Diving bradycardia is not correlated to the oculocardiac reflex

Both facial immersion in cold water and pressure on the eyeball cause reflex bradycardia. These reflexes are called diving reflex and oculocardiac reflex, respectively. The latter is sometimes used in diving medicine to estimate the risk of severe diving bradycardia. The purpose of this study was to quantify the effects of both reflexes on heart rate in 15 subjects. All subjects performed four tests: (1) breath-holding (2) breath-holding and facial immersion in water of 10 degrees, 15 degrees, and 20 degrees C; (3) facial immersion in water and snorkeling; (4) application of pressure of 30, 50, and 70 mmHg on the eyeball. In seven subjects an additional test was done: (5) eyeball pressures during breath-holding. It was shown that the intensity of the oculocardiac reflex is not a good indication of the bradycardia that can be expected during diving. It is proposed that breath-holding with facial immersion in water of 20 degrees C or colder during at least 10 s is a more appropriate test to assess the possibility of severe diving bradycardia and cardiac arrhythmias.