Respiratory sinus arrhythmia as a second order system

The presented study describes the influence of respiration on heart rate, under controlled respiration conditions. In addition, this study makes a comparison of a simple physical model, the spring-mass system, with the biophysics of respiration. It is possible to use the equations describing the behaviour of the respiratory system, under certain conditions, and analyse them in a way similar to the equations that describe the physical spring-mass system. The results of the heart rate and respiration measurements effected on 10 subjects at various respiration frequencies show us that the heart rate behaves as a second order system within the boundary conditions during a longer period of constant respiration. The results also show that the heart rate behaves as a second order system within the intermediate mode during short time intervals when there is no respiration.     

Respiratory sinus arrhythmia from two coupled pacemakers.

We reproduce global features of respiratory sinus arrhythmia (RSA), a prominent source of heart rate variability, from two signals coupled in alternate fashion so dominance periodically switches back and forth between them. We consider two different possibilities for this coupling and illustrate our method with numerical simulations that we contrast with the corresponding results from real data. We interpret our findings within the context of the two-pacemaker model of the heartbeat, an alternative to the single-pacemaker mechanism of pulse generation in the orthodox conduction model.     

Respiratory sinus arrhythmia alteration following training in endurance athletes.

Significant increases in maximum oxygen consumption (VO2max) were noted in nine young track athletes following an 8-week high-intensity running period (P less than 0.05). VO2max was measured, prior to and following the training program, using an on-line, open-circuit spirometry system. Parasympathetic activity was assessed using heart period variation (R-R interval in milliseconds) during carefully controlled breathing activity (R sinus arrhythmia). Following the training program, a 7.3% increase in aerobic capacity was associated with a 23.1% augmentation of efferent parasympathetic activity (P less than 0.01). These data suggest that enhanced aerobic capacity increases efferent parasympathetic tone.     

Respiratory sinus arrhythmia in freely moving and anesthetized rats.

Heart rate increases during inspiration and slows during postinspiration; this respiratory sinus arrhythmia helps match pulmonary blood flow to lung inflation and maintain an appropriate diffusion gradient of oxygen in the lungs. This cardiorespiratory pattern is found in neonatal and adult humans, baboons, dogs, rabbits, and seals. Respiratory sinus arrhythmia occurs mainly due to inhibition of cardioinhibitory parasympathetic cardiac vagal neurons during inspiration. Surprisingly, however, a recent study in anesthetized rats paradoxically found an enhancement of cardiac vagal activity during inspiration, suggesting that rats have an inverted respiratory sinus arrhythmia (Rentero N, Cividjian A, Trevaks D, Pequignot JM, Quintin L, and McAllen RM. Am J Physiol Regul Integr Comp Physiol 283: R1327-R1334, 2002). To address this controversy, this study examined respiratory sinus arrhythmia in conscious freely moving rats and tested whether the commonly used experimental anesthetics urethane, pentobarbital sodium, or ketamine-xylazine alter respiratory sinus arrhythmia. Heart rate significantly increased 21 beats/min during inspiration in conscious rats, a pattern similar to the respiratory sinus arrhythmia that occurs in other species. However, anesthetics altered normal respiratory sinus arrhythmia. Ketamine-xylazine (87 mg/kg and 13 mg/kg) depressed and pentobarbital sodium (60 mg/kg) abolished normal respiratory sinus arrhythmia. Urethane (1 g/kg) inverted the cardiorespiratory pattern so that heart rate significantly decreased during inspiration. Our study demonstrates that heart rate normally increases during inspiration in conscious, freely moving rats, similar to the respiratory sinus arrhythmia pattern that occurs in other species but that this pattern is disrupted in the presence of general anesthetics, including inversion in the case of urethane. The presence and consequences of anesthetics need to be considered in studying the parasympathetic control of heart rate.     

Respiratory sinus arrhythmia, cardiac vagal control, and daily activity

Ambulatory respiratory sinus arrhythmia (RSA) or high-frequency heart rate (HR) variability is frequently employed as an index of cardiac parasympathetic control and related to risk or severity of cardiovascular disease. However, laboratory studies indicate variations in physical activity and respiratory parameters of rate and tidal volume may confound estimation of vagal activity. Because little is known about these relations outside the laboratory, we examined ambulatory relations among RSA, respiration, physical activity, and HR during waking hours by employing a multichannel monitoring system. Forty healthy young-to-middle aged adults underwent daytime monitoring that included continuous registration of the ECG, respiration (inductance plethysmography), and accelerometry motion activity. Within-individual regression analyses were performed to examine minute-to-minute relations between RSA and respiration, HR, and indexes of physical activity (minute ventilation and motion). HR changes were assumed to be strongly related to within-individual variations of vagal tone. RSA adjusted for respiratory parameters and unadjusted RSA were compared for strength of prediction of other measures. Unadjusted RSA was related to respiratory parameters (R = 0.80) and moderately predicted minute-to-minute HR and activity variances (means = 56%, HR; 48%, minute ventilation; and 37%, motion). Adjusted RSA predicted significantly more HR and activity variance (means = 75%, 76%, and 57%, respectively) with narrower confidence intervals. We conclude that ambulatory RSA magnitude is associated with respiratory variations and physical activity. Adjustment for respiratory parameters substantially improves relations between RSA and significantly vagally mediated HR and physical activity. Concurrent monitoring of respiration and physical activity may enhance HR variability accuracy to predict autonomic control.     

Respiratory sinus arrhythmia: noninvasive measure of parasympathetic cardiac control.

The degree of parasympathetic heart rate control, PC, was defined as the decrease in average heart period (RR interval) caused by the elimination of parasympathetically mediated influences on the heart while keeping sympathetic activity unchanged. By reviewing published results on the interaction of sympathetic and parasympathetic heart rate control, the prediction was made that PC should be directly proportional to VHP, the peak-to-peak variations in heart period caused by spontaneous respiration. In sevel chloralose/urethan-anesthetized dogs the vagi were reversibly blocked by cooling, and PC (the difference between average heart period before and after cooling) and VHP (without cooling) were determined under a variety of conditions that included a) increasing vagal activity by elevating the blood pressure b) sympathetic blockade, and c) parasympathetic blockade. The relationship between VHP and PC was linear with an average correlation coefficient of 0.969 +/- 0.024 (SD) and a PC-axis intercept of 15.2 +/- 25.9 ms. In each dog the correlation coefficient between VHP and PC was higher than between VHP and the average heart period (avg correlation coef: 0.914 +/- 0.044). These results suggest that the degree of respiratory sinus arrhythmia may be used as a noninvasive indicator of the degree of parasympathetic cardiac control.     

Mechanisms of respiratory sinus arrhythmia in patients with mild heart failure

The high-frequency (HF) component of the heart rate variability (HRV) is regarded as an index of cardiac vagal responsiveness. However, when vagal tone is decreased, nonneural mechanisms could account for a significant proportion of the HF component. To test this hypothesis, we examined the HRV spectral power in 20 patients with mild chronic heart failure (CHF) and 11 controls before and during ganglion blockade with trimethaphan camsylate (3-6 mg/min iv). A small HF component was still present during ganglion blockade, and its amplitude did not differ between CHF patients and controls. The average contribution of nonneural oscillations to the HF component was 15% (range 1-77%) in patients with CHF and 3% (range 0. 7-30%) in healthy controls (P < 0.005). During controlled breathing at 0.16 Hz, however, it decreased to 1% (range 0.2-13%) in healthy controls and 5% (range 1-44%) in CHF patients. Our results indicate that the HF component can significantly overestimate cardiac vagal responsiveness in patients with mild CHF. This bias is improved by controlled breathing, since this maneuver increases the vagal contribution to HF without affecting its nonneural component.     

Respiratory sinus arrhythmia is associated with efficiency of pulmonary gas exchange in healthy humans

Respiratory sinus arrhythmia (RSA) may be associated with improved efficiency of pulmonary gas exchange by matching ventilation to perfusion within each respiratory cycle. Respiration rate, tidal volume, minute ventilation (.VE), exhaled carbon dioxide (.VCO(2)), oxygen consumption (.VO(2)), and heart rate were measured in 10 healthy human volunteers during paced breathing to test the hypothesis that RSA contributes to pulmonary gas exchange efficiency. Cross-spectral analysis of heart rate and respiration was computed to calculate RSA and the coherence and phase between these variables. Pulmonary gas exchange efficiency was measured as the average ventilatory equivalent of CO(2) (.VE/.VCO(2)) and O(2) (.VE/.VO(2)). Across subjects and paced breathing periods, RSA was significantly associated with CO(2) (partial r = -0.53, P = 0.002) and O(2) (partial r = -0.49, P = 0.005) exchange efficiency after controlling for the effects of age, respiration rate, tidal volume, and average heart rate. Phase between heart rate and respiration was significantly associated with CO(2) exchange efficiency (partial r = 0.40, P = 0.03). These results are consistent with previous studies and further support the theory that RSA may improve the efficiency of pulmonary gas exchange.     

Microgravity alters respiratory sinus arrhythmia and short-term heart rate variability in humans

We studied heart rate (HR), heart rate variability (HRV), and respiratory sinus arrhythmia (RSA) in four male subjects before, during, and after 16 days of spaceflight. The electrocardiogram and respiration were recorded during two periods of 4 min controlled breathing at 7.5 and 15 breaths/min in standing and supine postures on the ground and in microgravity. Low (LF)- and high (HF)-frequency components of the short-term HRV (< or =3 min) were computed through Fourier spectral analysis of the R-R intervals. Early in microgravity, HR was decreased compared with both standing and supine positions and had returned to the supine value by the end of the flight. In microgravity, overall variability, the LF-to-HF ratio, and RSA amplitude and phase were similar to preflight supine values. Immediately postflight, HR increased by approximately 15% and remained elevated 15 days after landing. LF/HF was increased, suggesting an increased sympathetic control of HR standing. The overall variability and RSA amplitude in supine decreased postflight, suggesting that vagal tone decreased, which coupled with the decrease in RSA phase shift suggests that this was the result of an adaptation of autonomic control of HR to microgravity. In addition, these alterations persisted for at least 15 days after return to normal gravity (1G).     

The effect of the tidal volume on the sinus arrhythmia of the heart]

The sinus arrhythmia of the human heart was investigated in its relation to the tidal volume under resting conditions in the course of the day, in voluntarily changed tidal volume, under atropine medication and during physical work. In resting conditions it was found that nearly 40% of the sinus arrhythmia is of respiratory origin, and that no respiratory influence is demonstrable any longer at sufficient doses of atropine. Under physical load, the sinus arrhythmia is diminished in spite of the enlarged tidal volume. In this case, an additional smoothing influence to the sinus arrhythmia has to be assumed.     

Respiratory sinus arrhythmia as an objective criterion for studying and optimizing speech breathing parameters

It was established that the use of respiratory sinus arrhythmia as an objective physiological criterion of intersystem integration is effective and justifiable for the study, optimization, diagnosis, and correction of human speech breathing, speech, and speech behavior.     

Individual differences in respiratory sinus arrhythmia

To investigate the interindividual differences in respiratory sinus arrhythmia (RSA), recordings of ventilation and electrocardiogram were obtained from 12 healthy subjects for five imposed breathing periods (T(TOT)) surrounding each individual's spontaneous breathing period. In addition to the spectral analysis of the R-R interval signal at each breathing period, RSA characteristics were quantified by using a breath-by-breath analysis where a sinusoid was fitted to the changes in instantaneous heart rate in each breath. The amplitude and phase (or delay = phase x T(TOT)) of this sinusoid were taken as the RSA characteristics for each breath. It was found that for each subject the RSA amplitude-T(TOT) relationship was linear, whereas the delay-T(TOT) relationship was parabolic. However, the parameters of these relationships differed between individuals. Linear correlation between the slopes of RSA amplitude versus T(TOT) regression lines and 1) mean breathing period and 2) mean R-R interval during spontaneous breathing were calculated. Only the correlation coefficient with breathing period was significantly different from zero, indicating that the longer the spontaneous breathing period the lesser the increase in RSA amplitude with increasing breathing period. Similarly, only the correlation coefficient between the curvature of the RSA delay-T(TOT) parabola and mean breathing period was significantly different from zero; the longer the spontaneous breathing period the larger the curvature of RSA delay. These results suggest that the changes in RSA characteristics induced by changing the breathing period may be explained partly by the spontaneous breathing period of each individual. Furthermore, a transfer function analysis performed on these data suggested interindividual differences in the autonomic modulation of the heart rate.     

The relationship between respiratory sinus arrhythmia and heart rate during anesthesia in rat.

During inspiration the heart rate (HR) increases and during expiration it decreases. Contribution of respiratory sinus arrhythmia (RSA) to spontaneous heart rate variability (HRV) can be measured as the high frequency (HF) component of variation in consecutive R-R intervals on ECG. In conscious rats, slowing of HR is associated with an increase in HF. The aim of this study was to investigate whether this relationship between HF and HR is preserved during anesthesia in rat. A 15 minutes long ECG signal was recorded from rats (N=15) under moderate chloral hydrate (CHL) anesthesia. Recordings were extended with 45 minutes to investigate the effect of atropine (N=3), against controls (N=3). Short term HRV was investigated in 30 seconds long epochs. HF was considered the frequency band between 0.8 and 1.6 Hz. RSA was quantified as the relative spectral power of the HF. Respiratory frequency (RF) was quantified as the mean spectral frequency within the HF band. One minute estimates of HR, RSA and HF were calculated by averaging 3 epochs of 30 seconds overlapped 50%. The average HR was 427 +/- 3 bpm. The magnitude of RSA was 45 +/- 1% at a RF of 71 +/- 1 rpm. We found that: (1) the decrease in HR that occurs during CHL anesthesia in rat correlates with an increase in RSA; (2) atropine reduces RSA and the time-dependent decrease in HR; (3) the time-dependent increase in RSA is preserved after atropine. We conclude that the correlation between RSA and HR reflects the cardio-pulmonary coupling under parasympathetic control.     

Self-regulation of respiratory sinus arrhythmia

Respiratory sinus arrhythmia (RSA) — the peak-to-peak variations in heart rate caused by respiration — can be used as a noninvasive measure of parasympathetic cardiac control. In the present study four strategies to increase RSA amplitude are investigated: (1) biofeedback of RSA amplitude, (2) biofeedback of RSA amplitude plus respiratory instructions, (3) respiratory biofeedback, and (4) respiratory instructions only. All four procedures produce a significant increase of RSA amplitude from the first physiological control trial compared to baseline. This increase is faster for the groups that received respiratory biofeedback and respiratory instructions only than for the two groups that received biofeedback of RSA amplitude, the increases being equivalent for the four groups in the third session. All subjects of the group that received biofeedback of RSA amplitude only reported respiratory strategies in order to achieve the increase in RSA. Possible clinical implications of these results for parasympathetic cardiac control and cardiovascular disorders are discussed.This research was supported by a grant to the first author from the University of Granada (Spain).     

Differences in stress reactivity of laboratory macaques measured by heart period and respiratory sinus arrhythmia

Some laboratory primates are more likely than others to react to anxiety-provoking stressors. Individuals that overreact to stressors may experience diminished psychological well-being and would be inappropriate for some experiments. The differences between reactive and nonreactive individuals may be reflected in heart period and respiratory sinus arrhythmia (RSA). Using surface electrodes and radio telemetry, we measured these two cardiac variables in seven male and ten female singly caged longtailed macaques (Macaca fascicularis) when they were exposed to two stressors, a sudden noise (whistle test) and an unfamiliar technician wearing capture gloves (glove test). Behavior was videotaped during both tests. For the whistle test, cardiac data were recorded before, during, and after two 1 min whistle blasts separated by 90 min. For the glove test, data were recorded in 1 min blocks every 8 min over 96 min before, during, and after 1 min exposure to the gloved technician. Heart period was decreased and RSA was suppressed during both the whistle and glove exposures. After the whistle test, the cardiac activity of most subjects returned to baseline levels within 10 min. The glove test produced more extended suppression, with greater individual differences, than the whistle test. There were greater individual differences in RSA than in heart period. These enhanced individual differences were used to define stress reactors that differed from nonreactors in their cardiac data profiles. Of 16 subjects that completed the glove test, five were identified as reactors. Am. J. Primatol. 45:245–261, 1998. © 1998 Wiley-Liss, Inc.