Research challenges link between quick EMS response and survival chances
By Margot Sanger-Katz
The Concord Monitor
CONCORD, N.H. — In 2002, emergency medicine specialists at the Denver Health Medical Center wanted to find out how much response times affected the survival of the major trauma patients they treated — often the victims of car crashes, gunshots and stabbings. They treated the kind of patients for whom every minute — they thought — would matter.
More Research Studies on the Response Interval – Clinical Outcome Relationship
By Mic Gunderson
This article from the Concord Monitor raises important questions about the relationship between response intervals and clinical outcomes. The author cited just one of several such studies that have been carried out over the past few years.
Here is a summary of some of the others from the peer-reviewed literature.
The Ontario Prehospital ALS (OPALS) Study Group publication entitled 'Optimal Defibrillation Response Intervals for Maximum Out-of-Hospital Cardiac Arrest Survival Rates1 ' examined 9,273 resuscitation cases from multiple cities in Ontario during a six-year time frame.
The average response interval was 6.2 minutes (90th percentile at 9.3 minutes). Their data correlated survival rates to 90th percentile compliance to various response intervals: 9 minutes (4.6percent survival), 8 minutes (5.9percent survival), 7 minutes (7.5percent survival), 6 minutes (9.5percent survival), and 5 minutes (12.0percent survival).
The investigators concluded that the 8-minute ambulance response interval target established in many communities is not supported by their data as the optimal EMS defibrillation response interval for cardiac arrest.
They suggest that EMS system leaders consider the effect of decreasing the 90th percentile defibrillation response interval to less than 8 minutes.
The study by Callaham and Madsen entitled 'Relationship of timeliness of paramedic advanced life support interventions to outcome in out-of-hospital cardiac arrest treated by first responders with defibrillators3' looked for the effect the interval between the arrival of first responder/defibrillators and paramedic advanced life support (ALS) interventions on outcome.
The study concluded that faster response by medics, or any individual ALS intervention other than first-responder defibrillation, demonstrated no benefit in this urban population with short intervals between responder arrivals. Aggressive ALS increased the number of survivors but also decreased their neurological outcome quality.
A study by Pons and colleagues entitled 'Paramedic response interval: Does it Affect Patient Survival3?' concluded that response intervals within 8 minutes were not associated with improved survival to hospital discharge after controlling for several important confounders, including level of illness severity for multiple condition types including cardiopulmonary arrest and trauma.
A survival benefit was identified when the response interval was within 4 minutes for patients with intermediate or high risk of mortality.
Adherence to the 8-minute response interval guideline in most patients who access out-of-hospital emergency services is not supported by these results.
1 The Ontario Prehospital ALS (OPALS) Study Group: Optimal Defibrillation Response Intervals for Maximum Out-of-Hospital Cardiac Arrest Survival Rates. Ann Emerg Med. 2003;42:242-250
2 Callaham MJ, Madsen CD: Relationship of timeliness of paramedic advanced life support interventions to outcome in out-of-hospital cardiac arrest treated by first responders with defibrillators. Ann Emerg Med. 1996 May;27(5):638-48
3 Pons PT, Haukoos JS, Bludworth W, Cribley T, Pons KA, Markovchick VJ: Paramedic response time: does it affect patient survival?. Acad Emerg Med. 2005 Jul;12(7):594-600
Blackwell and Kaufman published a paper entitled 'Response Time Effectiveness: Comparison of Response Time and Survival in an Urban Emergency Medical Services System.4'
The authors concluded that when emergency calls had response intervals of less than 5 minutes, they were associated with improved survival when compared with calls where response intervals exceeded 5 minutes.
They acknowledged that variables other than time may be associated with this improved survival, but there is little evidence in these data to suggest that changing the study system’s response interval specifications (10:59 for life threatening emergency cases and 12:59 for non-life threatening case) to times less than current, but greater than 5 minutes, would have any beneficial effect on survival.
The cases types included all cases types – including cardiopulmonary arrest and major trauma. Feero and colleagues’ study, 'Does out-of-hospital EMS time affect trauma survival?5' tried to determine if out-of-hospital emergency medical services time intervals are associated with unexpected survival and death in urban major trauma. Of 848 major trauma cases, there were 13 (1.5percent) unexpected survivors and 20 (2.4percent) unexpected deaths.
Of those patients with complete EMS times, the mean out-of-hospital response interval was significantly shorter for the unexpected survivors (3.5 +/- 1.2 minutes v 5.9 +/- 4.3 minutes; P = .04). The mean EMS on-scene time interval (7.8 +/- 4.1 minutes v 11.6 +/- 6.5 minutes; P = .06) and the mean transport time interval (9.5 +/- 4.4 minutes v 11.7 +/- 4.0 minutes; P = .17) also favored the unexpected survivor group. Overall, the total EMS time interval was significantly shorter for unexpected survivors (20.8 +/- 5.2 minutes v 29.3 +/- 12.4 minutes; P = .02).
It was concluded that a short overall out-of-hospital time interval may positively affect patient survival in selected urban major trauma patients. In a different study by Pons and colleagues entitled 'Eight minutes or less: does the ambulance response interval guideline impact trauma patient outcome?6', the authors concluded, that there was no difference in survival after traumatic injury when the 8 min ambulance response interval was exceeded.
There was also no significant difference in survival when patients were stratified by injury severity score group. The authors assert that exceeding the commonly used ambulance industry response interval criterion of 8 min does not affect patient survival after traumatic injury.
From my own perspective, I do not think there is much of any evidence to support choosing 8 minutes over any other particular number for an EMS response interval target in order to improve outcomes.
Looking at each of these studies in detail, I did not see data showing a particular time frame have any particular significance over any other. For the most truly time sensitive cases, such as a witnessed onset cardiac or respiratory arrest, the sooner the better is what the data shows – and even in urban systems, we probably do not get there fast.
That's because the costs go up exponentially as the response interval target gets shorter and shorter. So, where a community chooses to set the response interval target, it becomes more of a financial and political question than a clinical one.
4 Blackwell TH, Kaufman JS: Response Time Effectiveness: Comparison of Response Time and Survival in an Urban Emergency Medical Services System. Acad Emerg Med 2002; 9:288–295
5 Feero S, Hedges JR, Simmons E, Irwin L: Does out-of-hospital EMS time affect trauma survival? Am J Emerg Med. 1995 Mar;13(2):133-5
6 Pons PT, Markovchick VJ: Eight minutes or less: does the ambulance response time guideline impact trauma patient outcome? J Emerg Med. 2002 Jul;23(1):43-8
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