Cardiovascular responses elicited by simulated diving and their habituation in man.

The cardiovascular responses of 24 subjects were investigated under various simulated diving conditions. Muscle blood flow in forearm and calf, arterial pressure, heart rate and intrathoracic pressure were monitored. Breath holding with face immersion in water at 18 degrees C gave a typical diving response at intrathoracic pressure of 0 and 20 mmHg, (23% bradycardia, greater than 60% muscle vasoconstriction). Breath holding alone at 20 mmHg intrathoracic pressure resulted in vasoconstriction (50%) and bradycardia (4%). Breath holding at 0 mmHg intrathoracic pressure induced a muscle vasoconstriction (5%). These results indicate that both increased intrathoracic pressure and facial immersion can produce a typical diving response individually but that the full 'diving response' requires the presence of both conditions. Diving often activated two responses, the typical 'diving response' and a superimposed defence reaction. Cardiovascular components of the defence reaction (muscle vasodilatation and tachycardia) which was elicited in some divers masked the diving response. In those subjects in whom the diving response was initially absent during repetition of diving manoeuvres the cardiovascular components of the defence reaction were habituated and the characteristic diving response gradually emerged: the initial tachycardia diminished and was replaced by bradycardia, while vasodilatation in the forearm and calf was replaced by vasoconstriction.