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

Background: <br/>The common hippopotamus (Hippopotamus amphibius) shares a common terrestrial ancestor with whales (Cetacea) and has independently evolved similar physiological adaptations to their aquatic lifestyle. While several studies have explored the electrical signaling in whale hearts, the understanding of the conduction system and electrical activation of the hippopotamus heart remains sparse.<br/><br/>Aim: <br/>To map the conduction system within the hippopotamus heart and determine the sequence of the electrical activation, including the mean electrical axis of ventricular activation.<br/><br/>Methods: <br/>Histological samples were collected from three animals and electrocardiograms (ECGs) were recorded from three anesthetized hippopotamuses in both Einthoven and orthogonal configurations.<br/><br/>Results: <br/>The hearts of the hippopotamuses constituted approximately 0.3% of body mass and were situated in the thoracic cavity spanning from the 1st to the 4th intercostal space. The Purkinje fibers were large and ovoid in shape and the network of Purkinje fiber strands extended deep into the ventricular walls. Orthologous ECG recordings revealed a mean electrical axis pointing towards the neck of the animal, indicating that activation takes place in an apex-to-base direction. <br/> <br/>Conclusion: <br/>As in whales, the heart of the hippopotamus is situated more cranially in the thorax compared to most terrestrial mammals. The Purkinje fiber network extends deep into the ventricular wall, resulting in an apex-to-base direction of the electrical activation of the ventricles.