Three-dimensional angle between the QRS complex and T wave vectors is a known powerful
cardiovascular risk predictor. Nevertheless, several physiological properties of the angle are
unknown or poorly understood. These include, among others, intra-subject profiles and stability of
the angle relationship to heart rate, characteristics of angle/heart-rate hysteresis, and the changes
of these characteristics with different modes of QRS-T angle calculation. These characteristics were
investigated in long-term 12-lead Holter recordings of 523 healthy volunteers (259 females). Three
different algorithmic methods for the angle computation were based on maximal vector magnitude
of QRS and T wave loops, areas under the QRS complex and T wave curvatures in orthogonal leads,
and weighted integration of all QRS and T wave vectors moving around the respective 3-dimensional
loops. These methods were applied to orthogonal leads derived either by a uniform conversion
matrix or by singular value decomposition (SVD) of the original 12-lead ECG, giving 6 possible ways
of expressing the angle. Heart rate hysteresis was assessed using the exponential decay models. All
these methods were used to measure the angle in 659,313 representative waveforms of individual
10-second ECG samples and in 7,350,733 individual beats contained in the same 10-second samples.
With all measurement methods, the measured angles fitted second-degree polynomial regressions
to the underlying heart rate. Independent of the measurement method, the angles were found
significantly narrower in females (p<0.00001) with the differences to males between 10o and 20o
,
suggesting that in future risk-assessment studies, different angle dichotomies are needed for both
sexes. The integrative method combined with SVD leads showed the highest intra-subject
reproducibility (p<0.00001). No reproducible delay between heart rate changes and QRS-T angle
changes was found. This was interpreted as a suggestion that the measurement of QRS-T angle
might offer direct assessment of cardiac autonomic responsiveness at the ventricular level.