Ability of laypersons to estimate short time intervals in cardiac arrest.

STUDY
OBJECTIVE: Estimates of time intervals by bystanders are considered critical in cardiac arrest, and are often used in other disorders such as stroke and myocardial infarction. Because they have never been previously studied, we sought to determine their accuracy.
METHODS: This study was performed by prospective collection of bystander estimates (made at the time of the arrest) of the time from calling 911 to the arrival of urban fire department first responders, and comparison with actual measured response interval from computerized records, in all out-of-hospital cardiac arrests from January 1996 through June 1998.
RESULTS: The fire department responded to 1,015 patients in cardiac arrest during the study period. First responders arrived before advanced life support providers to 831 patients, who thus met study entry criteria. Bystander estimates were obtained in 497 of these 831 patients, who did not differ in key characteristics from those lacking estimates. The bystander's average estimated fire department response interval was 5.6 minutes (95% confidence interval [CI] 5.2 to 5.9 minutes) and the actual measured interval to the patient's side from computer records was 6.1 minutes (95% CI 5.9 to 6.4 minutes). However, the median error of the bystander estimate (1.3 minutes) was 32% of the median of the actual measured on-scene interval, and there was no correlation between the bystander estimates and the measured interval in individual cases (R </=0.14), regardless of which intervals were examined. Seventy-five percent of the bystander estimates erred by 20% or more. When bystanders estimated a response interval as excessively long, they were almost invariably wrong, but they also usually failed to identify intervals that actually were long.
CONCLUSION: Although many diagnostic and research conclusions are based on interval estimates from laypersons, we found no correlation between estimates and actual measured intervals in cardiac arrest. Current methodology may not be developed well enough to provide reliable data for research or quality assurance, and other clinical time estimates by patients and bystanders may be equally unreliable.