Abstract
OBJECTIVE
The Lewis lead is an EGC configuration used to detect atrial activity and its relationship with ventricular activity. However, no equivalent configuration has been proposed in veterinary medicine. Therefore, this study explored new bipolar lead configurations to evaluate atrial depolarization and investigated their characteristics by comparing them with conventional leads. The authors hypothesized that a lead with a high absolute value of the P wave (|P|) and a high ratio of the P wave to the QRS complex (|P|/|QRS|) might be an appropriate configuration.
ANIMALS
6 laboratory Beagles were used.
PROCEDURES
Electrocardiograms were recorded using bipolar limb leads (I, II, III, aVL, aVF, and aVR) and unipolar precordial leads (C2, C3, C4, C5, C6, M1, M2, M5, M6, 1st-R, CV6LL, and V10) in the standing position. The new bipolar leads were attached in the following configuration: the negative electrode was attached to M6, and the positive electrode was sequentially applied to M1 and M2. The leads were named M6M1 and M6M2.
RESULTS
The waveforms obtained using the new bipolar leads M6M1 and M6M2 showed significantly higher |P|/|QRS| and |P| values than those obtained using conventional limb leads and precordial leads.
CLINICAL RELEVANCE
The new leads achieved a specific enhancement of atrial activity conducted at a reduced ventricular amplitude and increased atrial amplitude, similar to the Lewis lead in human medicine. These findings suggest that M6M1 and M6M2 could be supplemental lead configurations to identify P waves without amplifying the QRS waves compared to conventional methods.
Accurate diagnosis of supraventricular arrhythmias, such as atrial fibrillation (AF), atrial flutter (AFL), and atrial tachycardia (AT), is challenging in clinical practice.1–3 In AF, chaotic activation is present on an ECG without regular P waves, irregular R-R intervals, and fibrillatory waves. On the other hand, AFL and AT ventricular cycles can be regular R-R intervals, and in AFL, sawtooth flutter waves are commonly observed on ECG.4 However, ventricular cycles are sometimes not regular due to a block in the atrioventricular node, and the surface ECG can mimic AF. Consequently, differentiating between AF and AFL/AT can be challenging in the ECG-based diagnosis of atrial arrhythmias, especially when observing brief periods of surface ECG.5
In human medicine, the Lewis lead, described by Sir Thomas Lewis,6 is an ECG configuration for identifying atrial depolarization waves in patients with AF and AFL by repositioning the limb leads to the precordial region. The lead system has also been used to detect atrial activity and its relationship with ventricular activity and, recently, to recognize P waves during wide and narrow QRS tachycardia.7–9 Also in veterinary practice, identifying the P waves is essential for differentiating between various types of supraventricular tachycardia, such as AFL, AT, orthodromic atrioventricular reciprocating tachycardia, and sinus tachycardia.10 However, there has not been an equivalent method to the Lewis lead proposed in veterinary medicine, and the Lewis lead configuration described in humans cannot be directly translated to dogs due to differences in the anatomical and electrical axes of the heart across species.11,12
Therefore, this study proposes new bipolar lead configurations to specifically evaluate atrial depolarization in dogs and investigate their characteristics by comparing them with conventional leads. Furthermore, the authors hypothesized that a lead depicting (1) the high absolute value of the P wave (|P|) and (2) the high ratio of the P wave to the QRS complex (|P|/|QRS|), maximal P-wave ratio and minimal QRS complex ratio, could be a functional lead system to specifically detect atrial activity.
The authors also hypothesized that P-wave amplitude enhancement could be obtained by attaching the electrodes to the following locations: the negative electrode is attached to where the P-wave polarity is negative, and the positive electrode is attached to where the P-wave polarity is positive. Takahashi11 conducted an in-depth study on dogs, recording EGC waveforms from 160 locations across the thorax. By utilizing x-rays to verify the lead placements, several waveforms with distinct EGC characteristics were identified. Additionally, potential distribution maps derived from the surface waveforms were examined to analyze the heart’s electrical longitudinal axis. Furthermore, the correspondence between the selected waveform lead positions and the anatomical heart location was studied using frozen specimens and model specimens and by labeling directions on the ventricular septum. As a result of this thorough research, Takahashi proposed both chest unipolar lead placements (C1 to C6) and supplementary lead placements (M1 to M6). In our study, M6 was chosen as the negative electrode while M1 and M2 were selected as positive electrodes to enhance the P-wave amplitude. We then compared these leads with conventional leads from previous literature,13–15 which include standard limb leads and unipolar precordial leads.
Methods
Animals
This study used 6 laboratory Beagles (intact males, 10.8 ± 0.8 kg, 70 ± 0 months old). All dogs underwent a physical examination and blood tests (determination of CBC, total protein, albumin, BUN, creatinine, total bilirubin, GGT, AST, ALT, ALP, lipase, sodium, potassium, chloride, calcium, phosphorus, and magnesium levels) to ensure their clinical health. The Laboratory Animal Experimentation Committee at Azabu University approved the study procedure (approval No. 210216-1).
ECG measurements
The procedures of ECG were performed in a nonsedated and standing position. A single observer (KK) recorded and evaluated all the ECG procedures. A commercial 12-lead ECG was used, and waves were measured using a digital caliper with a sweep speed of 50 mm/s. Electrodes were placed on the skin surface using animal ECG clips, and ECG paste was applied to maintain electrical contact with the skin. The ECG unit was set with a sampling frequency of 500 Hz for acquisition, a 50-Hz low-pass filter, a 30-Hz muscle filter, and a 0.36-Hz high-pass filter. Before starting the ECG recordings, we first verified that both the ECG waveforms and the sinus rhythm were within normal parameters.
For each lead, ECG variables were measured from 3, randomly selected, consecutive heart beats. The mean electrical axis was determined in the frontal plane as follows16: the mean electrical axis = arctan (Iamp, aVFamp) X 180/π, where Iamp and aVFamp are the algebraic sums of the Q, R, and S waves for leads I and aVF, respectively. Other standard ECG variables (heart rate, QRS complex duration, P-wave duration, PQ interval duration, QT interval duration, and T-wave amplitude) were evaluated for lead II. P-, Q-, R-, and S-wave amplitudes were measured for each lead. The absolute value of the P wave (|P|), the magnitude of the QRS complex (|QRS|), and the P wave-to-QRS complex ratio (|P|/|QRS) were calculated and used for the analysis. The absolute value of the P wave was calculated by removing any negative sign in front of the P-wave amplitude. The magnitude of the QRS complex was determined by combining the R-wave amplitude with the larger of the Q- or S-wave amplitudes. Additionally, the P-wave ratio to the QRS complex was defined as the absolute value of the P-wave ratio to the magnitude of the QRS complex. The QRS complex morphological pattern was assessed in italics using upper- and lowercase letters when the wave’s amplitude was ≥ 0.5 mV (ie, Q, R, S) or < 0.5 mV (ie, q, r, s), respectively.17,18 In this study, the new method values were compared with the conventional method values.
ECG electrode positioning
The conventional methods were recorded using bipolar limb leads (I, II, and III) and augmented unipolar leads (aVL, aVF, and aVR). The unipolar precordial leads (C2, C3, C4, C5, C6, M1, M2, M5, M6, 1st-R, CV6LL, and V10) were selected from previous studies11,14,15 and placed on the body surface and are shown (Table 1). The following procedure was performed for the new bipolar lead configurations from Takahashi’s unipolar precordial ECG region (Figure 1). First, the negative electrode was attached to M6. Then, the positive electrode was sequentially applied to M1 and M2. We carried out the measurements across 2 distinct sessions. In the first session, 6 unipolar electrodes were attached to the precordial region, and 4 bipolar electrodes served as the standard limb leads. During the subsequent session, we affixed 6 unipolar electrodes to the remaining 6 precordial regions and adjusted the positioning of the bipolar electrodes to implement the new bipolar method. Each recording session lasted for at least 1 uninterrupted minute.
The electrode locations for unipolar precordial leads.
| Electrode position | |||
|---|---|---|---|
| Lead | Left or right | Intercostal space | Level |
| M1 | Left | Third | The widest portion of the thorax |
| M2 | Left | Sixth | The widest portion of the thorax |
| M5 | Right | Seventh | The widest portion of the thorax |
| M6 | Right | Third | The widest portion of the thorax |
| C2 | Left | Second | The costochondral junction |
| C3 | Left | Fifth | The costochondral junction |
| C4 | Right | Seventh | The costochondral junction |
| C5 | Right | Fifth | The costochondral junction |
| C6 | Right | Third | The costochondral junction |
| 1st-R | Right | First | The costochondral junction |
| CV6LL | Left | Sixth | The sternal border |
| V10 | Dorsally on the spinous process of the seventh thoracic vertebra | ||
New bipolar lead configurations were developed by selecting 1 negative electrode (M6) and 1 positive electrode (M1 and M2) from the unipolar lead configurations. Then, 2 bipolar leads were named from the above electrodes: M6M1 and M6M2, respectively.
Conventional lead configurations (A) and new bipolar lead configurations (B). In the new method, the negative electrode was located at M6 (the right third intercostal space at the level of the widest portion of the thorax). Positive electrodes were located at M1 (the left third intercostal space at the level of the widest portion of the thorax) and M2 (the left sixth intercostal space at the widest portion of the thorax). This illustration was modified from a previous report (Takahashi M. Experimental studies on the electrocardiogram of the dog. Nihon Juigaku Zasshi. 1964;26(4):191–210).
Citation: American Journal of Veterinary Research 85, 1; 10.2460/ajvr.23.07.0151
Statistical analysis
Statistical analysis was performed using R version 4.0.4. The Shapiro-Wilk test was used to assess the data distribution of each continuous variable for normality. The data were expressed as the mean ± SD to summarize the results for normally distributed continuous variables, and the median [IQR] was used to summarize the results for nonnormally distributed continuous variables. In addition, if the data followed a normal distribution, a repeated measures ANOVA and Dunnett test were used to compare the values between the new and conventional methods. If the data did not meet the assumption, the Friedman test and Steel test were employed for the comparison. The difference was considered significant at a P value of < .050. Finally, the intraclass correlation coefficient, specifically the case 1 model, was employed in this study to evaluate the intrarater reliability of the measured values. The strength of the reliability was rated as poor (0.00 to 0.50), moderate (0.5 to 0.75), good (0.76 to 0.90), or excellent (0.91 to 1.00). A power analysis was conducted for our t test to determine the appropriate sample size for analyzing the |P|/|QRS| ratio derived from our preliminary experiment. Based on this analysis, approximately 5.09 dogs are needed in each group to detect a mean difference of 0.10, given an SD of 0.05, a significance level of 0.05, and a power of 0.80.
Results
The value of the intraclass correlation coefficient was > 0.75 for most of the leads; only the P wave of the 1st-R had moderate reliability, with a value of 0.74. The test reliability was considered sufficient for the analysis (Table 2).
Intraclass correlation coefficients for P-, Q-, R-, and S-wave amplitude.
| Lead | P wave | Q wave | R wave | S wave |
|---|---|---|---|---|
| M6M1 | 0.91 | 0.97 | 1.00 | 1.00 |
| M6M2 | 0.91 | 0.89 | 1.00 | 0.97 |
| I | 0.89 | 0.98 | 0.99 | 0.99 |
| II | 0.94 | 0.98 | 0.99 | 0.99 |
| III | 0.92 | 1.00 | 0.99 | 0.96 |
| aVL | 0.91 | 0.99 | 0.99 | 0.98 |
| aVF | 0.91 | 0.98 | 0.98 | 0.97 |
| aVR | 0.83 | NA | 0.96 | 1.00 |
| M1 | 0.81 | 0.97 | 0.99 | 0.98 |
| M2 | 0.81 | 0.99 | 0.99 | NA |
| M5 | 0.82 | 1.00 | 0.99 | 0.96 |
| M6 | 0.93 | 1.00 | 0.98 | 1.00 |
| C2 | 0.78 | 0.96 | 1.00 | 1.00 |
| C3 | 0.94 | 0.99 | 1.00 | 1.00 |
| C4 | 0.90 | 0.99 | 0.98 | 0.99 |
| C5 | 0.90 | NA | 0.98 | 0.99 |
| C6 | 0.96 | NA | 0.99 | 0.99 |
| 1st-R | 0.74 | NA | 1.00 | 0.99 |
| CV6LL | 0.89 | 0.99 | 0.98 | 0.99 |
| V10 | 0.99 | 0.99 | 0.99 | NA |
aVL, aVF, and aVRI = Unipolar leads. I, II, and III = Bipolar limb leads. NA = Intraclass correlation coefficients could not be calculated as values were 0 for all dogs.
Standard ECG variables were as follows: heart rate was 93 ± 14 beats/min, QRS complex duration was 52 ± 5 ms, P-wave duration was 48 ± 2 ms, PQ interval duration was 107 ± 7 ms, QT interval duration was 229 ± 18 ms, T-wave amplitude was −0.20 ± 0.40 mV, and mean electrical axis was 51 ± 11°.
The values of the P-, Q-, R-, and S-wave amplitudes for each lead are shown (Table 3). For each of the P, Q, R, and S waves, significant differences were confirmed across the groups with a P value of < .050 before post hoc tests (P < .001, < .001, < .001, and < .001, respectively). The P-wave amplitude tends to be higher in M6M1 compared to conventional leads, except for lead I. In M6M2, it was higher compared to conventional leads with the exception of leads I, II, aVF, and C3. The polarity of the P waves was negative in aVR, M6, C6, and 1st-R. The morphology of the QRS complexes was as follows: qR pattern in M6M1, I, II, M1, and M2; qRs pattern in M6M2, III, aVF, M5, C3, and CV6LL; qr pattern in aVL; Qr pattern in M6; rS pattern in aVR and 1st-R; Rs pattern in C2, C4, and C5; and RS pattern in C6. The values of |P|, |QRS|, and |P|/|QRS| are presented (Table 4). For each of the |P|, |QRS|, and |P|/|QRS|, significant differences were confirmed across the groups with a P value of < .050 before post hoc tests (P < .001, < .001, and < .001, respectively). The values of |P| in the new bipolar leads were significantly higher than those in the conventional leads (except for leads I, II, and aVR in M6M1 and leads I, II, aVR, and C3 in M6M2). The values of |QRS| were higher in leads II and C3 (vs M6M1) and lower in aVL, M5, and M6 (vs M6M2). The values of |P|/|QRS| in M6M1 were significantly higher than those of all conventional leads except M6, while the value of |P|/|QRS| in M6M2 was higher than that in C5, C6, and CV6LL.
P-, Q-, R-, and S-wave amplitude (mV) for M6M1, M6M2, and conventional leads.
| P wave | Q wave | R wave | S wave | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| P values vs | P values vs | P values vs | P values vs | |||||||||
| Lead | Values | M6M1 | M6M2 | Values | M6M1 | M6M2 | Values | M6M1 | M6M2 | Values | M6M1 | M6M2 |
| New bipolar lead configurations | ||||||||||||
| M6M1 | 0.28 ± 0.05 | – | – | 0.08 [0.09] | – | – | 1.50 [0.60] | – | – | 0.00 [0.05] | – | – |
| M6M2 | 0.27 ± 0.05 | 1.000 | – | 0.07 [0.10] | 1.000 | – | 1.78 [0.65] | .872 | – | 0.11 [0.18] | .968 | – |
| Conventional lead configurations | ||||||||||||
| I | 0.19 ± 0.04 | .038 | .119 | 0.36 [0.22] | .082 | .153 | 1.36 [0.79] | .999 | .891 | 0.00 [0.00] | .999 | .731 |
| II | 0.24 ± 0.08 | .877 | .997 | 0.46 [0.22] | < .001 | < .001 | 2.37 [0.81] | .005 | .724 | 0.00 [0.11] | 1.000 | .994 |
| III | 0.12 ± 0.05 | < .001 | < .001 | 0.17 [0.40] | .999 | .999 | 1.13 [0.13] | .929 | .076 | 0.23 [0.35] | .703 | .996 |
| aVL | 0.09 ± 0.04 | < .001 | < .001 | 0.17 [0.42] | .999 | .999 | 0.49 [0.31] | < .001 | < .001 | 0.00 [0.13] | 1.000 | .999 |
| aVF | 0.18 ± 0.04 | .015 | .054 | 0.27 [0.35] | .046 | .173 | 1.71 [0.48] | .592 | 1.000 | 0.09 [0.24] | .989 | 1.000 |
| aVR | 0.20 ± 0.05 | < .001 | < .001 | 0.00 [0.00] | .68 | .181 | 0.41 [0.18] | < .001 | < .001 | 1.79 [0.80] | < .001 | < .001 |
| M1 | 0.15 ± 0.05 | < .001 | .002 | 0.37 [0.16] | < .001 | < .001 | 1.04 [0.18] | .949 | .005 | 0.00 [0.00] | .769 | .185 |
| M2 | 0.15 ± 0.05 | < .001 | .002 | 0.24 [0.18] | .046 | < .001 | 1.23 [0.24] | .999 | .544 | 0.00 [0.06] | 1.000 | .822 |
| M5 | 0.05 ± 0.03 | < .001 | < .001 | 0.12 [0.27] | .992 | .941 | 0.61 [0.29] | .020 | < .001 | 0.07 [0.12] | .994 | .999 |
| M6 | 0.15 ± 0.07 | < .001 | < .001 | 0.71 [0.35] | .302 | .323 | 0.05 [0.19] | < .001 | < .001 | 0.00 [0.00] | .999 | .932 |
| C2 | 0.09 ± 0.03 | < .001 | < .001 | 0.00 [0.00] | .461 | .375 | 1.03 [0.35] | .988 | .418 | 0.19 [0.65] | .703 | .996 |
| C3 | 0.18 ± 0.08 | .015 | .054 | 0.12 [0.08] | .982 | .941 | 2.48 [1.30] | .304 | .987 | 0.06 [0.14] | .998 | .999 |
| C4 | 0.08 ± 0.04 | < .001 | < .001 | 0.00 [0.03] | .969 | .978 | 0.85 [0.21] | .213 | < .001 | 0.14 [0.05] | .658 | .999 |
| C5 | 0.05 ± 0.06 | < .001 | < .001 | 0.00 [0.00] | .169 | .181 | 0.77 [0.25] | .071 | .006 | 0.48 [0.38] | < .001 | < .001 |
| C6 | 0.09 ± 0.06 | < .001 | < .001 | 0.00 [0.00] | .169 | .181 | 0.87 [0.37] | .304 | < .001 | 1.54 [0.45] | < .001 | < .001 |
| 1st-R | 0.12 ± 0.04 | < .001 | < .001 | 0.00 [0.00] | .169 | .181 | 0.29 [0.19] | < .001 | < .001 | 1.45 [0.11] | < .001 | < .001 |
| CV6LL | 0.13 ± 0.03 | < .001 | < .001 | 0.05 [0.14] | 1.000 | 1.000 | 2.24 [1.13] | .424 | 0.998 | 0.20 [0.11] | .303 | .863 |
| V10 | 0.02 ± 0.08 | < .001 | < .001 | 0.83 [0.27] | < .001 | < .001 | 0.34 [0.24] | < .001 | < .001 | 0.00 [0.00] | .769 | .184 |
Data are presented as mean ± SD for normally distributed continuous variables and as median [IQR] for nonnormally distributed continuous variables. P values: the Dunnett test was applied to normally distributed continuous variables, while the Steel test was used for those not normally distributed.
Values of |P|, |QRS|, and |P|/|QRS| for M6M1, M6M2, and conventional leads.
| |P| | |QRS| | |P|/|QRS| | |||||||
|---|---|---|---|---|---|---|---|---|---|
| P values vs | P values vs | P values vs | |||||||
| Lead | Values | M6M1 | M6M2 | Values | M6M1 | M6M2 | Values | M6M1 | M6M2 |
| New bipolar lead configurations | |||||||||
| M6M1 | 0.28 ± 0.05 | – | – | 1.52 ± 0.53 | – | – | 0.20 ± 0.06 | – | – |
| M6M2 | 0.27 ± 0.05 | 1.000 | – | 2.00 ± 0.70 | 0.616 | – | 0.14 ± 0.03 | 0.075 | – |
| Conventional lead configurations | |||||||||
| I | 0.19 ± 0.04 | .015 | .060 | 1.84 ± 0.71 | .961 | 1.000 | 0.11 ± 0.03 | .001 | .858 |
| II | 0.24 ± 0.08 | .785 | .990 | 2.96 ± 0.63 | < .001 | .017 | 0.08 ± 0.02 | < .001 | .080 |
| III | 0.12 ± 0.05 | < .01 | < .001 | 1.52 ± 0.18 | 1.000 | .613 | 0.08 ± 0.03 | < .001 | .042 |
| aVL | 0.09 ± 0.04 | < .001 | < .001 | 0.79 ± 0.33 | .138 | < .001 | 0.12 ± 0.04 | .002 | .929 |
| aVF | 0.18 ± 0.04 | .005 | .022 | 2.12 ± 0.31 | .322 | 1.000 | 0.09 ± 0.03 | < .001 | .135 |
| aVR | 0.20 ± 0.05 | .035 | .127 | 2.33 ± 0.67 | .065 | .953 | 0.09 ± 0.02 | < .001 | .159 |
| M1 | 0.15 ± 0.05 | < .001 | < .001 | 1.47 ± 0.25 | 1.000 | .493 | 0.10 ± 0.03 | < .001 | .484 |
| M2 | 0.15 ± 0.05 | < .001 | < .001 | 1.57 ± 0.26 | 1.000 | .741 | 0.10 ± 0.04 | < .001 | .348 |
| M5 | 0.06 ± 0.02 | < .001 | < .001 | 0.86 ± 0.35 | .228 | .002 | 0.08 ± 0.03 | < .001 | .059 |
| M6 | 0.15 ± 0.07 | < .001 | < .001 | 0.92 ± 0.41 | .332 | .004 | 0.18 ± 0.08 | .989 | .516 |
| C2 | 0.09 ± 0.03 | < .001 | < .001 | 1.54 ± 0.68 | 1.000 | .668 | 0.06 ± 0.02 | < .001 | .006 |
| C3 | 0.18 ± 0.08 | .005 | .022 | 2.67 ± 0.83 | .002 | .216 | 0.07 ± 0.01 | < .001 | .010 |
| C4 | 0.08 ± 0.04 | < .001 | < .001 | 1.04 ± 0.30 | .632 | .016 | 0.10 ± 0.07 | < .001 | .318 |
| C5 | 0.07 ± 0.02 | < .001 | < .001 | 1.51 ± 0.56 | 1.000 | .596 | 0.05 ± 0.03 | < .001 | .001 |
| C6 | 0.10 ± 0.04 | < .001 | < .001 | 2.34 ± 0.21 | .063 | .947 | 0.04 ± 0.01 | < .001 | < .001 |
| 1st-R | 0.12 ± 0.04 | < .001 | < .001 | 1.76 ± 0.40 | .998 | .997 | 0.08 ± 0.04 | < .001 | .034 |
| CV6LL | 0.14 ± 0.04 | < .001 | < .001 | 2.40 ± 0.53 | .035 | .836 | 0.06 ± 0.00 | < .001 | .002 |
| V10 | 0.08 ± 0.02 | < .001 | < .001 | 1.19 ± 0.30 | .954 | .066 | 0.07 ± 0.03 | < .001 | .030 |
The data are expressed as the mean ± SD. P values were derived from the Dunnett test.
|P| = Absolute value of the P wave. |P|/|QRS| = P wave-to-QRS complex ratio. |QRS| = Magnitude of the QRS complex.
Discussion
The new bipolar leads M6M1 and M6M2 enhanced the P wave without amplifying the QRS complex. This finding suggests that these methods are functional in specifically identifying atrial activity during normal sinus rhythm in healthy dogs.
Over the years, several precordial lead methods have been proposed in veterinary medicine. In 1964, Takahashi11 first proposed a 12-lead ECG system for dogs by correlating electrical potentials at the heart surface with an ECG pattern derived from a lead on the body surface. The report11 proposed lead positions as C1 to C6 to the intercostal space and M1 to M6 at the level of the thorax’s widest portion or the xiphoid position. It also reported that negative P waves were observed at C1 and M6, similar to this study’s results. Next, Detweiler and Patterson13 proposed the modified Lannek system. In this system, the precordial leads were CV6LU (at the level of the costochondral junction in the left sixth intercostal space), CV6LL, CV5RL (the fifth right intercostal space at the sternal border), and V10. They demonstrated the P wave was positive in most leads except for negative waves in 38.6% of V10 and 10.0% of CV5RL. The present study’s results are consistent with those of a previous report; tiny P waves were observed in C6 (close to CV5RL). Recently, Kraus et al14 modified the Wilson precordial lead system in humans for dogs. Santilli et al15 developed the system to adapt to different thoracic conformations among dog breeds: the report suggested that negative P waves were shown at 1st-R, which was the case in the present study.
Previous studies19,20 using vector ECGs have shown that the electrical directions of the P and QRS loops are similar. Hence, the present study’s results did not appear to be due to electrical vector differences between the P and QRS waves. In essence, the authors assumed that the relationships between the anatomical positions of the heart and thorax and the distance between the electrodes from the heart surface affected the results. Moreover, a previous study15 with various thoracic formations has shown that the closer the ECG electrodes are to the basal heart or the narrower the thoracic structure, the higher the P waves tend to be.
In human medicine, the coexistence of AF and bundle branch block on an ECG can be diagnostically challenging. This is because the resulting wide QRS complexes can mimic the appearance of ventricular tachycardia.4 Using Lewis leads to explore the precordium can be instrumental in identifying P waves or magnifying fibrillation waves (f waves), which is essential for differential diagnosis. This method involves repositioning the standard ECG’s right and left arm electrodes along the sternal borders on the chest wall, while monitoring lead I to enhance the detection of P-wave activity.21 Furthermore, discerning ventriculoatrial dissociation in tachycardias with wide QRS complexes is among the most reliable criteria for distinguishing the tachycardia’s origin. Indeed, the Lewis lead has proven invaluable in conclusively diagnosing ventricular tachycardia and in visualizing fibrillation waves in real-world clinical scenarios involving humans.7–9,22–24
The present study shows that M6M1 and M6M2 exhibit a high atrial-to-ventricular amplitude ratio; the enhancement of atrial activity is achieved at a reduced ventricular amplitude and increased atrial amplitude. This finding is similar to a previous study22 of the modified Lewis lead system; hence, M6M1 and M6M2 could specifically observe atrial activity as well. Similarly, M6 had a high ratio of P waves, but because both P and QRS waves were lowered at this electrode, it was not considered helpful for detecting P waves. Furthermore, these bipolar lead configurations can be used with a standard 6-lead ECG by relocating the negative electrode (the right forelimb electrode) to M6 and the positive electrodes (the left forelimb electrode and the left hind limb electrode) to M1 and M2, respectively. Further analysis may allow this method to be widely applied in various scenes of the veterinary field, from general practice to emergency and critical care.
The present study had several limitations. First, this was an experimental study with healthy Beagles. As a result, it remains uncertain whether similar findings would apply to other breeds in a clinical setting, especially given that ECG results can vary based on thoracic morphology.15,25 Second, the study was conducted with the dogs in the standing position. Body position can potentially influence the electrical axis,16 and using precordial unipolar leads in the standing position might pose challenges in detecting P waves.26 Bearing these considerations in mind, further research is necessary to provide more comparative data on different body positions using our new bipolar lead configurations, which emphasize the clear identification of P waves. Additionally, the measurements were not blinded with respect to the lead system. While the enhancement of the P wave in this study aimed to maximize regular P-wave activity, it remains uncertain if similar findings would be observed in cases of AF, AFL, and AT. It is also worth noting that, even in dogs with normal cardiac status, the effects of sinus arrhythmia and a wandering pacemaker should be considered. Thus, further data from actual clinical cases are essential.
In conclusion, the waveforms obtained using the new bipolar leads, especially M6M1, showed higher values of |P|/|QRS| and |P| than those obtained using the conventional leads. This study’s findings suggest that M6M1 and M6M2 might be supplemental lead configurations for identifying P waves without amplifying the QRS complex.
Acknowledgments
The illustration was created by Mayu Okumura, a JASMINE Veterinary Cardiovascular Medical Center staff member, to whom we express our gratitude. We also thank Editage (www.editage.com) for the English language editing.
Disclosures
The progress of this study was presented at the 2021 ACVIM Forum.
No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
References
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