The reproducibility of cardiac baroreceptor activity assessed non-invasively by spectral sequence techniques

The Stability and Repeatability of Spontaneous Sympathetic Baroreflex Sensitivity in Healthy Young Individuals

Spontaneous sympathetic baroreflex sensitivity (BRS) is a valuable tool for assessing how well the baroreflex buffers beat-to-beat changes in blood pressure. However, there has yet to be a study involving appropriate statistical tests to examine the stability of sympathetic BRS within an experimental session and the repeatability between separate sessions. The aim of this study was to use intra-class correlations, ordinary least products regression, and Bland–Altman analyses to examine the stability and repeatability of spontaneous sympathetic BRS assessment. In addition, the influence of recording duration on values of BRS was assessed. In eighty-four healthy young individuals (49 males, 35 females), continuous measurements of blood pressure, heart rate and muscle sympathetic nerve activity (MSNA) were recorded for 10 min. In a subgroup of 13 participants (11 male, 2 female) the measurements were repeated on a separate day. Sympathetic BRS was quantified using MSNA burst incidence (BRS_inc) and total MSNA (BRS_total) for the first 5-min period, the second 5-min period, and a 2-min segment taken from the second 5-min period. Intra-class correlation coefficients indicated moderate stability in sympathetic BRS_inc and BRS_total between the first and second 5-min periods in males (BRS_incr = 0.63, BRS_totalr = 0.78) and females (BRS_incr = 0.61, BRS_totalr = 0.47) with no proportional bias, but with fixed bias for BRS_inc in females. When comparing the first 5-min with the 2-min period (n = 76), the intra-class correlation coefficient indicated poor to moderate repeatability in sympathetic BRS_inc and BRS_total for males (BRS_incr = -0.01, BRS_totalr = 0.70) and females (BRS_incr = 0.46, BRS_totalr = 0.39). However, Bland–Altman analysis revealed a fixed bias for BRS_total in males and proportional bias for BRS_total in females, with lower BRS values for 5-min recordings. In the subgroup, intra-class correlations indicated moderate repeatability for measures of BRS_inc (9 male, 2 female, r = 0.63) and BRS_total (6 male, 2 female, r = 0.68) assessed using 5-min periods recorded on separate days. However, Bland–Altman analysis indicated proportional bias for BRS_inc and fixed bias for BRS_total. In conclusion, measures of spontaneous sympathetic BRS are moderately stable and repeatable within and between testing sessions in healthy young adults, provided that the same length of recording is used when making comparisons.

Reproducibility of heart rate variability and baroreflex sensitivity measurements in children

Despite their extensive use, the reproducibility of cardiac autonomic measurements in children is not well-known. We investigated the reproducibility of short-term continuous measurements of heart rate (HR), heart rate variability (HRV, time and frequency domain), and spontaneous baroreflex sensitivity (BRS, frequency domain) in the supine and standing position in 57 children (11.2+/-0.7 years, 52.6% boys). Reproducibility between two sessions within a two-week interval was evaluated by intraclass correlation coefficients (ICCs), standard error of measurement, coefficients of variation (CVs), limits of agreement, and Bland-Altman plots. HR and HRV were moderately-to-highly (ICC=.63-.79; CV=5.7%-9.7%) and BRS moderately (ICC=.49-.63; CV=11.4%-14.0%) reproducible. While the BRS measurements were slightly less reproducible than the HR and HRV measurements, all can be reliably applied in research, thus implicating sufficient capacity to detect real differences between children. Still, clinical studies focusing on individual changes in cardiac autonomic functioning need to address the considerable random variations that may occur between test-retest measurements.

Analysis of blood pressure-heart rate feedback regulation under non-stationary conditions: beyond baroreflex sensitivity

The feedback regulation of blood pressure and heart rate is an important indicator of human autonomic function usually assessed by baroreflex sensitivity (BRS). We suggest a new method yielding a higher temporal resolution than standard BRS methods. Our approach is based on a regression analysis of the first differences of inter-heartbeat intervals and blood pressure values. Data are recorded from 23 patients with hypertension and sleep apnoea, 22 patients with diabetes mellitus and 23 healthy subjects. Using the proposed method for 3 min data segments, we obtain average regression coefficients of 9.1 and 3.5 ms mmHg(-1) for healthy subjects in supine and orthostatic positions, respectively. In patients with hypertension, we find them to be 3.8 and 2.6 ms mmHg(-1). The diabetes patients with and without autonomic neuropathy are characterized by 3.1 and 6.1 ms mmHg(-1) in the supine position compared with 1.7 and 3.3 ms mmHg(-1) in the orthostatic position. The results are highly correlated with conventional BRS measures; we find r > 0.9 for the dual sequence method. Therefore, we suggest that the new method can quantify BRS. It is superior in distinguishing healthy subjects from patients both in supine and orthostatic positions for short-term recordings. It is suitable for non-stationary data and has good reproducibility. Besides, we cannot exclude that other regulatory mechanisms than BRS may also contribute to the regression coefficients between the first differences.

Advances in the cardiovascular assessment of patients with chronic kidney disease

Cardiovascular mortality is grossly elevated in patients with chronic kidney disease (CKD), and is associated with a wide variety of structural and functional abnormalities. These issues have driven additional attempts to further characterise these abnormalities to elucidate the pathophysiology involved, assess individual risk and/or target and monitor therapies specifically directed at the cardiovascular (CV) system. This review aims to assess the techniques that are currently available for the study of the CV system. This includes conventional assessments of the whole CV system from heart to peripheral microcirculation (although not deal with VC assessment), as well as the key functional consequences relating to stress induced cardiovascular reserve, perfusion and vasoregulation. In addition this review will introduce a variety of techniques aiming to expand the envelope of conventional measurements.

Pathological baroreceptor sensitivity in patients suffering from somatization disorders: do they correlate with symptoms?

AIM

We conducted a study to investigate whether patients with somatization disorders (ICD-10, F45.0) show abnormal values in autonomic testing.

METHOD

35 patients with a diagnosis of somatization disorder (SP) were matched to 35 healthy volunteers (HV). International standardized autonomic testing based on heart rate variation and continuously measured blood pressure signals was used to assess autonomic activity and establish baroreceptor sensitivity (BRS). Three different statistical procedures were used to confirm the reliability of the findings.

RESULTS

There were no statistical differences between the 2 groups in age, BMI, systolic and diastolic blood pressures, and spectral values (total power, low, and high frequency power). However, heart rate was higher (p=0.044) and baroreceptor sensitivity was lower (p=0.002) in the patients compared to the healthy volunteers. Median BRS (+/-S.E.M.) of patients was 9.09+/-0.65 compared to 12.04+/-0.94 ms/mmHg in healthy volunteers. Twenty-two of the 35 patients had a BRS of -1.0S.D. below the mean of HV. SP with lower values differed from SP with normal BRS in values of total power, low-, mid-, and high-frequency bands (p<0.01 to <0.0001). No differences in psychometric testing were found between patients with lower or higher BRS. In addition, no correlation whatsoever was found in relation to autonomic variables between HV and SP, except for a higher LF/HF quotient in the latter (p<0.05).

CONCLUSION

Autonomic regulation was impaired in 62% of patients with a somatization disorder. Severity of clinical symptoms measured by psychometric instruments did not preclude autonomic function impairment. Accordingly, autonomic dysfunction may constitute an independent somatic factor in this patient group.

Baroreflex sensitivity measured by spectral and sequence analysis in cerebrovascular disease : methodological considerations

Baroreflex sensitivity (BRS) is impaired and of prognostic value in cerebrovascular disease. However, no studies to date have been published on the reproducibility of current methods of measuring BRS in this group. The reproducibility of sequence and spectral analysis methods were therefore assessed in subjects with cerebrovascular disease. A total of 14 subjects were assessed on 2 separate occasions at least 2 weeks apart, and beat-to-beat blood pressure (BP) and ECG trace were recorded for three 5-minute periods. These traces were then analyzed by spectral analysis using Fast Fourier Transform and sequence analysis. Reproducibility was calculated as the coefficient of variation (CV) and reproducibility coefficient (RC). There were no significant differences in heart rate, BP or BRS derived by either method between visits. Reproducibility was CV 22.2%, RC 6.04 ms/mmHg with spectral analysis, and CV 26.3%, RC 7.48 ms/mmHg for sequence analysis. There was close agreement between sequence and spectral derived BRS (r = 0.90). We have demonstrated that the use of spectral and sequence analysis to measure BRS is reproducible in subjects with cerebrovascular disease. These techniques are suitable for follow-up and intervention studies of BRS in this patient group.

Reproducibility of methods for assessing baroreflex sensitivity and temporal QT variability in end-stage renal disease and healthy subjects

Spontaneous baroreflex sensitivity (BRS), the reflex heart rate modulation in response to blood pressure changes (predominantly an index of cardiac vagal activity) and temporal QT variability (an index of myocardial repolarization) have been demonstrated to convey important prognostic information. The information about reproducibility of BRS and temporal QT variability is limited and there is lack of information regarding patients with cardiovascular diseases. We investigated reproducibility of spontaneous BRS using the sequence technique and temporal QT variability index (QTVI) in terms of intra-, interexaminer and within-subject variability in end-stage renal disease patients (ESRD, n=17, age 55+/-14 years) and healthy subjects (HS, n=29, age 32+/-12 years, P<0.01). ECG and blood pressure (Portapres) were recorded on two separate days and BRS and QTVI were evaluated by two independent examiners. The mean heart rate was similar in ESRD patients in comparison to healthy controls, whereas the mean arterial pressure was 13 % higher in ESRD patients (P<0.01). Spontaneous BRS was 62% lower (P<0.01) and QTVI was 41% higher in ESRD patients (P<0.01) compared to healthy subjects, respectively. Coefficient of variation (CV) of within-subject reproducibility of BRS and QTVI measurements was moderate (BRS: 33 % for ESRD, 27% for HS; QTVI: 40% for ESRD, 18% for HS). The 95% limit of within-subject reproducibility of BRS measurements was 3.8 ms/mm Hg for ESRD patients and 8.1 ms/mm Hg for healthy subjects; whereas the 95% limit of reproducibility of within-subject reproducibility of QTVI measurements was 0.73 for ESRD patients and 0.55 for healthy subjects. Concordance correlation coefficients of within-subject variability of BRS and QTVI were between 0.74 and 0.83 in both groups. CV of intra- and inter-examiner reproducibility of BRS and QTVI measurements in both groups ranged between 1 and 11%. In conclusion, the intra- and inter-examiner reproducibility/agreement of BRS and QTVI were high, whereas the within-subject reproducibility of these two methods was moderate, in both ESRD patients and healthy subjects. Thus, small differences in BRS and QTVI in longitudinal/interventional studies should be interpreted with caution.

Effects on baroreflex sensitivity measurements when different protocols are used to induce regular changes in beat-to-beat intervals and systolic pressure

Baroreflex sensitivity is becoming an important clinical measurement. Nevertheless there is no recommend standard measurement protocol. This study assessed the ability of eight protocols to induce regular changes in cardiac beat-to-beat interval and systolic pressure (SP), and the effect each protocol had on baroreflex sensitivity (BRS). Twelve subjects had changes in cardiac beat-to-beat intervals and SP levels induced at 8 times a minute by following 8 different protocols, each for 3 min. These comprised breathing in a supine and standing posture, breathing through a resistance, breathing into a closed orifice (the breathing protocols), and performing handgrip exercises, being rocked, having legs raised and lowered, and being presented with mental arithmetic questions (the non-breathing protocols). Induction success of each protocol was determined by the percentage of cardiac beat-to-beat interval and SP level signals with a peak at 8 times per minute in their frequency spectra. The consistency of the induced changes was measured by a signal-to-noise ratio (SNR). BRS was calculated from the frequency spectra. The induction success was 85% for breathing and 31% for non-breathing protocols. The consistency of cardiac beat-to-beat interval changes was highest with supine breathing (SNR = 1.6 +/- 0.3) and resistance breathing (SNR = 1.5 +/- 0.5) protocols. The consistency of SP level changes was highest with resistance breathing (SNR = 1.0 +/- 0.3) and breathing into a closed orifice (SNR = 1.0 +/- 0.5) protocols. BRS values in the supine breathing protocol (24 +/- 10 ms mmHg(-1)) and the handgrip protocol (32 +/- 3 ms mmHg(-1)) were significantly greater (p < 0.05) than for standing breathing (11 +/- 5 ms mmHg(-1)), resistance breathing (17 +/- 8 ms mmHg(-1)) or breathing into a closed orifice (12 +/- 5 ms mmHg(-1)) protocols. Different protocols have different induction successes and degrees of effectiveness in inducing cardiac beat-to-beat and SP level changes. BRS is affected by the induction protocol used, highlighting the need for a standard measurement protocol.

Oxidative stress and baroreflex sensitivity in healthy subjects and patients with mild-to-moderate hypertension

Decreased baroreflex sensitivity (BRS) is a prognostic marker in essential hypertension. Animal experiments suggest that decreased BRS is related to increased oxidative stress. Our study was aimed at testing whether oxidative stress, estimated by isoprostane 15-F(2t)-IsoP urinary levels, is correlated to BRS variation in healthy subjects as well as in patients suffering from essential hypertension. Urinary 15-F(2t)-IsoP levels and BRS were evaluated in two groups of subjects: healthy volunteers (n=64) and patients with untreated mild-to-moderate hypertension (n=33). Data were analysed in 61 and 31 subjects, respectively, BRS analysis being impossible in three and two subjects, respectively. 15-F(2t)-IsoP levels were measured using gas chromatography/mass spectrometry. BRS was measured using the sequence method [PS+/RR+ and PS-/RR-] and crossspectral analysis (CSP) (MF gain) at rest, lying down. No significant correlation was found between basal urinary 15-F(2t)-IsoP levels and BRS (sequence method and CSP) in either healthy controls or hypertensive patients. Our study shows that oxidative stress is not involved in interindividual variations of BRS in healthy subjects and patients suffering from mild-to-moderate hypertensionJournal of Human Hypertension (2004) 18, 517-521. doi:10.1038/sj.jhh.1001684 Published online 12 February 2004

Autonomic Dysfunctions in Patients with Anxiety Throughout Therapy

Abstract Patterns of autonomic cardiovascular regulation were studied in patients with anxiety throughout the course of an intensive inpatient rehabilitation therapy at rest, and in response to an anxiety provocation, in order to measure reactivity changes. Patients had to meet the ICD-10 criteria: F40.0 (agoraphobia), F40.00 (agoraphobia without panic attacks), F40.01 (agoraphobia with panic attacks), or F41 (panic disorder). Thirty-eight patients (13 males, 25 females) were examined after recruitment (T1) and at the end of treatment (T2). Each laboratory session consisted of a resting baseline (5min) and two stress tests of 8min (presentation of word pairs with either neutral or with anxiety-related content). A continuous assessment of ECG, blood pressure, breathing pattern, and thoracic impedance allowed the calculation of heart rate variability (power spectra), baroreflex sensitivity, stroke volume, vascular compliance, and total peripheral resistance. In general, therapy had little impact on the parameters of autonomic cardiovascular regulation, neither on resting level nor on reactivity. Diminished baroreflex sensitivity values in our patients with anxiety were observed; in particular, 15% of the patients had values below 3ms/mmHg, which indicates an increased risk for cardiovascular mortality. Low baroreflex sensitivity in this group goes along with further changes within the cardiovascular system (higher heart rate, diminished heart rate variability, lower stroke volume, higher vascular compliance). This general effect describes altered sympathetic/parasympathetic balance and may be the consequence of illness-related changes in behavior. Our results suggest that a careful cardiovascular examination of patients with anxiety will detect patients with clinically significant cardiovascular alterations. This might lead to a more complex intervention in these patients including, for example, exercise training.

A method for heart rate-corrected estimation of baroreflex sensitivity

OBJECTIVE

The relationship between the prevailing heart rate (HR) and the baroreflex sensitivity (BRS) is described in the present study together with a method for individual HR-corrected estimations of BRS.

DESIGN

HR and BRS, determined with the sequence method, were measured in ten young healthy subjects during rest, stress, standing and bicycle exercise, i.e. at a wide range of HRs.

RESULTS

BRS decreased exponentially with increasing HR. The relationship between the natural logarithm of BRS and HR was linear in each individual and could be described by the equation of a straight line. The equation describing the individual BRS-HR relationship could be derived either from BRS and HR measured during steady-state conditions or from the slope and average HR of the individual sequences occurring throughout the experimental protocol. The latter method was preferable since it did not require recordings during steady-state conditions. In order to eliminate the influence of differences in HR on BRS when comparing BRS between subjects, the equation describing the individual BRS-HR relationship was used to calculate BRS at a HR of 60 bpm, BRS(60), which ranged from 9.5 to 30.1 ms/mmHg for the 10 subjects.

CONCLUSIONS

Considering the dramatic effect of a small difference in HR on BRS, especially at lower HRs, BRS should be estimated at a wide range of HRs in order to determine the HR-corrected BRS from the individual HR-BRS relationship. Otherwise, comparisons of BRS between different individuals, study groups or following drug treatment or other interventions would be highly dependent on differences in HR and thereby easily misinterpreted.