Summary Configuration and amplitude values of the QRS complex of the electrocardiogram were studied with standard Einthoven leads in 173 healthy Spanish‐bred (Andalusian) horses, aged between 1 month and 17 years. Animals which were 1 month old had a predominantly negative QRS complex, whereas a predominantly positive complex direction was found in the rest of the animals. Statistically significant variations were found for the Q‐wave and QRS main vector between the different age groups, with highest values for the Q‐wave and most negative values for the direction of the QRS main vector in animals up to 2 months of age. The effect of age on the form and amplitude values of the QRS complex could be related to the changes in size and position of the heart in the thorax during growth. This study provides standard electrocardiographic values for the Andalusian horse at different ages.

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