanations for this finding require additional exploration. We speculate that accumulation of small defects in care (less time for feedings and for developmental care, more untimely patient interruptions, less rapid weaning from a ventilator or from parenteral nutrition, etc) may have resulted in reduced weight gain.
It is possible that PNRs influenced quality of care and outcomes of more premature infants in the same NICUs. Because illness severity is higher in these infants, it is conceivable they would be more vulnerable to the consequences of higher workload.
In other areas of medicine an increased nursing workload has been associated with an increased risk of morbidity and mortality. For example, Pronovost et al2
found a nearly twofold increase in the risk for postoperative complications after abdominal surgery in patients who received care in ICUs with high PNRs (3–4:1). Tarnow-Mordi et al4
similarly found that the adjusted mortality rate was more than doubled in patients exposed to high versus low ICU workloads, defined by average nursing requirement per occupied bed and peak occupancy. Other authors have substantiated these results.15,16
In neonatology, the evidence for an association between nurse-staffing and patient outcomes has been contradictory. In a previous study, we found a significant correlation between a different quantitative measure of workload (NICU census) and the decision to discharge preterm infants.17
Low NICU census resulted in discharge of fewer-than-expected infants and vice versa. Most neonatal staffing-related research has focused on the effects on very low birth weight infants. These studies have yielded inconclusive results, and those conducted abroad may not easily translate to the United States because of differences in nurse training and staffing arrangements. Findings from the United Kingdom Neonatal Staffing Study of>13 000 infants at 54 NICUs showed a strong correlation between NICU occupancy and mortality. Although the authors of that study did not find a relation between absolute PNR and mortality for the whole cohort, the ranked percentiles of PNR in every NICU showed an increase in mortality rate with an increase in the PNR.7
Hamilton et al6
studied patients from 7 Scottish and 2 Australian neonatal units and found that the odds of risk-adjusted mortality increased by nearly 80% with assignment of >1.7 infants per nurse per shift. Conversely, the authors of a single-center Australian study reported that the adjusted odds of mortality improved by 82% with the PNR above the highest tercile for this NICU, suggesting improved survival with the highest PNR.8
Contradictory results might be related to differences in study design and national models of care provision, yet these findings certainly warrant cautious interpretation and additional study.
The results of our study must be viewed within the context of the study design. One limitation of this study is the potential for transfer bias, which may be introduced around the time of birth or later in the course of an infant when transfer occurs for convalescent, chronic, or acute care. However, sensitivity analyses demonstrated no significant influence on study conclusions. Sampling bias because of enrollment at discharge and exclusion of infants transferred to a nonparticipating center may have affected our findings. However, given that 7 of 10 facilities were level III NICUs, and given that our previous work with this study population demonstrated an association between higher NICU census and greater number of discharges,17
we think our sample was relatively enriched with sicker infants. In this case, bias favors finding clinically significant associations of staffing ratios with clinical outcomes.
Observational studies are subject to confounding from unmeasured variables across NICUs, such as differences in patient mix, patient-care practices, staffing, and organizational make-up. For example, the PNR is merely a quantitative measure of nurse-staffing. We had no information regarding the quality of nursing, which might vary among NICUs and might influence patient outcomes.18
However, this study is strengthened by several efforts to control bias and confounding, including the use of sampling strategies intended to correct for sitespecific differences in recruitment, adjustment for clinical risk by using previously validated tools, and correction for hospital-level effects.