Idioventricular low frequency oscillation in QT interval responds univocally to RR confusing kinds of mental stress.

Seventeen male subjects, aged nineteen to twenty, went through a protocol including, while supine, relaxation at rest (10 min) and mental stress (MS) by a Kraepelin (arithmetic) test (5 min), as part of a larger study. With a polygraphic analog recording set-up we also collected a 1 ms - digital facsimile of a lead II-like thoracic ECG with maximum T-wave (Codas, Dataq Instr). Twenty-three stress responses were assigned to three classes according to known cardiotacho-, plethysmo-, and pneumo-graphic marks of "concentrated attention mainly," "emotion," or still "high emotion." During each setting the most stationary 3 min RR epoch in cardiotachogram was selected for joint RR & QT beat-by-beat variability study. RR and QT intervals were detected using a published algorithm. Conventional RR and QT Fourier autospectra were computed, while using RR*QT mean square coherence spectrum we detached the RR-independent, idioventricular (IV) fraction of QT low frequency (LF: 0.04-0.15 Hz) power of variability (IV QT-LF). IV QT-LF responded consistently to varieties of mental stress that confuse RR-LF or let QT-LF unchanged, best witnessing the cortically-issued ventricular adrenergic strain. Indeed, while emotion propels the same way all spectral variables above, concentrated attention increased (Wilcoxon) significantly IV QT-LF only (0.54-0.80 ms2) and decreased RR-LF (715-463 ms2). Findings hold promise of a non-invasive, high resolution Holter based monitoring of sympathetic status of myocardium, robust vis-à-vis of confusion caused by the autonomic interplay at sino-atrial node.

Mental stress enhances the sympathetic fraction of QT variability in an RR-independent way.

In 10 healthy male volunteers aged 19-20, spectral power of the beat-by-beat QT interval was measured at the Traube-Herring-Mayer (THM) band (0.05 to 0.15 Hz) when the subject was at rest and during atrial pacing. After resting in dorsal decubitus for 10 minutes, right atrial pacing was performed at a slight elevation above sinus rhythm as well as at 100 or at 110 beats per minute for 7 minutes each. In addition, during pacing at 100 or 110 bpm, the subject was required to perform a Kraepelin Arithmetic test. There was a statistically significant increase in QT spectral power at the THM band, while heart rate was maintained unchanged during the periods of mental stress versus rest. This experiment confirms the potential independence of the QT interval from heart rate and suggests that THM fraction of QT spectral power has a double subordination: during relaxed rest it follows mostly RR-fluctuation; during stress an RR-independent contribution is added, which presumably reflects the supra-normal sympathetic drive on the ventricles.

Hypobaric hypoxia: dual sympathetic control in the light of RR and QT spectra.

ECGs of 21 candidate-pilots and 19 pilots were recorded during: 1) exposure to 5500 m hypobaric hypoxia (HH) while sitting, in the sequence: 1a) initial 7 min of adaptation (A); 1b) later 7 min of recovery (R) after short but intense tread-mill effort; and 2) final 7 min baseline (B), while sitting and requested to relax, at "0 m altitude" in the hypobaric room. RR and QT short-term variability were studied using spectral powers within Traube-Hering-Mayer (THM: 0.05-0.15 Hz) and respiratory (RESP: 0.2-0.4 Hz) bands. Mean RR proved highest capability to aggregate individual response-profiles: 15 pilots and 9 candidates entered the main (normal) cluster, featured by a comparison "triangle" set as expected: A > R < B > A. QT-THM power closely followed: 10 subjects (ss) in very normal cluster, defined as: A < R > B < A, while secondary clusters in candidates and pilots were interpreted by not-successful relaxation and exaggerated start-effects, respectively. Subjects with QT-THM normal clusters (A < R > B < A has group averages, p < = 0.05), also showed a quasi-normal "triangle" for mean RR, (A = R < B > A). During adaptation to hypoxia, pilots' QT-THM was higher than candidates' one (p < 0.02, Wilcoxon test). Study supports the emerging capability of QT-THM spectral power to index ventricular sympathetic control. Exposure to hypobaric hypoxia proved to be in these subjects a psycho-physiologic rather than a purely physiologic test.