Cardiac conduction system and the electrocardiogram of the common hippopotamus (Hippopotamus amphibius)

The common hippopotamus (Hippopotamus amphibius) shares a common terrestrial ancestor with whales (Cetacea) and has independently evolved similar physiological adaptations to their aquatic lifestyle. Although several studies have explored the electrical signalling in whale hearts, the understanding of the conduction system and electrical activation of the hippopotamus heart remains sparse. We set out to map the conduction system within the hippopotamus heart and determine the sequence of electrical activation, including the mean electrical axis of ventricular activation. ECGs were recorded from three anaesthetized hippopotamuses. Histological samples were collected from two of these animals and from an additional animal. The hearts of the hippopotamuses constituted similar to 0.3% of body mass and as in whales, the hearts were situated more cranially in the thoracic cavity compared to most terrestial mammals, and were spanning from the first to the fourth intercostal space. The network of Purkinje fibre strands extended deep into the ventricular walls and consisted of large, ovoid cells. Orthogonal ECG recordings revealed a mean electrical axis pointing towards the neck of the animal, indicating that electrical activation takes place in an apex-to-base direction.