Open Veterinary Journal, (2026), Vol. 16(4): 2164-2179

Original Research Article

10.5455/OVJ.2026.v16.i4.19

A 12-lead or 10-lead electrocardiogram in cats? An analysis comparing 6-lead and 4-lead precordial systems

Laura Gonçalves Nascimento^1*, Mariana de Resende Coelho¹, Diego Ribeiro³, Isa Lúcia Sousa Resende², Fernando Yoiti Kitamura Kawamoto¹, Luiz Eduardo Duarte de Oliveira⁴, Jullia de Almeida Lima⁴ and Claudine Botelho de Abreu¹

¹Veterinary Clinics Complex, University Center of Lavras (UNILAVRAS), Lavras, Brazil

²Department of Veterinary Clinics, Faculty of Veterinary Medicine and Animal Science, Federal University of Lavras (UFLA), Lavras, Brazil

³Department of Veterinary Clinics, School of Veterinary Medicine and Animal Science, São Paulo State University (UNESP), Botucatu, Brazil

⁴Department of Veterinary Clinics, School of Veterinary Medicine, Federal University of Minas Gerais, Minas Gerais, Brazil

*Corresponding Author: Laura Gonçalves Nascimento. Postgraduate Student in Feline Medicine, Instituto PAV, São Paulo, Brazil. Email: lauragoncalvs [at] outlook.com

Submitted: 15/11/2025 Revised: 25/02/2026 Accepted: 09/03/2026 Published: 30/04/2026

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ABSTRACT

Background: Electrocardiography is widely used in feline cardiology, but precordial lead placement remains poorly standardized for this species. Although six-lead precordial systems have been validated in dogs, their applicability to cats remains uncertain. This is due to the species’ distinct thoracic conformation and the potential stress associated with electrode placement. A simplified precordial arrangement may improve patient comfort, but its diagnostic equivalence has not been fully investigated.

Aim: To compare six (6P) and four (4P) precordial lead systems in feline electrocardiography and evaluate their clinical applicability, impact on wave measurements, and patient tolerance.

Methods: Forty healthy, non-sedated adult cats were included. All animals underwent two electrocardiographic examinations in right lateral recumbency. The 6P system was applied as standardized in dogs. The 4P system consisted of one electrode positioned in the first right intercostal space and three electrodes spaced proportionally in the sixth left intercostal space. Amplitudes and durations of P, R, S, and T waves were recorded and compared within and between systems. Associations with demographic variables were assessed. Observational assessment of patient tolerance was conducted during both examinations.

Results: Wave morphology varied slightly between systems. R-wave amplitude was consistently higher in the 6P system and showed a progressive decrease from V2 toward caudodorsal leads. The 4P system produced slightly longer P-wave durations. In contrast, R-, S-, and T-wave amplitudes tended to be higher in the 6P recordings. No significant associations were identified between electrocardiographic measurements and age, sex, or body weight. Cats demonstrated visibly greater tolerance to the 4P system, with fewer attempts to reposition or resist handling.

Conclusion: Both the 6P and 4P precordial lead systems provide reliable electrocardiographic tracings in cats and can be used in clinical practice. However, the improved patient tolerance observed with the 4P arrangement suggests that it may be preferable in routine examinations. This approach maintains diagnostic usefulness while enhancing feline comfort.

Keywords: Cardiology, Electrocardiography, Felines, Thorax.

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Introduction

The electrocardiogram (ECG) is used to identify and quantify cardiac arrhythmias. It is also useful for assessing heart rate variability, an important marker of cardiovascular diseases (Walker et al., 2022). Cardiac electrical activity is recorded using a lead system. Frontal or limb leads record the electrical activity in the frontal plane (Santilli et al., 2018). Precordial leads record electrical activity in the horizontal plane from different locations on the thoracic wall (Santilli et al., 2019). They complement frontal lead analysis by indicating chamber enlargement, detecting conduction abnormalities and identification of P waves (Santilli et al., 2018).

Several precordial lead systems have been proposed for use in dogs (Lannek, 1949; Takahashi, 1964; Detweiler, 1993; Kraus et al., 2002; Santilli et al., 2019). In feline patients, the model most commonly employed adheres to the current ECG standards established for dogs (Santilli et al., 2019). Nevertheless, cats have a thoracic conformation that differs significantly from that of dogs. This raises questions regarding the applicability of existing precordial lead systems to cats (Santilli et al., 2019). Moreover, the restraint required for electrode placement can cause stress in cats. This may compromise the reliability of the analyses (Chen et al., 2024).

In feline patients, four precordial leads are typically employed, rather than the six used in dogs. However, there is no standardized precordial system for cats. Therefore, the aim of this study was to compare the use of six versus four precordial leads in feline electrocardiography and to establish reference intervals for this species. The hypothesis is that placing six electrodes results in overlapping precordial leads due to their close proximity on the small feline thorax. This would lead to no significant differences between them. In contrast, using four leads allows greater spacing between electrodes and may yield distinct values.

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Materials and Methods

Study design and inclusion criteria

The study included apparently healthy, non-sedated cats aged at least 1 year. Cats of various breeds, sexes, and weights were enrolled. Health status was assessed based on owner-reported history and cardiac auscultation. Exclusion criteria included the presence of a heart murmur and abnormal electrocardiographic findings. These included arrhythmias or alterations in the duration and/or amplitude of waves and intervals in lead II, according to Santilli et al. (2018).

Electrocardiographic analyses

Electrocardiographic recordings were obtained using a 12-lead veterinary electrocardiograph system^a. The cats were gently restrained in right lateral recumbency on an electrically insulated rubber mat. The limbs were extended and kept parallel to the body. Two examinations were performed, each lasting for 3 minutes. In both, four electrodes were placed on the limb leads. Two were positioned on the left and right humeroradioulnar joints, and two on the left and right femorotibiopatellar joints. For the first examination, to assess the precordial leads, electrode V1 was positioned in the first right intercostal space at the costochondral junction. The remaining electrodes were placed in the left sixth intercostal space. V2 was positioned near the sternum, and V4 at the costochondral junction. V3 was placed midway between V2 and V4. Electrodes V5 and V6 were placed dorsal to V4, maintaining proportional distances between V2–V3 and V3–V4 (Fig. 1A). For the second examination, electrodes V5 and V6 were removed. The following adjustments were made: V3 was positioned at the costochondral junction, and V4 was placed dorsal to it, maintaining the same distance as between V2 and V3 (Fig. 1B). A 70% alcohol solution was applied to each electrode to improve electrical conduction. An example of ECG tracings obtained from the same cat using both methods is shown in Fig. 2. Recordings were analyzed offline by an experienced observer.

Fig. 1. Six-lead (A) and four-lead (B) precordial placement in a apparently healthy and non-sedated cat. For further information, refer to the text.

Fig. 2. Electrocardiographic tracings obtained with six-lead (A) and four-lead (B) precordial systems from the same cat of Figure 1. Speed of 50 mm/s and gain of 10 mm/mV.

Statistical analysis

Statistical analyses were performed using GraphPad Prism^b (version 5.0). A p-value < 0.05 was considered statistically significant. Data normality was assessed with the Shapiro-Wilk test. Variables with a normal distribution were expressed as mean ± standard deviation. Variables without a normal distribution were expressed as median (25th–75th percentiles). Comparisons among precordial leads within each method (6P or 4P) were conducted using the Kruskal–Wallis test with Dunn's post hoc test and Bonferroni correction for multiple comparisons (Tables S1 and S2). Agreement between the two methods (6P versus 4P) was evaluated using Bland–Altman analysis. The mean difference and the 95% confidence interval (CI) were calculated. After ensuring that the differences were normally distributed, a one-sample t test was applied to determine whether the mean difference was significantly different from zero. Proportional bias was assessed using simple linear regression. Comparisons of precordial electrocardiographic variables between males and females were performed using an unpaired Student's t test or the Mann–Whitney U test for parametric and non-parametric data, respectively. Associations between demographic variables (age, sex, weight, and hair type) and electrocardiographic variables were assessed using Spearman’s rank correlation coefficient (r). Correlations were considered strong if r ≥ 0.70.

Intra- and interobserver variability were determined using the coefficient of variation. It was calculated as the standard deviation of the differences between paired measurements divided by the mean of the pooled measurements (de Bruyne et al., 1998). A coefficient of variation > 30% was considered indicative of high data dispersion and, consequently, high variability (Jekova et al., 2016). Because the Q and S waves were absent in most precordial leads, they were excluded from the comparison. The only exception is the S wave in lead V1 of both ECG systems. Intraobserver analysis was performed by a single experienced observer with a one-month interval between evaluations. Interobserver analysis was conducted through blinded assessments by two independent, experienced observers. Both analyses were performed in 20% of the animals, which were randomly selected. Reference intervals, along with 90% CI, were calculated using Reference Value Advisor® in Microsoft Excel, following the standard method (Geffré et al., 2011).

Ethical approval

All procedures involving animals were conducted in compliance with international guidelines for animal welfare. This study was reviewed and approved by the Animal Ethics Committee of University Center of Lavras (UNILAVRAS) under protocol no. 039/2023.

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Results

Patient inclusion

Of the 76 cats evaluated, 30 were excluded for the following reasons: increased wave duration and amplitude (20), QRS axis deviation (5), premature ventricular complexes (4), and poor-quality tracings (1). Consequently, 40 cats were included in the study, consisting of 20 females (50%) and 20 males (50%). All cats were mixed-breed, with a median age of 3.5 (2–4) years, and the median weight was 4.1 (4–5) kg. Of these, 29 (72.5%) cats had short hair and 11 (27.5%) had long hair.

Electrocardiographic findings

The electrocardiographic variables obtained from the standard lead II recording were as follows: P-wave duration (P_dur=34, 32–36 ms), P-wave amplitude (P_amp=0.1 ± 0.03 mV), and PQ interval (PQ_int=73.2 ± 9.33 ms), QRS duration (QRS_dur=38, 36–40 ms), R-wave amplitude (R_amp=0.33 ± 0.15 mV), QT interval (QT_int=156.4 ± 17 ms), corrected QT interval (QTc_int=172.1 ± 16 ms) and T-wave amplitude (T_amp=0.08 ± 0.04 mV). Amplitudes of the Q and S-waves (Q_amp and S_amp), as well as ST-segment deviation (ST_seg), were not observed. The T wave exhibited positive polarity in most animals (38/40).

Precordial leads, assessed using both the six-lead and four-lead methods, were reported as reference intervals with 90% CI (Tables 1 and 2). In both precordial systems (Fig. 3), P_amp was lowest in lead V1. The Q wave was typically absent in leads V1–V4, but variably present in V5 and V6. In the four-lead precordial system (4P), the Q wave was observed only in lead V4. In the six-lead precordial system (6P), the S wave was variably present in leads V1–V3. In the four-lead system (4P), the S wave was observed in leads V1 and V2.

R and T waves were present in all precordial leads in both systems. The lowest amplitudes were recorded in lead V1. All leads showed an R/S ratio > 1, except for lead V1. Most animals (36/40) showed an R/S < 1 in lead V1 in both systems.

In all cats, the P wave exhibited negative polarity in lead V1 and positive polarity in the remaining precordial leads. In most animals, the T wave showed negative polarity in V1 (6P=30/40; 4P=28/40) and positive polarity in the other leads. Wave durations and intervals did not differ significantly across leads within the same system.

Comparison between systems

Significant differences were found when comparing corresponding leads between the two systems (6P versus 4P) (Figs. 4 and 5). In lead V1, P_dur was longer in the 4P system (34, 32 to 36 ms) than in the 6P system (32, 28.5–34 ms) (p < 0.001). In lead V2, S_amp was larger in the 6P system (0.00, 0.00 to −0.08 mV) than in the 4P system (0.00, 0.00 to −0.10 mV) (p=0.041).

In lead V3, the 4P system showed longer P_dur (38, 32– 40 ms) and PQ_int (74.55 ± 10.53 ms) compared to the 6P system (P_dur=34, 30–40 ms; p=0.004) (PQ_int=71.80 ± 9.67 ms; p=0.037). Also in lead V3, the 6P system exhibited a higher R_amp (0.25 ± 0.12 mV) than the 4P system (0.22 ± 0.11 mV; p=0.003).

In lead V4, the Q_amp was lower in the 6P system (0.00, 0.00–0.00 mV) than in the 4P system (0.00, 0.00 to −0.05 mV) (p < 0.001). For the same lead, R_amp and T_amp were larger in the 6P system (R_amp=0.22 ± 0.11 mV; T_amp=0.07, 0.05–0.11 mV) than in the 4P system (R_amp=0.20 ± 0.09 mV; T_amp=0.05, 0.03–0.08 mV), with p-values of 0.030 and < 0.001, respectively.

Proportional bias was identified in the following variables: P_amp in lead V1 (p=0.040); Q_amp in leads V1 (p=0.027), V2 (p < 0.001), and V3 (p < 0.001); and S_amp in lead V4 (p < 0.001).

Comparison between sexes

In the comparison between sexes, P_dur was significantly longer in males than in females in leads V2, V3, and V4 of the 6P system (males: V2=37 ± 7 ms, V3=37 ± 6 ms, and V4=37 ± 6 ms; females: V2=32 ± 5 ms, V3=33 ± 5 ms, and V4=32 ± 5 ms), with corresponding p-values of 0.028, 0.017, and 0.021, respectively.

Table 1. Reference intervals with a 90% confidence interval* for the electrocardiographic precordial leads from V₁ to V₆ in 40 apparently healthy and non-sedated cats.

Table 2. Reference intervals with a 90% confidence interval* for the electrocardiographic precordial leads from V₁ to V₄ in 40 apparently healthy and non-sedated cats.

Fig. 3. Boxplot comparing of the P, Q, R, S, T waves amplitudes in the six-lead (A, C, E, G, I) and four-lead (B, D, F, H, J) precordial systems in 40 apparently healthy and non-sedated cats.

Fig. 4. Bland-Altman analyses of the differences of waves and intervals electrocardiographic duration between six-lead and four-lead precordial systems in 40 apparently healthy and non-sedated cats. Dashed line indicates the bias (average difference), straight lines indicate upper and lower limits of agreement.

In leads V2 and V3 of the 4P system, P_dur was also significantly longer in males (V2=39, 32–44 ms; V3=39 ± 8 ms) than in females (V2=34, 30–35 ms; V3=35 ± 5 ms), with p-values of 0.019 and 0.048, respectively.

Additionally, P_amp in lead V2 of the 4P system was higher in males (0.06 ± 0.02 mV) than in females (0.05 ± 0.02 mV) (p=0.043).

In contrast, S_amp in leads V3 and V4 of the 6P system was larger in females (V3=0, 0 to −0.08 mV; V4=0, 0 to −0.04 mV) than in males (both 0mV), with p-values of 0.033 and 0.048, respectively.

Fig. 5. Bland-Altman analyses of the differences of waves electrocardiographic amplitude between six-lead and four-lead precordial systems in 40 apparently healthy and non-sedated cats. Dashed line indicates the bias (average difference), straight lines indicate upper and lower limits of agreement.

Demographic associations

In the correlation analysis, no associations were detected between electrocardiographic variables and age, sex, weight, or hair type using either method.

Intra and interobserver variability

Intra- and interobserver variability were low for P_dur (5%–13%; 7%–20%), PQ_int (5%–11%; 2%–12%), QRS_dur (6%–14%; 6%–20%), QT_int (3%–6%; 5%–12%), QTc_int (3–%6%; 5%–12%), R_amp (3%–28%; 4%–26%), and S_amp (21%–30%; 10%–13%); and high for P_amp (15%–45%; 9%–33%) and T_amp (8%–44%; 13%–38%).

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Discussion and Conclusion

The precordial system in apparently healthy, non-sedated cats showed no significant differences between six or four leads. However, the animals exhibited less discomfort when fewer electrodes were placed on the thorax. Although the methodology was based on canine studies (Santilli et al., 2019), feline thoracic morphology differs considerably. This is the first study to evaluate different precordial lead systems in cats and to provide species-specific reference intervals.

The absence of an association between demographic and electrocardiographic variables indicates that age, sex, body weight, and hair type do not affect precordial lead values. Therefore, the reference intervals obtained may be applicable to the general feline population, although variations related to sex should still be considered. Male cats exhibited longer P_dur, larger P_amp, and lower S_amp than females. In humans, longer P_dur has also been reported in males and is attributed to larger cardiac size, electrophysiological characteristics, and hormonal influences (Bocchi et al., 2020).

In both systems (6P and 4P), Q_amp and S_amp followed a pattern similar to that previously described in dogs (Santilli et al., 2019). The Q wave was absent in the initial leads and gradually appeared in leads V4, V5, or V6. In contrast, the S wave was more prominent in lead V1 and progressively decreased until it disappeared in leads V4, V5, or V6.

This pattern can be explained by the position of the heart within the thoracic cavity, with the right ventricle located anteriorly and medially and the left ventricle positioned posteriorly and laterally (Thaler, 2022). Thus, lead V1 is positioned directly over the right ventricle; leads V2 and V3 over the interventricular septum; lead V4 over the apex of the left ventricle; and leads V5 and V6 over the lateral wall of the left ventricle (Thaler, 2022).

Since the Q wave represents the beginning of ventricular depolarization, which originates at the cardiac apex (Santilli et al., 2018), it is recorded only from lead V4 onward. Lead V1 typically records a small R wave followed by a deep S wave (Thaler, 2022), resulting in an R/S < 1, as observed. This occurs because the depolarization vector associated with right ventricular activation is directed rightward, with a posterior-to-anterior and inferior-to-superior orientation (Santilli et al., 2018). The presence of the S wave in leads V2 and V3 is usually due to these leads representing a transitional zone (interventricular septum) (Thaler, 2022).

In lead V1, the P wave exhibited negative polarity in all cats and had a lower amplitude compared to the other precordial leads, similar to findings reported in dogs (Santilli et al., 2019). This pattern can be explained by the depolarization vector of the right atrium, which is directed slightly leftward, posterior-to-anterior, and superior-to-inferior (Santilli et al., 2018, 2019). Therefore, the precordial lead systems used in this study appear to have accurately captured the electrical activity of the right cardiac chambers in the cats evaluated.

However, the characteristic pattern of R-wave progression described in both dogs (Santilli et al., 2019) and humans (Thaler, 2022) was not observed. Typically, R-wave amplitude increases from right to left across the precordial leads, with lead V1 showing the smallest R wave and lead V5 the largest, while V6 is often slightly smaller than V5 (Thaler, 2022). In our study, the R wave was less prominent in lead V1 and significantly larger in lead V2. It then progressively decreased in amplitude towards leads V4, V5, and V6, where the smallest amplitudes were observed. This finding may be attributed to the smaller thoracic dimensions or reduced left ventricular size and mass in cats, as previously reported in humans (Kim et al., 2009).

The T wave showed a decreasing amplitude pattern similar to that of the R wave. Although Santilli et al. (2019) did not describe the T-wave amplitude pattern in dogs, such a behaviour is expected, since the R-wave represents ventricular depolarization and the T-wave corresponds to ventricular repolarization (Santilli et al., 2018). The T_amp would be expected to follow a pattern similar to that of the R wave across the precordial leads. In this study, the polarity of the T wave varied across all precordial leads, unlike in dogs, where it is consistently positive in lead V1 (Santilli et al., 2018).

When comparing the two systems (6P versus 4P), T_amp in lead V4 was larger in the 6P system than in the 4P system. A similar pattern was observed for the R_amp in leads V3 and V4. In the 4P system, leads V3 and V4 were positioned where leads V4 and V6, respectively, would be located in the 6P system, that is, more dorsally on the thorax. As noted earlier, R_amp and T_amp progressively decrease with increasingly dorsal electrode placement, which explains the observed differences.

Additionally, Q_amp in lead V4 was lower in the 6P system than in the 4P system. This difference is attributed to lead V4 in the 4P system being positioned more dorsally on the thorax. Thus, Q_amp becomes more prominent as the leads are placed farther from the sternum.

In contrast, P_dur and PQ_int in lead V3 were longer in the 4P system. In dogs, P_dur can vary depending on lead placement within the thoracic cavity (Santilli et al., 2019). Although leads V1 and V2 remained in the same positions in both systems, differences were still observed. Specifically, P_dur in V1 and S_amp in V2 were lower in the 6P system compared to the 4P system. However, these differences were subtle and consistent with the normal variation in ECG wave amplitudes. Since the precordial recordings were obtained at different times, it was not possible to assess the same cardiac cycle in both systems. Therefore, such minor differences are plausible and do not significantly affect the overall results.

Although agreement was found between the two systems for P_amp (V1), Q_amp (V1, V2, V3), and S_amp (V4), a proportional bias was identified. In other words, the differences were not randomly distributed around the mean but tended to cluster either above or below it. This suggests a systematic tendency of one method to consistently overestimate or underestimate these variables.

The reproducibility (interobserver variability) of duration variables was higher than that of amplitude variables, as previously described in humans (de Bruyne et al., 1998; Kligfield et al., 2007). The highest reproducibility was also found for the QTc_int and PQ_int (de Bruyne et al., 1998). In contrast, greater variability in ECG duration variables between observers has been reported in cats (Hayasaki Porsani et al., 2020).

Regarding repeatability (intraobserver variability), a variability of 10%–20% in ECG interpretation is expected when the same recording is analyzed at different times (Grauer et al., 1987), as was observed in our study. The higher variability observed in P and T wave amplitudes is likely due to beat-to-beat biological variation in cardiac electrical activity and to the influence of respiratory variation (Kligfield et al., 2007).

An important limitation of this study was the lack of echocardiographic evaluation. Cats with asymptomatic structural or functional cardiac alterations may have been included. However, the comparison of precordial leads was performed within the same cat, thereby minimizing the potential confounding effect of underlying cardiac disease.

In addition, the body condition score was not assessed, which could have influenced the data obtained. Nevertheless, the cats showed little variation in body condition. Finally, the lack of thoracic radiography and thoracic diameter measurements precluded the calculation of the cardiothoracic ratio and its correlation with precordial ECG variables. Such data would have provided a better understanding of the influence of thoracic size on wave amplitudes, particularly R_amp.

The decreasing progression in R_amp within each system indicated that both methods produced distinct precordial leads, with no overlap between them. Although wave amplitudes—particularly those of the R wave—were slightly higher with the 6P system, no significant overall differences were observed between the use of six or four precordial leads in feline electrocardiography. Cats, however, showed greater comfort with the 4P system.

Therefore, the reference intervals obtained in our study can be applied to the general feline population, while considering that male cats may exhibit higher P-wave values. Further studies are required to explore the clinical relevance of different precordial lead systems in cats.

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Glossary

Conflict of interest

The Author(s) declare(s) that there is no conflict of interest.

Author contributions

LGN was responsible for data collection, data analysis, interpretation, drafting the manuscript, and translation. CBA contributed to the study concept and design, data collection, data analysis, interpretation, and manuscript writing. MRC performed data collection and data analysis. ILSR contributed to data collection. DR carried out data collection and translation. FYKK contributed to data collection. LEDO critically revised the manuscript for important intellectual content. JAL contributed to data collection. All authors have read and approved the final version of the manuscript.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data availability

All data supporting this study’s findings are available within the manuscript.

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Supplementary data for the statistical analysis

Table S1. Comparison of electrocardiographic variables from six precordial leads obtained in 40 apparently healthy cats.

Table S2. Comparison of electrocardiographic variables from four precordial leads obtained in 40 apparently healthy cats.