RR Interval Length Research Articles

Autonomic Integrative Function During Head-up Tilt In Children And Young Adults

PURPOSE: To investigate differences in cardiovascular responses and autonomic integrative function in response to head-up tilt between children and young adults. METHODS: We measured beat-to-beat blood pressure (BP) via finger photoplethysmography and RR-intervals via continuous ECG in 7 children (8.0 ± 1.0 yrs) and 10 young adults (23.0 ±1.0 yrs) at rest and during 10 min of head-up tilt. Spectral analysis of the RR-intervals was used to determine cardiovagal control (high-frequency power, HF (msec2): 0.15–0.4 Hz). Autonomic integrative function was evaluated using the sequence technique [up-up (Seq+) and down-down (Seq-) sequences] and frequency-domain analysis of the RR-interval and systolic BP spectra. Frequency-domain analysis was used to calculate the coefficient (αLF) and the cross-spectral transfer function (LFgain) at low frequency ranges (0.05–0.15 Hz), which are indices of baroreflex gain. All spontaneous and frequency domain indices are presented as natural logarithms (ln). RESULTS: Systolic BP and the length of RR-intervals significantly decreased from rest to head-up tilt independent of group (p<0.05). In addition, there was a significant group-by-time interaction in the RR-intervals (p<0.05). Children had significantly higher lnHF power during tilt compared to adults (p<0.05). Further, there were no significant group-by-time interactions for the lnSeq+, lnSeq− (p=0.059), lnαLF, and lnHF power (p>0.05). There was, however, a significant time-by-group interaction for lnLFggain. Post-hoc t test revealed that children had higher inLFgain compared to adults during tilt (p<0.05).TableCONCLUSION: Head-up tilt results in a higher heart rate in children compared to adults while there is no difference in the systolic BP responses between groups. Children also have higher baroreflex gain and cardiovagal activity during tilt compared to adults. This suggests that autonomic control in response to head-up tilt may differ between adults and children.

Reflection of heart rate regulation on linear and nonlinear heart rate variability measures

The aim of this study was to examine the dependence of heart rate variability (HRV) measures on RR interval length and to find out relationships between linear and nonlinear measures. The spectral powers in very low (VLF), low (LF) and high (HF) frequency regions, short-term scaling exponent α1 and sample entropy (SampEn) were determined. All spectral powers increased with increasing RR interval length until they reached a plateau. Neighbouring spectral powers were strongly correlated. The largest fraction of the spectrum consisted of VLF (from about 40 to 95%) and the smallest of HF, although HF was most sensitive to changes in RR interval length. SampEn is also increased with increasing RR interval, reaching a plateau. The dependence of α1 on RR showed a deflection point at 0.5 s. Nonlinear measures can be expressed by spectral powers: α1 by a linear function of ln(LF/HF) and SampEn by a quadratic function of ln HF. We concluded that during the day an increase of HR up to 120 beats min−1 was reflected in a reduction in HF and LF and to a smaller extent in VLF and by decreased complexity and increased correlation in RR interval series. In sleep, HRV measures are at a plateau. We suggest that below intrinsic value, HR is regulated mainly by changes of parasympathetic activity, reflected in linear and nonlinear HRV measures.

Factors affecting heart rate variability and heart rate responses to tilting in infants aged 1 and 3 months.

Heart rate variability (HRV) and heart rate (HR) responses following a 60 degree head-up tilt were measured in 60 infants at 1 and 3 mo of age to investigate the effects on these of age, sleep state, sleep position, and mother's smoking status. HRV was determined from Poincaré plots of 500 sequential RR intervals to measure overall variability derived from the SDRR of this plot, and instantaneous variability derived from the SDdeltaRR. HR responses to the tilt were measured as changes in RR interval length from rest to immediately following the tilt and again once a stable pattern was reached. SDRR and SDdeltaRR increased 20 and 40%, respectively, with age (p < 0.0001), SDRR was higher in active sleep (AS) than quiet sleep (QS, +72%, p < 0.0001) but both measures of variability (SDRR and SDdeltaRR) were lower in the prone position compared with supine (-18%, p < 0.0001). However, several findings were dependent on the basal RR interval, thus the age effect disappeared once RR interval was taken into account, sleep state remained an important factor and the lower variability when prone now became a difference of -3% (p = 0.034). The tilt generally provoked a reflex tachycardia followed by a bradycardia and settling to a stable HR level below, at, or above baseline within 30 s. The more unusual responses were no HR change, sustained tachycardia or sustained bradycardia (15% of total). These were more likely to occur in younger infants (p = 0.008) and in AS (p < 0.0001). No changes were seen in any of the cardiac indices related to maternal smoking status. The findings confirm several reports indicating that prone sleeping damps some physiologic responses. The data emphasize the need to consider basal heart rate, and sleep position as well as sleep state in autonomic function testing during infant sleep.

Which indexes of filling behavior should be used to characterize left ventricular diastolic function when changes in heart rate and atrioventricular delay occur?

The routine use of the peak early–to–peak atrial velocity, early velocity integral–to–atrial velocity integral, and early velocity integral–to–the total filling velocity integral ratios are limited because they are influenced by heart rate and atrioventricular delay. Hence, we sought to establish whether these ratios could be normalized to account for the differences in cycle length (RR interval) and diastolic filling period when heart rate and atrioventricular delay were altered in 18 patients with programmable dual-chamber pacemakers. We further explored whether these and other parameters of the mitral velocity profile could be used to characterize the mitral filling pattern during isoproterenol and methoxamine infusions—interventions that are likely to change both heart rate and left ventricular filling. The early velocity integral–to–atrial velocity integral and early velocity integral–to–the total filling velocity integral ratios were more sensitive to minor variations in heart rate and atrioventricular delay than the peak early–to–peak atrial velocity ratio. The early velocity integral–to–atrial velocity integral and early velocity integral–to–total filling velocity integral ratios could not be normalized to account for differences in RR interval or diastolic filling period, whereas the peak early–to–peak atrial velocity ratio became less sensitive to changes in heart rate when it was divided by the RR interval, or diastolic filling period, or √diastolic filling period. Because the diastolic filling period is affected by atrioventricular delay independent of changes in the RR interval, these ratios were also corrected for the functional cycle length, defined as the interval from R-wave of the electrocardiogram to the end of the diastolic filling period. When corrected for either the functional cycle length or diastolic filling period or √diastolic filling period, only the peak early–to–peak atrial velocity ratio became less sensitive to variations in the atrioventricular delay. The ratio of diastolic filling period expressed as a proportion of RR interval or functional cycle length changed significantly when heart rate and atrioventricular delay were altered and did not improve when diastolic filling period was divided by the √RR or √functional cycle length. However, when the √diastolic filling period was divided by the RR interval or functional cycle length, the effects of heart rate and atrioventricular delay were not apparent. Of all the ratios, the ratio of √diastolic filling period expressed as a proportion of RR interval or functional cycle length was the most useful to differentiate the confounding effects of heart rate (±atrioventricular delay) from the effects of isoproterenol and methoxamine on left ventricular filling. Hence, this ratio appeared to be a heart rate– and atrioventricular delay–independent index of left ventricular diastolic function.

Effect of Sotalol on RR Interval Variability During Induced Ventricular Tachycardia

The effect of sotalol on the rate and the RR interval variability of induced ventricular tachycardia was studied in 26 patients. Sotalol increased the mean cycle length and irregularity of RR intervals, which may affect detection of ventricular tachycardia by implantable cardioverter-defibrillator devices.

Value of QRS alteration in determining the site of origin of narrow QRS supraventricular tachycardia.

To determine the value of alternation of QRS morphology in determining the site of origin of sustained narrow QRS supraventricular tachycardia (SVT), we retrospectively studied 163 distinct tachycardias in 161 patients (ages 4 to 91 years) in whom the site of origin of SVT was proven by intracardiac electrophysiologic study. Sustained SVT was defined as lasting longer than 30 sec. Narrow QRS was defined as QRS width less than 0.12 sec. Atrial fibrillation and flutter were excluded. The presence or absence of QRS alternation was judged at least 10 sec after initiation of SVT. Circus movement tachycardia with anterograde AV node conduction and a retrograde accessory AV pathway was seen in 89 patients (58 with Wolff-Parkinson-White syndrome, 31 with concealed accessory pathway); intra-AV nodal reentrant tachycardia (AVNT) was present in 57 cases, and 17 tachycardias were atrial in origin. QRS alternation was present in 36 of 163 cases (22%). In only eight of these 36 did RR interval length alternation accompany alternation in QRS morphology. Thirty-three of 36 (92%) tachycardias with QRS alternation were circus movement tachycardias. Two were atrial in origin and one was AVNT. We conclude that the presence of QRS alternation during sustained narrow QRS SVT is highly indicative of a retrograde accessory AV pathway in the tachycardia circuit.

Analysis of heart rate and beat-to-beat variability: Interval difference index

The relationship between the RR interval length (RR) and the RR interval-to-interval differences (RR differences) was investigated. Data on heart rate were obtained from 11 newborn infants in well-defined behavioral states undergoing polygraphy. It was found that the RR differences were strongly dependent on the RR interval length in all behavioral states. The relationship between the RR differences (y) and the interval length (RR) could be described by the equation y = α(RR-320)1.5 for the interval length in the investigated range from 375 to 605 msec. On the basis of this relationship the interval difference index (ID index) was constructed for the quantification of beat-to-beat heart rate variability. The ID index showed good independence from the long-term irregularity index (LTI index). Other statistical parameters proposed for the quantification of beat-to-beat heart rate variability are discussed and compared with the ID index.