National Trends in Ambulance Use by Patients With Stroke, 1997-2008

To the Editor: Thrombolytic therapy improves outcomes after ischemic stroke, but most patients are ineligible because they do not present in time.¹ This has prompted efforts to educate people to call 911 for signs of stroke because ambulance transportation results in faster arrival at the emergency department (ED).² Regional studies have suggested suboptimal ambulance use among patients with stroke,³ but none has examined a nationally representative population or temporal trends since the approval of thrombolysis.

We analyzed data collected by the National Hospital Ambulatory Medical Care Survey (NHAMCS) between 1997 and 2008.⁴ A nationally representative random sample of 340 to 408 EDs was surveyed annually, reflecting a participation rate of 87% to 98% and constituting approximately 10% of US EDs. Staff used structured forms to collect data about a systematic random sample of patients over a random 4-week period. Analysis of this publicly available deidentified data set was exempt from evaluation by our institutional review boards.

We included patients with a primary diagnosis of ischemic stroke, defined by International Classification of Diseases, Ninth Revision codes that have been validated for identifying patients with acute stroke and used in other studies.⁵ Additionally, we included patients with subarachnoid hemorrhage, intracerebral hemorrhage, and transient ischemic attack because these can present similarly to ischemic stroke. Our outcome was arrival at the ED via ambulance. We used survey visit weights provided by the NHAMCS to estimate the national proportion of patients diagnosed with stroke in the ED each year who arrived by ambulance. We examined trends within subgroups defined by characteristics associated with ambulance use: age, sex, race, payment source, geographic region, and stroke subtype.⁶ We performed sensitivity analyses limited to ischemic stroke and excluding patients not admitted to the hospital or with additional ED diagnoses besides stroke.

A survey-weighted χ² test for trend was used to examine the statistical significance of changes in ambulance use over time. We used multiple logistic regression to analyze yearly trends in ambulance use for stroke while controlling for covariates. The threshold of statistical significance was a 2-sided α level of .05. Statistical analysis was performed with Stata SE version 11 (StataCorp).

Overall, 19% (95% CI, 18%-19%) of adults nationwide presented to the ED via ambulance, with no significant change between 1997 and 2008 (P = .18). Based on 1605 cases, 51% (95% CI, 48%-54%) of patients with stroke arrived at the ED via ambulance. This proportion did not change significantly between 1997 and 2008 (49% vs 51%, P = .92; Figure). This was true in all subgroups, with the exception of downward trends among young patients and patients with a payment source other than private insurance, Medicare, or Medicaid (Table). Multiple logistic regression confirmed that overall rates of ambulance use did not change over time (odds ratio [OR] per year, 0.99; 95% CI, 0.96-1.03). Results were similar in analyses limited to patients with ischemic stroke (OR per year, 0.99; 95% CI, 0.95-1.04), without any secondary ED diagnoses (OR per year, 0.98; 95% CI, 0.93-1.04), or admitted to the hospital (OR per year, 1.00; 95% CI, 0.96-1.05).

Using data from a nationally representative sample of ED visits, we found that the proportion of patients with stroke who present via ambulance has not significantly changed over the past decade. Our analysis lacked power to detect temporal changes in subgroups. We could not determine what proportion of stroke diagnoses represented miscoding or mimics of stroke; however, our results were robust across sensitivity analyses addressing possible misclassification of cases.

Several factors may explain static ambulance use since the approval of time-sensitive therapy for ischemic stroke. Educational efforts may not be adequately addressing poor public knowledge about stroke, additional behavioral barriers may remain among those with adequate knowledge, or the response of health care providers to patients with stroke symptoms may be imperfect.³ Our findings suggest that national efforts to address barriers to ambulance use among patients with stroke need to be intensified or adjusted.

Author Contributions: Drs Kamel and Fahimi had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Kamel, Navi, Fahimi.

Acquisition of data: Fahimi.

Analysis and interpretation of data: Kamel, Fahimi.

Drafting of the manuscript: Kamel.

Critical revision of the manuscript for important intellectual content: Navi, Fahimi.

Statistical analysis: Kamel, Fahimi.

Administrative, technical or material support: Kamel, Navi.

Study supervision: Navi.