Central vagotonic effects of atropine modulate spectral oscillations of sympathetic nerve activity

Quantifying cardiac sympathetic and parasympathetic nervous activities using principal dynamic modes analysis of heart rate variability

The ratio between low-frequency (LF) and high-frequency (HF) spectral power of heart rate has been used as an approximate index for determining the autonomic nervous system (ANS) balance. An accurate assessment of the ANS balance can only be achieved if clear separation of the dynamics of the sympathetic and parasympathetic nervous activities can be obtained, which is a daunting task because they are nonlinear and have overlapping dynamics. In this study, a promising nonlinear method, termed the principal dynamic mode (PDM) method, is used to separate dynamic components of the sympathetic and parasympathetic nervous activities on the basis of ECG signal, and the results are compared with the power spectral approach to assessing the ANS balance. The PDM analysis based on the 28 subjects consistently resulted in a clear separation of the two nervous systems, which have similar frequency characteristics for parasympathetic and sympathetic activities as those reported in the literature. With the application of atropine, in 13 of 15 supine subjects there was an increase in the sympathetic-to-parasympathetic ratio (SPR) due to a greater decrease of parasympathetic than sympathetic activity ( P = 0.003), and all 13 subjects in the upright position had a decrease in SPR due to a greater decrease of sympathetic than parasympathetic activity ( P < 0.001) with the application of propranolol. The LF-to-HF ratio calculated by the power spectral density is less accurate than the PDM because it is not able to separate the dynamics of the parasympathetic and sympathetic nervous systems. The culprit is equivalent decreases in both the sympathetic and parasympathetic activities irrespective of the pharmacological blockades. These findings suggest that the PDM shows promise as a noninvasive and quantitative marker of ANS imbalance, which has been shown to be a factor in many cardiac and stress-related diseases.

Central cholinergic modulation of blood pressure short-term variability

The role of neurally born acetylcholine in the central modulation of cardiovascular short-term variability was assessed using a pharmacological probe physostigmine, a cholinesterase inhibitor that can act centrally also. Experiments were performed in instrumented conscious rats. Equidistant sampling at 20 Hz of systolic arterial pressure (SAP), diastolic arterial pressure (DAP) and heart rate (HR) allowed direct spectral analysis. Spectra were analysed in the whole, very-low frequency (VLF), low-frequency (LF) and high-frequency (HF) domains. Physostigmine, but not neostigmine, increased SAP, LF SAP and HF SAP variability while neostigmine, but not physostigmine, decreased HR without affecting HR variability. Atropine methyl nitrate prevented neostigmine-induced bradycardia and potentiated the effects of physostigmine on DAP, LF DAP and HF DAP variability. Atropine sulphate, hexamethonium, phentolamine and metoprolol inhibited physostigmine-induced increase of SAP and LF SAP. Pre-treatment of rats by quinapril prevented physostigmine-induced increase of SAP, but not of LF SAP, while the V(1a) antagonist prevented the increase of HF SAP. The results suggest that central cholinergic neurons facilitate but do not create LF SAP and HF SAP variability. The effect of physostigmine on LF SAP seems to be mediated via central muscarinic sites and the peripheral sympathetic system, while non-muscarinic central sites and vasopressin pathways subserve the increase of HF SAP.

Point: cardiovascular variability is/is not an index of autonomic control of circulation

PURPOSE AND SCOPE OF THE POINT:COUNTERPOINT DEBATES

This series of debates was initiated for the Journal of Applied Physiology because we believe an important means of searching for truth is through debate where contradictory viewpoints are put forward. This dialectic process whereby a thesis is advanced, then opposed by an antithesis, with a synthesis subsequently arrived at, is a powerful and often entertaining method for gaining knowledge and for understanding the source of a controversy. Before reading these Point:Counterpoint manuscripts or preparing a brief commentary on their content (see below for instructions), the reader should understand that authors on each side of the debate are expected to advance a polarized viewpoint and to select the most convincing data to support their position. This approach differs markedly from the review article where the reader expects the author to present balanced coverage of the topic. Each of the authors has been strictly limited in the lengths of both the manuscript (1,200 words) and the rebuttal (400). The number of references to publications is also limited to 30, and citation of unpublished findings is prohibited.

An investigation into the physiological relevance of the vagal tachycardia in the anaesthetized dog

AIM

Our aim was primarily to assess whether or not a vagal tachycardia can be elicited in vivo without administration of atropine, and secondly to evaluate whether the dose of atropine, a muscarinic antagonist, determines the magnitude of the tachycardia.

METHODS

Experiments were carried out in the presence of atenolol (2 mg kg(-1)). The vagal tachycardia requires high vagal activity which was induced by noradrenaline infusion (20 microg min(-1)). Two techniques were then used to elicit a tachycardia, vagal section and atropine administration.

RESULTS

The increase in blood pressure caused heart rate to fall to 60 +/- 7 beats min(-1) (mean +/- SEM). When the vagi were sectioned (n = 5) heart rate increased by 9 +/- 2 beats min(-1) above the intrinsic rate which was 108 beats min(-1), this increase was not significant. In contrast atropine given (9-20 microg kg(-1)) (n = 5) during high vagal activity increased heart rate by 81 +/- 22 beats min(-1) above the intrinsic rate (P < 0.05). To assess if the dose of atropine affects the magnitude of the vagal tachycardia, the right vagus was stimulated electrically at increasing frequencies (2, 4, 8, 16, 32 Hz) before and after increasing doses of atropine (0.02, 0.05, 1 mg kg(-1)). This reduced the magnitude of the bradycardia; however, the magnitude of the vagal tachycardia was unaffected.

CONCLUSION

The vagal tachycardia cannot be elicited without atropine suggesting that it does not play a significant physiological role.

Carotid-Cardiac Baroreflex Function Does Not Influence Blood Pressure Regulation during Head-Up Tilt in Humans

The influence of the carotid-cardiac baroreflex on blood pressure regulation was evaluated during supine rest and 40 degrees head-up tilt (HUT) in 9 healthy young subjects with and without full cardiac vagal blockade. The carotid baroreflex responsiveness, or maximal gain (G(MAX)), was assessed from the beat-to-beat changes in heart rate (HR) and mean arterial pressure (MAP) by the variable neck pressure and suction technique ranging in pressure from +40 to -80 Torr, with and without glycopyrrolate (12.0 +/- 1.0 microg/kg body weight; mean +/- SE). In the supine position, glycopyrrolate increased the HR to 91 +/- 3 bpm, from 54 +/- 3; MAP to 89 +/- 2 mmHg, from 76 +/- 2; and cardiac output to 6.8 +/- 0.3 l.min(-1), from 4.9 +/- 0.3 (P < 0.05). The G(MAX) of the carotid baroreflex control of HR was reduced to -0.06 +/- 0.01 bpm.mmHg(-1), from -0.30 +/- 0.02 (P < 0.05) with no significant effect on the G(MAX) of the carotid baroreflex control of MAP. During HUT the carotid baroreflex control of MAP was unchanged, though the G(MAX) of the carotid baroreflex control of HR was increased (P < 0.05). During HUT, central blood volume, assessed by electrical thoracic admittance, and total vascular conductance were decreased with and without glycopyrrolate. Furthermore, glycopyrrolate reduced G(MAX) of the carotid baroreflex control of HR during HUT (P < 0.05) with no significant effect on G(MAX) of the carotid baroreflex control of MAP. These data suggest that during supine rest and HUT-induced decreases in central blood volume, the carotid baroreflex control of HR is mediated primarily via parasympathetic activity. Furthermore, the maintenance of arterial blood pressure during postural stress is primarily mediated by arterial and cardiopulmonary reflex regulation of sympathetic activity and its effects on the systemic vasculature.

Heart rate variability and spontaneous baroreflex sequences in supine healthy volunteers subjected to nasal positive airway pressure

To determine the dynamic effects of short-term nasal positive airway pressure (nPAP) on cardiovascular autonomic control, continuous recordings of noninvasively obtained hemodynamic measurements and heart rate variability (HRV) were obtained in 10 healthy subjects during frequency-controlled breathing (between 0.20 and 0.24 Hz) in supine posture under different pressures of nPAP ranging from 3 to 20 cmH(2)O. HRV was assessed using spectral analysis of the R-R interval. The slope of the regression line between spontaneous systolic blood pressure and pulse interval changes was taken as an index of the sensitivity of arterial baroreflex modulation of heart rate (sequence method). Application of nPAP resulted in a pressure-dependent decrease of cardiac output and stroke volume (P < 0.05, ANOVA) and in an increase in total peripheral resistance (P < 0.03, ANOVA). Hemodynamic changes under increasing nPAP were accompanied by a decrease in total power of HRV despite mean R-R interval remaining unchanged. The overall decrease in HRV was accompanied by a reduction across all frequency bands when absolute units were used (P < 0.01). When the power of low frequency and high frequency was calculated in normalized units, a diminished high frequency and an increased low-to-high frequency ratio were observed (P < 0.05). Compared with low levels of nPAP, pressure levels of >10 cmH(2)O were associated with a significant decline in the mean slope of spontaneous baroreceptor sequences (P < 0.04). These findings indicate that short-term administration of nPAP in normal subjects exerts significant alterations in R-R interval variability and spontaneous baroreflex modulation of heart rate.

Frequency-dependent baroreflex modulation of blood pressure and heart rate variability in conscious mice

The goal of this study was to determine the baroreflex influence on systolic arterial pressure (SAP) and pulse interval (PI) variability in conscious mice. SAP and PI were measured in C57Bl/6J mice subjected to sinoaortic deafferentation (SAD, n = 21) or sham surgery ( n = 20). Average SAP and PI did not differ in SAD or control mice. In contrast, SAP variance was enhanced (21 ± 4 vs. 9.5 ± 1 mmHg2) and PI variance reduced (8.8 ± 2 vs. 26 ± 6 ms2) in SAD vs. control mice. High-frequency (HF: 1–5 Hz) SAP variability quantified by spectral analysis was greater in SAD (8.5 ± 2.0 mmHg2) compared with control (2.5 ± 0.2 mmHg2) mice, whereas low-frequency (LF: 0.1–1 Hz) SAP variability did not differ between the groups. Conversely, LF PI variability was markedly reduced in SAD mice (0.5 ± 0.1 vs. 10.8 ± 3.4 ms2). LF oscillations in SAP and PI were coherent in control mice (coherence = 0.68 ± 0.05), with changes in SAP leading changes in PI (phase = −1.41 ± 0.06 radians), but were not coherent in SAD mice (coherence = 0.08 ± 0.03). Blockade of parasympathetic drive with atropine decreased average PI, PI variance, and LF and HF PI variability in control ( n = 10) but had no effect in SAD ( n = 6) mice. In control mice, blockade of sympathetic cardiac receptors with propranolol increased average PI and decreased PI variance and LF PI variability ( n = 6). In SAD mice, propranolol increased average PI ( n = 6). In conclusion, baroreflex modulation of PI contributes to LF, but not HF PI variability, and is mediated by both sympathetic and parasympathetic drives in conscious mice.

Effect of remifentanil with and without atropine on heart rate variability and RR interval in children

Remifentanil can cause bradycardia either by parasympathetic activation or by other negative chronotropic effects. The high frequency (HF) component of heart rate variability (HRV) is a marker of parasympathetic activity. This study aimed to evaluate the effect of remifentanil on RR interval and on HRV in children. Forty children ASA I or II were studied after approval by the human studies committee and informed parental consent was obtained. After stabilisation at sevoflurane 1 MAC, they were randomly divided into two groups: one received a 20 microg.kg(-1) atropine injection (AT + REMI) and the other ringer lactate solution (REMI). Three minutes later, a 1 microg.kg(-1) bolus of remifentanil was administered over 1 min, followed by a continual infusion at 0.25 microg.kg(-1).min(-1) for 10 min increased to 0.5 microg.kg(-1).min(-1) for a further 10 min. A time varying, autoregressive analysis of RR sequences was used to estimate classical spectral parameters: low (0.04-0.15 Hz; LF) and high (0.15-0.45 Hz; HF) frequency, whereas the root mean square of successive differences of RR intervals (rmssd) was derived directly from the temporal sequence. Statistical analyses were conducted by means of the multiple correspondence analysis and with non parametrical tests. Remifentanil induced an RR interval lengthening, i.e. bradycardia, in both groups compared to pretreatment values and was associated with an increase of HF and rmssd only for the REMI group. The parasympathetic inhibition by atropine did not totally prevent remifentanil's negative chronotropic effect. A direct negative chronotropic effect of remifentanil is proposed.

Circadian variation of cardiac autonomic nervous profile is affected in Japanese ambulance men with a working system of 24-h shifts

OBJECT

The purpose of this study was to compare the stress levels of Japanese ambulance men between on-duty and off-duty days, by using the physiological indices of heart rate variability (HRV) and cortisol in urine, measured over each 24-h period.

METHODS

Measurements were made during one on-duty and one off-duty day for each subject. The participants were monitored for 24 h with a Holter recording system and a parameter reflecting overall stress levels was obtained by measuring the cortisol level in urine collected over 24 h.

RESULTS

The circadian variation of cardiac autonomic nervous system activity was affected when the subjects were on duty. The low-frequency/high-frequency power ratio (=low-frequency power/high-frequency power: LF/HF), which is a useful parameter that reflects the balance of cardiac autonomic nervous activity, differed significantly between the waking and sleeping times on the off-duty day (P=0.03), while it did not differ between these two states on the on-duty day (P=0.56). Similarly, the normalized high-frequency power [=high-frequency/(high-frequency+low-frequency) power: HF/(HF+LF)] ratio, which is a useful measure of the activity of the parasympathetic nervous system, differed significantly between these two states on the off-duty day (P=0.04), while there was no significant difference in the ratio between the two states on the on-duty day (P=0.13).

CONCLUSION

These results show that the diurnal balance of the cardiac autonomic nervous system is affected on the on-duty day, even though it is possible for ambulance men to sleep regular hours.

Negative chronotropic response to low-dose atropine is associated with parasympathetic nerve-mediated cardiovascular response in postoperative patients with congenital heart disease

OBJECTIVES

To investigate the negative chronotropic response (NCR) to low-dose atropine in postoperative patients with congenital heart disease (CHD).

BACKGROUND

Low-dose atropine causes a NCR through the central nervous system muscarinic receptor and is attenuated in adult heart failure patients. It has never been evaluated in CHD patients.

METHODS

NCR corrected for basal heart rate (HR) (minimal HR/basal HR=cNCR) was determined after low-dose atropine (3 microg/kg) administration in 124 postoperative CHD patients (97 biventricular repair and 27 Fontan patients) and 11 controls and was compared with the cardiac autonomic nervous and functional status.

RESULTS

The cNCR in simple CHD (post atrial or ventricular septal defect closure), complex biventricular CHD, and Fontan patients were 0.92+/-0.04, 0.94+/-0.04 and 0.96+/-0.04, respectively, and higher than in controls (0.87+/-0.03, p<0.001). In the complex CHD patients, higher cNCR was mainly associated with the lower pharmacologically determined cardiac parasympathetic nervous tone (PST), HR variability, high atrial natriuretic peptide, and lower right ventricular ejection fraction (p<0.0001). In Fontan patients, the lower beta sensitivity of the sinus node and the PST mainly determined the higher cNCR (p<0.01) and the cNCR did not correlate with either hemodynamics or exercise capacity.

CONCLUSIONS

NCR is attenuated in proportion to the impaired cardiac parasympathetic nervous system and hemodynamics in postoperative complex biventricular CHD patients. In addition to PST, beta sensitivity of the sinus node significantly influences the NCR in Fontan patients.

Low-dose atropine amplifies cardiac vagal modulation and increases dynamic baroreflex function in humans

It has been previously known that low-dose atropine (LDA) enhances vagal outflow to the heart. To demonstrate the importance of vagal cardiac modulation in arterial blood pressure (ABP) stability, we evaluated the effect of vagal cardiac stimulation with administration of LDA on ABP fluctuation during dynamic hypertensive and hypotensive stimuli. We assessed changes in RR interval (RRI), ABP, power spectral densities of heart rate variability (HRV) and ABP variability, and spontaneous baroreflex sensitivity (BRS) in 16 healthy volunteers before and after administration of LDA (2 microg/kg). Transient hypertension was induced by phenylephrine (2 microg/kg), whereas hypotension was induced by bilateral thigh cuff deflation after a 3-min suprasystolic occlusion. LDA elicited bradycardia and significantly increased high-frequency (HF, 0.15-0.4 Hz) power of HRV and spontaneous BRS, as determined by transfer function analysis. The increase in systolic blood pressure (SBP) after phenylephrine administration was significantly attenuated by LDA (16+/-2 to 11+/-3 mmHg, P<0.005) and was associated with the augmented reflex bradycardia, whereas the decrease in SBP after cuff deflation was not affected (14+/-5 to 13+/-5 mmHg) with the augmented reflex tachycardia. Increases of HF HRV were correlated significantly and negatively with the increased SBP induced by phenylephrine before and after LDA (r=-0.502, P<0.05). These data suggest that the increased vagal cardiac function induced by LDA augments HR buffering effects, and is important in minimizing arterial pressure fluctuation during dynamic hypertensive stimuli.

Autonomic cardiovascular responses are impaired in women with irritable bowel syndrome

GOALS

This study characterizes cardiovascular autonomic function in women with irritable bowel syndrome (IBS), using standardized techniques.

BACKGROUND

Autonomic dysfunction is believed to contribute to abnormal gastrointestinal motility and visceral hypersensitivity in IBS. There is mounting evidence of generalized impairment of autonomic activity in patients with IBS.

STUDY

Thirty women aged 39 years (95% C.I. 25-53 years) diagnosed with IBS, and 30 age-matched healthy women were studied. The ratio of low frequency to high frequency heart rate variability domains (LF:HF ratio) was used to represent cardiac sympathovagal activity, and orthostatic testing and sustained isometric handgrip exercise were used as sympathetic stimuli. Parasympathetic activity was represented by the expiratory to inspiratory R-R interval (E:I) ratio during deep breathing at 6 minutes.

RESULTS

LF:HF responses to handgrip exercise (316%, C.I. 134% to 498% vs. 107%, C.I. 15% to 153%; P < 0.05) and orthostatic testing (648%, C.I. 520% to 904% vs. 330%, C.I. 140% to 520%; P < 0.05) were higher in IBS patients than controls, and the E:I ratio was significantly lower (1.47, C.I. 1.33-1.61 vs. 1.20, C.I. 1.14-1.26; P < 0.01).

CONCLUSIONS

Autonomic cardiovascular function is impaired in IBS, manifest as attenuated cardio-vagal tone, and relative sympathetic excess during stimulated conditions.

Heart rate and blood pressure variability in subjects with vasovagal syncope

Autonomic nervous system control in subjects with vasovagal syncope is controversial. In the present study, we used short-term spectral analysis to evaluate autonomic control in subjects with recurrent vasovagal syncope. We assessed the ability of spectral indices of HR (heart rate) variability to predict tilt-test responses. A series of 47 outpatients with recurrent vasovagal syncope and with positive responses to head-up tilt testing underwent a further study of RR variability during controlled breathing at rest and during tilt testing. During controlled breathing, RR interval variability of total power (TPRR; P<0.001), low-frequency power (LFRR; P<0.05), high-frequency power (HFRR; P<0.001) and HF expressed in normalized units (HFnuRR; P<0.001) were all higher, and LF expressed in normalized units (LFnuRR) and LF/HF ratio were lower in subjects with vasovagal syncope than in controls (P<0.001). To assess the ability of spectral components of RR variability to predict tilt-test responses, we prospectively studied 109 subjects with recurrent vasovagal syncope. The two normalized measures, HFnuRR and LFnuRR, determined during controlled breathing alone predicted a positive tilt-test response (sensitivity, 76%; specificity, 99%; positive predictive value, 96%; and negative predictive value, 90%). During tilting, subjects with vasovagal syncope had lower SBP (systolic blood pressure; P<0.05), LF component of peak SBP variability (LFSBP) and LFnuRR than controls, and higher TPRR, HFRR, HFnuRR and α HF (P<0.001). These spectral data indicate that vagal sinus modulation is increased at rest in subjects with vasovagal syncope. Spectral analysis of RR variability during controlled breathing, a procedure that predicts tilt-test responses, could be a useful guide in choosing the method of tilt testing.

Serial assessment of cardiovascular control shows early signs of developing pre-eclampsia

PURPOSE

To evaluate whether differences in autonomic cardiovascular control between normal pregnant women and women who develop pre-eclampsia later in pregnancy can be detected even before or early in pregnancy.

DESIGN

We studied 42 women, 21 multigravid with a history of pre-eclampsia and 21 primigravid, before pregnancy, at 6, 8, 12, 16, 20 and 32 weeks gestation and 15 weeks after delivery.

METHODS

The outcome of pregnancy was classified after delivery as normal pregnancy (NP group) or pre-eclampsia (PE group). Continuous heart rate and blood pressure were recorded by Portapres (TNO, Amsterdam, The Netherlands) during orthostatic stress, during rest in a supine and sitting position, and during paced breathing for periods of 1 min at breathing frequencies of 6, 10 and 15 breaths/min. Baroreflex gain from heart rate and blood pressure variability and the phase angle between both signals at low (approximately 0.1 Hz) and high frequency (respiratory rate) were analyzed by spectral analysis.

RESULTS

Eight women were diagnosed with pre-eclampsia. Subgroups did not differ in age, weight or height. The PE group showed a significantly higher mean arterial pressure before and during pregnancy [analysis of variance (ANOVA), P = 0.001], a significantly larger initial blood pressure drop to orthostatic stress before and in the first half of pregnancy (ANOVA, P = 0.002) and a significantly larger negative phase difference during supine rest at low frequency from 8 weeks onward (ANOVA P = 0.003).

CONCLUSIONS

These findings are compatible with increased resting sympathetic activity and decreased circulating volume, already present before and early in pregnancy, in women who will later develop pre-eclampsia.

Spectral analysis of muscle sympathetic nerve activity in heat-stressed humans

Whole body heating increases muscle sympathetic nerve activity (MSNA); however, the effect of heat stress on spectral characteristics of MSNA is unknown. Such information may provide insight into mechanisms of heat stress-induced MSNA activation. The purpose of the present study was to test the hypothesis that heat stress-induced changes in systolic blood pressure variability parallel changes in MSNA variability. In 13 healthy subjects, MSNA, electrocardiogram, arterial blood pressure (via Finapres), and respiratory activity were recorded under both normothermic and heat stress conditions. Spectral characteristics of integrated MSNA, R-R interval, systolic blood pressure, and respiratory excursions were assessed in the low (LF; 0.03-0.15 Hz) and high (HF; 0.15-0.45 Hz) frequency components. Whole body heating significantly increased skin and core body temperature, MSNA burst rate, and heart rate, but not mean arterial blood pressure. Systolic blood pressure and R-R interval variability were significantly reduced in both the LF and HF ranges. Compared with normothermic conditions, heat stress significantly increased the HF component of MSNA, while the LF component of MSNA was not altered. Thus the LF-to-HF ratio of MSNA oscillatory components was significantly reduced. These data indicate that the spectral characteristics of MSNA are altered by whole body heating; however, heat stress-induced changes in MSNA do not parallel changes in systolic blood pressure variability. Moreover, the reduction in LF component of systolic blood pressure during heat stress is unlikely related to spectral changes in MSNA.

Effects of aerobic training on heart rate dynamics in sedentary subjects

This study was designed to assess the effects of moderate- and high-volume aerobic training on the time domain and on spectral and fractal heart rate (HR) variability indexes. Sedentary subjects were randomized into groups with moderate-volume training (n = 20), high-volume training (n = 20), and controls (n = 15). The training period was 8 wk, including 6 sessions/wk at an intensity of 70-80% of the maximum HR, lasting for 30 min/session in the moderate-volume group and 60 min/session in the high-volume group. Time domain, frequency domain, and short-term fractal scaling measures of HR variability were analyzed over a 24-h period. Mean HR decreased from 70 +/- 7 to 64 +/- 8 beats/min and from 67 +/- 5 to 60 +/- 6 beats/min (P < 0.001 for both) for the moderate- and high-volume training groups, respectively. The normalized high-frequency spectral component increased in both groups (P < 0.05). The normalized low-frequency component decreased significantly (P < 0.05), resulting in a marked decrease in low frequency-to-high frequency ratio in both groups. In addition, short-term scaling exponent decreased in both groups (P < 0.001). There were no significant differences in the changes of HR variability indexes between groups. Aerobic training in sedentary subjects results in altered autonomic regulation of HR toward vagal dominance. A moderate training volume is a sufficient intervention to induce these beneficial effects.

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).

Non-invasive model-based estimation of the sinus node dynamic properties from spontaneous cardiovascular variability series

A non-invasive model-based approach to the estimation of sinus node dynamic properties is proposed. The model exploits the spontaneous beat-to-beat variability of heart period and systolic arterial pressure and the sampled respiration, thus surrogating the information from direct measures of neural activity. The residual heart period variability not related to baroreflex, to direct effects of respiration and to low frequency influences independent of baroreflex, is interpreted as the effect of the dynamic properties of the sinus node and modelled as a regression of the RR interval over its previous value. Therefore the sinus node transfer function is modelled by means of a filter with a real pole z = mu (and a zero in the origin). It was found that: first, in young healthy subjects the nodal tissue responded as a low-pass filter with mu = 0.76 +/- 0.12 (mean +/- SD); secondly, ageing did not significantly modify either its shape or gain at 0 Hz; thirdly, in heart transplant recipients, the dynamic transduction properties were lost (all-pass filter, p = 0.06 +/- 0.16, p < 0.001); fourthly, low-dose atropine left the sinus node dynamic properties unmodified; fifthly, high-dose atropine affected the dynamic transduction properties by increasing the gain at 0 Hz and rendering steeper its roll-off (the percent increase of mu with respect to baseline was 18.3 +/- 22.3, p < 0.05).

Non-invasive assessment of autonomic cardiovascular control in normal human pregnancy and pregnancy- associated hypertensive disorders: a review

PURPOSE

Pre-eclampsia is a major complication of pregnancy. Although the disorder usually becomes apparent only in the third trimester of pregnancy, evidence is available that underlying pathophysiological abnormalities are already present early in pregnancy. The association between alterations in autonomic cardiovascular control and the development of hypertension in pregnancy has been investigated for some time. Non-invasive methods are especially of interest, since they have the advantage of minimal risk for the mother and the conceptus and enable repeated measurements during pregnancy. If non-invasive tests for autonomic cardiovascular control could demonstrate the increased sympathetic activity, as observed by microneurography than this method is a candidate for early identification of pre-eclampsia. Therefore, the literature on non-invasive testing of autonomic cardiovascular control in normal pregnancies and pre-eclampsia was summarized.

DATA IDENTIFICATION AND SELECTION

Medline was searched and 36 articles on autonomic cardiovascular control in human pregnancy by non-invasive test methods were reviewed. For each test method, data of different studies were summarized to evaluate if the method could discriminate between healthy pregnancy and pre-eclampsia.

CONCLUSION

Although small differences have been observed between normal pregnancy and pre-eclampsia in individual studies using non-invasive methods, the consistency in the available data is insufficient to discriminate between normal pregnancy and pre-eclampsia. The failure to demonstrate the increased sympathetic activity, as observed by direct microneurography, might be due to methodological factors of the non-invasive studies. Alternatively, sympathetic activity to resistance vessels in skeletal muscle may not be a proper reflection of autonomic cardiovascular control in pregnancy. Well-designed longitudinal research could be useful to test these suppositions.

Mechanism of blood pressure and R-R variability: insights from ganglion blockade in humans

Spontaneous blood pressure (BP) and R-R variability are used frequently as 'windows' into cardiovascular control mechanisms. However, the origin of these rhythmic fluctuations is not completely understood. In this study, with ganglion blockade, we evaluated the role of autonomic neural activity versus other 'non-neural' factors in the origin of BP and R-R variability in humans. Beat-to-beat BP, R-R interval and respiratory excursions were recorded in ten healthy subjects (aged 30 +/- 6 years) before and after ganglion blockade with trimethaphan. The spectral power of these variables was calculated in the very low (0.0078-0.05 Hz), low (0.05-0.15 Hz) and high (0.15-0.35 Hz) frequency ranges. The relationship between systolic BP and R-R variability was examined by cross-spectral analysis. After blockade, R-R variability was virtually abolished at all frequencies; however, respiration and high frequency BP variability remained unchanged. Very low and low frequency BP variability was reduced substantially by 84 and 69 %, respectively, but still persisted. Transfer function gain between systolic BP and R-R interval variability decreased by 92 and 88 % at low and high frequencies, respectively, while the phase changed from negative to positive values at the high frequencies. These data suggest that under supine resting conditions with spontaneous breathing: (1) R-R variability at all measured frequencies is predominantly controlled by autonomic neural activity; (2) BP variability at high frequencies (> 0.15 Hz) is mediated largely, if not exclusively, by mechanical effects of respiration on intrathoracic pressure and/or cardiac filling; (3) BP variability at very low and low frequencies (< 0.15 Hz) is probably mediated by both sympathetic nerve activity and intrinsic vasomotor rhythmicity; and (4) the dynamic relationship between BP and R-R variability as quantified by transfer function analysis is determined predominantly by autonomic neural activity rather than other, non-neural factors.

Short-term cardiovascular oscillations in man: measuring and modelling the physiologies

Research into cardiovascular variabilities intersects both human physiology and quantitative modelling. This is because respiratory and Mayer wave (or 10 s) cardiovascular oscillations represent the integrated control of a system through both autonomic branches by systemic haemodynamic changes within a fluid-filled, physical system. However, our current precise measurement of short-term cardiovascular fluctuations does not necessarily mean we have an adequate understanding of them. Empirical observation suggests that both respiratory and Mayer wave fluctuations derive from mutable autonomic and haemodynamic inputs. Evidence strongly suggests that respiratory sinus arrhythmia both contributes to and buffers respiratory arterial pressure fluctuations. Moreover, even though virtual abolition of all R-R interval variability by cholinergic blockade suggests that parasympathetic stimulation is essential for expression of these variabilities, respiratory sinus arrhythmia does not always reflect a purely vagal phenomenon. The arterial baroreflex has been cited as the mechanism for both respiratory and Mayer wave frequency fluctuations. However, data suggest that both cardiac vagal and vascular sympathetic fluctuations at these frequencies are independent of baroreflex mechanisms and, in fact, contribute to pressure fluctuations. Results from cardiovascular modelling can suggest possible sources for these rhythms. For example, modelling originally suggested low frequency cardiovascular rhythms derived from intrinsic delays in baroreceptor control, and experimental evidence subsequently corroborated this possibility. However, the complex stochastic relations between and variabilities in these rhythms indicate no single mechanism is responsible. If future study of cardiovascular variabilities is to move beyond qualitative suggestions of determinants to quantitative elucidation of critical physical mechanisms, both experimental design and model construction will have to be more trenchant.

Heart rate to arterial pressure impulse response during one lung ventilation

BACKGROUND

Although one lung ventilation (OLV) is commonly used, little is known about the modulation of the autonomic nervous system with OLV while under general anesthesia. As the frequency domain and time domain analyses are powerful analytic tools, we investigated their modulation during OLV.

METHODS

Patients undergoing thoracic surgery were classified into two groups: those who did (group A, n=8) and those who did not (group N, n=8) receive atropine. After a double lumen tube was placed endotracheally, mechanical ventilation of both lungs (BLV) was established at 18 min(-1) while under isoflurane anesthesia. Electrocardiogram, systolic arterial pressure (SAP), and inspiratory flow (Finsp) were digitally recorded as follows: awake before anesthesia; BLV after anesthesia; BLV after intravenous 10 microg kg(-1) of atropine (group A) or not (group N); left OLV; and right OLV. Power spectral analyses of heart rate (HR) and SAP were computed by determining low-(LF, 0.04-0.15 Hz) and high-frequency (HF, 0.15-0.40 Hz) components, and impulse response analysis was executed among HR, SAP, and Finsp. Impulse responses were assessed by the maximum values in the time domain.

RESULTS

In frequency domain analysis, atropine depressed LF and LF/HF but not HF in HR variability, while no difference was observed between right OLV and left OLV. The heart rate to SAP impulse response was maintained at a significantly higher level in group A than in group N (905+/-360 vs. 425+/-375 mmHg beats(-1)min(-1)) at right OLV. A significant difference was also observed between left and right OLV within group N.

CONCLUSION

Impulse response analysis demonstrated that there is a greater effect on autonomic nervous system modulation during right OLV than in left OLV, which mainly results from a parasympathetic neural linkage origin.

Heart rate dynamics in monoamine oxidase-A- and -B-deficient mice

Heart rate (HR) dynamics were investigated in mice deficient in monoamine oxidase A and B, whose phenotype includes elevated tissue levels of norepinephrine, serotonin, dopamine, and phenylethylamine. In their home cages, spectral analysis of R-R intervals revealed more pronounced fluctuations at all frequencies in the mutants compared with wild-type controls, with a particular enhancement at 1-4 Hz. No significant genotypic differences in HR variability (HRV) or entropies calculated from Poincaré plots of the R-R intervals were noted. During exposure to the stress of a novel environment, HR increased and HRV decreased in both genotypes. However, mutants, unlike controls, demonstrated a rapid return to baseline HR during the 10-min exposure. Such modulation may result from an enhanced vagal tone, as suggested by the observation that mutants responded to cholinergic blockade with a decrease in HRV and a prolonged tachycardia greater than controls. Monoamine oxidase-deficient mice may represent a useful experimental model for studying compensatory mechanisms responsible for changes in HR dynamics in chronic states of high sympathetic tone.

Intravenous amiodarone modifies autonomic balance and increases baroreflex sensitivity in conscious rats

Amiodarone is an antiarrhythmic agent commonly used to treat cardiac arrhythmias. This study was designed to investigate the effects of intravenous amiodarone on the neural control of heart rate and arterial pressure and spontaneous baroreflex sensitivity (BRS). Experiments were carried out on conscious freely moving normotensive Wistar (WR) and spontaneously hypertensive rats (SHR). Arterial pressure was continuously monitored before and after amiodarone (50 mg/kg i.v.) or vehicle for 30 min. Heart rate (expressed as the pulse interval, PI) and systolic arterial pressure (SAP) variabilities were assessed using autoregressive spectral analysis. BRS was calculated as the alpha-index (the square root of the ratio between the PI and SAP powers). Amiodarone induced bradycardia and hypotension in both strains, with these effects being more intense in SHR. The variability profile of PI was characterized by a significant reduction of normalized low frequency (LF) and LF/HF ratio, while the high frequency (HF) component both in absolute and normalized units (nu) was increased in both WR and SHR strains. A significant decrease in SAP variance and its LF oscillation was observed. In addition, BRS was also increased in both groups, being more intense in SHR. In both WR and SHR, intravenous amiodarone had a considerable effect on heart rate variabilities (HRV), shifting cardiac sympathovagal balance toward a sympathetic inhibition and/or vagal activation, which were associated with an increase in spontaneous BRS. Decreases of SAP variance and LF(SAP) suggest sympatholytic effects on peripheral vessels. Besides the direct ion channel effects, these changes in the autonomic balance could contribute to the antiarrhythmic action of the intravenous amiodarone.

Neural influences on cardiovascular variability: possibilities and pitfalls

Altered variability in the cardiovascular system is associated with a range of cardiovascular diseases and increased mortality. Because blood pressure and heart rate show distinct low-frequency oscillations that appear to be affected by either vagal or sympathetic activity, it has been hoped that measurement of the strength of these oscillations could be used as an index of autonomic tone and thus form the basis of a diagnostic test. This review focuses on recent research that has examined the fundamental origin of variability associated with respiration and a slow oscillation at 0.1 Hz in the human. A new hypothesis is proposed to account for the slow oscillation in heart rate and blood pressure that incorporates components of the central nervous system, other reflex pathways regulating sympathetic activity, and resonance in the baroreflex control of blood pressure. Whereas it is clear that sympathetic activity and arterial baroreflexes are critical elements in producing cardiovascular variability, there is also evidence that other factors, including the ability of the vasculature to respond to sympathetic activity, appear to play a role in determining the strength of oscillations. Given the potential impact of other nonbaroreflex or nonautonomic pathways in affecting cardiovascular variability, it is proposed that one must use care in relating changes in the strength of an oscillation in blood pressure and heart rate as definitively due to a change in autonomic control.

Vagal cardiac function and arterial blood pressure stability

This study was designed to investigate the importance of vagal cardiac modulation in arterial blood pressure (ABP) stability before and after glycopyrrolate or atropine treatment. Changes in R-R interval (RRI) and ABP were assessed in 10 healthy young (age, 22 +/- 1.8 yr) volunteers during graded lower body negative pressure (LBNP) before and after muscarinic cholinergic (MC) blockade. Transient hypertension was induced by phenylephrine (1 microg/kg body wt), whereas systemic hypotension was induced by bilateral thigh cuff deflation after a 3-min suprasystolic occlusion. Power spectral densities of systolic [systolic blood pressure (SBP)] and diastolic ABP variability were examined. Both antimuscarinic agents elicited tachycardia similarly without significantly affecting baseline ABP. The increase in SBP after phenylephrine injection (+14 +/- 2 mmHg) was significantly augmented with atropine (+26 +/- 2 mmHg) or glycopyrrolate (+27 +/- 3 mmHg) and associated with a diminished reflex bradycardia. The decrease in SBP after cuff deflation (-9.2 +/- 1.2 mmHg) was significantly greater after atropine (-15 +/- 1 mmHg) or glycopyrrolate (-14 +/- 1 mmHg), with abolished reflex tachycardia. LBNP significantly decreased both SBP and RRI. However, after antimuscarinic agents, the reduction in SBP was greater (P < 0.05) and was associated with less tachycardia. Antimuscarinic agents reduced (P < 0.05) the low-frequency (LF; 0.04-0.12 Hz) power of ABP variability at rest. The LF SBP oscillation was significantly augmented during LBNP, which was accentuated (P < 0.05) after antimuscarinic agents and was correlated (r = -0.79) with the decrease in SBP. We conclude that antimuscarinic agents compromised ABP stability by diminishing baroreflex sensitivity, reflecting the importance of vagal cardiac function in hemodynamic homeostasis. The difference between atropine and glycopyrrolate was not significant.

Effects of pharmacological adrenergic and vagal modulation on fractal heart rate dynamics

Breakdown of short-term fractal-like behaviour of HR indicates an increased risk for adverse cardiovascular events and mortality, but the pathophysiological background for altered fractal HR dynamics is not known. Our aim was to study the effects of pharmacological modulation of autonomic function on fractal correlation properties of heart rate (HR) variability in healthy subjects. Short-term fractal scaling exponent (alpha1) along with spectral components of HR variability were analysed during the following pharmacological interventions in healthy subjects: (i) noradrenaline (NE) infusion (n=22), (ii) NE infusion after phentolamine (PHE) (n=8), (iii) combined NE + adrenaline (EPI) infusion (n=12), (iv) vagal blockade with high dose of atropine (n=10), (v) and vagal activation by low dose of atropine (n=10). Then alpha1 decreased progressively during the incremental doses of NE (from 0.85 +/- 0.250 to 0.55 +/- 0.23, P<0.0001). NE also decreased the average HR (P<0.001) and increased the high frequency spectral power (P<0.001). Vagal blockade with atropine increased the alpha1 value (from 0.82 +/- 0.22 to 1.24 +/- 0.41, P<0.05). Combined NE + EPI infusion and vagal activation with a low dose atropine did not result in any changes in alpha1, and alpha-adrenergic blockade by PHE did not completely reverse the effects of NE on alpha1. Increased levels of circulating NE result in reduction of short-term correlation properties of HR dynamics. The results suggest that coactivation of cardiac vagal outflow at the time of high levels of a circulating sympathetic transmitter explains the breakdown of fractal-like behaviour of human HR dynamics.

Autonomic dysregulation in essential hypertension: insight from heart rate and arterial pressure variability

Essential hypertension is the most prevalent cardiovascular disorder, affecting more than 50 million people in the USA. Hypertension-related mortality and morbidity figures have been greatly improved over the last three decades by major advances in prevention. Detection and operative suggestions for practicing physicians are available from several guideline treatments derived from grouped data obtained in numerous well-conducted studies on large numbers of patients. However, the disappointing results of some forms of antihypertensive therapies, particularly in preventing coronary artery disease, has shed some doubts on traditional approaches to managing hypertensive patients. At present, in spite of extensive investigations, the exact causal mechanism(s) are far from being fully understood, and consequently, essential hypertension is still managed using a heuristic approach. Persistent elevations in arterial pressure imply some disturbances in the complex and multifactorial cardiovascular control mechanisms. In this context, neurohumoral disturbances might play a special role, in view of the demonstration that an elevated sympathetic drive seems essential in hypertensive patients. In this review, we follow the hypothesis that other allied methods capable of quantitatively assessing some aspects of the regulatory system might support and integrate the usual dichotomous diagnostic procedure based on arterial pressure determinations. In prior studies, we reported that parameters obtained by spectral analysis of heart rate variability (HRV) might furnish useful information on autonomic normal and abnormal nervous system regulation. In the foregoing, we summarize our experience using this approach in the clinical management of hypertensive patients. It is our tenet that spectral analysis of mono or multivariate cardiovascular beat-by-beat variabilities provides potentially important information on alterations in neural control of the circulation accompanying essential hypertension. In spite of an ongoing debate on the interpretation of specific aspects of HRV spectral components, overall, it appears that the available evidence supports the hypothesis that in essential hypertension, there is an increased sympathetic and reduced vagal cardiac drive coupled with an enhancement of vasomotor sympathetic modulation. Prospective studies on large populations, rendered more easy to perform, thanks to improvements in technology and telemedicine applications, might provide an answer to the still open question of how to apply spectral analysis of HRV to a better mechanistic understanding of essential hypertension, and to more satisfactory individually tailored antihypertensive treatments.

Bimodal dose-dependent effect on autonomic, cardiac control after oral administration of Atropa belladonna

This single-blind placebo-controlled study was designed to investigate the dose-dependent vagolytic and vagotonic effects after a single oral administration of Atropa belladonna tincture (ABT, 0.1 mg/ml alkaloid concentration, atropine/scopolamine = 20:1). In eight healthy young subjects, heart rate and noninvasive arterial finger blood pressure were recorded simultaneously over 4 h after oral application of four different doses of ABT (day 1: 2 ml, day 2: placebo, day 3: 5 ml, day 4: 1 ml). On each day, 14 20-min sequences under controlled experimental conditions were performed. Among others, mean RR interval (RR), high-frequency spectral power of heart rate variability (HF), and noninvasive baroreflex sensitivity (BRS) were calculated during metronome breathing in supine position. These parameters were robust markers of vagal activity. One hour after 5ml ABT, RR, HF and BRS decreased clearly in six of eight subjects. This effect was interpreted as vagolytic response. After 1 and 2 ml ABT, and after placebo, RR and HF increased markedly. The increase after ABT was much higher than the increase solely due to adaptation after placebo administration, and it could be clearly identified as an augmentation of vagal cardiac activity caused by low-dose ABT. In conclusion, low doses of orally administered ABT can be effectively used to stimulate parasympathetic activity in man. The mode of vagal activation changes between 2 and 5 ml ABT from vagotonic to vagolytic. ABT has no or very little effect on blood pressure control.

Sympathetic rhythms and cardiovascular oscillations

Spectral analysis of heart rate and arterial pressure variabilities is a powerful noninvasive tool, which is increasingly used to infer alterations of cardiovascular autonomic regulation in a variety of physiological and pathophysiological conditions, such as hypertension, myocardial infarction and congestive heart failure. A most important methodological issue to properly interpret the results obtained by the spectral analysis of cardiovascular variability signals is represented by the attribution of neurophysiological correlates to these spectral components. In this regard, recent applications of spectral techniques to the evaluation of the oscillatory properties of sympathetic efferent activity in animals, as well as in humans, offer a new approach to a better understanding of the relationship between cardiovascular oscillations and autonomic regulation.

Time delay of vagally mediated cardiac baroreflex response varies with autonomic cardiovascular control

To examine whether changes in autonomic activity have an effect on the latency of the vagally mediated cardiac baroreflex response in humans, we investigated the effects of neck suction fluctuating sinusoidally at 0.2 Hz on R-R intervals (known to be mediated mainly by vagal activity) in the supine position, during 15 degrees head-down tilt and 60 degrees head-up tilt, and during vagotonic (2 microg/kg) and vagolytic (10 microg/kg) doses of atropine while the subjects breathed at 0.25 Hz. The phase shift between fluctuations in neck chamber pressure and in R-R interval was calculated by complex transfer function analysis and was used as a measure of the time delay between carotid baroreceptor stimulation and cardiac effector response. Cardiac baroreflex responsiveness increased significantly during low-dose atropine and decreased during head-up tilt or 10 microg/kg atropine. With increasing tilt angle, the time delay between cyclic baroreceptor stimulation and oscillations in R-R interval increased from 0.32 +/- 0.27 s (head down), to 0.59 +/- 0.25 s (supine position, P < 0.05 vs. head down), and to 0.86 +/- 0.27 s (head up, P < 0.01 vs. supine). Low-dose atropine had a similar effect to head-down tilt on baroreflex latency, whereas 10 microg/kg atropine increased the time delay markedly to 1.24 +/- 0.30 s. Our results demonstrate that changes in autonomic activity, generated either by gravitational stimulus or by atropine, not only affect baroreflex responsiveness but also have a major influence on the latency of the vagally mediated carotid baroreceptor-heart rate reflex. The prolonged baroreflex latency during decreased parasympathetic function may contribute to an unstable regulation of heart rate in patients with cardiac disease.

The sleep apnoea/hypopnoea syndrome depresses waking vagal tone independent of sympathetic activation

The modest daytime hypertension and sympathetic upregulation associated with the sleep apnoea/hypopnoea syndrome (SAHS), does not explain the relatively large increased risk of cardiac morbidity and mortality in the SAHS patients population. Therefore, efferent vagal and sympathetic activity was evaluated during wakefulness in SAHS subjects and matched healthy controls, in order to determine if vagal downregulation may play a role in the aetiology of cardiac disease in the SAHS. The awake autonomic nervous system function of 15 male subjects, with mild-to-moderate SAHS was compared to that of 14 healthy controls matched for age, body mass index, gender and blood pressure. All subjects were free from comorbidity. Vagal activity was estimated from measurements of heart rate variability high frequency power (HF) and sympathetic activity was measured from urine catecholamine excretion. The %HF power was significantly (p < 0.03) reduced in SAHS patients (10+/-1.6 (mean+/-SEM)) as compared to controls (17 +/- 3). In addition, HF power correlated with the apnoea/hypopnoea index in the SAHS subjects (R = -0.592, p = 0.02). There was no statistically significant difference in the daytime excretion of nonadrenaline between control (242 +/- 30 nmol x collection(-1)) and SAHS (316 +/- 46 nmol x collection(-1)) subjects (p = 0.38). In these sleep apnoea/hypopnoea syndrome patients there was limited evidence of increased waking levels of urine catecholamines. The principal component altering waking autonomic nervous system function, in the sleep apnoea/hypopnoea syndrome subjects, was a reduced daytime efferent vagal tone.

Evidence for central organization of cardiovascular rhythms

Spectral analysis of heart rate and arterial pressure variabilities is a powerful noninvasive tool that is increasingly used to infer alterations of cardiovascular autonomic regulation in a variety of physiological and pathophysiological conditions such as hypertension, myocardial infarction, and congestive heart failure. A most important methodological issue to properly interpret the results obtained by the spectral analysis of cardiovascular variability signals is represented by the attribution of neurophysiological correlates to these spectral components. In this regard, recent application of spectral techniques to the evaluation of the oscillatory properties of sympathetic efferent activity in animals as well as in humans offers a new approach to a better understanding of the relationship between cardiovascular oscillations and autonomic regulation. The data so far collected seem to suggest the presence of a centrally organized neural code, characterized by excitatory and inhibitory neural mechanisms subserving the genesis and the regulation of cardiovascular oscillations concerning the major variables of autonomic regulation.

Prediction of short cardiovascular variability signals based on conditional distribution

A new approach measuring the predictability of a process is proposed. The predictor is defined as the median of the distribution conditioned by a sequence of L - 1 previous samples (i.e., a pattern). A function referred to as the corrected mean squared predictor error is defined to prevent the perfect adequacy to the data (i.e., the decrease to zero of the prediction error), thus avoiding to divide the whole set of data in learning and test sets. This function exhibits a minimum and this minimum is taken as a measure of predictability of the series. The use of the minimization procedure avoids to fix a priori the pattern length L. This approach permits one a reliable measure of predictability on short data sequences (around 300 samples). Moreover, this method, in connection with a surrogate data approach, is useful to detect nonlinear dynamics. The analysis indicates that, in simulated and real data, predictability and nonlinearity measures provide different information. The application of this approach to the analysis of cardiovascular variability series of the heart period (RR interval) and systolic arterial pressure (SAP) shows: 1) SAP series is more predictable than RR interval series; 2) predictability of the RR interval series is larger during tilt, during controlled respiration at 10 breaths/min (bpm) and after high-dose administration of atropine; 3) SAP series is dominated by linear correlation; 4) RR interval series exhibits nonlinear dynamics during controlled respiration at 10 bpm and after low-dose administration of atropine, while it is linear during sympathetic activation produced by tilt and after peripheral parasympathetic blockade caused by high-dose administration of atropine.

Interpreting oscillations of muscle sympathetic nerve activity and heart rate variability

Computer analysis of spontaneous cardiovascular beat-by-beat variability has gained wide credibility as a means of inferring disturbances of autonomic cardiovascular regulation in a variety of cardiovascular conditions, including hypertension, myocardial infarction and heart failure. Recent applications of spectral analysis to muscle sympathetic nerve activity (MSNA) offer a new approach to a better understanding of the relationship between cardiovascular oscillations and autonomic regulation. However, areas of uncertainty and unresolved debates remain, mostly concerning different methodologies and interpretative models that we will consider in this article. Perusal of all available literature suggests that average sympathetic nerve activity and its oscillatory components, although correlated to some extent, are likely to provide different types of information. In addition, the specific experimental context is of paramount importance, as the rules that seem to govern the relationship between average and oscillatory properties of MSNA appear to be different in usual conditions and in conditions of extremes of activation or disease. In general, dynamic experiments, such as with graded tilt or with vasoactive drugs, are more suited to investigations of the complexity of autonomic regulation than are static comparisons. In addition, because the information is spread across variables and is affected by a potentially large error, it appears that several different techniques should be perceived as complementary rather than as mutually exclusive. Available evidence suggests that low-frequency and high-frequency oscillations in peripheral signals of variability might have a predominantly central, rather than a peripheral, origin and that this applies in particular to low-frequency oscillations. A crucial point in the assessment of the meaning of spectral components relates to consideration of the varying level of very-low-frequency noise, and the mathematical manipulation of derived indices, particularly using a normalization procedure. This appears easier to obtain with auto-regressive than with fast Fourier techniques. With this approach, discrepant interpretations seem to be resolved, provided adequate care is taken in separating direct physiological data from derived meaning, which relates to hidden information and neural codes; in the case of sympathetic discharge, the latter display greater complexity than simple average spike activity per unit time. Accordingly we believe, in conclusion, that the judicious use of spectral methodology, in addition to other techniques, might provide unprecedented, useful insights into autonomic cardiovascular regulation, in both physiopathological and clinical circumstances.

Autonomic control of heart period in duodenal ulcer patients insights from spectral analysis of heart rate variability

Beyond the fundamental pathogenetic importance of Helicobacter Pylori a possible additional role of vagal innervation in favouring or modulating the clinical history of duodenal ulcer (DU) has been suggested by old studies employing invasive methodologies. Aim of this study was to assess whether vagal prevalence in autonomic modulation was present in healed DU patients (n=20) as compared to controls,(n=50), using a validated non-invasive methodology, based on spectral analysis of cardiovascular variability. This approach provides markers of the sympathetic and vagal modulations of the SA node, respectively by way of the normalized low frequency (LF(RR)) and high frequency (HF(RR)) components of RR interval variability; LF/HF ratio furnishes a marker of sympatho-vagal balance. In addition, sham feeding (SF) provided a means to assess, in DU patients, neurally mediated acid secretion, as the SF acid output (SAO) to basal acid output (BAO) ratio (SAO/BAO). Results showed that LF(RR) was smaller in DU patients than in controls (40.3+/-3.9 vs. 52.3+/-2.3 normalized units, nu; P<0.05). On the contrary, HF(RR) was greater (52.1+/-3.7 vs. 35.7+/-2.3 nu; P<0.05). Conversely the LF component of SAP variability, a marker of sympathetic vasomotor modulations, and the index alpha, a measure of baroreflex control of the SA node, as well as respiratory patterns, were similar in the two groups. SAO/BAO ratio was significantly correlated with markers of autonomic control of the SA node (r = -0.67, P<0.0083 with HF(RR)). In conclusion results suggest an enhanced vagal modulation of heart period in DU patients at rest, that appears linked to indices of neurally mediated gastric acid secretion response.

Orthostatic hypotension in aging humans

We tested the hypothesis that hypotension occurred in older adults at the onset of orthostatic challenge as a result of vagal dysfunction. Responses of heart rate (HR) and mean arterial pressure (MAP) were compared between 10 healthy older and younger adults during onset and sustained lower body negative pressure (LBNP). A younger group was also assessed after blockade of the parasympathetic nervous system with the use of atropine or glycopyrrolate and after blockade of the beta(1)-adrenoceptor by use of metoprolol. Baseline HR (older vs. younger: 59 +/- 4 vs. 54 +/- 1 beats/min) and MAP (83 +/- 2 vs. 89 +/- 3 mmHg) were not significantly different between the groups. During -40 Torr, significant tachycardia occurred at the first HR response in the younger subjects without hypotension, whereas significant hypotension [change in MAP (DeltaMAP) -7 +/- 2 mmHg] was observed in the elderly without tachycardia. After the parasympathetic blockade, tachycardiac responses of younger subjects were diminished and associated with a significant hypotension at the onset of LBNP. However, MAP was not affected after the cardiac sympathetic blockade. We concluded that the elderly experienced orthostatic hypotension at the onset of orthostatic challenge because of a diminished HR response. However, an augmented vasoconstriction helped with the maintenance of their blood pressure during sustained LBNP.

Detection of low- and high-frequency rhythms in the variability of skin sympathetic nerve activity

Spectral analysis of skin blood flow has demonstrated low-frequency (LF, 0.03-0.15 Hz) and high-frequency (HF, 0.15-0.40 Hz) oscillations, similar to oscillations in R-R interval, systolic pressure, and muscle sympathetic nerve activity (MSNA). It is not known whether the oscillatory profile of skin blood flow is secondary to oscillations in arterial pressure or to oscillations in skin sympathetic nerve activity (SSNA). MSNA and SSNA differ markedly with regard to control mechanisms and morphology. MSNA contains vasoconstrictor fibers directed to muscle vasculature, closely regulated by baroreceptors. SSNA contains both vasomotor and sudomotor fibers, differentially responding to arousals and thermal stimuli. Nevertheless, MSNA and SSNA share certain common characteristics. We tested the hypothesis that LF and HF oscillatory components are evident in SSNA, similar to the oscillatory components present in MSNA. We studied 18 healthy normal subjects and obtained sequential measurements of MSNA and SSNA from the peroneal nerve during supine rest. Measurements were also obtained of the electrocardiogram, beat-by-beat blood pressure (Finapres), and respiration. Spectral analysis showed LF and HF oscillations in MSNA, coherent with similar oscillations in both R-R interval and systolic pressure. The HF oscillation of MSNA was coherent with respiration. Similarly, LF and HF spectral components were evident in SSNA variability, coherent with corresponding variability components of R-R interval and systolic pressure. HF oscillations of SSNA were coherent with respiration. Thus our data suggest that these oscillations may be fundamental characteristics shared by MSNA and SSNA, possibly reflecting common central mechanisms regulating sympathetic outflows subserving different regions and functions.

Information domain analysis of cardiovascular variability signals: evaluation of regularity, synchronisation and co-ordination

A unifying general approach to measure regularity, synchronisation and co-ordination is proposed. This approach is based on conditional entropy and is specifically designed to deal with a small amount of data (a few hundred samples). Quantitative and reliable indexes of regularity, synchronisation and co-ordination (ranging from 0 to 1) are derived in a domain (i.e. the information domain) different from time and frequency domains. The method is applied to evaluate regularity, synchronisation and co-ordination among cardiovascular beat-to-beat variability signals during sympathetic activation induced by head-up tilt (T), during the perturbing action produced by controlled respiration at 10, 15 and 20 breaths/min (CR10, CR15 and CR20), and after peripheral muscarinic blockade provoked by the administration of low and high doses of atropine (LD and HD). It is found that: (1) regularity of the RR interval series is around 0.209; (2) this increases during T, CR10 and HD; (3) the systolic arterial pressure (SAP) series is more regular (0.406) and its regularity is not affected by the specified experimental conditions; (4) the muscle sympathetic (MS) series is a complex signal (0.093) and its regularity is not influenced by HD and LD; (5) the RR interval and SAP series are significantly, though weakly, synchronised (0.093) and their coupling increases during T, CR10 and CR15; (6) the RR interval and respiration are coupled (0.152) and their coupling increases during CR10; (7) SAP and respiration are significantly synchronised (0.108) and synchronisation increases during CR10; (8) MS and respiration are uncoupled and become coupled (0.119) after HD; (9) the RR interval, SAP and respiration are significantly co-ordinated (0.118) and co-ordination increases during CR10 and CR15; (10) during HD the co-ordination among SAP, MS and the respiratory signal is larger than that among the RR interval, SAP, MS and the respiratory signal, thus indicating that the RR interval contributes towards reducing co-ordination.

Cardiac autonomic adjustments to normal human pregnancy: insight from spectral analysis of R-R interval and systolic arterial pressure variability

OBJECTIVE

To assess the adaptation in autonomic control mechanisms that accompanies the marked haemodynamic changes, such as increases in cardiac size and output, that occur in the course of normal human pregnancy.

DESIGN

We studied 14 healthy pregnant women (aged 30+/-1 years) before the 6th week (early stage) and within weeks 32-34 (late stage) of pregnancy, while they were at rest or in a state of active orthostatism (standing), which enhances sympathetic activity.

METHODS

We used echocardiography to assess cardiac volumes and mass, and spectral analysis of the R-R interval and systolic arterial pressure variability to obtain indices of autonomic regulation of the circulation. This non-invasive methodology, recently validated with direct recordings of muscle sympathetic nerve activity, furnishes quantitative markers of sympathetic modulation of the sino-atrial node (low frequency component, LF in normalized units, nu), vagal modulation (high frequency component, HF in normalized units, nu) and the overall arterial pressure-heart rate baroreflex gain (alpha index).

RESULTS

Late pregnancy was characterized by an increase in cardiac size and volumes and by a reduction of R-R interval, R-R interval variance and the alpha index, together with an increase in the LF/HF ratio (from 1.4+/-0.4 to 5.6+/-1.9). Changes in markers of autonomic modulation of the sino-atrial node normally induced by the standing position were blunted.

CONCLUSIONS

The late stage of normal human pregnancy appears to be characterized by alterations in the autonomic control of the circulation and by attenuated responsiveness to active standing, possibly as a consequence of the accompanying increase in cardiac size.