Ventriculophasic response in a horse with atrioventricular block.

The article talks about a horse with a heart condition called atrioventricular block, or AVN block, displaying a particular response known as ventriculophasic response, characterized by certain changes in the heart rhythm. This response, normally observed in humans and dogs, is investigated here in the context of horses for the first time.

Understanding Ventriculophasic Response and AVN Block

• Ventriculophasic response (VR) is an occurrence that happens when the heart experiences an atrioventricular nodal (AVN) block. This block is a condition where the electrical signals that control the heart’s rhythm are partially or completely blocked between the heart’s upper (atria) and lower chambers (ventricles).
• VR is identified in an electrocardiogram (ECG) by a shortening of the sinoatrial cycle length when a QRS complex (a graphical deflection seen on an ECG) appears between two P waves (another graphical representation on an ECG corresponding to the depolarization of the atria).
• While studies have recorded this in humans and dogs, this is the first time it has been documented in horses.

The Case Study and Clinical Implications

• The paper presents a case where a horse with a second-degree AVN block showed a 5:3 conduction variation and an expected VR at rest, though it was less consistent after performing submaximal exercise.
• Three mechanisms have been suggested to explain VR: the inhibition of the Bezold-Jarisch reflex due to reduced end-systolic volume during ventricular contraction; vagal inhibition due to increased intra-atrial pressure (known as the Bainbridge reflex); and more rapid sinus node discharge owing to changes in the blood supply to the sinus node during ventricular contraction.
• From the ECGs taken for this case, the horse showed traits of the first two mechanisms, thus explaining the changes in the PP interval (the time between two P waves) and the suspected VR.
• The study sheds light on how horses often show physiological AVN blocks. Therefore, equine clinicians should take VR into consideration when interpreting ECGs taken from horses, thus furthering their understanding of heart conditions in equines.