ECG scores for a triage of patients with acute myocardial infarction transported by the emergency medical system

Journal of Electrocardiology Vol. 33 Supplement 2000

ECG Scores for a Triage of Patients With A c u t e Myocardial Infarction Transported by the E m e r g e n c y Medical S y s t e m Robert J. Zalenski, MD, MS *r Mary Grzybowski, PhD, MPH, *~ Michael A. Ross, MD, *g Nate Blaustein, MD, II and Brooks Bock, MD*

A b s t r a c t : Prehospital triage of cardiac patients for bypass from c o m m u n i t y hospitals to cardiac centers m a y i m p r o v e survival. This articIe d e t e r m i n e s if electrocardiogram (ECG)-based scoring triage m e t h o d s (Aldrich MI scoring, QRS distortion, and the TIMI classification) and location of infarct (via 12 lead ECG) are associated with mortality before and after adjusting for age, sex, and race. It is a retrospective study of 291 AMI adult patients transported by a m b u l a n c e to c o m m u n i t y hospitals or cardiac centers. Patients w i t h an ED chief complaint of chest pain or dyspnea, presence of MI as defined by ECG findings of 0.1 mV of ST s e g m e n t elevation in two leads or positive CPK-MB w e r e eligible for the study. The p r i m a r y o u t c o m e variable was 2-year mortality as d e t e r m i n e d w i t h a m e t r o p o l i t a n Detroit tri-county death index. Logistic regression was used to calculate the unadjusted and adjusted odds ratios (with 95% CIs) of the predictor variables w i t h mortality. Of the initial p o p u l a t i o n selected for the study (n - 291), 229 patients w e r e eligible for the analysis. The m e a n age was 66 years (SD of 14.4) with 63.8% being male and 54% being white. The overall mortality point estimate was 21.3% (95% CI of 15.2 to 27.3%). Aldrich scores and QRS distortion (yeslno) w e r e not associated w i t h mortality. Patients classified as a "high risk" for AMI per TIMI status w e r e almost 3 times m o r e likely to die than those at "low risk" and reached borderline statistical significance (P = .06) after adjusting for the covariates. Having an anterior infarct, as opposed to an inferior infarct, was significantly associated with death before and after adjusting for the covariates (Unadjusted OR = 2.6, Adjusted OR - 2.8). Properly training e m e r g e n c y medical system professionals in this area m a y p r o v e useful for identifying h i g h e r risk AMI patients in the prehospitaI setting. K e y w o r d s : Electrocardiogram, myocardial infarction, triage, risk factor, cardiac mortality.

From the * Department of Emergency Medicine, Wayne State University School of Medicine, Detroit, MI; t Department qf Medicine, John D. Dingell Veterans Hospital, Detroit, MI,"~ Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor, MI; ~ Department of Emergency Medicine, William Beaumont Royal Oak Hospital Royal Oak, MI; and IIDepartment of Emergency Medicine, Grace Hospital, Detroit, ML Reprint requests: Robert J. Zalenski, MD, Department of Emergency Medicine, Wayne State University School of Medicine, 6G University Health Center, 4201 St. Antoine, Detroit, MI 48201; Phone: 313-9964 7679; Fax: 313-993-7703; e-marl: [email protected] Copyright 9 2000 by Churchill Livingstone | 0022-0736/00/330S-0040535.00/0 doi: 10.1054/jelc.2000.20298

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Journal of Electrocardiology Vol. 33 Supplement 2000

The identification of patients with suspected acute myocardial infarction (AMI) in the a m b u lance or e m e r g e n c y medical system would provide doctors with helpful information. Patients at a low risk of cardiac death could be allowed to proceed to the conveniently located closest c o m m u n i t y hospital, the location m a n d a t e d in most e m e r g e n c y m e d ical systems. Patients with a high risk of death could be bypassed to a fully staffed cardiac center, w h e r e e m e r g e n c y invasive cardiac procedures could be p e r f o r m e d w h e n e v e r n e e d e d by an experienced team. Although the retriage to invasive hospitals of h e m o d y n a m i c a l l y unstable patients with an AMI transported by a m b u l a n c e is r e c o m m e n d e d by the American College of Cardiology (1), to date, there are no validated, specific criteria for the identification of such high risk patients, or definite proof of inferior o u t c o m e s in local hospitals (2). In this article we evaluated 4 ECG based tools w h i c h could potentially be of value in such risk stratification: the Aldrich Score for AMI (3), the distortion of the terminal QRS (4), the location of AMI, and the classification used in the Thrombolysis in Myocardial Infarction (TIMI) study (5). We e x a m i n e the ability of each of these tools to predict cardiac death in the follow-up period.

Materials and Methods Design, Setting, and Patient Population The sample for this retrospective study consisted of all patients coded by the E m e r g e n c y D e p a r t m e n t s (EDs) as AMI and t r a n s p o r t e d by the e m e r g e n c y medical system (EMS) in 3 m e t r o p o l i t a n Detroit area hospitals b e t w e e n J a n u a r y l, 1996 and Dec e m b e r 31, 1997. Charts for all of the patients in the sampling f r a m e w e r e further e x a m i n e d to assess their meeting the following eligibility criteria: a) Chief complaint of chest pain or dyspnea and b) The presence of MI as defined by ECG findings of 0.1 mV of ST-segment elevation in 2 leads or positive creatine p h o s p h o k i n a s e - M B (CPK-MB) on presentation to the ED. Patients with left bundle branch block were excluded. The p r i m a r y o u t c o m e or d e p e n d e n t variable was cardiovascular mortality as d e t e r m i n e d during the surveillance period by review of a tricounty death index with its associated ICD-9 codes. Because the study utilized existing data only, the research was e x e m p t from institutional review b o a r d (IRB) review.

Independent Variables The i n d e p e n d e n t variables e x a m i n e d w e r e the Aldrich Score (3), presence or absence of a distorted terminal QRS (4), high or low TIMI risk status (5), and anterior or inferior location of AMI. The Aldrich scores were calculated with the following formulae: 1. Anterior Infarct Aldrich Score = 3 [1.5 n u m b e r leads with ST-segment elevation) - .41 2. Inferior Infarct Aldrich Score = 3[.6 (sum of ST-segment elevation in leads II, III, AvF) +

2.01 The terminal portion of the QRS was rated as distorted if, in 2 or m o r e consecutive leads, there was the e m e r g e n c e of the J point a b o v e the lower half of the R w a v e or the disappearance of the S w a v e in inferior AMI in leads V2-V4. The TIMI risk assessment was considered positive (indicating higher risk of death) if a n y of the following criteria w e r e present: age > 7 0 years, previous AMI, anterior AMI, atrial fibrillation, hypotension, a n d sinus tachycardia. These criteria are modifications of the original TIMI criteria (5), as not all of the findings (such as tales m o r e that 89 of the lungs) were believed to be reliable in the EMS setting. The last criterion, infarction location, was coded as anterior if the anterior leads (V1-V4) had ST-segment elevation, and coded as inferior if there w e r e not anterior ST-segment elevation. All first p e r f o r m e d ED ECGs were interpreted and scored by readers blinded to clinical outcomes.

Dependent Variable Cardiac mortality within 2 years of index ED p r e s e n t a t i o n served as the d e p e n d e n t variable. D e a t h status and date of death were collected by using the Detroit m e t r o p o l i t a n tricounty d e a t h index.

Statistical Methods The point estimate and 95% confidence interval (CIs) of death within 2 years was determined. Patients w e r e divided into 2 groups based on their survival status: survived or died. D e m o g r a p h i c and ECG characteristics are presented for the entire study p o p u l a t i o n and by survival status. Race was collapsed into a binary variable ( w h i t e / n o n w h i t e ) . All characteristics w e r e c o m p a r e d b e t w e e n the 2 groups. For age and Aldrich scores, m e a n s and standard deviations are presented. The Student's

The ECG in the Triage of AMI in the Ambulance

t-test w a s u s e d to d e t e r m i n e if d i f f e r e n c e s in t h e continuous variables existed between patients who survived and died within the follow-up period. P o i n t b i s e r i a l c o r r e l a t i o n s to assess t h e s t r e n g t h of the association between each Aldrich (anterior and i n f e r i o r ) score a n d m o r t a l i t y w e r e d e t e r m i n e d (6). T h e c h i - s q u a r e test w a s u s e d to d e t e r m i n e frequency differences between those who survived a n d t h o s e w h o died. U n a d j u s t e d o d d s ratios (ORs) a n d 9 5 % CIs w e r e c a l c u l a t e d to d e t e r m i n e t h e m a g n i t u d e of a s s o c i a t i o n b e t w e e n t h e c a t e g o r i c a l p r e d i c t o r v a r i a b l e a n d t h e o u t c o m e of i n t e r e s t . By u s i n g m u l t i p l e logistic r e g r e s s i o n , a d j u s t e d ORs a n d 9 5 % CIs w e r e c a l c u l a t e d for QRS d i s t o r tion, TIMI risk status a n d i n f a r c t l o c a t i o n . All a d j u s t e d ORs, e x c e p t for t h e TIMI p r e d i c t o r v a r i a b l e , w e r e a d j u s t e d for age, g e n d e r , a n d r a c e to d e t e r m i n e if t h e i r m a g n i t u d e of a s s o c i a t i o n w i t h d e a t h c h a n g e d . W e also c a l c u l a t e d t h e sensitivity, specificity, p o s i t i v e a n d n e g a t i v e p r e d i c t i v e v a l u e s , a c c u racy, a n d p o s i t i v e a n d n e g a t i v e l i k e l i h o o d ratios for e a c h of t h e t h r e e p r e d i c t o r v a r i a b l e s . F o r all statistical analysis, P - v a l u e s less t h a n .05 w e r e c o n s i d e r e d significant. A n a l y s e s w e r e per~ f o r m e d u s i n g t h e Statistical A n a l y s i s S y s t e m (SAS I n s t i t u t e Inc, Cary, NC) (7).

Results T h e i n i t i a l s a m p l e of A M I p a t i e n t s t r a n s p o r t e d b y a m b u l a n c e to the EDs of t h e 3 h o s p i t a l s listed c o m p r i s e d 2 9 I p a t i e n t s . Of t h e s e , 252 ( 8 6 . 6 % ) h a d a chief c o m p l a i n t of c h e s t p a i n (n = 220) o r d y s p n e a (n = 32) w h o s e ECGs d i d n o t s h o w left

~ Zalenski et al.

247

b u n d l e b r a n c h b l o c k . A m o n g t h e 252 p o t e n t i a l l y eligible p a t i e n t s , 229 p a t i e n t s ( 9 0 . 9 % ) m e t t h e criteria for t h e p r e s e n c e of m y o c a r d i a l i n f a r c t i o n (MI) as d e f i n e d b y E C G f i n d i n g s of 0.1 m V of S T - s e g m e n t e l e v a t i o n in 2 l e a d s or p o s i t i v e C P K - M B u p o n p r e s e n t a t i o n to t h e ED. Specifically, 2 1 . 4 % (n = 49) h a d S T - s e g m e n t e l e v a t i o n a n d a n e g a t i v e C P K - M B , 3 0 . 6 % {n = 70) b a d n o STs e g m e n t e l e v a t i o n a n d p o s i t i v e CPK-MB, 4 8 . 0 % (n = I 1 0 ) h a d b o t h S T - s e g m e n t e l e v a t i o n a n d a p o s i t i v e CPK-MB. The m e a n age of t h e final s t u d y p o p u l a t i o n (n = 174) w a s 66 (SD of 14.4). F e m a l e s w e r e , o n a v e r age, 7.8 y e a r s o l d e r t h a n t h e i r m a l e c o u n t e r p a r t s (P -< .001). T h e r e w a s g r e a t e r p r o p o r t i o n of m e n in t h e s t u d y ( 6 3 . 8 % vs. 3 7 . 8 % , P -< .01). F i f t y - f o u r p e r c e n t of t h e p o p u l a t i o n w e r e w h i t e , 2 8 % w e r e black, 1.0% w a s H i s p a n i c , 1.5% w e r e r e c o r d e d as " o t h e r " , a n d 1 5 . 3 % w e r e u n k n o w n . The o v e r a l l 2 - y e a r c a r d i a c m o r t a l i t y w a s 2 1 . 3 % (n = 37, 9 5 % CI [ c o n f i d e n c e i n t e r v a l ] : 15.2 to 27.3). Table 1 s h o w s t h e d e m o g r a p h i c a n d ECG c h a r a c teristics for t h e e n t i r e s t u d y p o p u l a t i o n a n d b y s u r v i v a l status. T h e r e w e r e n o d i f f e r e n c e s in m e a n a n t e r i o r a n d i n f e r i o r i n f a r c t A I d r i c h scores b y survival status. The p o i n t b i s e r i a l c o r r e l a t i o n for t h e a n t e r i o r a n d i n f e r i o r i n f a r c t A l d r i c h scores w e r e - 0 . 1 4 a n d 0.11, r e s p e c t i v e l y ( d a t a n o t s h o w n ) . A g r e a t e r p r o p o r t i o n of p a t i e n t s w h o w e r e classified as h i g h risk via t h e TIMI classification s y s t e m d i e d t h a n p a t i e n t s classified as l o w risk ( 8 6 . 5 % vs. 6 9 . 3 % ; OR = 2.8; P < .05}. Of t h e ECG data, o n l y l o c a t i o n of i n f a r c t w a s s i g n i f i c a n t l y d i f f e r e n t b y s u r v i v a l status ( 5 1 . 1 % s u r v i v e d a n d 7 3 . 0 % died; OR = 2.6; P --< .05). N o s i g n i f i c a n t d i f f e r e n c e s between survivors and nonsurvivors were observed

T a b l e 1. Demographic and ECG Characteristics of the Study Population (n - 174) and b y Survival Status Characteristic Demographic Variable Age (years) Gender: maIe Race: white ECG Data Anterior Aldrich Score Inferior Aldrich Score Distorted QRS Wave Anterior Infarct High TIMI Classification Ischemic ST-Depression* Ischemic T-Wave Inversion* Other ST-Abnormality

Study Population (n = 174)

Survived (n = 137)

Died (n = 37)

P-value*

66 (I4.4) 38% 54%

63.8 (13.7) 63.5% 63.8%

74.3 (14.2) 37.8% 55.6%

<.0001 <.01 NS

11.9 (9.3) 20 (10.3) 23% 55.7% 73% 52 % I3.5% 13.5 %

12.7 (9.1) 19.6 (10.3) 24.1% 5 i. i % 69.3% 50.7 % 15.4% 13.2%

9.8 (9.5) 22,8 (10.7) 18.9% 73.0% 86.5% 57.1% 5.7% 14.3%

NS NS NS <.05 <.05 NS NS NS

* >1 mV on 2 contiguous leads. * Represents difference between patients who survived and did not survive within a 2-year surveillance period. NS, not significant.

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Journal of Electrocardiology Vol. 33 Supplement 2000

Table 2. UnadJusted and Adjusted Odds Ratios for ECG Predictor Variables* and Mortality Within 2 Years of ED Presentation EIectroc~diogram Predictor variable Distorted QRS wave Anterior infarct Iligh risk TIMI classification w

Unadjusted OR (95"/o CI)

Adjusted OR (95% CI)

and 48.9% specific. By using the TIMI risk status, the sensitivity and specificity w o u l d be 86.5% and 30.7%, respectively. The positive and negative likelihood ratios for both location of infarct and TIMI risk status, ranging from 0.439 to 1.430 depict {hat there would be no clinically m e a n i n g f u l change in the likelihood of disease.

Difference in OR (%)

0.7 2,6

0.3-1.8 1,2 5.7 t

0,7 2.8

0.2-1.8 1.2-6.8 t

0% ~7.7%

2,8

1.1-7.8 t

2.5

0.9-7.2 a

$10.7%

ECG, electrocardiogram; ED, emergency department; OR, odds ratio; CI, confidence interval. * Adjusted for age, race, and gender. t p < 0.05; * P 0,062 -~TIMI is not adjusted for age because age is included in the equation for classification.

for distorted QRS wave, ischemic ST-depression, ischemic T-wave inversion or other ST-segment abnormality. Distorted QRS wave, TIMI risk status and location of infarct, after controlling for potentially confounding variables, s h o w e d a slight a d j u s t m e n t of the odds ratios or n o n e at all (distorted QRS had a zero percent change). Table 2 shows the relationship b e t w e e n location of infarct and mortality after adjusting for age, g e n d e r and race. An anterior infarct remained a significant predictor of t w o - y e a r cardiac mortality (Adjusted OR = 2.8, 95% CI: 1.2-6.8; P --< .05); the OR increased by 7.7%. After we controlled for gender and race for the TIMI risk status, the adjusted odds ratio achieved only borderline significance (OR = 2.5; 95% CI: 0.9-7.2; P = .06). Table 3 presents the sensitivity, specificity, positive and negative predictive values, accuracy, and positive and negative likelihood ratios of each ECG variable for d e t e r m i n i n g cardiac mortality within two years of index ED presentation. For example, if a physician w e r e to predict cardiac death via location of infarct, classifying a patient as having an anterior infarct (yes/no) would be 73% sensitive

Discussion The study show that in the a m b u l a n c e transported AMI patients, b o t h the location of the AMI and the related TIMI risk association score predicted mortality in the 2 year follow-up period. The strongest predictor of risk was simply the location of infarction. This suggests that this simple m e a s u r e m i g h t be incorporated into the EMS triage system. W h e n used in conjunction with d e m o g r a p h i c and presenting clinical information, an a d e q u a t e l y sensitive and specific m o d e l m a y be derived (8). Prospective derivation and validation studies are n o w w a r r a n t e d (9). Such a study ks justified because of the potential trade-oils f r o m the patient's higher risks of longer time out of hospital (10) and the potentially greater benefits of hospitals with invasive capability (i I). The study was limited because of several factors. First, this was a retrospective design. Second, the study included only AMI patients, not patients with suspected AMI as would have to be used in the EMS system study. Second, the ECG data used w e r e from the e m e r g e n c y d e p a r t m e n t s rather t h a n an ECG in the ambulance. Prior w o r k has s h o w n that the a m b u l a n c e ECG and the first ED ECG are not equivalent, although the differences are probably small. Patients in the study group w e r e elderly; therefore, they m a y h a v e died within a 2-year period from their index ED p r e s e n t a t i o n due to cardiovascular disease unrelated to the index hospitalization. Finally, the ECGs w e r e read by a single

Table 3. Sensitivity, Specificity, Positive Predictive Value, and Negative Predictive Value of Having a Distorted QRS Wave and Anterior Infarct via ECG and Being Classified as High Risk per TIMI Classification ECG Characteristic Distorted Q-Wave Anterior infarct TIME High risk

Sensitivity 95% CI

Specificity 95% CI

PPV 95% CI

NPV 95% CI

Accuracy 95% CI

18.9% 6.3-31.5% 73% 58.7 87.3% 86.5% 75,5-97.5%

75.9% 68,8-83.1% 48,9% 40.5-57.3% 30.7% 22.9-38.8%

17.5% 5.7-29,3% 27.8% 18.9-36,8% 25.2% 17.6-32.7%

77.6% 70.6-84.7% 87% 79.5-94.5% 89.9% 80.5-98.2%

63.2% 5 ] .6-70.4% 54,0% 46.6 61.4% 42.5% 35.2-49.8%

+LR

-LR

0,784

1.068

t.430

0,552

1.248

0.439

ECG, electrocardiogram; PPV, positive predictive value; NPV, negative predictive value; CI, confidence interval; +LR, posiIive likelihood ratio; -LR, negative likelihood ratio.

The ECG in the Triage of AMI in the Ambulance r e a d e r , a n d w e do n o h a v e i n t e r - r a t e r reliability assessments.

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