Tachycardia caused by hypoxia is due to:-

The primary reflex evoked by hypoxic stimulation of carotid chemoreceptor leads to excitation of the medullary vagal center. This will decrease the hea rate There is also a secondary reflex. Hyperventilation caused by hypoxic stimulation of chemoreceptor initiates pulmonary inflation reflex. Stretch receptors in the thorax stimulated by hyperventilation initiate this reflex which will inhibit medullary vagal center. This will result in tachycardia. There is also the effect of hypocapnia produced by hyperventilation; it will also add to the secondary reflex-induced tachycardia.