Aetiology and natural history of Parkinson's disease.

understanding of the reflex basis of bradycardia is in procedures used for terminating bouts of supraventricular tachycardia. Since breathing inhibits the reflexes that evoke bradycardia all attempts to increase vagal tone-whether by carotid sinus massage, pressure on the eyes, or applying water to the face-should be attempted while the patient voluntarily holds his breath. I The obligatory holding of the breath when the face is completely immersed gives the method a physiological advantage over carotid sinus massage or eye pressure10 unless the patient voluntarily holds his breath during those procedures. Ideally, breath holding should occur in the end-expiratory phase. But as reflex inhibition from intrapulmonary receptors is phasic, and lasts only a few seconds when the lungs are held inflated,3 breath holding in the inspiratory position should carry no great disadvantage. On the other hand, where bradycardia becomes excessive, as in simple vasovagal faints or during manipulations in ophthalmic and nasopharyngeal surgery," it should be preventable by voluntarily increased breathing or by increasing the frequency and amplitude of imposed lung inflations. Even the glass of water given to someone who feels faint makes good physiological sense, for swallowing is known to be associated with intense bursts of firing in central inspiratory neurones, and this central inspiratory activity inhibits vagal excitation and relieves bradycardia.'