Abstract & Commentary
ECG Frontal Plane QRS-T Angle
By Michael H. Crawford, MD, Editor
Source: Walsh JA, et al. Prognostic value of frontal QRS-T angle in patients without clinical evidence of cardiovascular disease (from the Multi-Ethnic Study of Atherosclerosis). Am J Cardiol 2013;112:1880-1884.
The difference in degrees between the QRS axis and the T-wave axis has been shown to predict cardiovascular disease (CVD) events and mortality. However, most studies have used the spatial QRS-T angle derived from vectorcardiography, which is not available on a routine ECG. Thus, these investigators from the Multi-Ethnic Study of Atherosclerosis (MESA) evaluated the frontal plane QRS-T axis from standard 12-lead ECGs and its association with CVD in this patient population. MESA subjects were 45-84 years old and free of apparent CVD. The frontal plane QRS-T angle was the absolute difference between the QRS axis and the T-wave axis in degrees. They excluded subjects with paced rhythms, WPW, or bundle branch block. The subjects were followed at 9-12 month intervals for up to 8 years. The primary endpoint was a composite of CVD events: CV death, myocardial infarction, angina, or heart failure. The subjects were divided into three groups depending on their QRS-T angle: normal (< 75th percentile), borderline (75th-95th percentile), or abnormal (> 95th percentile). The borderline and abnormal groups were older, and had more comorbidities and other ECG abnormalities. Also, they were more likely to have evidence of subclinical atherosclerosis by CT scan and carotid ultrasound. When adjusted for age, race, and sex, the borderline and abnormal groups had more CVD events (hazard ratio [HR], 1.37; 95% confidence interval [CI], 1.10-1.70; and HR, 2.2; 95% CI, 1.63-2.97, respectively). Modest attenuation of these results occurred with adjustments for clinical covariants, but after adjustment for other T-wave abnormalities, the association of QRS-T angle with CVD events was lost. The authors concluded that an abnormal frontal plane QRS-T angle predicts CVD events in a multiethnic population and that this increased risk is mainly due to T-wave abnormalities.
Commentary
This study demonstrates that more complex epidemiologic data (spatial T waves) can be applied to routine ECG interpretation. The excellent predictive ability of frontal plane T-wave axis can be explained by T-wave abnormalities that can be discerned by inspection. Other studies have shown that T-wave abnormalities are related to structural heart disease and electrical perturbations of ion channels (e.g., hypokalemia). The authors propose that since the frontal plane QRS-T angle is a continuous variable, it may detect abnormalities earlier before there are obvious T-wave abnormalities. This hypothesis is not directly tested in this study. However, the QRS-T angle is readily calculated on most computer-interpreted ECGs by subtracting the T-wave axis from the QRS axis and recording the absolute value. If the QRS-T angle is > 180°, then the following formula is used: QRS-T angle = 360° absolute value. Once the value is obtained, all that remains is to see whether it is normal, borderline, or abnormal. This study uses definitions from the NHANES literature, which is reported in percentiles, but this paper does not give you the actual QRS-T angle values that correspond to the percentiles. For this, you have to read the NHANES III paper1 and there you learn that the QRS-T angle varies with sex. Also, the actual cutoff values are not simple round numbers that are easy to remember. In general, men have larger values than women, but rough cutoff numbers for both sexes are < 45° is normal, < 90° is borderline, and > 90° is abnormal. These values should work for most ethnicities; however, the study was underpowered for Asians.
REFERENCE
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Whang W, et al. Am J Cardiol 2012;109:981-987.
