Autonomic modulation of the atrial cycle length by the head up tilt test: non-invasive evaluation in patients with chronic atrial fibrillation

Differential hemodynamic adaptations to tilt test in patients with idiopathic atrial fibrillation

The hemodynamic response during the transition from the supine to standing position in idiopathic atrial fibrillation (AF) patients is not completely understood. This study aimed to analyze the hemodynamic changes that occur during the head-up tilt test in idiopathic AF patients. We investigated the hemodynamic changes during the head-up tilt test with impedance cardiography in 40 AF patients (12 with AF rhythm-AFr and 28 with sinus rhythm-AFsr) and 38 non-AF controls. Patients with AFr had attenuated SVI decrease after standing when compared to AFsr and non-AF [ΔSVI in mL/m2: -1.3 (-3.4 to 1.7) vs. -6.4 (-17.3 to -0.1) vs. -11.8 (-18.7 to -8.0), respectively; p < 0.001]. PVRI decreased in AFr but increased in AFsr and non-AF [ΔPVRI in dyne.seg.m2/cm5: -477 (-1148 to 82.5) vs. 131 (-525 to 887) vs. 357 (-29 to 681), respectively; p < 0.01]. Similarly, compared with non-AF patients, AFr patients also had a greater HR and greater CI increase after standing. The haemodynamic response to orthostatic challenge suggests differential adaptations between patients with AF rhythm and those reverted to sinus rhythm or healthy controls. Characterizing the hemodynamic phenotype may be relevant for the individualized treatment of AF patients.

The frequency of atrial fibrillatory waves is modulated by the spatiotemporal pattern of acetylcholine release: a 3D computational study

In atrial fibrillation (AF), the ECG P-wave, which represents atrial depolarization, is replaced with chaotic and irregular fibrillation waves (f waves). The f-wave frequency, F f, shows significant variations over time. Cardiorespiratory interactions regulated by the autonomic nervous system have been suggested to play a role in such variations. We conducted a simulation study to test whether the spatiotemporal release pattern of the parasympathetic neurotransmitter acetylcholine (ACh) modulates the frequency of atrial reentrant circuits. Understanding parasympathetic involvement in AF may guide more effective treatment approaches and could help to design autonomic markers alternative to heart rate variability (HRV), which is not available in AF patients. 2D tissue and 3D whole-atria models of human atrial electrophysiology in persistent AF were built. Different ACh release percentages (8% and 30%) and spatial ACh release patterns, including spatially random release and release from ganglionated plexi (GPs) and associated nerves, were considered. The temporal pattern of ACh release, ACh(t), was simulated following a sinusoidal waveform of frequency 0.125 Hz to represent the respiratory frequency. Different mean concentrations ( A C h ¯ ) and peak-to-peak ranges of ACh (ΔACh) were tested. We found that temporal variations in F f, F f(t), followed the simulated temporal ACh(t) pattern in all cases. The temporal mean of F f(t), F ¯ f , depended on the fibrillatory pattern (number and location of rotors), the percentage of ACh release nodes and A C h ¯ . The magnitude of F f(t) modulation, ΔF f, depended on the percentage of ACh release nodes and ΔACh. The spatial pattern of ACh release did not have an impact on F ¯ f and only a mild impact on ΔF f. The f-wave frequency, being indicative of vagal activity, has the potential to drive autonomic-based therapeutic actions and could replace HRV markers not quantifiable from AF patients.

Respiratory Induced Modulation in f-Wave Characteristics During Atrial Fibrillation

The autonomic nervous system (ANS) is an important factor in cardiac arrhythmia, and information about ANS activity during atrial fibrillation (AF) may contribute to personalized treatment. In this study we aim to quantify respiratory modulation in the f-wave frequency trend from resting ECG. First, an f-wave signal is extracted from the ECG by QRST cancelation. Second, an f-wave model is fitted to the f-wave signal to obtain a high resolution f-wave frequency trend and an index for signal quality control ( S ). Third, respiratory modulation in the f-wave frequency trend is extracted by applying a narrow band-pass filter. The center frequency of the band-pass filter is determined by the respiration rate. Respiration rate is estimated from a surrogate respiration signal, obtained from the ECG using homomorphic filtering. Peak conditioned spectral averaging, where spectra of sufficient quality from different leads are averaged, is employed to obtain a robust estimate of the respiration rate. The envelope of the filtered f-wave frequency trend is used to quantify the magnitude of respiratory induced f-wave frequency modulation. The proposed methodology is evaluated using simulated f-wave signals obtained using a sinusoidal harmonic model. Results from simulated signals show that the magnitude of the respiratory modulation is accurately estimated, quantified by an error below 0.01 Hz, if the signal quality is sufficient ( S > 0 . 5 ). The proposed method was applied to analyze ECG data from eight pacemaker patients with permanent AF recorded at baseline, during controlled respiration, and during controlled respiration after injection of atropine, respectively. The magnitude of the respiratory induce f-wave frequency modulation was 0.15 ± 0.01, 0.18 ± 0.02, and 0.17 ± 0.03 Hz during baseline, controlled respiration, and post-atropine, respectively. Our results suggest that parasympathetic regulation affects the magnitude of respiratory induced f-wave frequency modulation.

Evaluation of late atrial enhancement by cardiac magnetic resonance imaging in patients with obstructive sleep apnea

INTRODUCTION

Atrial fibrillation (AF) is a growing public health problem especially due to its association with thromboembolic phenomena. Among its risk factors, obstructive sleep apnea (OSA) has increased in incidence and is often under diagnosed. OSA increases the risk of AF by mechanisms not fully known, but it may lead to remodeling and structural alteration of the atria. Cardiac magnetic resonance (CMR), in addition to assessing heart morphology, allows the identification of areas of fibrosis, including the atrium, by the late gadolinium enhancement technique (LGE) and could identify cases of OSA with potential atrial instability.

OBJECTIVE

To evaluate the relationship of LGE atrial by CMR in patients with atrial fibrillation with OSA.

METHODS

We selected 81 patients who were divided into four groups: Group 1: 20 OSA patients without AF, Group 2: 20 OSA and AF patients, Group 3: 21 patients with only atrial fibrillation without OSA and Group 4: 20 healthy patients without associated comorbidities. All underwent CMR for morphofunctional evaluation and LGE research.

RESULTS

The average age was 57.1+-10.59 years. Clinical variables such as hypertension (p = 0.24) and Diabetes Mellitus (p = 0.20) were not predictors of AF in OSA patients. Of the 40 cases with OSA, 18, 45% had severe obstructive disorder, and in this group AF was more prevalent. The mean left ventricular ejection fraction was 62.9% (+-7.46) and it did not differ between groups (p = 0.2). Patients with concomitant OSA and AF had significantly larger left atria (p < 0.001). Cases of OSA with AF showed significantly more atrial LGE (95% vs. 30%, p < 0.001), being an independent predictor in multivariate analysis (P < 0,001).

CONCLUSION

Atrial LGE is independently associated with the presence of AF in patients with OSA. These elements may help to identify cases of higher risk for developing AF in OSA patients in clinical practice.

Cardiovascular exercise and burden of arrhythmia in patients with atrial fibrillation - A randomized controlled trial

Background

Physical activity at moderate-high intensity is recommended to prevent lifestyle diseases. Patients with atrial fibrillation are at risk of a sedentary lifestyle due to fear of exercise-induced episodes of atrial fibrillation. The burden of arrhythmia can be reduced by physical exercise. The effect of exercise intensity on burden of atrial fibrillation needs to be studied further.

Methods and results

In a 12-week randomized controlled trial, 76 patients with paroxysmal/persistent atrial fibrillation were allocated to perform exercise at either low intensity or high intensity (50% and 80% of maximal perceived exertion, respectively). Primary outcome was burden of AF measured by daily electrocardiography-reporting during 12 weeks. Secondarily, change in maximal oxygen uptake (peak VO₂) and 1-year hospitalization was compared between low and high intensity exercise. Sixty-three patients completed the follow-up. In the intention-to-treat analysis, we found no statistical difference in burden of atrial fibrillation between low and high intensity exercise (incidence rate ratio 0.742, 95% CI 0.29–1.91, P = 0.538). No serious adverse events were reported and there was no difference in hospitalization between the two exercise groups. Both exercise groups improved significantly in peak VO₂ (low intensity: 3.62 mL O₂/kg/min, SD 3.77; high intensity: 2.87 mL O₂/kg/min, SD 4.98), with no statistical difference between-groups (mean difference: 0.76 mL O₂/kg/min, 95% CI -3.22–1.7).

Conclusions

High intensity physical exercise was not superior to low intensity physical exercise in reducing burden of atrial fibrillation. HI exercise was well tolerated; no evidence of an increased risk was found for HI compared to LI exercise. Larger studies are required to further prove our findings.

Trial registration

ClinicalTrials.gov NCT01817998

Autonomic influence on atrial fibrillatory process: head-up and head-down tilting

BACKGROUND

Changes in the autonomic nervous system (ANS) tone are present before, during, and after episodes of atrial fibrillation (AF). Atrial fibrillatory rate (AFR, the inverse of the atrial cycle length) has been used as a surrogate marker for local refractoriness and is a key characteristic of the fibrillatory process in patients with AF. Aim of this study is to assess changes in AFR, as an effect of autonomic balance change.

METHODS

Forty patients undergoing cardiac cardioversion for symptomatic persistent AF were included in the study. Surface ECG was recorded during rest, head-down (HDT, -30°), and head-up tilt (HUT, +60°). A median value of AFR was computed in each phase of the protocol.

RESULTS

AFR decreased during HDT compared to the baseline (B) condition in all patients but three (median AFR_B = 391 fpm vs. AFR_HDT = 377 fpm, p < .0001). HUT increased AFR, making it significantly higher than HDT and baseline conditions (median AFR_HUT = 396 fpm, p < .0001 vs. B and HDT). Heart rate (HR) increased during HUT, but had a heterogeneous behavior in the population during HDT: about one third of the patients had an HR lower during HDT than during baseline, whereas the remaining two third had an increase in HR during HDT.

CONCLUSIONS

Dominant sympathetic/vagal tone during HUT/HDT significantly affects AFR, increasing/decreasing in respect to baseline. It may be worth exploring the possibility that patients with AF of shorter duration can convert to sinus rhythm during HDT.

Increased heart rate variability but no effect on blood pressure from 8 weeks of hatha yoga - a pilot study

BACKGROUND

Yoga exercises are known to decrease stress and restore autonomic balance. Yet knowledge about the physiological effects of inversion postures is limited. This study aimed to investigate the effects of inversion postures (head below the heart) on blood pressure (BP) and heart rate variability (HRV).

METHODS

Twelve healthy women and men took part in an 8-week yoga program (60 min once a week). BP was measured with an automatic Omron mx3 oscillometric monitoring device and HRV with a Holter 24-hour ECG at baseline and 8 weeks after the intervention.

RESULTS

There was no significant effect of inversion postures on BP. Nine out of 12 participants showed a significant increase in HRV (p < 0.05) at night (2 hours) on pNN50% (12.7 ± 12.5 to 18.2 ± 13.3). There were no significant changes in other HRV measures such as NN50, LF, HF, LF/HF ratio, LF normalized units (n.u.), HF n.u. and RMSSD.

CONCLUSION

Eight weeks of hatha yoga improved HRV significantly which suggests an increased vagal tone and reduced sympathetic activity.

Quantification of anaesthetic effects on atrial fibrillation rate by partial least-squares

The mechanism underlying atrial fibrillation (AF) remains poorly understood. Multiple wandering propagation wavelets drifting through both atria under hierarchical models are not understood. Some pharmacological drugs, known as antiarrhythmics, modify the cardiac ionic currents supporting the fibrillation process within the atria and may modify the AF propagation dynamics terminating the fibrillation process. Other medications, theoretically non-antiarrhythmic, may slightly affect the fibrillation process in non-defined mechanisms. We evaluated whether the most commonly used anaesthetic agent, propofol, affects AF patterns. Partial least-squares (PLS) analysis was performed to reduce significant noise into the main latent variables to find the differences between groups. The final results showed an excellent discrimination between groups with slow atrial activity during the propofol infusion.

Determinants of a reduced heart rate variability in chronic atrial fibrillation

BACKGROUND

We aimed to evaluate whether clinical factors, which influence heart rate variability (HRV) in the presence of undisturbed sinus rhythm, have any associations with HRV in patients with permanent atrial fibrillation (AF).

METHODS

One hundred ninety-seven consecutive patients with permanent AF were included (122 males, 75 females, aged 64 ± 11 years, range 25-85). In each patient a 24-hour electrocardiographic recording was performed and an HRV fraction (HRVF)-the index based on scatter plot numerical processing-was calculated. Additionally, standard HRV measures were analyzed. Reduced HRVF was defined as its value lower than lower normal limit. Demographic and clinical factors were examined for their association with a reduced HRVF by means of a univariate and multivariate logistic regression analysis.

RESULTS

The reduced HRVF was associated with advanced age, clinical diagnosis of a previous MI or dilated cardiomyopathy, presence of diabetes, depressed left ventricular function, NYHA class > II, treatment regimen, use of digoxin, diuretics or antiarrhythmic agents, nonuse of beta-blockers, and increased heart rate. The independent determinants that sustained after multivariate analysis were: heart rate (per 10 bpm increase, odds ratio 2.77 [1.88-4.07]), age (per 5 years increase 1.43 [1.1-1.85]), depressed left ventricular EF (<30% vs higher 2.26 [1.19-4.31]), and presence of diabetes (3.45 [1.1-10.85]). The HRVF correlated moderately with standard HRV measures. This index showed also the strongest correlation with left ventricular ejection fraction.

CONCLUSION

We concluded that advanced age, left ventricular systolic dysfunction, increased heart rate, and presence of diabetes are cofactors of a reduced HRV in AF patients. Thus, the determinants of heart rate variability in the presence of atrial fibrillation are the same as those in sinus rhythm.

Circadian variation in dominant atrial fibrillation frequency in persistent atrial fibrillation

Circadian variation in atrial fibrillation (AF) frequency is explored in this paper by employing recent advances in signal processing. Once the AF frequency has been estimated and tracked by a hidden Markov model approach, the resulting trend is analyzed for the purpose of detecting and characterizing the presence of circadian variation. With cosinor analysis, the results show that the short-term variations in the AF frequency exceed the variation that may be attributed to circadian. Using the autocorrelation method, circadian variation was found in 13 of 18 ambulatory ECG recordings (Holter) acquired from patients with long-standing persistent AF. Using the ensemble correlation method, the highest AF frequency usually occurred during the afternoon, whereas the lowest usually occurred during late night. It is concluded that circadian variation is present in most patients with long-standing persistent AF though the short-term variation in the AF frequency is considerable and should be taken into account.

Analysis of surface atrial signals: time series with missing data?

Uncovering of the atrial signal for patients undergoing episodes of atrial fibrillation is usually obtained from surface ECG by removing waves induced by ventricular activities. Once earned the atrial signal, the detection of the dominant fibrillation frequency is often the main (and only) goal. In this work we verified if subtraction of the ventricular activity might be avoided by performing spectral analysis on those ECG segments where ventricular activity is absent, (i.e. the T-Q intervals). While the approach might seem crude, in here the question was recast into a problem of missing data in a long time series and proper methods were applied: the Lomb periodogram and the iterative Singular Spectrum Analysis. The two methods were tested on both simulated signals and "realistic" atrial signals constructed using the ECG recordings provided by the 2004 Computers in Cardiology competition. The results obtained showed that both techniques were able to provide a reliable quantification of the dominant oscillation, with a slightly superior performance of the iterative Singular Spectrum Analysis. Absolute errors larger than 1.0 Hz were unlikely (p < 0.05) up to 130-140 bpm. Such level of agreement is consistent with similar comparative works where techniques for separating the atrial signal from ventricular waves were considered.

Sleep-disordered breathing and chronic atrial fibrillation

BACKGROUND

Little has been known about the prevalence of sleep apnea in patients with atrial fibrillation (AF). Studies have suggested that the prevalence of AF is increasing in patients with sleep-disordered breathing. We hypothesize that the prevalence of OSA is higher in chronic persistent and permanent AF patients than a sub-sample of the general population without this arrhythmic disorder.

OBJECTIVE

Evaluate the frequency of Obstructive Sleep Apnea in a sample of chronic AF compared to a sub-sample of the general population.

METHODS

Fifty-two chronic AF patients aged (60.5 +/- 9.5, 33 males) and 32 control (aged 57.3 +/- 9.6, 15 males). All subjects were evaluated by a staff cardiologist for the presence of medical conditions and were referred for polysomnography. The differences between groups were analyzed by ANOVA for continuous variables, and by the Chi-square test for dichotomous variables. Statistical significance was established by alpha=0.05.

RESULTS

There were no differences in age, gender, BMI, sedentarism, presence of hypertension, type 2 diabetes mellitus, abdominal circumference, systolic and diastolic blood pressure, and sleepiness scoring between groups. Despite similar BMI, AF patients had a higher neck circumference compared to control group (39.9cm versus 37.7cm, p=0.01) and the AF group showed higher percentage time of stage 1 NREM sleep (6.4% versus 3.9%, p=0.03). Considering a cut-off value for AHI >= 10 per hour of sleep, the AF group had a higher frequency of OSA compared to the control group (81.6% versus 60%, p=0.03). All the oxygen saturation parameters were significantly worse in the AF group, which had lower SaO(2) nadir (81.9% versus 85.3%, p=0.01) and mean SaO(2) (93.4% versus 94.3%, p=0.02), and a longer period of time below 90% (26.4min versus 6.7min, p=0.05).

CONCLUSION

Sleep-disordered breathing is more frequent in chronic persistent and permanent AF patients than in age-matched community dwelling subjects.

Exercise testing for non-invasive assessment of atrial electrophysiological properties in patients with persistent atrial fibrillation

AIMS

Experimental studies suggest that the autonomic nervous system modulates atrial refractoriness and conduction velocity in atrial fibrillation (AF). These modulatory effects are, however, difficult to assess in the clinical setting. This study sought to non-invasively characterize in patients with persistent AF, the influence of autonomic modulation induced by exercise on atrial fibrillatory rate as marker of atrial refractoriness and to identify clinical and electrocardiographic predictors of atrial rate response.

METHODS AND RESULTS

In 24 patients (16 males, mean age 60 +/- 13 years) with persistent AF (16 +/- 25 months), continuous ECGs were recorded during bicycle exercise testing. Fibrillatory rate (in fibrillations per minute, fpm) was assessed at baseline and immediately after termination of exercise with spatiotemporal QRST cancellation and time-frequency analysis. Ventricular response was characterized by time-domain HRV indices. Exercise had no influence on mean fibrillatory rate (409 +/- 42 vs. 414 +/- 43 fpm, P = NS). Seven patients responded to exercise with an increase in fibrillatory rate (26 +/- 10 fpm, P < 0.001 and three with a decrease (-21 +/- 8 fpm, P < 0.001), while the remaining 14 patients did not show a response. Responders' HRV indices changed in response to exercise similarly to that of non-responders. Their baseline fibrillatory rate was, however, lower than that of non-responders (387 +/- 18 vs. 425 +/- 48 fpm, P = 0.028). No other clinical or echocardiographic variable was associated with fibrillatory rate response. Twelve weeks after cardioverson, responders were more likely to remain in sinus rhythm than non-responders (88 vs. 46 %, P = 0.04).

CONCLUSIONS

Exercise-induced autonomic activation produces changes in atrial electrophysiological properties that can be detected by time-frequency analysis. Higher baseline fibrillatory rates are associated with an impaired atrial response to exercise that suggests advanced electrical remodelling and reduced sensitivity to autonomic stimuli.

Analysis of surface electrocardiograms in atrial fibrillation: techniques, research, and clinical applications

Atrial fibrillation (AF) is the most common arrhythmia encountered in clinical practice. Neither the natural history of AF nor its response to therapy is sufficiently predictable by clinical and echocardiographic parameters. The purpose of this article is to describe technical aspects of novel electrocardiogram (ECG) analysis techniques and to present research and clinical applications of these methods for characterization of both the fibrillatory process and the ventricular response during AF. Atrial fibrillatory frequency (or rate) can reliably be assessed from the surface ECG using digital signal processing (extraction of atrial signals and spectral analysis). This measurement shows large inter-individual variability and correlates well with intra-atrial cycle length, a parameter which appears to have primary importance in AF maintenance and response to therapy. AF with a low fibrillatory rate is more likely to terminate spontaneously and responds better to antiarrhythmic drugs or cardioversion, whereas high-rate AF is more often persistent and refractory to therapy. Ventricular responses during AF can be characterized by a variety of methods, which include analysis of heart rate variability, RR-interval histograms, Lorenz plots, and non-linear dynamics. These methods have all shown a certain degree of usefulness, either in scientific explorations of atrioventricular (AV) nodal function or in selected clinical questions such as predicting response to drugs, cardioversion, or AV nodal modification. The role of the autonomic nervous system for AF sustenance and termination, as well as for ventricular rate responses, can be explored by different ECG analysis methods. In conclusion, non-invasive characterization of atrial fibrillatory activity and ventricular response can be performed from the surface ECG in AF patients. Different signal processing techniques have been suggested for identification of underlying AF pathomechanisms and prediction of therapy efficacy.

Patients with persistent atrial fibrillation taking oral verapamil exhibit a lower atrial frequency on the ECG

BACKGROUND

While there is agreement that verapamil attenuates the AF- induced refractory period shortening when given before AF induction, controversy exists regarding its effects when given after the onset of persistent AF. This study aimed to compare atrial fibrillatory frequency obtained from the surface ECG in patients with persistent atrial fibrillation (AF) with oral verapamil treatment to those without this treatment.

METHODS AND RESULTS

Surface ECG recordings were performed in 57 patients (34 male, 23 female, mean age 59 +/- 11 years) with persistent AF (> 7 days). The frequency content of the fibrillatory baseline was quantified using digital signal processing (filtering, QRST complex averaging and subtraction, Fourier transformation). In 27 patients with verapamil treatment (120 or 240 mg/day for at least 7 days) mean fibrillatory frequency measured 6.4 +/- 0.2 Hz, compared to 7.0 +/- 0.4 Hz (P = 0.012) in 30 patients without verapamil. In a subset of 20 randomly selected patients (10 with, 10 without verapamil treatment) a 24-hour Holter ECG recording was performed and fibrillatory frequency determined at 4 PM, 10 PM, 4 AM, and 10 AM. While there was a significant frequency reduction in the verapamil treated patients at night (P = 0.011), it remained constant over time in the other patients.

CONCLUSION

In patients with persistent AF, fibrillatory frequency assessed by spectral analysis of the surface ECG is lower in patients taking verapamil.

Rapid fluctuations in atrial fibrillatory electrophysiology detected during controlled respiration

Heart rate during sinus rhythm is modulated through the autonomic nervous system, which generates short-term oscillations. The high-frequency components in these oscillations are associated with respiration, causing sinus arrhythmia, mediated by the parasympathetic nervous system. In this study, we evaluated whether slow, controlled respiration causes cyclic fluctuations in the frequency of the fibrillating atria. Eight patients (four women; median age 63 yr, range 53-68 yr) with chronic atrial fibrillation (AF) and third-degree atrioventricular block treated by permanent pacemaker were studied. ECG was recorded during baseline rest, during 0.125-Hz frequency controlled respiration, and finally during controlled respiration after full vagal blockade. We calculated fibrillatory frequency using frequency analysis of the fibrillatory ECG for overlapping 2.5-s segments; spectral analysis of the resulting frequency trend was performed to determine the spectrum of variations of fibrillatory frequency. Normalized spectral power at respiration frequency increased significantly during controlled respiration from 1.4 (0.76-2.0) (median and range) at baseline to 2.7 (1.2-5.8) (P = 0.01). After vagal blockade, the power at respiration frequency decreased to 1.2 (0.23-2.8) (P = 0.01). Controlled respiration causes cyclic fluctuations in the AF frequency in patients with long-duration AF. This phenomenon seems to be related to parasympathetic modulations of the AF refractory period.

Atrial Fibrillatory Wave Characteristics on Surface Electrogram:

INTRODUCTION

Fibrillatory waves on the surface ECG have been scrutinized to allow inferences about underlying mechanisms and pathophysiology, based on the premise that fibrillatory waves do not vary "randomly" but provide a consistent reflection of the underlying state of the atria in an individual patient. This premise is untested.

METHODS AND RESULTS

Ten standard ECGs were recorded over a 24-hour period in each of 20 clinically stable inpatients with atrial fibrillation. After QRS-T cancellation, the remainder fibrillatory waves were analyzed. Interpatient versus intrapatient differences in fibrillatory wave characteristics were evaluated by analysis of variance (ANOVA). The fibrillatory wave peak-to-peak amplitude of all the patients ranged from 0.06 to 0.35 mV, whereas 1 SD of the amplitude for each patient ranged from 0.004 to 0.053 mV. Short-term peak frequencies of all the patients ranged from 4.6 to 8.0 Hz, whereas 1 SD for each patient varied from 0.2 to 0.5 Hz. For these and all other parameters tested, interpatient differences were significantly greater compared to intrapatient differences (P < 0.0001).

CONCLUSION

Fibrillatory wave characteristics are repeatable from ECG to ECG over 24 hours for clinically stable patients, whereas substantial differences are present between patients. Further study of the relationship of such characteristics to pathophysiology and management decisions is valid and warranted.

Frequency analysis of atrial fibrillation from the surface electrocardiogram

Atrial fibrillation (AF) is the most common arrhythmia encountered in clinical practice. Neither the natural history of AF nor its response to therapy are sufficiently predictable by clinical and echocardiographic parameters. Atrial fibrillatory frequency (or rate) can reliably be assessed from the surface electrocardiogram (ECG) using digital signal processing (filtering, subtraction of averaged QRST complexes, and power spectral analysis) and shows large inter-individual variability. This measurement correlates well with intraatrial cycle length, a parameter which appears to have primary importance in AF domestication and response to therapy. AF with a low fibrillatory rate is more likely to terminate spontaneously, and responds better to antiarrhythmic drugs or cardioversion while high rate AF is more often persistent and refractory to therapy. In conclusion, frequency analysis of AF seems to be useful for non-invasive assessment of electrical remodeling in AF and may subsequently be helpful for guiding AF therapy.

Prevalence of sleep apnea syndrome in lone atrial fibrillation: a case-control study

BACKGROUND

According to several studies, obstructive sleep apnea predisposes to cardiac arrhythmias, but the prevalence of sleep apnea in specific arrhythmias has not been determined. Our case-control study assesses prevalence of sleep apnea syndrome (SAS) in lone atrial fibrillation (AF).

METHODS

Patients with AF (n = 59; 48 men and 11 women; mean age, 59 years; age range, 25 to 84 years) without evident cardiovascular diseases, and their 56 gender-matched, age-matched, and cardiovascular morbidity-matched community control subjects underwent an overnight sleep study.

RESULTS

Prevalence of SAS in the AF group was 32%, which did not differ from that in control subjects (29%, p = 0.67). In men, mean neck circumference was higher in the AF group (40.9 cm vs 39.5 cm, p = 0.01) than in control subjects. In men, after adjusting for body mass index and waist circumference, neck circumference was independently related to AF, with an odds ratio (OR) of 1.8 (95% confidence interval, 1.3 to 2.5) per 1-cm increase, and an OR of 5.2 (95% confidence interval, 1.6 to 17.0) for values > 40 cm. Compared to control subjects, the AF group reported more daily/almost-daily tiredness (29% vs 4%, p < 0.001), daily/almost-daily sleepiness (27% vs 7%, p = 0.005), and nightly/almost-nightly breathing pauses during sleep (12% vs 2%, p = 0.03).

CONCLUSIONS

SAS seems to be common in lone AF. Nevertheless, we could not show SAS to be more common in patients with AF than in gender-matched, age-matched, and cardiovascular morbidity-matched community control subjects. Compared to control subjects, men with AF seem to have thicker necks, and patients with lone AF report more daytime tiredness, daytime sleepiness, and breathing pauses during sleep.

Experimental study of the effect of the vagus nerve on atrial electrical remodeling

Recent studies have shown that rapid atrial activation causes atrial electrical remodeling (AER), which recovers quickly following withdrawal of stimulation. The underlying mechanisms, however, are incompletely understood. The purpose of the present study, therefore, was to characterize the effect of the vagus on AER as well as define possible mechanisms of the phenomenon. Eight dogs were used in the study for 3 consecutive protocols. In the first, the dogs were subjected to atrial pacing at 800 ppm for 7 hours. Every hour, pacing was interrupted for a short time and atrial effective refractory period (AERP) was measured at 6 sites. The rapid atrial pacing was then discontinued and the electrophysiological study was repeated every hour for another 7 hours. Time-domain parameters of heart rate variability (HRV) were also computed 1 hour before pacing as well as each of 7 hours after the rapid atrial pacing protocol. The second program was performed two weeks after the first; 0.04 mg/kg of atropine was administered intravenously 30 min before pacing, and then 0.007 mg/kg was added at each hour. Parameters of HRV were not evaluated. Finally, the 8 dogs were subjected to the third protocol 2 weeks after completion of the second; 0.2 mg/kg of propranolol was given intravenously 30 min before pacing, and 0.04 mg/kg was added at each hour. The dispersion of AERP (dAERP) was calculated as, maximum AERP minus minimum AERP. There was a prompt decrease in AERP as the result of pacing (P<.05), but dAERP did not change significantly. The AERP recovered quickly, and dAERP increased from 21 +/-5.3 ms to 40 +/- 7.4 ms (P<.05) after cessation of pacing. At the same time, the parameters of HRV increased (P<.05) after cessation of pacing. The AERP increased from 128 +/- 12 ms to 135 +/- 12 ms and from 127 +/- 12 ms to 142 +/- 14 ms (P<.05) after vagal and autonomic blockade. However, AERP decreased during pacing (P<.05) with vagal or autonomic blockade, but dAERP did not change significantly during or after pacing. These results suggest that vagal and autonomic blockade can not prevent AER, but a high vagal tone is associated with a high dAERP during recovery from AER, indicating that the vagus and sympathetic have a synergistic effect on the refractory period.

Evidence for electrical remodelling of the atrial myocardium in patients with atrial fibrillation. A study using the monophasic action potential recording technique

Experimental studies have shown that remodelling of the atrial myocardium is linked to the occurrence and perpetuation of atrial fibrillation (AF). Clinical evidence, however, is insufficient. We recorded monophasic action potentials (MAP) during AF from one to three sites in the right atrium in seven patients with chronic AF (CAF) and in 11 patients with paroxysmal AF (PAF). The fibrillatory (FF) interval between two consecutive upstrokes of the MAP was measured using a computer-assisted manual method. The mean, median, 15th, 10th, 5th percentile and shortest FF intervals were calculated in each patient and used as estimates of the local atrial effective refractory period (AERP) during AF. In three patients burst pacing at 400 and 500 beats min(-1) was delivered during the MAP recording. In nine patients, the AERP was also tested using the extra stimulus technique during sinus rhythm.

RESULTS

Thirty-eight recordings were obtained. The shortest FF interval was significantly shorter in patients with CAF as compared with that in patients with PAF (50+/-13 vs. 72+/-31 ms, P<005). Similar differences were seen in the mean, median, 15th, 10th, and 5th percentile FF interval. The AERP during sinusrhythm was significantly longer than the estimated AERPs (P<0 05 to P<0.01) in the nine patients. There was no significant difference in FF interval before and after the burst pacing in the three patients.

CONCLUSION

The AERP was significantly shortened during AF, as compared with that during sinus rhythm, and the AERP shortening was more marked in patients with CAF than in patients with PAF. These clinical findings support the connection between the electrical remodelling and the occurrence and/ or perpetuation of the AF.

Characterization of atrial fibrillation using the surface ECG: time-dependent spectral properties

Time-frequency analysis is considered for characterizing atrial fibrillation in the surface electrocardiogram (ECG). Variations in fundamental frequency of the fibrillatory waves are tracked by using different time-frequency distributions which are appropriate to short- and long-term variations. The cross Wigner-Ville distribution is found to be particularly useful for short-term analysis due to its ability to handle poor signal-to-noise ratios. In patients with chronic atrial fibrillation, substantial short-term variations exist in fibrillation frequency and variations up to 2.5 Hz can be observed within a few seconds. Although time-frequency analysis is performed independently in each lead, short-term variations in fibrillation frequency often exhibit a similar pattern in the leads V1, V2 and V3. Using different techniques for short- and long-term analysis, it is possible to reliably detect subtle long-term changes in fibrillation frequency, e.g., related to an intervention, which otherwise would have been obscured by spontaneous variations in fibrillation frequency.

Circadian variations in atrial fibrillatory frequency in persistent human atrial fibrillation

Atrial fibrillatory frequency reflects the atrial refractory period during AF. This study was conducted to investigate noninvasively the diurnal fluctuations of fibrillatory frequency in persistent human atrial fibrillation and to determine the relationship between changes in ventricular rate and fibrillatory frequency. Ambulatory ECGs were recorded in 30 patients (18 men, 12 women, mean age 60 +/- 11 years) with persistent AF (> 24 hours). AF frequency was measured in 1-minute ECG segments by subtracting averaged QRST complexes and applying Fourier analysis to the resulting signals at 4 PM, 10 PM, 4 AM, and 10 AM. Peak frequency was determined in the 3-12 Hz frequency band. Mean fibrillatory frequency measured 6.6 +/- 0.6 Hz (range 5.0-7.8 Hz). Two different frequency patterns were distinguished comparing maximal diurnal versus nocturnal fibrillatory frequency. In six (20%) patients an increase (P = 0.045) in nocturnal fibrillatory frequency (type I) was found. In the remaining 24 (80%) patients a decrease (P < 0.001) in fibrillatory frequency occurred (type II). Type I AF showed a strong inverse correlation between relative changes (percent) in ventricular rate and fibrillatory frequency obtained from two consecutive measurement points (r = -0.88 to -.97, P < 0.01), whereas in type II AF a moderate positive correlation (r = 0.36 to 0.41, P < 0.05) was detected. These data indicate a circadian pattern in AF frequency that concurs with ventricular rate changes suggesting a modulating influence of the autonomic nervous system on atrial electrophysiology in persistent human AF.