Review of: Feldman M, Verbeek P, Lyons D, et al: "Comparison of the Medical Priority Dispatch System to an Out-of-hospital Patient Acuity Score." Academic Emergency Medicine 13(9):954-960, 2006.
Canadian ambulances and emergency departments have prioritized patients using the Canadian Triage and Acuity Scale (CTAS), a five category system, for several years. This study attempted to validate the priority assigned to 9-1-1 callers using the Medical Priority Dispatch System (MPDS). In MPDS, patients are assigned low to high priority in four levels, which are alpha, bravo, charlie, and delta, with an additional echo level for those with obvious life threats such as cardiac arrest, hanging, etc.
The authors divided all calls into low priority, CTAS 4 and 5, MPDS alpha and bravo, and high priority, CTAS 1, 2, and 3, MPDS charlie, delta, and echo. The MPDS priority was assigned by dispatch and the CTAS score was assigned by EMS.
In this study, 102,582 met inclusion criteria. The overall sensitivity of MPDS was 68.2% (95% confidence interval [CI] = 67.8% to 68.5%), with a specificity of 66.2% (95% CI = 65.7% to 66.7%). The most sensitive protocol for detecting high acuity of illness was the breathing-problem protocol, with a sensitivity of 100.0% (95% CI = 99.9% to 100.0%), whereas the most specific protocol was the one for psychiatric problems, with a specificity of 98.1% (95% CI = 97.5% to 98.7%).
The best performing protocol overall was the cardiac-arrest protocol. The protocol with the overall poorest performance was the one for unknown problems. Sixteen of the 32 protocols performed no better than chance alone at identifying high acuity patients.
This is one of the largest evaluations of MPDS to accurately provide the right level of resources to the patients based on the information the 9-1-1 dispatchers receive. Of course, the biggest limitation to this study is that CTAS was applied by EMS who knew the MPDS dispatch level. It's not clear how this may have changed the EMS category assignment. Because the patients were transported to 20 different facilities, the logistics of having a blinded third party assign a CTAS category, as well as examining outcomes such as admission rate, length of stay and even diagnosis, were not possible.
Despite these limitations, the MPDS was found to have both strengths and weaknesses. For the patients with high acuity complaints such as shortness of breath, chest pain, and cardiac arrest, MPDS performed well. For a significant number of protocols, MPDS could not reliably determine which patients should receive ALS or BLS response. For those of us who use MPDS, Unknown Medical Problem, is one that is frequently used because the dispatchers have difficulty in determining what is the patient's primary complaint. Hopefully, this study and others like it will aid further refinement of the MPDS questions so that a more specific and accurate protocol will be used.
In systems with tiered BLS and ALS response having quality dispatch is critical to using EMS resources more efficiently. Another component of safe EMS operations is proper use of lights and sirens response modes. Further improvements in priority dispatch will have significant impacts on both of these.