The effect of pain on involuntary and voluntary capture of attention.

BACKGROUND: There is converging evidence for the notion that pain affects a broad range of attentional domains. This study investigated the influence of pain on the involuntary capture of attention as indexed by the P3a component in the event-related potential derived from the electroencephalogram. METHODS: Participants performed in an auditory oddball task in a pain-free and a pain condition during which they submerged a hand in cold water. Novel, infrequent and unexpected auditory stimuli were presented randomly in a series of frequent standard and infrequent target tones. P3a and P3b amplitudes were observed to novel, unexpected and target-related stimuli, respectively. RESULTS: Both electrophysiological components were characterized by reduced amplitudes in the pain compared with the pain-free condition. Hit rate and reaction time to target stimuli did not differ between the two conditions presumably because the experimental task was not difficult enough to exceed attentional capacities under pain conditions. CONCLUSIONS: These results indicate that voluntary attention serving the maintenance and control of ongoing information processing (reflected by the P3b amplitude) is impaired by pain. In addition, the involuntary capture of attention and orientation to novel, unexpected information (measured by the P3a) is also impaired by pain. Thus, neurophysiological measures examined in this study support the theoretical positions proposing that pain can reduce attentional processing capacity. These findings have potentially important implications at the theoretical level for our understanding of the interplay of pain and cognition, and at the therapeutic level for the clinical treatment of individuals experiencing ongoing pain.

The disruptive effect of chronic pain on mismatch negativity.

OBJECTIVE: To investigate the effect of chronic pain on processes that generate the mismatch negativity (MMN). METHODS: Twelve participants with a diagnosis of chronic intractable pain were tested before and after pain treatment. During testing, event-related potentials were recorded while participants performed tasks of varying difficulty. RESULTS: The amplitude of the MMN was found to be greater following a nerve block procedure compared to MMN amplitude when participants were experiencing chronic pain. This effect was found to occur in the MMN for difficult-to-detect tones elicited while participants were performing a simultaneous cognitively demanding visual task. MMN amplitude was found to be greater with attention to difficult-to-detect deviants during pain but not in no pain conditions. CONCLUSIONS: These results provide an electrophysiological correlate of previous findings that high levels of pain disrupt cognition during the performance of demanding tasks.

Value and limitations of an inverse solution for two equivalent dipoles in localising dual accessory pathways.

Investigations were carried out into whether an equivalent generator consisting of two dipoles could be used to detect dual sites of ventricular activity. A computer model of the human ventricular myocardium was used to simulate activation sequences initiated at eight different pairs of sites positioned on the epicardial surface of the atrio-ventricular ring. From these sequences, 117-lead body surface potentials (covering the anterior and posterior torso), 64-lead magnetic field maps (above the anterior chest) and 128-lead magnetic field maps (above the anterior and posterior chest) were simulated and were then used to localise dual accessory pathways employing pairs of equivalent dipoles. Average localisation errors were 12 mm, 12 mm and 9 mm, respectively, when body surface potentials, 64-lead and 128-lead magnetic fields were used. The results of the study suggest that solving the inverse problem for two dipoles could provide additional information on dual accessory pathways prior to electrophysiological study.

Noninvasive characterisation of multiple ventricular events using electrocardiographic imaging.

Distributions of epicardial potentials, calculated from body surface electrocardiograms (ECGs), were investigated to determine if they could enable detection of multiple sites of ventricular activity. An anatomical model of the human ventricular myocardium was used to simulate activation sequences initiated at nine different ventricular pairs of sites. From these sequences, body surface ECGs were simulated at 352 sites on the torso surface and then used to reconstruct epicardial potentials at 202 sites. The criterion for detection of dual ventricular events was the presence of two distinct primary potential minima in the reconstructed epicardial potentials. The shortest distance between the two events in the right ventricle that resulted in the reconstruction of epicardial potential patterns, featuring two minima, was 27 mm; the distance between the two events in the left ventricle was 23 mm. When Gaussian white noise in the simulated body surface potentials was increased from 3 microV to 15 microV and 50 microV, dual events became more difficult to distinguish. Findings indicate that calculated epicardial potentials provide useful visual information about the presence of multiple ventricular events that is not apparent in features of body surface ECGs, and could be particularly helpful in optimising mapping procedures during difficult or unsuccessful radiofrequency ablations of accessory pathways.

Electrocardiographic imaging.

Electrocardiographic imaging (ECGI) is a useful noninvasive modality for exploring the spread of electrical activation within the ventricular wall. In this model study, we seek to determine whether ECGI could detect changes in patterns of pacing-generated epicardial potentials. An anatomical model of the human ventricular myocardium is used to simulate activation sequences initiated at 116 endocardial pacing sites distributed over the left ventricular free wall. From these realistic sequences, we simulate extracardiac potentials at epicardial (202 sites) and torso surfaces (352 sites) using boundary element model of the human torso. ECGI is applied to compute epicardial potentials and unipolar electrograms (202 sites). Inversely computed electrograms correlate well with those simulated by an anatomical model (r>0.9 at 71% of sites). Features of the calculated epicardial potential patterns provide direct visual information about the site of pacing. The results also demonstrate that ECGI provides detailed spatio-temporal reconstruction of patterns of myocardial activation.

Low-frequency component of body surface potential maps identifies patients at risk for ventricular tachycardia.

AIMS: To investigate the ability of spectral features of signal-averaged body-surface potential maps in identifying post-infarction patients who are at risk of developing ventricular tachycardia. METHODS AND RESULTS: We recorded 120 lead body surface potential maps during sinus rhythm in 135 subjects (45 patients with healed myocardial infarction but no history of ventricular tachycardia, 45 patients with both healed myocardial infarction and at least one episode of sustained ventricular tachycardia, and 45 normal subjects) and analysed spectral features of body surface potential maps selected on the basis of isoharmonic maps for given bands of the frequency spectrum. We found that in the low-frequency band (1-11 Hertz), the group-mean power spectra of leads located at isoharmonic map maxima were significantly different (P<0.0001) between the two groups of myocardial infarction patients. We estimated that this single feature alone can prospectively identify myocardial infarction patients at risk for ventricular tachycardia with a predictive accuracy of 74+/-6%. CONCLUSION: Our results suggest that the bulk of diagnostic information associated with arrhythmogenicity resides in the low-frequency band of the power spectrum. This finding is at variance with the established notion that only the high-frequency component of signal-averaged electrocardiograms carries such information.

Arrhythmia vulnerability assessment using magnetic field maps and body surface potential maps.

Magnetic field maps and body surface potential maps can be used to measure cardiac activity. The ability of magnetic and potential body surface maps to identify patients' vulnerable to recurrent sustained ventricular arrhythmia (VA) were compared. Magnetic field maps (MFM) and body surface potential mapping (BSPM) were obtained from 76 normal (N) subjects, 15 myocardial infarct (MI) patients, and 15 VA patients. QRST integral maps were calculated for each subject and nondipolar content was determined using Karhunen-Loeve transform eigen-maps. Although differences in nondipolar content were significant between the normal and patient groups (P = 2.4 x 10(-5) for BSPM and P = 6.0 x 10(-8) for MFM), differences in nondipolar content between MI and VA patients using QRST integral BSPM and MFM maps were not significant. The trajectory of the location of the maxima and minima on the map area during the QRS and ST-T intervals were also constructed. Discrimination between MI and VA patients was based on intergroup differences in the amount of fragmentation of the trajectory plots. The ST-T trajectory plots were significantly more fragmented (P < 0.0001 and P < 0.05 for MFM and BSPM, respectively) for VA than for MI patients. The ST-T interval MFM and BSPM trajectory plots enabled separation of MI and VA patients with accuracies of 83% and 73%, respectively. These results suggest that repolarization MFM and BSPM extrema trajectory plots can be used effectively as a means of identifying patients at risk for VA.

Assessment of spatial resolution of pace mapping when using body surface potentials.

Using computer simulations and statistical methods, the resolution of pace mapping when used in combination with body surface potentials was systematically investigated. In an anatomical model of the human ventricular myocardium, pre-excitation sequences were initiated at 69 sites positioned along the atrioventricular (AV) ring and corresponding body surface potential maps (BSPMs) were calculated at 32 leads placed on the anterior torso. For each time after the onset of pre-excitation (every 4 ms to 40 ms) and each root-mean-square (RMS) noise level (5, 10, 20 and 50 microV), BSPMs were cros-correlated and the spatial resolution defined as the largest pacing site separation at which the differences in correlation coefficients were not statistically significant (level p > or = 0.05). The findings indicate that when random RMS noise of 5 microV was added to the simulated BSPMs, average spatial resolution over all 60 sites was at 20 ms after the onset of pre-excitation within 3.5 +/- 0.9 mm. The results provide theoretical evidence that statistical analysis of BSPMs obtained during pace mapping can offer improved means for subcentimetre identification of accessory pathways located along the AV ring.

Spatial and temporal variations in the magnetic fields produced by human gastrointestinal activity.

Magnetoenterography (MENG) is a new, non-invasive technique that measures gastrointestinal magnetic signals near the body surface. This study was undertaken to evaluate the temporal and spatial characteristics of the magnetic signals generated by gastric and duodenal slow wave activity. The gastrointestinal magnetic fields of eight normal subjects were measured for 60 minutes in both the fasting and fed state using 36 magnetic sensors simultaneously. The results were displayed as a succession of maps over time showing the temporal evolution of the spatial distribution of the signal over the upper abdomen. In all subjects, slow wave activity of the stomach centred at 3.0 +/- 0.5 cycles min-1 in both the fasting and fed state was observed. The duodenal signal at 11.0 +/- 1.0 cycles min-1 was observed in four subjects. The spatial distribution of these two signals is distinctly different. The observed spatial and temporal variations are described in terms of a model used previously to explain the potentials observed in electrogastrography (EGG).

Accuracy of single-dipole inverse solution when localising ventricular pre-excitation sites: simulation study.

Different factors are investigated that may affect the accuracy of an inverse solution that uses a single-dipole equivalent generator, in a standardised inhomogeneous torso model, when localising the pre-excitation sites. An anatomical model of the human ventricular myocardium is used to simulate body surface potential maps (BSPMs) and magnetic field maps (MFMs) for 35 pre-excitation sites positioned on the epicardial surface along the atrioventricular ring. The sites of pre-excitation activity are estimated by the single-dipole method, and the measure for the accuracy of the localisation is the localisation error, defined as the distance between the location of the best-fitting single dipole and the actual site of pre-excitation in the ventricular model. The findings indicate that, when the electrical properties of the volume conductor and lead positions are precisely known and the 'measurement' noise is added to the simulated BSPMs and MFMs, the single-dipole method optimally localises the pre-excitation activity 20 ms after the onset of pre-excitation, within 0.71 +/- 0.28 cm and 0.65 +/- 0.30 cm using BSPMs and MFMs, respectively. When the standard torso model is used to localise the sites of onset of the pre-excitation sequence initiated in four individualised torso models, the maximum errors are as high as 2.6-3.0 cm (even though the average error, for both the BSPM and MFM localisations, remains within the 1.0-1.5 cm range). In spite of these shortcomings, it is thought that single-dipole localisations can be useful for non-invasive pre-interventional planning.

Spatial resolution of body surface potential maps and magnetic field maps: a simulation study applied to the identification of ventricular pre-excitation sites.

The spatial resolution of body surface potential maps (BSPMs) and magnetic field maps (MFMs) is investigated by means of an anatomically accurate computer model of the human ventricular myocardium. BSPMs and MFMs are calculated for the simulated activation sequences initiated at 35 pre-excitation sites located along the atrioventricular (AV) ring of the epicardium. Changes in the BSPMs and MFMs corresponding to different pre-excitation sites are quantified in terms of the correlation coefficient r. The spatial resolution (selectivity) for a given pre-excitation site is defined as the half-distance between those neighbouring locations at which morphological features of maps, in terms of r, become distinct (r < 0.95). It is found that, at 28 ms after the onset of pre-excitation and with no noise added, this distance +/- SD, for all sites along the AV ring for the 117-lead BSPMs, is 0.83 +/- 0.32 cm, and for the 64-lead and 128-lead MFMs it is 1.54 +/- 0.84 cm and 1.15 +/- 0.43 cm, respectively. The findings suggest that, when features of non-invasively recorded electrocardiographic and magnetocardiographic map patterns are used for identifying accessory pathways in patients suffering from WPW syndrome, BSPMs are likely to provide more detailed information for guiding the ablative treatment than MFMs. For some sites MFMs provide more information. Both modalities may provide additional assistance to the cardiologist in locating the site of the accessory pathway.

Factors affecting the accuracy of the boundary element method in the forward problem--I: Calculating surface potentials.

A comprehensive review of factors affecting the accuracy of the boundary element method (BEM) for calculating surface potentials is presented. A relative-error statistic is developed which is only sensitive to calculation errors that could affect the inverse solution for source position, and insensitive to errors that only affect the solution for source strength. The factors considered in this paper are: numerical approximations intrinsic to the BEM, such as constant-potential versus linear-potential basis functions and sharp-edged versus smooth-surfaced volumes; aspects of the volume conductor including the volume shape, density of surface elements, and element shape; source position and orientation; and effects of "refinements" in the numerical methods. The effects of these factors are considered in both smooth-shaped (spheres and spheroids) and sharp-edged (cubes) volume conductors. This represents the first attempt to assess the effects of many of these factors pertaining to the numerical methods commonly used in fields such as electrocardiography (ECG) and electroencephalography (EEG). Strategies for obtaining the most accurate solutions are presented.

Body surface potential mapping of a patient with Wolff-Parkinson-White syndrome with two accessory pathways and two atrial pacemaker complexes.

As part of an ongoing research protocol, a patient with Wolff-Parkinson-White syndrome underwent body surface potential mapping and electrophysiologic studies before radiofrequency ablation therapy. Careful analysis of the body surface potential mapping data made it possible to distinguish four different map sequences representing four different cardiac complexes. Analysis of these maps is consistent with two accessory pathways, with the additional pathology of two distinct atrial pacemaker sites. A right anterosuperior pathway was found to conduct continuously. The second pathway is consistent with a right inferior pathway conducting intermittently. The analysis demonstrates the type of information that can be extracted from body surface potential maps, even in the presence of complex pathologies.

Comparison between electrocardiographic and magnetocardiographic inverse solutions using the boundary element method.

The accuracy of imaging cardiac sources using electrocardiographic and magnetocardiographic signals is influenced by thoracic inhomogeneities, e.g. the lungs and cardiac blood masses. The effects is investigated of such inhomogeneities on the body-surface potential maps (BSPM) and magnetic-field maps (MFM) inverse solutions for a single moving dipole as the source model and a realistic torso model as the volume conductor, by employing a node-based boundary element method. Using the same number and placement of the body-surface potential and magnetic field leads, a comparison is obtained of the numerical accuracy of body-surface potential and magnetic field leads. The results show that, with no noise added, the body-surface potential solution is less sensitive to the exclusion of the inhomogeneities than the magnetic field solution. The influence of noise on the BSPM and MFM localization is comparable for x (left-right) and y (foot-head) oriented dipoles, and the BSPM localisation is more accurate than the MFM localisation for z (anterior-posterior) oriented dipoles.

A complete linear discretization for calculating the magnetic field using the boundary element method.

An analytic solution is derived for the magnetic field generated by current sources located in a piecewise homogeneous volume conductor. A linear discretization approach is used, whereby the surface potential is assumed to be a piecewise linear function over each tessellated surface defining the regions of differing conductivity. The magnetic field is shown to be a linear combination of vector functions which are strictly dependent on the geometry of the problem, the surface tesselation, and the observation point.

The effect of measurement conditions on MCG inverse solutions.

A magnetic inverse solution that uses a single current dipole in a homogeneous volume conductor with realistic torso shape was tested numerically to establish the effect of magnetic noise, number of measurement points, and torso size on the localization accuracy. Seven different sites of cardiological interest were selected as locations for the source dipole. The three components of the magnetic field were calculated as if measured by second order gradiometers, Gaussian noise was added, and Monte Carlo tests performed for inverse solutions using a single field component, or all three combined. It was found that for any of the single component solutions, and a signal-to-noise ratio of 100, 25 measuring points are sufficient for good accuracy; just 12 points are needed if all three components are used together. If, however, the torso size of the inverse solution is different from that of the field data by 10 or 20%, a larger error occurs, even for 56 measurement points and no noise. In this case, the field component orthogonal to the measurement grid, Bz, yields better results than the other two components, or even all three combined. We conclude that a multichannel system measuring the z component of the magnetic field in about 30 locations would be the best choice to locate a dipolar source, provided the torso of the field data is closely matched by the model used in the inverse solution. To this effect, scaling of the torso model can easily be included in the computation. Imaging techniques could be used to accommodate different torso shapes.

Discrimination between myocardial infarct and ventricular tachycardia patients using magnetocardiographic trajectory plots and iso-integral maps.

Magnetocardiograms were recorded from 30 normal (N) subjects, 15 myocardial infarct (MI) patients, and 15 ventricular tachycardia (VT) patients. Discrimination between the groups was affected by iso-integral magnetic field mapping (MFM) and trajectory plotting of MFM extrema. Iso-integral MFM for the QRST, QRS, and ST-T intervals was created for each test group member. A polarity score, based on the number of extrema features present, was assigned to each iso-integral MFM. Differences in group mean integral of QRST map polarity scores were significant (p less than 0.05) between MI and N, between VT and N (p less than 0.005), and between MI and VT (p less than 0.05) subjects. integral of ST-T map polarity scores were significantly (p less than 0.0001) different between VT and N and between MI and VT (p less than 0.001) subjects. Discrimination between MI and VT patients, based on polarity score difference, was 56% accurate using integral of QRS maps and 73% accurate using integral of ST-T maps. For each subject, time-normalized MFM was used to construct trajectory plots of the maxima and minima in the QRS and ST-T intervals. Discrimination between MI and VT patients was based upon intergroup differences in fragmented trajectory plots. When the number of discrete trajectories and/or the total number (F) of trajectory points at which discrete trajectories coexist were considered, QRSmin trajectory plots were significantly (p less than 0.05) different for VT and N, but not for MI and N subjects. The significant (p less than 0.05) difference between MI and VT trajectory plots enabled 76% accuracy for MI and VT identification. ST-Tmax trajectory plots show significantly (p less than 0.0001) higher F values for VT patients facilitating accurate (87%) discrimination between MI and VT patients. These results suggest that the abnormalities of repolarization processes, displayed by MFM as multipolar integral of ST-T maps and/or as fragmented trajectory plots of ST-T extrema, may be useful indicators of the arrhythmia substrate/processes that characterize VT and vulnerable MI patients.

Orthogonal expansions: their applicability to signal extraction in electrophysiological mapping data.

The applicability of orthogonal expansions (singular-value decomposition, Karhunen-Loève transform and principal-component analysis) for the purpose of identifying source distributions associated with definite electrophysiological events in the heart and brain is explored with a current dipole source model. By definition, the expansion eigenvectors are orthogonal, and as such will extract the features of one specific source only if all other secondary signals are orthogonal to that first source. The number of significant eigenvectors can be related to the number of original components forming a signal, but there is not a one-to-one correspondence between these eigenvectors and the individual components. Furthermore, many eigenvectors may be needed to faithfully represent even a single source, if that source is nonstationary. We conclude that generally it would be inappropriate to ascribe any physiological significance to the data resulting from such expansions.