Effect of electrode impedance in improved buffer amplifier for bioelectric recordings

We analysed the effects of electrode impedance on the transfer response of a one-stage improved buffer amplifier. The electrode DC resistance (R(d)) modifies the one-stage buffer transfer response. We found a limit electrode resistance (R(d(lim))) which depends on the transfer damping factor (epsilon). If R(d) is lower than 86.5 komega, the transfer response of the buffer fulfils American Heart Association (AHA) recommendations, but when R(d) is greater than R(d(lim)) it must be cautiously weighed up because its influence in the transfer response becomes appreciable. The maximum R(d) that can be driven by the buffer is 1.2 Momega. Higher values do not fulfil AHA recommendations. Therefore, electrodes with higher impedance should not be used with this kind of buffer. In contrast, when this buffer is used to build in an instrumentation amplifier (IA) for bipolar recording, the common-mode rejection ratio (CMRR) is sensitive to the electrode type used.

Differential modulation of electrocardiographic indices of ventricular repolarization dispersion depending on the site of pacing during premature stimulation

INTRODUCTION

Dispersion of ventricular repolarization has been shown to increase with premature stimulation. Moreover, a straight correlation between the amount of dispersion of repolarization and the vulnerability to ventricular fibrillation was reported. On the other hand, differences between right ventricular (RV) and left ventricular (LV) fibrillation threshold have been reported. However, no data exist regarding the influence of the site of stimulation on modulation of dispersion of repolarization.

METHODS AND RESULTS

In the present study, several ECG indices of dispersion of repolarization, as a function of the coupling interval and the site of stimulation, were evaluated in a modified Langendorff-perfused rabbit heart (n = 12), with a 5 x 8 array of a simulated body surface unipolar lead system. As the coupling interval was shortened, a biphasic modulation of dispersion of repolarization was found when stimuli were elicited at the LV. In contrast, when the heart was paced from the RV, the dispersion increased monotonically as coupling interval was shortened.

CONCLUSION

A differential behavior of the modulation of dispersion of repolarization was found as a function of the site of stimulation.

Sex differences on the electrocardiographic pattern of cardiac repolarization: possible role of testosterone

BACKGROUND

Cardiac repolarization has been shown to be shorter and faster in men compared with women. In this study, we examined the electrocardiographic pattern of repolarization in patients with abnormal plasma levels of testosterone to gain insight into the role that this hormone plays in modulating repolarization.

METHODS AND RESULTS

Repolarization was measured in 27 castrated men, 26 women with virilization, and 53 control subjects pair-matched for age and sex. Repolarization in castrated men was slower and longer than that of normal men. Women with virilization exhibited a shorter and faster repolarization than normal women and castrated men. These differences are the opposite of those found in the normal population. Finally, the changes observed in castrated men may be reverted by testosterone.

CONCLUSIONS

We conclude that testosterone plays an important role in modulating cardiac repolarization.

Sex-dependent electrocardiographic pattern of cardiac repolarization

BACKGROUND

Although sex-dependent differences in cardiac repolarization have been known for many years, few attempts have been carried out to define the individual contribution of each electrocardiographic (ECG) repolarization variable to the sex-dependent pattern.

METHOD AND RESULTS

We analyzed several ECG repolarization variables that reflect both the duration and the rate of repolarization in 500 normal healthy subjects between the ages of 20 and 80 years (250 men and 250 women) and distributed into 5 groups according to age. Cardiac repolarization, measured in the precordial lead exhibiting the highest T-wave amplitude, was found to be shorter and faster in normal men compared with normal women (P <.001). The parameters with the highest individual weight in determining these sex differences were the J point and the ST angle, as indicated by nonlinear (logistic) multivariate regression analysis.

CONCLUSION

We conclude that changes in the duration and rate of early repolarization are determinants for these sex-dependent ECG pattern differences.

Effects of amiodarone and desethylamiodarone on the inward rectifying potassium current (IK1) in rabbit ventricular myocytes

We examined the effects of amiodarone (AMI) and desethylamiodarone (DAM) on whole-cell inward rectifying potassium current (IK1) in freshly isolated adult rabbit ventricular myocytes by using the whole-cell voltage-clamp technique, as an index of their effects on resting membrane resistance (Rm). Under control conditions, the current showed a strong inward rectification with a maximal inward current measured at -130 mV of -26.4 +/- 1.3 pA/pF and a maximal outward current measured at -50 mV of 3.5 +/- 0.3 pA/pF The current also exhibit a time-dependent activation, with a time constant of activation (tau(a)) that increased with depolarization. The maximal slope conductance normalized to cell capacitance was 0.509 +/- 0.019 nS/pE After exposure to both DAM (50 microM; n = 8) and AMI (50 microM; n = 7), rapid decrease in inward IK1 was observed. Block was restricted almost exclusively to the inward component. DAM caused a significant reduction of the maximal inward current (-20.0 +/- 2.0 pA/pF; p < 0.05), whereas AMI induced an even greater reduction of the same component (-14.1 +/- 1.2 pA/pF; p < 0.05 with respect to control and to DAM). The outward component of IK1 was not changed by either AMI or DAM (4.0 +/- 0.3 pA/pF and 3.4 +/- 0.4 pA/pF, respectively). AMI and DAM also decreased the maximal slope conductance significantly (0.297 +/- 0.019 nS/pF and 0.421 +/- 0.038 nS/pF, respectively). In addition, AMI but not DAM significantly increased the tau(a). However, the voltage dependence of the acceleration of tau(a) remained unchanged after both AMI and DAM exposure. These results allow us to conclude that AMI may induce a greater increase in the resting Rm than its main metabolite. This effect may counterbalance, at least in part, the conduction slowing due to its sodium channel-blocking properties.

Influence of filtering techniques on the time-domain analysis of signal-averaged P wave electrocardiogram

INTRODUCTION

The advent of signal-averaged ECG (SAECG) systems for P wave analysis has made it important to determine if the use of different filtering techniques in these systems is diagnostically equivalent.

METHODS AND RESULTS

Three different high-pass filtering techniques and two cutoff frequency values were used: 29- and 40-Hz Butterworth bidirectional filter (BB29, BB40), 29- and 40-Hz Butterworth unidirectional filter (UB29, UB40), and 29- and 40-Hz least mean square filter (LMS29, LMS40). Normal healthy volunteers (n = 36) and patients with documented paroxysmal atrial fibrillation (n = 23) were analyzed. A custom-built SAECG system and standard bipolar orthogonal leads were used. Noise was reduced to < 0.3 microV. P wave total duration, root mean square voltage of the terminal 20, 30, and 40 msec of the filtered vector magnitude, and the area under the curve between the onset and offset of averaged unfiltered and filtered P wave vector magnitude were analyzed. Only the duration of the P wave showed statistically significant differences between groups, being longer in the PAF group for all filters and cutoff frequencies studied. A bias increment of approximately 20 msec was detected in unidirectional and least mean square filters as compared to the bidirectional filter. Sensitivity, specificity, and predictive accuracy were > 70% for all filters; the BB40 filter yielded the best performance.

CONCLUSION

The normality limits derived from one filter cannot be applied directly to recordings obtained from the other filters. Critical limits must be established individually for different software settings.

Chronic versus acute effects of amiodarone on the Vmax-conduction velocity relationship and on the space constant in canine myocardium

Isolated tissue experiments (canine ventricular epicardium) and computer simulations were used to characterize the relationship between changes in maximum rate of depolarization during upstroke (Vmax) and the conduction velocity (theta) induced after long-term therapy with amiodarone and to compare these data with those obtained after acute superfusion with either desethylamiodarone or the parent compound. After chronic amiodarone, the changes in Vmax were linearly related to the square of the changes in theta during longitudinal propagation (LP) (slope = 0.93, r = 0.93, p = NS with respect to slope = 1), whereas during transverse propagation (TP), the slope of the relationship between both variables was slightly decreased (slope = 0.88, r = 0.96, p < 0.05 with respect to slope = 1). Similar results were observed after acute superfusion with desethylamiodarone (LP), (slope = 1, r = 0.05, p = NS with respect to slope = 1). In contrast, a significant increase in slope (p < 0.05 with respect to slope = 1) was observed after acute superfusion (slope = 1.45, r = 0.85 and slope = 1.48, r = 0.75 during LP and TP, respectively). In addition, the space constant (lambda) after chronic amiodarone (1.05 +/- 0.06 mm) was not significantly different from control (0.98 +/- 0.04 mm), but was slightly though significantly increased after acute amiodarone administration (1.07 +/- 0.03 mm, p < 0.03). Data are mean +/- SEM. Experimental data from chronic amiodarone were well fitted in a one-dimensional Beeler-Reuter-based discrete cable by reducing sodium conductance (GNa) exclusively. In contrast, data from acute superfusion were fitted only when junctional resistance (rj) and GNa were simultaneously reduced. These data suggest that acute amiodarone may modify both active and passive membrane properties whereas chronic amiodarone appears to alter only the active properties; the data further indicate that desethylamiodarone may play an important role in the mechanism of action of chronic amiodarone treatment.

Relationship between Vmax and conduction velocity in uniform anisotropic canine ventricular muscle: differences between the effects of lidocaine and amiodarone

The purpose of these experiments was to determine if Vmax in anisotropic myocardium varies approximately as the square of the conduction velocity (theta) after the addition of lidocaine (6.5 micrograms/ml) and amiodarone (20 micrograms/ml). We measured Vmax and theta in 16 epicardial strips of uniform anisotropic ventricular muscles, over a wide range of stimulation frequencies. The relationship of Vmax to theta 2 was evaluated by linear regression analysis. We found that the decrease in Vmax was proportional to the square of the decrease in theta in the presence of lidocaine both during longitudinal (LP) and transverse (TP) propagation (mean slope +/- SEM: 0.961 +/- 0.047 and 0.918 +/- 0.068, respectively). The changes in Vmax, in the presence of amiodarone, were not predicted by the quadratic changes in theta during TP. However, during LP, the changes in Vmax and theta were well fitted by the predicted relationship. The slope was significantly different from that of lidocaine (2.399 +/- 0.673 vs. 0.961 +/- 0.047, p less than 0.05). On the other hand, the predicted values of theta, assuming theta = square root of Vmax x k, were significantly more depressed than the measured values. We conclude that in uniform anisotropic ventricular muscle, when the changes in Vmax and theta are solely due to a decrease in sodium conductance, a quadratic relationship between the changes in both variables is seen and the slope of the regression line should be 1, such as we have shown for lidocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of amiodarone on Vmax and conduction velocity in anisotropic myocardium

Despite its widespread clinical use, the precise mechanism of action of amiodarone (AMI) has not been completely defined. We examined the effects of AMI (20 micrograms/ml) on Vmax and on conduction velocity (theta) during longitudinal (LP) and transverse (TP) propagation with respect to fiber orientation, in 10 strips of uniform anisotropic epicardial muscle obtained from the left ventricle of adult canine hearts. Mean values +/- SEM (standard error of the mean) were calculated as normalized values (beat 50/beat 1) after 4 h of AMI superfusion at five different basic cycle lengths (BCL). Vmax decreased from 0.99 +/- 0.01 at a BCL of 5,000 ms to 0.43 +/- 0.03 at a BCL of 300 ms during LP. During TP, Vmax decreased from 0.99 +/- 0.01 at a BCL of 5,000 ms to 0.54 +/- 0.05 at a BCL of 300 ms. The differences in the relative changes between both directions at a BCL of 300 ms, as well as at intermediate values of 1,000, 500, and 400, were significant (p less than 0.01). theta during LP (theta L) was depressed from 0.99 +/- 0.01 at a BCL of 5,000 ms to 0.80 +/- 0.04 at a BCL of 300 ms. In contrast, theta during TP (theta T) did not change as the BCL was decreased. In consequence, theta L was significantly more depressed than theta T at BCLs shorter than 1,000 ms (p less than 0.05). Moreover, theta T after AMI was not statistically different from control at any BCL studied. The lack of depression of theta T associated with a marked depression of Vmax during either LP or TP suggests that in addition to its sodium channel blocking properties, AMI could produce a decrease in the effective axial resistivity.

Effects of lidocaine on Vmax and conduction velocity in uniform anisotropic canine ventricular muscle: possible role of its binding-rate constants

We evaluated the effect of lidocaine (5 micrograms/ml) on Vmax and conduction velocity during longitudinal and transverse propagation to fiber orientation in strips of uniform anisotropic ventricular muscle from adult canine hearts. Tissues were markedly anisotropic, with conduction velocities 3.2 times faster during longitudinal propagation than during transverse propagation to the long axis of the fibers, and with the greatest values of Vmax associated with the slowest conduction velocities. After addition of lidocaine, Vmax (normalized values with respect to control for each cycle length, expressed as mean +/- SEM) decreased from 0.99 +/- 0.02 at a cycle length of 1,000 ms to 0.86 +/- 0.02 at a cycle length of 300 ms during longitudinal propagation. During transverse propagation, Vmax decreased from 0.99 +/- 0.03 at a cycle length of 1,000 ms to 0.87 +/- 0.03 at a cycle length of 300 ms. The differences in the relative changes between both directions at these cycle lengths, as well as with intermediate values of 500, 400, and 350 ms, were not significant. Similar results were obtained for conduction velocity. We conclude from these findings that under these experimental conditions the effects of lidocaine are characterized by a relative change both in Vmax and conduction velocity that is almost the same during longitudinal and transverse propagation at all cycle lengths explored. Moreover, the rapid binding-rate constants reported for lidocaine may play a significant role in determining the characteristics of Vmax and conduction velocity depressions in both directions.

Effect of amiodarone on ventricular fibrillation and defibrillation thresholds in the canine heart under normal and ischemic conditions

The main goal of this project was to study the effect of amiodarone upon ventricular fibrillation and defibrillation thresholds (VFT and VDT) in the canine heart under normal and ischemic conditions. Both parameters were assessed in 11 dogs, each experiment consisting of three consecutive phases: (a) control, which resulted in VFT = 27.9 (S.D. = 15.5) and VDT = 43.9 (S.D. = 4.5); (b) drug, with VFT = 63.5 (S.D. = 32.8) and VDT = 57.8 (S.D. = 11.3), and (c) coronary occlusion, with VFT = 44.7 (S.D. = 21.6) and VDT = 57.1 (S.D. = 11.0). These values are overall means scaled to ventricular weight (microA/g for VFT and mA/g for VDT). Both VFT and VDT in (b) and (c) were, on the average, greater than in (a) and these differences were statistically significant (P less than 0.01, paired t-test). The animals were kept normothermic (37.3 degrees C, S.D. = 0.6) within normal values for the acid-base state. We concluded that amiodarone increased the VFT significantly. Similarly, it increased VDT. However, although the latter change was statistically significant, we believe it would not be important from a physiological or clinical point of view.

The occurrence of phase-4 block in the anomalous bundle of patients with Wolff-Parkinson-White syndrome

Conduction in the Kent bundle was studied in 17 patients with WPW syndrome, before and after administration of ajmaline and amiodarone. Before drug administration, only one patient showed phase-3 block and only one other showed phase-4 block. After administration of ajmaline to 11 patients and amiodarone to 16 patients (each patient received at least one of the drugs and the two drugs were given independently to 10 patients), the occurrence of phase-3 block alone in the Kent bundle was documented in 9 patients of the 17, phase-4 block alone in 1, and phase-3 and phase-4 block together with a typical accodion in 3. In 4 patients conduction in the Kent bundle was not modified. The phase-3 and phase-4 block as well as the accordion effect in the Kent bundle were similar to the same phenomena previously described in patients with diseased or in dogs with injured intraventricular conducting fascicles. The fact that the Kent bundle can show the same physiological and pharmacological behavior as a diseased His bundle or bundle branches is highly significant, first, because it may indicate that the anomalous bundle is also composed of Purkinje or Purkinje-like fibers and, secondly, because it may help to distinguish a normal from a diseased Kent bundle, and this may become essential for determining the prognosis and treatment of patients with WPW syndrome. The Kent bundle can be compared with a normal or diseased extra-His bundle.