<p><strong>BACKGROUND: </strong>Early warning scores (EWSs) assign points to clinical observations and generate scores to help clinicians identify deteriorating patients. Despite marginal predictive accuracy in retrospective datasets and a paucity of studies prospectively evaluating their clinical effectiveness, pediatric EWSs are commonly used.</p>

<p><strong>OBJECTIVE: </strong>To identify mechanisms beyond their statistical ability to predict deterioration by which physicians and nurses use EWSs to support their decision making.</p>

<p><strong>DESIGN: </strong>Qualitative study.</p>

<p><strong>SETTING: </strong>A children's hospital with a rapid response system.</p>

<p><strong>PARTICIPANTS: </strong>Physicians and nurses who recently cared for patients with false-positive and false-negative EWSs (score failures).</p>

<p><strong>INTERVENTION: </strong>Semistructured interviews.</p>

<p><strong>MEASUREMENTS: </strong>Themes identified through grounded theory analysis.</p>

<p><strong>RESULTS: </strong>Four themes emerged among the 57 subjects interviewed: (1) The EWS facilitates safety by alerting physicians and nurses to concerning changes and prompting them to think critically about deterioration. (2) The EWS provides less-experienced nurses with vital sign reference ranges. (3) The EWS serves as evidence that empowers nurses to overcome barriers to escalating care. (4) In stable patients, those with baseline abnormal physiology, and those experiencing neurologic deterioration, the EWS may not be helpful.</p>

<p><strong>CONCLUSIONS: </strong>Although pediatric EWSs have marginal performance when applied to datasets, clinicians who recently experienced score failures still considered them valuable to identify deterioration and transcend hierarchical barriers. Combining an EWS with a clinician's judgment may result in a system better equipped to respond to deterioration than retrospective data analyses alone would suggest. Future research should seek to evaluate the clinical effectiveness of EWSs in real-world settings.</p>

<p><strong>IMPORTANCE: </strong>Rapid response systems aim to identify and rescue deteriorating hospitalized patients. Previous pediatric rapid response system implementation studies have shown variable effectiveness in preventing rare, catastrophic outcomes such as cardiac arrest and death.</p>

<p><strong>OBJECTIVE: </strong>To evaluate the impact of pediatric rapid response system implementation inclusive of a medical emergency team and an early warning score on critical deterioration, a proximate outcome defined as unplanned transfer to the intensive care unit with noninvasive or invasive mechanical ventilation or vasopressor infusion in the 12 hours after transfer.</p>

<p><strong>DESIGN, SETTING, AND PARTICIPANTS: </strong>Quasi-experimental study with interrupted time series analysis using piecewise regression. At an urban, tertiary care children's hospital in the United States, we evaluated 1810 unplanned transfers from the general medical and surgical wards to the pediatric and neonatal intensive care units that occurred during 370,504 non-intensive care patient-days between July 1, 2007, and May 31, 2012.</p>

<p><strong>INTERVENTIONS: </strong>Implementation of a hospital-wide rapid response system inclusive of a medical emergency team and an early warning score in February 2010.</p>

<p><strong>MAIN OUTCOMES AND MEASURES: </strong>Rate of critical deterioration events, adjusted for season, ward, and case mix.</p>

<p><strong>RESULTS: </strong>Rapid response system implementation was associated with a significant downward change in the preintervention trajectory of critical deterioration and a 62% net decrease relative to the preintervention trend (adjusted incidence rate ratio = 0.38; 95% CI, 0.20-0.75). We observed absolute reductions in ward cardiac arrests (from 0.03 to 0.01 per 1000 non-intensive care patient-days) and deaths during ward emergencies (from 0.01 to 0.00 per 1000 non-intensive care patient-days), but these were not statistically significant (P = .21 and P = .99, respectively). Among all unplanned transfers, critical deterioration was associated with a 4.97-fold increased risk of death (95% CI, 3.33-7.40; P &lt; .001).</p>

<p><strong>CONCLUSIONS AND RELEVANCE: </strong>Rapid response system implementation reversed an increasing trend of critical deterioration. Cardiac arrest and death were extremely rare at baseline, and their reductions were not statistically significant despite using nearly 5 years of data. Hospitals seeking to measure rapid response system performance may consider using valid proximate outcomes like critical deterioration in addition to rare, catastrophic outcomes.</p>