IHD Journal Watch: Ambulance Diversion and Cardiac Access

November 30, 2015


Ambulance diversion occurs when an emergency department (ED) is temporarily closed to incoming ambulance traffic. Temporary ED closures can be triggered by overcrowding and a lack of available resources, as well as other reasons. Numerous studies have documented the adverse effects of ED overcrowding and, to a lesser degree, the negative effects of ambulance diversion; however, the mechanisms through which diversion affects patients have been less well studied. Proper identification of these mechanisms is critical as policy makers strive to implement solutions to improve quality of care for all populations, particularly those experiencing the poorest outcomes. The potential value of exploring these mechanisms is to determine whether exceptions to ambulance diversion for a small but extremely sick subset of patients could significantly improve outcomes.

Using 100% of Medicare claims and daily ambulance diversion logs from 26 California counties between 2001–11, researchers investigated the potential mechanisms through which ambulance diversion leads to poorer patient outcomes. We analyzed changes in access, treatment and outcomes when patients were exposed to different levels of diversion. Based on the conceptual pathway described below, researchers performed these analyses to understand the overall (that is, net) effects, as well as to evaluate the contribution of the intermediary mechanisms.


Patient data were extracted from the 100% Medicare Provider Analysis and Review (MedPAR) file, linked with vital files that contained date of death, between 2001–11. Researchers also linked the data with the Healthcare Cost Report Information System and the American Hospital Association annual surveys to obtain additional hospital-level information.

To identify which ED was closest to each patient, researchers supplemented hospital data with longitude and latitude coordinates of the hospital’s physical address or heliport (if one existed). They obtained actual driving distance from the patient’s ZIP code centroid (that is, center point of each ZIP code polygon) to the nearest hospital’s latitude and longitude coordinates based on Google Maps, using automation codes developed in Stata software.

To identify a hospital’s daily ambulance diversion hours, the researchers used daily ambulance diversion logs from California’s local emergency medical services agencies. Our logs contained data for 17 of the 23 local emergency medical services agencies that did not ban diversion for the years 2001–11 (actual coverage dates vary for each agency). The 17 local emergency medical services agencies covered 26 of California’s 58 counties and 88% of the state’s population.


After controlling for hospital capability and treatment received, researchers observed that high levels of diversion were associated with an increased one-year mortality rate by 2.38 percentage points, representing an 8.2% relative increase in mortality when compared to patients across the diversion categories with comparable technology access and treatment patterns. The lack of physical access to the technology is associated with a 4.6% relative reduction in the likelihood of catheterization or PCI and a 9.8% increase in one-year mortality. Researchers did not observe significant differences in other health outcomes.


Ambulance diversion is a contentious issue. Those in support of ED diversion contend that the practice decreases ambulance wait time, thereby increasing ambulance resource availability in the field. Detractors of the practice cite the risks of time lost while ambulances navigate various secondary resources when the closest, most appropriate hospital is on diversion status, and the delay in high-acuity patients receiving timely critical interventions.

The underlying issue, from a policy issue, is ED overcrowding, which leads to the perceived need for EDs to go on diversion status. A recent report from George Washington University cites the following finding from a 2014 study on EDs:1

  • Between 1995–2010, annual ER visits in the U.S. grew by 34%;
  • The number of EDs in the United States has dropped by 25% in the past two decades;
  • There are 130 million ED visits annually in the U.S.;
  • 20% of adults visit the ED one or more times;
  • The average ED wait time in 2003 was 46.5 minutes;
  • The average ED wait time in 2009 was 58.1 minutes (62.4 minutes in urban EDs);
  • 50% of EDs operate at or above capacity;
  • 90% of EDs report patient boarding while awaiting inpatient beds;
  • 500,000 ambulances are diverted annually due to overcrowding;
  • 88% of patients in the ED in 2010 were triaged as urgent/less urgent/nonurgent.

When you compare the findings from the Health Affairs analysis with the findings from GWU, it seems the best way to reduce ED diversions and the increased mortality findings is for the healthcare system to take an assertive, proactive approach to better integrate care for patients using the ED for nonemergent reasons. An additional recommendation would be for emergency care systems to develop protocols and procedures to eliminate diversion of patients requiring critical interventions such as PCI.


1. Newhook E. State of Emergency: Overcrowding in the ER [Infographic]. George Washington University,

Journal Source

Shen YC, Hsia RY. Ambulance diversion associated with reduced access to cardiac technology and increased one-year mortality. Health Aff (Millwood), 2015 Aug; 34(8): 1,273–80.

Matt Zavadsky, MS-HSA, EMT, is the public affairs director at MedStar Mobile Healthcare, the exclusive emergency and non-emergency EMS/MIH provider for Fort Worth and 14 other cities in North Texas and the recipient of the EMS World/NAEMT 2013 Paid EMS system of the Year.