An integrated approach based on uniform quantization for the evaluation of complexity of short-term heart period variability: Application to 24 h Holter recordings in healthy and heart failure humans.

We propose an integrated approach based on uniform quantization over a small number of levels for the evaluation and characterization of complexity of a process. This approach integrates information-domain analysis based on entropy rate, local nonlinear prediction, and pattern classification based on symbolic analysis. Normalized and non-normalized indexes quantifying complexity over short data sequences ( approximately 300 samples) are derived. This approach provides a rule for deciding the optimal length of the patterns that may be worth considering and some suggestions about possible strategies to group patterns into a smaller number of families. The approach is applied to 24 h Holter recordings of heart period variability derived from 12 normal (NO) subjects and 13 heart failure (HF) patients. We found that: (i) in NO subjects the normalized indexes suggest a larger complexity during the nighttime than during the daytime; (ii) this difference may be lost if non-normalized indexes are utilized; (iii) the circadian pattern in the normalized indexes is lost in HF patients; (iv) in HF patients the loss of the day-night variation in the normalized indexes is related to a tendency of complexity to increase during the daytime and to decrease during the nighttime; (v) the most likely length L of the most informative patterns ranges from 2 to 4; (vi) in NO subjects classification of patterns with L=3 indicates that stable patterns (i.e., those with no variations) are more present during the daytime, while highly variable patterns (i.e., those with two unlike variations) are more frequent during the nighttime; (vii) during the daytime in HF patients, the percentage of highly variable patterns increases with respect to NO subjects, while during the nighttime, the percentage of patterns with one or two like variations decreases.

[Syncope and work. STePS study (Short Term Prognosis of Syncope)]. / La sincope in età lavorativa. Studio multicentrico prospettico STePS.

BACKGROUND: Recurrent syncope is a common medical problem responsible for 3-5% of emergency department (ED) accesses and 1-6% of hospital admissions. If syncope occurs in a subject working in a critical safety task, the consequences of this event might be very dangerous for the worker, colleagues, others or for the environment. Therefore, syncope management is a major problem for occupational medicine, converning the general safety at work. AIMS: To evaluate the syncope events in a group of potential workers aged 18 to 65 years; to evaluate the symptoms preceding syncope and the presence of associated illnesses and recurrent events. POPULATION AND RESULTS: This study is part of the prospective study STePS (Short Term Prognosis of Syncope), and included 305 consecutive patients (aged 18-65 years, female 56%) who had syncope as a main symptom and presented at ED of four general hospitals in the Milan area, Italy, between the 23rd of January and 30th of June 2004. The 24% of subjects were hospitalized. In 21% the syncope occurs suddenly without any preceding symptom. The 67% of subjects didn't have any important illness at the time. 50% of subjects had recurrent syncope. In four subjects another syncope occurred in a 10 day follow-up. CONCLUSIONS: occupational medicine should consider syncope scrupulously. Proper diagnostic management is important to permit a correct evaluation of work safety issues.

Automatic classification of interference patterns in driven event series: application to single sympathetic neuron discharge forced by mechanical ventilation.

This study proposes a method for the automatic classification of nonlinear interactions between a strictly periodical event series modelling the activity of an exogenous oscillator working at a fixed and well-known rate and an event series modelling the activity of a self-sustained oscillator forced by the exogenous one. The method is based on a combination of several well-known tools (probability density function of the cyclic relative phase, probability density function of the count of forced events per forcing cycle, conditional entropy of the cyclic relative phase sequence and a surrogate data approach). Classification is reached via a sequence of easily applicable decision rules, thus rendering classification virtually user-independent and fully reproducible. The method classifies four types of dynamics: full uncoupling, quasiperiodicity, phase locking and aperiodicity. In the case of phase locking, the coupling ratio (i.e. n: m) and the strength of the coupling are calculated. The method, validated on simulations of simple and complex phase-locking dynamics corrupted by different levels of noise, is applied to data derived from one anesthetized and artificially ventilated rat to classify the nonlinear interactions between mechanical ventilation and: (1) the discharges of two (contemporaneously recorded) single postganglionic sympathetic neurons innervating the caudal ventral artery in the tail and (2) arterial blood pressure. Under central apnea, the activity of the underlying sympathetic oscillators is perturbed by means of five different lung inflation rates (0.58, 0.64, 0.76, 0.95, 1.99 Hz). While ventilation and arterial pressure are fully uncoupled, ventilation is capable of phase locking sympathetic discharges, thus producing 40% of phase-locked patterns (one case of 2:5, 1:1, 3:2 and 2:2) and 40% of aperiodic dynamics. In the case of phase-locked patterns, the coupling strength is low, thus demonstrating that this pattern is sliding. Non-stationary interactions are observed in 20% of cases. The two discharges behave differently, suggesting the presence of a population of sympathetic oscillators working at different frequencies.

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).

Analysis of cardiac left-ventricular volume based on time warping averaging.

The cardiac left-ventricular (LV) volume signal, obtained by acoustic quantification, is affected by noise and respiratory modulation, resulting in a large beat-to-beat variability that affects the computation of LV function indices. A new method is proposed to improve the evaluation of LV indices by applying a signal averaging technique based on dynamic time warping to consecutive LV volume waveforms. Volume signals obtained from ten normal young (NY) subjects (mean age +/- SD: 25+/-5 years) were used to evaluate the performance of this algorithm. To evaluate its clinical utility, the effects of ageing and pharmacologically induced changes on LV function were assessed by studying, respectively, ten normal (N) adult subjects (age 64+/-8 years) and ten patients with dilated cardiomyopathy during a control and low-dose dobutamine (10 microg kg(-1) min(-1)) study. Indices of LV function were highly consistent, with a variability of less than 8%, even when only 16 beats were averaged, independently of their selection inside the whole recording. When compared with beat-to-beat measures, the averaging of 16 beats significantly reduced (by more than 50%) the interbeat variability of all indexes. Expected alterations in both diastolic and systolic function were evidenced both with ageing (peak filling atrial contraction and ejection rates: from 275+/-77 mls(-1), 76+/-30 ml s(-1) 230+/-70 mls(-1), respectively, in NY, to 160+/-33 mls(-1), 125+/-39 mls(-1), 163+/-54 mls(-1) in N) and with dobutamine (peak filling and ejection rates from 160+/-72 mls(-1) and 183+/-86 mls(-1) respectively, in control, to 253+/-75 mls(-1) and 251+/-105 mls(-1) with dobutamine). Signal averaging with time warping allows fast and improved assessment of LV function.

Quantifying the strength of the linear causal coupling in closed loop interacting cardiovascular variability signals.

The coherence function measures the amount of correlation between two signals x and y as a function of the frequency, independently of their causal relationships. Therefore, the coherence function is not useful in deciding whether an open-loop relationship between x and y is set (x acts on y, but the reverse relationship is prevented) or x and y interact in a closed loop (x affects y, and vice versa). This study proposes a method based on a bivariate autoregressive model to derive the strength of the causal coupling on both arms of a closed loop. The method exploits the definition of causal coherence. After the closed-loop identification of the model coefficients, the causal coherence is calculated by switching off separately the feedback or the feedforward path, thus opening the closed loop and fixing causality. The method was tested in simulations and applied to evaluate the degree of the causal coupling between two variables known to interact in a closed loop mainly at a low frequency (LF, around 0.1 Hz) and at a high frequency (HF, at the respiratory rate): the heart period (RR interval) and systolic arterial pressure (SAP). In dogs at control, the RR interval and the SAP are highly correlated at HF. This coupling occurs in the causal direction from the RR interval to the SAP (the mechanical path), while the coupling on the reverse causal direction (the baroreflex path) is not significant, thus pointing out the importance of the direct effects of respiration on the RR interval. Total baroreceptive denervation, by opening the closed loop at the level of the influences of SAP on RR interval, does not change these results. In elderly healthy men at rest, the RR interval and SAP are highly correlated at the LF and the HF. At the HF, a significant coupling in both causal directions is found, even though closed-loop interactions are detected in few cases. At the LF, the link on the baroreflex pathway is negligible with respect to that on the reverse mechanical one. In heart transplant recipients, in which SAP variations do not cause RR interval changes as a result of the cardiac denervation, the method correctly detects a significant coupling only on the pathway from the RR interval to the SAP.

Sequential modulation of cardiac autonomic control induced by cardiopulmonary and arterial baroreflex mechanisms.

BACKGROUND: Nonhypotensive lower body negative pressure (LBNP) induces a reflex increase in forearm vascular resistance and muscle sympathetic neural discharge without affecting mean heart rate. We tested the hypothesis that a reflex change of the autonomic modulation of heartbeat might arise during low intensity LBNP without changes of mean heart rate. METHODS AND RESULTS: Ten healthy volunteers underwent plasma catecholamine evaluation and a continuous recording of ECG, finger blood pressure, respiratory activity, and central venous pressure (CVP) during increasing levels of LBNP up to -40 mm Hg. Spectrum and cross-spectrum analyses assessed the changes in the spontaneous variability of R-R interval, respiration, systolic arterial pressure (SAP), and CVP and in the gain (alpha(LF)) of arterial baroreflex control of heart rate. Baroreceptor sensitivity was also evaluated by the SAP/R-R spontaneous sequences technique. LBNP began decreasing significantly: CVP at -10, R-R interval at -20, SAP at -40, and the indexes alpha(LF) and baroreceptor sensitivity at -30 and -20 mm Hg, compared with baseline conditions. Plasma norepinephrine increased significantly at -20 mm Hg. The normalized low-frequency component of R-R variability (LF(R-R)) progressively increased and was significantly higher than in the control condition at -15 mm Hg. CONCLUSIONS: Nonhypotensive LBNP elicits a reflex increase of cardiac sympathetic modulation, as evaluated by LF(R-R), which precedes the changes in the hemodynamics and in the indexes of arterial baroreflex control.

Entropy, entropy rate, and pattern classification as tools to typify complexity in short heart period variability series.

An integrated approach to the complexity analysis of short heart period variability series (approximately 300 cardiac beats) is proposed and applied to healthy subjects during the sympathetic activation induced by head-up tilt and during the driving action produced by controlled respiration (10, 15, and 20 breaths/min, CR10, CR15, and CR20 respectively). The approach relies on: 1) the calculation of Shannon entropy (SE) of the distribution of patterns lasting three beats; 2) the calculation of a regularity index based on an entropy rate (i.e., the conditional entropy); 3) the classification of frequent deterministic patterns (FDPs) lasting three beats. A redundancy reduction criterion is proposed to group FDPs in four categories according to the number and type or of heart period changes: a) no variation (0V); b) one variation (1V); and c) two like variations (2LV); 4) two unlike variations (2UV). We found that: 1) the SE decreased during tilt due to the increased percentage of missing patterns; 2) the regularity index increased during tilt and CR10 as patterns followed each other according to a more repetitive scheme; and 3) during CR10, SE and regularity index were not redundant as the regularity index significantly decreased while SE remained unchanged. Concerning pattern analysis we found that: a) at rest mainly three classes (0V, 1V, and 2LV) were detected; b) 0V patterns were more likely during tilt; c) 1V and 2LV patterns were more frequent during CR10; and d) 2UV patterns were more likely during CR20. The proposed approach based on quantification of complexity allows a full characterization of heart period dynamics and the identification of experimental conditions known to differently perturb cardiovascular regulation.

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.

Selective impairment of excitatory pressor responses after prolonged simulated microgravity in humans.

The haemodynamic and autonomic effects of prolonged exposure to simulated microgravity were assessed non-invasively in seven healthy volunteers completing a 42-day -6 degrees head down tilt. Before, during and after head down tilt, subjects were exposed to moderate excitatory stimuli (mental arithmetic and static handgrip) to gauge possible progressive impairment of pressor responses. Before and after head down tilt, subjects were also exposed to orthostatic stress, to assess influences of simulated microgravity on orthostatic defence. Simple haemodynamics (heart rate and systolic arterial blood pressure), linear (i.e., oscillatory) components of beat-by-beat variability, non-linear properties (i.e., corrected conditional entropy (CCE)) of RR interval variability, and baroreflex slope furnished a non-invasive evaluation of autonomic regulatory mechanisms. Pressor responses to mental arithmetic and to handgrip were markedly impaired after 42 days head down tilt, whereas responses in markers of autonomic regulation were not modified. Standing, performed 8 days after head down tilt to limit the risk of syncope, still induced a variable degree of hypotension, with signs of progressively greater sympathetic activation than before head down tilt. Simulated microgravity-induced reduction of pressor responses, in spite of largely maintained autonomic activation, favours the hypothesis of a peripheral impairment of cardiovascular homeostasis. rights reserved.

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.

[Neural control of the circulation in normal and pathologic conditions]. / Il controllo nervoso della circolazione in condizioni normali e patologiche.

The concept of sympatho-vagal balance is particularly useful to explore some functional features of cardiovascular neural regulation. The variability of signals like RR interval or systolic arterial pressure, assessed with power spectrum analysis, has provided for the first time, simultaneously, markers of sympathetic and vagal modulation, and hence of their interaction. The relative power of low frequency (LF) oscillation quantifies sympathetic modulation, while the high frequency (HF) oscillation quantifies vagal modulation. LF and HF components are also detectable in the variability of muscle sympathetic nerve activity recorded in man. In numerous physiological and pathophysiological conditions these two oscillations undergo a reciprocal relationship similar to that characterizing sympatho-vagal balance. An increase in sympathetic modulation has been found during different physiological stimuli and in abnormal states such as recovery from myocardial infarction, essential arterial hypertension, and congestive heart failure in its early stage. A correct use of this methodology is prevented by a drastic decrease in variance, often accompanying pathophysiological reductions in responsiveness of target functions. In these cases the decrease in LF power yields an important negative prognostic value.

Importance of ventilation in modulating interaction between sympathetic drive and cardiovascular variability.

Chemoreflex stimulation elicits both hyperventilation and sympathetic activation, each of which may have different influences on oscillatory characteristics of cardiovascular variability. We examined the influence of hyperventilation on the interactions between changes in R-R interval (RR) and muscle sympathetic nerve activity (MSNA) and changes in neurocirculatory variability, in 14 healthy subjects. We performed spectral analysis of RR and MSNA variability during each of the following interventions: 1) controlled breathing, 2) maximal end-expiratory apnea, 3) isocapnic voluntary hyperventilation, and 4) hypercapnia-induced hyperventilation. MSNA increased from 100% during controlled breathing to 170 +/- 25% during apnea (P = 0.02). RR was unchanged, but normalized low-frequency (LF) variability of both RR and MSNA increased markedly (P < 0.001). During isocapnic hyperventilation, minute ventilation increased to 20.2 +/- 1.4 l/min (P < 0.0001). During hypercapnic hyperventilation, minute ventilation also increased (to 19.7 +/- 1.7 l/min) as did end-tidal CO(2) (both P < 0.0001). MSNA remained unchanged during isocapnic hyperventilation (104 +/- 7%) but increased to 241 +/- 49% during hypercapnic hyperventilation (P < 0.01). RR decreased during both isocapnic and hypercapnic hyperventilation (P < 0.05). However, normalized LF variability of RR and of MSNA decreased (P < 0.05) during both isocapnic and hypercapnic hyperventilation, despite the tachycardia and heightened sympathetic nerve traffic. In conclusion, marked respiratory oscillations in autonomic drive induced by hyperventilation may induce dissociation between RR, MSNA, and neurocirculatory variability, perhaps by suppressing central genesis and/or inhibiting transmission of LF cardiovascular rhythms.

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.

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.

Modifications of cardiac autonomic profile associated with a shift schedule of work.

BACKGROUND: Shift work is associated with an increased rate of cardiovascular diseases and accidents. Discordance between circadian rhythms of stress-related biological variables and the work-sleep schedule might explain the reduced efficiency of work. It is not clear whether a shift schedule of work may induce similar discordance in the 24-hour oscillatory pattern of the cardiac autonomic control in respect to the work-sleep periods. METHODS AND RESULTS: Twenty-two healthy male blue-collar shift workers underwent 24-hour ECG recordings during each of the 3 different shifts (first, 6 AM to 2 PM; second, 2 to 10 PM; third, 10 PM to 6 AM). Spectral analysis of heart rate variability over 24 hours provided the normalized markers of cardiac sympathetic (LF(nu)) and vagal (HF(nu)) modulation of the sinoatrial node activity and of the sympathovagal balance (LF/HF). LF(nu) and LF/HF exhibited 24-hour oscillations with different times of maximum and minimum in accordance with the working and sleeping periods, respectively. Lower values of LF(nu) and LF/HF suggestive of a reduced cardiac sympathetic modulation were present when the job task was performed at night compared with the values observed when the work was performed during morning and evening. CONCLUSIONS: Continuous weekly changes of time of maximum and minimum in the cardiac sympathetic and vagal autonomic control may play a role in the excessive rate of cardiovascular diseases in shift workers. The reduced values of the indexes of cardiac sympathetic modulation during night work might be related to the presence of sleepiness or diminished alertness, which in turn could facilitate errors and accidents.

Assessing baroreflex gain from spontaneous variability in conscious dogs: role of causality and respiration.

A double exogenous autoregressive (XXAR) causal parametric model was used to estimate the baroreflex gain (alpha(XXAR)) from spontaneous R-R interval and systolic arterial pressure (SAP) variabilities in conscious dogs. This model takes into account 1) effects of current and past SAP variations on the R-R interval (i.e., baroreflex-mediated influences), 2) specific perturbations affecting R-R interval independently of baroreflex circuit (e.g., rhythmic neural inputs modulating R-R interval independently of SAP at frequencies slower than respiration), and 3) influences of respiration-related sources acting independently of baroreflex pathway (e.g., rhythmic neural inputs modulating R-R interval independently of SAP at respiratory rate, including the effect of stimulation of low-pressure receptors). Under control conditions, alpha(XXAR) = 14.7 +/- 7.2 ms/mmHg. It decreases after nitroglycerine infusion and coronary artery occlusion, even though the decrease is significant only after nitroglycerine, and it is completely abolished by total arterial baroreceptor denervation. Moreover, alpha(XXAR) is comparable to or significantly smaller than (depending on the experimental condition) the baroreflex gains derived from sequence, power spectrum [at low frequency (LF) and high frequency (HF)], and cross-spectrum (at LF and HF) analyses and from less complex causal parametric models, thus demonstrating that simpler estimates may be biased by the contemporaneous presence of regulatory mechanisms other than baroreflex mechanisms.

Comparison of low-dose dobutamine ventriculography with low-dose dobutamine echocardiography for predicting regional improvement in left ventricular function after coronary artery bypass grafting.

The demonstration of a contractile reserve during low-dose dobutamine echocardiography (LDDE) identifies viable myocardium and predicts recovery of left ventricular (LV) function after myocardial revascularization in patients with chronic coronary artery disease. However, a technically difficult transthoracic visualization may limit the use of LDDE, thus requiring an alternative diagnostic procedure. The present study compares LDDE with low-dose dobutamine ventriculography (LDDV) in predicting an improvement in regional LV function after surgical revascularization. We studied 18 patients with coronary artery disease and LV dysfunction who were to undergo coronary artery bypass grafting. Preoperatively, all patients were evaluated for the presence of viable myocardium using LDDE and LDDV. Follow-up echocardiography at rest and left ventriculography were performed 4 months after successful revascularization to assess recovery of LV function. The sensitivity and specificity of LDDE to identify dysfunctional segments capable of recovering function were 63% and 71%, respectively, with a diagnostic accuracy of 68%. The sensitivity, specificity, and diagnostic accuracy of LDDE improved to 81%, 72%, and 76% when patients with optimal transthoracic evaluation were selected, whereas they were 30%, 77%, and 57%, respectively, in those who underwent suboptimal evaluation. The sensitivity, specificity, and diagnostic accuracy of LDDV were 66%, 75%, and 71%, respectively, with no difference in subgroups of patients. This study demonstrates that LDDV can be considered a useful technique for identifying the presence of myocardial viability and may provide an advantage over LDDE in patients with suboptimal echocardiographic visualization.

Heart rate variability is encoded in the spontaneous discharge of thalamic somatosensory neurones in cat.

1. We studied the spontaneous discharge variability of thalamocortical somatosensory neurones in the awake cat in order to disclose its possible information content. The presence of slow (0.09-1.39 Hz) regular fluctuations in the discharge rate of these cells during the waking state has been previously reported. Oscillations in a similar frequency range are known to characterize the activity of central and peripheral neurones pertaining to the autonomic nervous system and the variability of heart period (RR interval variability). 2. A surrogate data test, performed on our database, confirmed the presence of slow (0.05-1 Hz) non-random fluctuations in firing rate. 3. Linear regression detected the presence of an inverse relationship between the values of RR interval and the concurrent levels of neural discharge. 4. Frequency domain analysis indicated that a significant coupling between the two variability signals preferentially occurred in two frequency bands: in the frequency of the respiratory sinus arrhythmia and in correspondence with a slower rhythm (0.07-0.3 Hz), the two signals being in phase opposition in most of the cases. 5. Coherent fluctuations could also be observed when epochs of evoked activity were analysed, while coupling between the two variability signals appeared to be disrupted after sleep onset. 6. We conclude that RR interval variability, an internally generated dynamic related to basic visceral regulation, is encoded in the discharge of single somatosensory thalamocortical neurones during wakefulness. A possible interaction with the transmission of somatosensory information has to be evaluated.