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Timing Is Everything: Do We Need to Redesign the Afferent Limb in Rapid Response?

Critical Care Medicine www.ccmjournal.org 2247 Rapid response systems (RRSs) have been evolving over the last four decades, from the initial development of specialized cardiac arrest response teams (CAT) to the increasingly prevalent medical emergency team (MET) and rapid response team models in place to provide critical care interventions in the presence of unexpected physiological instability (1). RRSs are a “safety net” based on the detection of deterioration (afferent limb) to drive time-sensitive interventions by the RRS responders (efferent limb), which are typically a team of critical care trained clinicians. When the RRS is activated, the responding clinician(s) assesses the patient’s condition within minutes to coordinate immediate needs, such as fluid administration or central venous catheter insertion. The afferent limb of RRSs is challenging to measure and evaluate. Traditionally, rapid response activations rely on clinical assessments by nursing staff to identify patient deterioration through vital sign derangements (e.g., systolic blood pressure < 90 mm Hg) or nursing concern about the patient’s condition. However, although there is evidence that vital sign abnormalities are warning signs of physiologic instability, a universal set of activation criteria (triggers) that have adequate sensitivity and specificity to warrant the activation of rapid response remains undefined (2, 3). To further complicate rapid response outcome measurement, the structure (team composition, staffing) and processes (activation criteria) of RRSs can vary substantially between hospitals and change over time, which limits the ability to compare and/or pool results across studies. As the only major multicenter, cluster-randomized controlled trial designed to evaluate rapid response outcomes, the Medical Early Response Intervention and Therapy (MERIT) trial (2004–2005) remains a rich source of data (4). In this issue of Critical Care Medicine, Chen et al (5) report their MERIT trial findings in more detail to explore the relationship between afferent limb failures of delayed RRS activations and patient outcomes. The MERIT trial includes 727,153 hospitalizations from 23 hospitals (11 randomized to maintain cardiac arrest teams and 12 randomized to add MET to existing cardiac arrest teams) in Australia over a 12-month period. Of these, 3,135 hospitalizations with CAT or MET activations were analyzed to compare patients with timely rapid response activations versus delayed activations (> 15 min) to identify if there were differences in mortality, cardiac arrests, or unplanned ICU transfers between groups. This study expands on the original MERIT trial results reporting afferent limb failure in more than 70% of admissions (8,133/11,268) with an ICU transfer, cardiac arrest, or death that did not result in any type of rapid response, by examining the influence of timing on hospitalizations that did include a rapid response activation. Delayed activations (> 15 min) were common, representing 30% of the rapid response activations. When adjusted for hospital cluster effect and hospital-, patient-, and activation-related characteristics, delayed activations were associated with an increased risk of death in all hospitals combined (odds ratio [OR] = 1.79; 95% CI, 1.43–2.27; p < 0.001). In addition, patients with delayed calls were 1.56 times more likely to have an unplanned ICU transfer. Characteristics associated with delayed activations included hospitalization in a rural hospital, being older than 75 years, and MET activations for altered mental status or hypotension. Although the MERIT trial data were collected a decade ago, in some instances, the large-scale multicenter cluster-randomized findings provide validation to more recent single-study cohort studies, while in others they are contradictory. For example, hypotension and oxygen saturation less than 90% were the main triggers associated with delayed calls in a single-center prospective observational study in Brazil, but unlike the MERIT trial data, the same study found no association between delayed activations (> 30 min) and the patient’s age (6). Chen et al (5) reported delays dichotomously by grouping all delayed activations as less than or greater than 15 minutes, so the magnitude and dispersion of the delayed activations associated with poorer outcomes are unclear. If many activations included substantial delays (> 8 hr), it could inform different strategies to resolving afferent limb failures. When comparing this subset of MERIT trial findings that essentially describe the birth of RRS programs with Copyright © 2015 by the Society of Critical Care Medicine and Wolters Kluwer Health, Inc. All Rights Reserved. DOI: 10.1097/CCM.0000000000001239 *See also p. 2059.