A fundamental relationship between intraventricular conduction and heart rate.

BACKGROUND: Existence of a relationship between the electrocardiographic QRS interval duration and the diurnally varying heart rate, of consistent sign and magnitude, is controversial and the relationship has not been fully characterized in normal populations. METHODS AND RESULTS: We analyzed the QRS-RR interval relationship in 884 Holter recordings in 410 normal subjects participating in 5 clinical trials. The slope of the linear regression of QRS on RR was positive in 93% of subjects with an average slope of 0.0125, which indicates an increase in QRS duration of 1.25msec for an increase in RR interval of 100msec. The increase was 15% larger in women than in men. Age had no significant effect on the slope. CONCLUSIONS: In two populations of normal subjects we observed a robust, direct relationship between the spontaneously changing RR interval and intraventricular conduction time represented by the duration of the QRS interval. As heart rate increases, QRS duration decreases. The change is larger in women. These observations have important physiological and clinical implications.

Comparison of HRV analysis methods during orthostatic challenge: HRV with respiration or without?

Heart Rate Variability (HRV) analysis has become an important tool in assessing human Autonomic Nervous System (ANS) activity in recent years. Orthostatic challenge is one of the most common tests to detect ANS dysfunction. In this study we looked at the changes in ANS activity of normal subjects to orthostatic challenge and compared the results of 3 different HRV analysis methods: Time-Domain Methods, HRV spectral analysis without respiratory analysis (RA) and with RA. Although all three methods have indicated an increase in sympathetic activity and a decrease in parasympathetic activity from baseline to stand, the only significant increase in sympathetic activity was observed in HRV with RA method. Additional information from RA enables isolating the sympathetic and parasympathetic branches in HRV signals and therefore reflects ANS changes more accurately. On the other hand, sympathetic and parasympathetic power may not be separated properly if respiration-dependent fluctuations in HRV are ignored. It is expected that the differences between methods would be very clear with low respiratory rates. However, we focused on studies with normal respiratory rates and have also found significant differences among the methods.

Effect of respiration in heart rate variability (HRV) analysis.

Beat-to-beat changes in cardiac signals or heart rate variability (HRV) are controlled by the two branches of autonomic nervous system (ANS) in a very complex manner. Although traditional HRV (tHRV) analysis has shown to provide information on cardiac ANS control, it often fails to isolate the effect of two branches in HRV signals. This problem becomes more obvious especially at low respiratory rates since parasympathetic activity shifts into lower frequencies and overlaps the frequency interval where sympathetic region is defined. To investigate the effect of respiration in HRV analysis we have analyzed the data of 17 healthy subjects while they were performing normal breathing (NB) and deep breathing (DB). Data were analyzed and compared using both tHRV analysis and enhanced HRV (eHRV) analysis where we used respiration to locate the frequency interval of parasympathetic activity in HRV signal. eHRV analysis provided proper isolation and more accurate detection of parasympathetic and sympathetic control of the heart.

Orthonormal-basis partitioning and time-frequency representation of cardiac rhythm dynamics.

Although a number of time-frequency representations have been proposed for the estimation of time-dependent spectra, the time-frequency analysis of multicomponent physiological signals, such as beat-to-beat variations of cardiac rhythm or heart rate variability (HRV), is difficult. We thus propose a simple method for 1) detecting both abrupt and slow changes in the structure of the HRV signal, 2) segmenting the nonstationary signal into the less nonstationary portions, and 3) exposing characteristic patterns of the changes in the time-frequency plane. The method, referred to as orthonormal-basis partitioning and time-frequency representation (OPTR), is validated using simulated signals and actual HRV data. Here we show that OPTR can be applied to long multicomponent ambulatory signals to obtain the signal representation along with its time-varying spectrum.

Detecting instabilities of cardiac rhythm.

Diminished beat-to-beat variations in cardiac cycle lengths (CLs) are associated with poor prognosis after acute myocardial infarction and in patients with heart failure. Short-long-short sequences of cardiac cycles, or ultra-short rhythm instabilities, precede initiation of ventricular tachyarrhythmias in some patients. However, little is known about clinical or prognostic significance of abrupt short-term instabilities in CL (AICL) that occur minutes to hours before the event, in part because appropriate analytical methods are lacking. Although various techniques have been used to analyze CL changes, methods for analysis of AICL are limited. We compared performance of time domain, spectral, nonlinear, and pattern recognition techniques with respect to the detection and quantification of AICL. Because of high intra- and inter-subject variability of CL, pattern recognition techniques compared favorably to other studied methods. In continuous ambulatory ECG recordings, AICL occurred hours before spontaneous initiation of sustained atrial and ventricular arrhythmias in different patient populations. AICL were also found prior to the onset of spontaneous ventricular arrhythmias in a mouse model of congestive heart failure. To quantify AICL, we used the number of unstable orthogonal projection coefficients; this number gradually increased hours before the event. Removal of ectopic beats reduced but did not eliminate AICL. To illustrate potential physiological effects and temporal evolution of AICL, we used a simple, continuous, two-dimensional model of cardiac tissue governed by the Morris-Lecar equations. Computer simulations in this model showed that AICL may lead to gradual accumulation of spatial irregularities of the propagation wavefront giving rise to the initiation of reentry. Time-frequency analysis of the most significant eigenvectors of cardiac rhythm in subjects undergoing head-up tilt showed that AICL could indicate instabilities and unsuccessful adaptation of autonomic nervous system activity to physiological stimuli.

Direct mechanical stimulation of brainstem modulates cardiac rhythm and repolarization in humans.

Natural mechanical stimulation of the brainstem area by the blood pressure waves propagating in the adjacent arteries plays an important role in the homeostasis of the brainstem centers of cardiovascular control. However, effects of direct mechanical stimulation of this area on the cardiac elecrophysiology have never been studied in humans. In 12 patients (age: 54 +/- 13 years, 5 females) undergoing microvascular decompression, the left (9 patients) or the right (3 patients) side of the ventro-lateral surface of the medulla oblongata was exposed during the surgery, and a mechanical stimulation (duration: 1 min, frequency: 1-2 Hz) of the roots of the cranial nerves and the surface of the brainstem was performed at 3-7 sites using a 2-mm metallic ball. Spatial changes in cardiac repolarization were examined using the 32-lead/192 site electrocardiographic body surface potential maps. Blood pressure was monitored using intra-arterial line. The intervals between the onset of the Q-wave and the offset of the T-wave (QTe) and between the onset of the Q-wave and the peak of the T-wave (QTp), the activation-recovery intervals (ARi), the peak T-wave amplitude, and the QRS and STT integrals were measured using custom software. During the stimulation between the caudal rootlets of the 10th nerve, the peak T-wave amplitude decreased 22% (range: 6-50%) and RR-intervals decreased from 923 +/- 190 to 794 +/- 111 ms compared to the recordings obtained before the stimulation (P =.025 and.063, respectively), whereas QTe, QTp, Ari, and the QRS- and the STT-integrals did not change. Decreased T-wave amplitudes and unchanged QT-intervals suggest that brainstem stimulation might evoke spatially inhomogenious repolarization changes. Stimulation of a localized region surrounding the caudal rootlets of the 10th nerve elicits pronounced effects on cardiac rhythm and repolarization.