SnorkelingThe cardiologist is rarely approached by the apnea diver.

This is unfortunate because cardiovascular interactions are major in this sport and the cardiologist could sometimes be good advice.

Snorkeling involves several activities, including underwater hockey and synchronized swimming, but it is mainly underwater hunting and snorkeling that are of concern to us because of the risks and accidents recorded in this area. Indeed, the statistics of the Regional Operational Surveillance and Rescue Centers (CROSS) mention, for 2002, no less than 68 interventions involving 84 apneists or underwater hunters. There were 12 deaths and 17 deaths in the previous year. Unfortunately, these statistics are far below reality, as they do not take into account the numerous interventions of the SAMU, firefighters or beach safety. These accidents occur mainly in the PACA region and more particularly in the Var, department most frequented by snorkelers.

According to the CROSS 2002 statistics, underwater sports remain far away, with scuba diving (8 deaths in 2002), the leading cause of water sports mortality, in front of swimming (14 deaths) or pleasure boating engine (9 deaths).

Many of these accidents occur outside of any framed structure and practitioners often do not receive proper medical follow-up.

Indeed, as regards underwater hunting, its practice is governed in France by Decree No. 90-618 of 11 July 1990 and the ministerial decree of 1 December 1960 as amended. It is prohibited at least 16 years and has several administrative restrictions, but does not require any medical certificate. The practice of hunting, apart from any structure, “exempts” the application for a license and therefore a medical certificate. Although hunters are mostly trained and knowledgeable athletes, they are not immune to asymptomatic congenital heart disease and even less aging.

The role of the physician would be to be able to detect a potentially dangerous pathology and that of the cardiologist, to advise physicians and sportsmen in his field. It is therefore necessary to recall some physiological elements concerning apnea diving.

Each of the phases of snorkeling brings its solicitations and its answers, so we will be interested in the immersion, the descent, the stay at the bottom, the rise and the recovery. But first we will describe the effects of apnea by itself on the cardiovascular system.

The effects of apnea on the cardiovascular system:

Adaptation mechanisms:

Like all diving animals, man develops mechanisms of adaptation to snorkeling. These hemodynamic changes are now grouped under the term “diving reflex”, a term initially used by Hong in 1967. However, these adaptations are not isolated and interact with others, especially ventilatory ones , to improve the quality of apnea.

The diving reflex:

The phylogenesis of apnea has been practiced almost exclusively in the field of diving, so it is not surprising that apnea is in itself capable of involving in humans the “diving reflex” which associates :

– a slowing of the heart rate: it is mainly of haemodynamic origin and adapts to the decrease of the venous return secondary to the thoracic hypertension, generated by apnea on closed glottis; however, a vagal response is also impaired by stimulation of thoracic mechanical receptors;

– peripheral vasoconstriction: it predominates on the limbs, with a decrease of the blood flow up to more than 65%, it can also touch the viscera, but spares the neurological and cardiac vascularization; it decreases the oxygen consumption and is more intense in the event of initial apnea hypoxemia;

– a decrease in cardiac output: it depends on the decrease of the frequency, but also on the associated vasoconstriction;

– splenic contraction: it was the last described aspect and it is an active phenomenon, more pronounced in immersion (30%) than in dry (10%), which generates an increase of the hematocrit (about 6%), capable of prolonging the duration of apnea and seems to extend about 60 minutes after apnea.

During the prolongation of apnea, these adaptive mechanisms are gradually attenuated by the appearance of hypoxia and hypercapnic acidosis, secondary to anaerobic metabolism.

The effects of immersion:

It is of course not suitable for snorkeling and entails some hemodynamic adjustments depending on the submerged area.

The immersion of the body:

The immersion of the body causes a redistribution of the blood mass, comparable to that of the decubitus and which is done to the detriments of the declivities towards the thorax. This increase in venous return, which increases the volume of systolic ejection by Starling phenomenon, is responsible for an increase in cardiac output of 15 to 20%, but also for a secretion of atrial natriuretic factor, with an increase in the diuresis, well known to divers.

The immersion of the face:

* Skin receptors:

The immersion of the face is very important because of the richness of the skin receptors on this sensitive area.

* Aqueous contact Aqueous contact, and especially cold, are responsible for intense vagal stimulation, which slows down the heart rate. This phenomenon can reach very high proportions in the entrained freedman, realized when the face is exposed to ice water.

Bradycardia is the more pronounced (<30 per minute), the water is cold and the subject is young and trained.

The immersion of the face also generates a peripheral vasoconstriction, reinforced by the contact of the rest of the body with a cold liquid. The snorkeler protects itself very little and is therefore very exposed to the high thermal conductivity of the water (25 times that of the air). This vasomotor response is such that true cardiac failures have been described during launching.

Immersion therefore tends to increase the hemodynamic effects of the diving reflex.

Finally, immersion decreases the volume of expiratory reserve (30%), thus promoting alveolar hypoventilation and a tendency to hypercapnia, especially since the tuba doubles the volume of dead space for an adult.


Pressure variations :

Recall that the increase in pressure is one atmosphere (1 ATA) every 10 meters approximately. At 10 meters depth, the pressure has already doubled (2 ATA) and it must reach 30 meters so that it has doubled again (4 ATA). The pressure variations are the strongest near the surface and this explains that many accidents occur in this area.Moreover, the partial pressures of the gases of the organism evolve parallel to the ambient pressure and will be doubled to 10 meters of background, which is the case of the oxygen pressure in oxygen (PaO 2 ).

A zone of depression:

During the descent, the hydrostatic pressure applies throughout the body, but the thoracic cage, locked on a sternum (unlike the mammal diving), remains little deformable and will therefore create an intra-thoracic area of ​​relative depression.

The “blood shift”:

The displacement of the diaphragm upwards is insufficient to balance the pressures and the intra-thoracic region will behave like a real reservoir, gradually sucking in a large blood mass and storing it in the only available circulatory area: the pulmonary circulation.

Blood shift is estimated to be around one liter at 30 m depth and will help to stiffen the thorax to resist hydrostatic pressure.

Vagal stimulation:

All this happens without a tachycardial reaction, even though the sequestration of the blood significantly increases the preload. Indeed, vagal stimulation is intense and the systemic resistances become very high due to apnea, cold and hyperoxia due to the increase in pressure.

In elite divers, arterial tensions between 220/110 and 290/150 (and up to 345 mmHg systolic) were observed in simulated caisson dives between 40 and 50 m in water yet at 25 ° C.

Bradycardia and vasoconstriction:

The descent leads to a situation of bradycardia and intense systemic vasoconstriction, with pulmonary blood sequestration.

Hemodynamic accidents are rare during the descent and the majority is mainly represented by barotrauma of the ORL region. But we can wonder about the capabilities of the body to respond to the very rapid variations of pressure generated by very high descent rates (> 1.5m.s-1), reached during competitions especially in “No Limit “.

Stay at the bottom:

A “soft nitrogenous narcosis”:

More or less prolonged depending on the goal (hunting, record …), it is generally a moment of well-being.

Physical exertion is minimized, hemodynamic balance is probably rapidly obtained, metabolic deficiencies are not yet felt, even a “soft nitrogenous narcosis” can enhance the stay of the deepest divers.

The hemodynamic situation:

In fact, there is little data available on the hemodynamic situation, but it seems that the absence of a change in pressure conditions leads to a state of equilibrium by means of a more or less important “blood shift”. Bradycardia remains unchanged, which is in favor of persistent peripheral vasoconstriction.

However, basal stay consumes oxygen and all peripheral zones without vascularization adopt an anaerobic metabolism with on-site storage of many acid ions. Moreover, the carbon dioxide produced is rapidly diffusible in the tissues and does not cause any consequences during the first minutes.

Ventricular rhythm disorders:

In fact, there is no incentive to go up, and this illusory comfort can lead to a brutally dangerous situation if the diver does not know himself well.

Training is fundamental in this area.

Stay at the bottom may, however, cause unsupported ventricular rhythm disturbances in trained divers and, at significant depths (> 40 m), they are favored by intense vagal hypertonia, with bradycardia below 35, but also by the large right ventricular overload and are aggravated by the cold.

The CO 2:

The signal of the return is finally given by several elements, with uncontrolled movements of the diaphragm and a pressing need to breathe, which depends in large part on the increase in CO 2 .

It is possible to artificially lower this rate and thus prolong the apnea by prior hyperventilation. In this case, there is a delay to the rising signal with a risk of accident. Hyperventilation should therefore be formally discouraged in the apneist. Finally, it has been recently considered that an endurance effort, prior to apnea, is likely to result in a minimization of apoptosis by preferential use of lipid catabolism and therefore a decrease in respiratory quotient. the capnie and to aggravate the hypoxia of the apneists.

The rise:

The lift is really the most dangerous phase for the freediver. It is rapid and has multiple obstacles that can be summarized.

* The emptying of the pulmonary “blood shift”, which occurs while the peripheral vasoconstriction remains intense, putting the left ventricle in a situation of high precharge and postcharge.

* The persistence of bradycardia has been demonstrated by numerous records.

* The sudden drop in ambient pressure, which will immediately affect the PaO 2 , divided by 2 between 30 and 10 m, and again by 2 between 10 m and the surface. A very satisfactory pressure of 100 mmHg at 30 m will end with a PaO 2 of 25 mmHg on the surface, with a clear risk of hypoxic loss of knowledge. This mechanism is most frequently mentioned for the “syncopes” of the ascent and occurs logically near the surface. These accidents are all the more dramatic because they are not preceded by any prodrome.

* Finally, the arrival on the surface and the rupture of the apnea will trigger a sudden rupture of the peripheral vasoconstriction and lead to a real tide of a cold and acidic venous return, corresponding to the release of the aerobic debt.

On arrival at the surface, the heart has to manage a large hemodynamic overload while it is hypoxia, acidosis, and exposed to cold.

All these conditions, added to the expiratory effort of emptying the tuba, explain that the majority of the rhythm disorders are encountered during the ascent or the first 10 seconds of return to the surface.

They involve more than half of the divers, in the form of frequent supraventricular extrasystoles or bursts of supraventricular tachycardia, or even more rarely ventricular rhythm disorders.


It is a fundamental period because it restores the hemodynamic and biochemical equilibrium. Unfortunately, it is still too often neglected by divers, especially hunters, who have good reasons to chain the apneas.

If the hemodynamic equilibrium can be obtained quickly, in less than three minutes, it is not the same for the catch up of the oxygen debt and even less for the removal of dissolved CO 2 , which may require more than 15 minutes depending on previous dives.

The accumulation of insufficient recovery periods can:

– aggravate the risk of hypoxic loss of consciousness;

– to promote the occurrence of a carbonarcose on the occasion of an upcoming descent;

– or even cause actual decompression accidents, linked to the accumulation of nitrogen and described in some deep hunters.

The role of the physician:

All in all, these schematic descriptions of snorkeling show that the organism implements adaptive strategies, but that these may be very restrictive with respect to the cardiovascular system.

These different considerations mean that, unlike scuba diving, where certain cardiovascular imperfections can be tolerated (asymptomatic cardiopathy, stabilized arterial hypertension), it is not at all the same for snorkeling. The role of the physician is therefore to detect, by interrogation and a rigorous clinical examination, any pathology likely to decompensate. Snorkeling, as we have envisaged (sporting or underwater hunting at more than 5 meters) must therefore be contra-indicated to any known cardiac, but also to those who do not know.

Finally, the advice that can be given in this area is essentially:

– no diving alone;

– being two is sufficient, provided that it means “a pair”, of which a diver always remains on the surface; for hunting, a second rifle is then too much;

– no hyperventilation before descending;

– no dive when you do not feel “well on your plate”.

Training is of course fundamental in this area and can improve performance. A daily training of 15 days can improve not only the apnea times but also the quality of the diving reflex.

Unfortunately, it is not a guarantee of safety and statistics are stubborn: the majority of severe accidents in the apneists occur in young trained divers and remain, for the most part, totally unpredictable and without prodromes.

In conclusion:

Although many unknowns persist in the knowledge of the physiology of snorkeling, it is certain that the cardiovascular stresses are major and that it is not a sport to propose to the susceptible myocardiums.

Any symptomatic pathology, or not, must be considered a priori as a definite contraindication to snorkeling. The role of the cardiologist is therefore fundamental in the detection of asymptomatic cardiopathy, provided that it is solicited in this field.

In this sense, an evolution of the legislation would be desirable, in order to consider underwater hunting not as an administrative approach but as a real sport, justifying a prior medical examination.

Finally, although training is essential in this area, it does not guarantee safety.

In snorkeling, the “pair” must remain the rule.