The aim of this study is to demonstrate the effect of the voluntary neck extension in the occurrence of an increase in the pulsatility index of the right internal carotid artery in the amateur diver, realizing an apnea of two minutes or more.

In literature concerning apnoea, accidents have been described during neck extension.

Accidental drowning are frequent on the French coast every year and many of them concern apneists victims of apnoeic blackout.

Neck extension is thus suspected to possibly produce an apnoeic blackout at the end of the dive since 1965, when Sir Sciarli (diving medicine pioneer) suspected this hypothesis.

Physiologically, during a dive in apnea, the human cardiovascular system is subjected to a parasympathetic dominance, via the apnea reflex and the diving reflex. This parasympathetic dominant could be reinforced during a neck extension at the end of snorkeling during the ascent.

Hypothesis: the neck extension at the end of apnea contributes to increase the pulsatility index of the right internal carotid artery by vagal component and thus causes a decrease in cerebral blood flow and may lead to the initiation of syncope in the healthy amateur free diver or underwater fisherman.

Inclusion Criteria:

• Major (≥18 years), under the age of 51
• Male
• Can hold an apnea of 2 minutes or more
• Practicing for less than 2 years the apnea at the rate of at least 10 sessions of apnea in the year
• Not having a known cardiovascular history : any form of heart rhythm disorder, Ischemic heart disease, angina, arteriopathy, any valvular pathology, high blood pressure ...
• No known respiratory history (asthma, Chronic Obstructive Pulmonary Disease, bronchiectasis, pulmonary emphysema, pneumothorax, pleurisy, pulmonary infection, lobectomy ...)
• No neurological history of type: epilepsy, deficit syndrome, iterative loss of knowledge, recent head trauma with loss of consciousness

Exclusion Criteria:

• - Minors and persons over 51 years of age
• Person under curators or trusteeship
• Person with a history of cardiovascular, respiratory or epilepsy as cited by the French Federation of Study and Submarine Sports, which poses as a contra-indication to the practice of apnea: Cardiac insufficiency, Obstructive Cardiomyopathy, syncopal risk pathology (valvulopathies type aorting narrowing or mitral narrowing), paroxysmal tachycardia, non-paired auriculo ventricular block 2/3, stroke, uncontrolled hypertension after stress test, recent infarction, angina, pericarditis. Asthma stage 3, severe Chronic Obstructive Pulmonary Disease, bronchiectasis, pulmonary emphysema, pneumothorax, pleurisy, pulmonary infection, lobectomy ...) Epilepsy, deficit syndrome, iterative loss of knowledge, recent head trauma with loss of consciousness in difficulty, symptomatic cervical or lumbar disc herniation
• Patient with medical treatment affecting the cardiovascular or respiratory system (for example: ventolin, betablocker ...)