Failure of the afferent limb: a persistent problem in rapid response systems

The prognosis of in-hospital cardiac arrest (IHCA) is poor. Survival rates at hospital discharge reported in most of major studies do not exceed 20%. For this reason, prevention of IHCA is essential, and it should be based on prompt recognition of signs of deterioration, the most common being respiratory problems, impaired level of consciousness and haemodynamic instability. Those signs are present in up to 84% of IHCA during the 8 h preceding the arrest but unfortunately they are often not acted on, especially in general wards. Rapid response systems (RRSs) have been proposed as a measure to detect and treat deteriorating patients and reduce the occurrence of adverse events such as in-hospital cardiac arrests, emergency admissions to Intensive Care Unit (ICU) and unexpected deaths. RRSs include an afferent limb and an efferent limb. The afferent limb is committed to the ward personnel, who should detect critical patients on the basis of predetermined criteria and activate the efferent limb, i.e. a response team. Response teams have been differently referred as Medical Emergency Team (MET), Rapid Response Team (RRT) or Critical Care Outreach (CCO) according to their staffing (e.g. physician-led or nurse-led) or to their range of activities, but all have in common the task of obtaining rapid assessment, stabilization, and triage of deteriorating patients to a higher level of care. A series of single-center, before-and-after studies suggested that RRS could reduce the incidence of unplanned ICU admissions, cardiac arrests, and deaths. Unfortunately, the only randomized multicenter trial conducted so far, the MERIT study, did not demonstrate any benefit from the introduction of a MET-based RRS in a cluster of hospitals compared with control hospitals. A series of reasons have been advocated to explain this disappointing result. These include low power and a possible contamination of the control arm. However, failure of the afferent limb is another important reason to be considered. In fact, among 313 patients who had documented MET calling criteria more than 15 min before an unplanned ICU admission, the MET was actually called by the ward staff in only 95 cases (30%). With such a low utilization rate, any potential benefit from RRS would have been difficult to identify.