Development and validation of a novel 9-lead electrocardiographic system for cats unaffected by age-related cardiac positional changes

Objective: To evaluate the accuracy of recordings of atrial and ventricular depolarizations in 9-lead ECG of cats, with V1 placed at the right first intercostal space at the costochondral junction and V2 and V4 at the fifth or the sixth intercostal space (adjacent to the sternum and at the costochondral junction), and to assess the effect of age-related cardiac positional changes on ECG parameters. Methods: 60 healthy client-owned cats were planned to be enrolled from January 1, 2024, through December 31, 2024. Each cat underwent physical examination, thoracic radiography, 9-lead ECG, and echocardiography. Cats were age-grouped (< 5, 5 to 10, and > 10 years). The radiographic heart-sternum angle (HSA) in right lateral view was measured and correlated with ECG findings. Results: The V1 location enabled recording of negative P waves and QRS complexes with R/S ≤ 1, with significant intergroup differences (0.68 ms [IQR, 0.42], 0.37 ms [IQR, 0.32 ms], and 0.48 ms [IQR, 0.65 ms]; P = .04). In V2 and V4 in both sites, P waves were predominantly positive, and QRS complexes showed R/S > 1, with minimal intergroup differences. Aging was associated with a significant HSA reduction (r = -0.72) but did not affect any clinically relevant ECG variables. Conclusions: This 9-lead system, with V1, V2, and V4 positioned as in dogs, enabled accurate P waves and QRS recording in cats. Aging reduced HSA without major effects on wave morphology. Clinical Relevance: This system will support detailed arrhythmias analysis in feline patients.