Vectorcardiography

The spatial QRS-T angle (SA) is derived from a vectorcardiogram, which is a three-dimensional representation of the 12-lead electrocardiogram (ECG) created with a computerized matrix operation. The SA is the angle of deviation between two vectors; the spatial QRS-axis representing all of the electrical forces produced by ventricular depolarization and the spatial T-axis representing all the electrical forces produced by ventricular repolarization.^[8] The SA is indicative of the difference in orientation between the ventricular depolarization and repolarization sequence.^{[citation needed]}

In healthy individuals, the direction of ventricular depolarization and repolarization is relatively reversed; this creates a sharp SA.^[9] There is high individual variability and gender difference in the magnitude of the SA. The mean, normal SA in healthy young adult females and males is 66° and 80°, respectively,^[9] and very similar magnitudes are found in the elderly population (65 years and older).^[10] In ECG analysis, the SA is categorized into normal (below 105°), borderline abnormal (105–135°) and abnormal (greater than 135°).^[11] A broad SA results when the heart undergoes pathological changes and is reflected in a discordant ECG. A large SA indicates an altered ventricular repolarization sequence, and may be the result of structural and functional myocardial changes that induce regional shortening in action potential duration and impaired ion channel functioning.^[12]

Current standard ECG markers of repolarization abnormalities include ST depression, T wave inversion and QT prolongation. Many studies have investigated the prognostic strength of the SA for cardiac morbidity and mortality compared to these and other ECG parameters. In treated hypertensive patients, the SA was significantly larger in patients with elevated blood pressure compared to those with lower blood pressure values and a discrimination between patients with high and low blood pressure could not be detected using other ECG parameters.^[13] In the Rotterdam Study with men and women aged 55 years and older, having an abnormal SA significantly increased the hazard ratios for cardiac death, sudden cardiac death, non-fatal cardiac events (infarction, coronary interventions) and total mortality. Independently, the SA was a stronger risk indicator of cardiac mortality compared to the other cardiovascular and ECG risk factors analyzed.^[11] The Women's Health Initiative study concluded that a wide SA was the strongest predictor for incident coronary heart failure risk and a dominant risk factor for all cause mortality compared to several other ECG parameters.^[12] The SA also increases accuracy of diagnosing left ventricular hypertrophy (LVH). Using only conventional ECG criteria to diagnose LVH the diagnostic accuracy was 57%, however the inclusion of the SA significantly improved the diagnostic accuracy to 79%.^[14]

The SA is not routinely measured in clinical ECG examination even though the computerized vectorcardiography software is widely available, efficient and is not affected by observational biases unlike other ECG parameters.^[13] The SA is a sensitive marker of repolarization aberrations and with further research support the SA will likely become clinically applied in predicting cardiac morbidity and mortality.^{[citation needed]}

A simplified criteria in using the vectorcardiogram has the ability to identify patients with a diaphragmatic infarction not apparent in the electrocardiogram. ^[15]

• Electrocardiography