Background

A consequence of the growing access to scuba diving is an increase in the number of followers of this recreational, fun and contemplative athletic activity. According to the latest recommendations by the French Federation of Undersea Studies and Sports (FFESSM) published in 2012 , congenital heart disease (CHD) is a formal and final contraindication to scuba diving. On the other hand, with the progress made in their surgical as well as percutaneous catheter management, the prognosis of patients with CHD has improved considerably . As a result, the population of adolescent and adult patients continues to grow at a rate of 5% per year, surpassing the number of children for the past 10 years . In France, approximately 150,000 adults currently have CHD. Besides the increase in their number, their quality of life has improved, enabling them to attempt–and often succeed–in engaging in physical and sports endeavours, which are well known for conferring general health and psychological benefits .

Because the issue of aptitude to engage in scuba diving presents itself regularly, we felt the need to reflect on the matter, with a view to contributing decisive elements for or against the practice of this activity. These thoughts address recreational diving exclusively, and exclude all commercial and military divers, whose medical supervision is specific. Furthermore, some valvular diseases, systemic hypertension and coronary heart disease, independent of their possible congenital origin, are often definitive contraindications to the practice of scuba diving. Therefore, we refer the readers to the existing recommendations for these disorders .

The anatomical presentation of CHDs is highly variable; they may, consequently, be responsible, alone or in combination, for shunts, valvular stenosis or regurgitation, myocardial disease, pre- or postcapillary (or mixed) pulmonary arterial hypertension, dilatation of the aorta, pulmonary artery or both, and atrial or ventricular arrhythmias or conduction disorders . This variability complicates the formulation of recommendations based on the usual and traditional classification of CHD as shunts, left or right stenosis, valvular diseases and others. Within each group of CHD, the anatomical and, consequently, clinical presentations are highly heterogeneous, which mandates for each patient a precise evaluation of each constitutional malformation. Inspired by Budts et al. , we formulated our recommendations on the basis of elements that are common to all CHDs, from which each individual can be separately evaluated. The importance of considering, for each patient, several extracardiac factors, including pulmonary, thoracic and haematological disorders, as well as therapeutic interventions, cannot be overemphasized .

Types of scuba diving included in these recommendations

These recommendations are limited to recreational open-circuit scuba diving. The gas used is room air or a < 40% enriched nitrox-like mixture and a ≤ 1.6 bar oxygen (O ₂ ) partial pressure. The use of recycler systems and complex gas mixtures, including trimix, is excluded from this discussion. The dives must observe the prerogatives of each level, according to the rules published in the technical training manual issued by the National Technical Commission of the FFESSM , using a validated decompression procedure (MN90 tables or dive computer) and a limit of no more than two dives daily. Divers with CHD are expected to follow the medical instructions. A state-designated physician is responsible for the delivery of a certificate confirming the absence of contraindications based on specific considerations (detailed below). The possible restrictions, with respect to the diver’s prerogatives, must be explicitly visible on the certificate handed to the patient, specifically applying the model prepared by the FFESSM , which includes a “special observations” section. Depending on the type of heart disease and possible sequelae, nitrox dives might be authorized upon the advice of a state-designated physician.

Cardiovascular effects of underwater dives

Underwater dives cause haemodynamic, thermal and ventilatory changes, which may cause the decompensation of a well-tolerated pre-existent heart disease . As soon as immersion begins, most of the hydrostatic pressure exerted over the entire immersed body is transmitted by the tissues to the adventitial side of the vascular walls, with a passive transfer of blood toward the central circulation, causing a rapid increase in left ventricular (LV) preload . The increase in preload increases cardiac output via the Frank-Starling mechanism. The atrial and ventricular distension resulting from this sudden increase in preload may promote the development of arrhythmias during the dive . A compensatory increase in the secretion of natriuretic peptides in response to this relative hypervolaemia, caused by dilatation of the cardiac chambers, gradually increases the diuresis, resulting in a relative hypovolaemia at the end of the dive. Furthermore, the immersion exposes the trunk and extremities to the cold, increasing peripheral vasoconstriction, systemic blood pressure and ventricular afterload . Vasoconstriction also promotes the central redistribution of the blood volume, further increasing the immersion diuresis. These specific thermal limitations must be taken into consideration when setting individual restrictions.

Ventilation is also modified during dives in autonomous scuba diving. The regulator equalizes the gas and the ambient hydrostatic pressures, which strive to overcome strong inspiratory and expiratory resistances, in the presence of large differences between inspiratory and expiratory pressures. As the dive deepens, the volumetric mass of inhaled gas increases, increasing the dynamic resistance to flow across the airways and the ventilatory effort, which might promote hypercapnoea from alveolar hypoventilation by increasing the end-expiratory volume.

During the period of desaturation, bubbles circulating in the pulmonary vascular network increase pulmonary vascular resistance, further augmenting the stress imposed on the right ventricle . While these bubbles are usually silent and harmless, they might be the source of air embolism in case of excessively rapid decompression. An atrial or ventricular septal defect with right-to-left shunt, present in some CHDs, increases the risk of paradoxical air embolisms.

In conclusion, the cardiovascular system must undergo multiple adaptations during immersion and hyperbaric conditions. The stress involved depends on the kind of dive, the work accomplished, the means of decompression, the equipment used and the amount of exposure to cold. These specific factors must be addressed by the state-appointed physician and mentioned on the certificate of clearance.

Amount of physical activity (or exercise capacity) involved in underwater diving

The intensity of physical activity and energy expenditure can be measured by O ₂ consumption, expressed in metabolic equivalents (MET), compared with the baseline metabolism (at rest, 1 MET = 3.5 mL/kg/min of O ₂ in men and 3.2 mL/kg/min of O ₂ in women) .

The energy consumed in underwater diving remains controversial. A capacity of approximately 4 MET is required for simple recreational diving, while 6 MET provides a safe and desirable margin for this activity . However, the energy consumed varies widely with the water temperature, the equipment available, the currents encountered, the surface swimming as well as the environmental conditions . In the “Compendium of Physical Activities” compiled by the University of Arizona , underwater diving was attributed 7 MET, a value found among the publications listed on that site and, therefore, adopted by the authors.

A physical activity sustained under stable aerobic conditions for more than 50–60 minutes must be performed below the first ventilatory threshold, usually near 50% of peak O ₂ consumption (VO ₂ ), which can be measured during cardiopulmonary exercise . The first ventilatory threshold of a candidate diver should, therefore, be ≥ 6 MET in order to remain below that threshold during a dive performed under stable physical conditions . This generally corresponds to a ≥ 12 MET maximum exercise capacity .

Based on the article published by Mitchell et al. , this exclusively recreational activity could be authorized by the state-appointed physician under strictly limited and clearly defined conditions, in patients whose first ventilatory threshold is > 4 MET (peak VO ₂ > 28 mL/kg/min in men and > 25.6 mL/kg/min in women). This proposal would contribute some flexibility and a range of decisions applicable according to the context.

Published recommendations on the practice of physical activities in patients with CHD

The virtues of physical activity and training in patients with CHD are well known . A routine particularly adapted to a physical activity increases exercise capacity without adverse or fatal consequences . After the institution of a retraining programme, quality of life scores are improved.

Recommendations were formulated in 2004 by Maron et al. regarding the physical activity and the recreational and competitive sports practiced by patients with cardiovascular disorders of exclusively genetic origin . Distinctions were made between the practice of recreational versus competitive sports, emphasising the greater difficulty in formulating recommendations pertaining to recreational activities. In that publication, scuba diving was included among low-intensity (< 4 MET) activities, while swimming was classified as a moderate-intensity (4–6 MET) activity.

In 2005, the Task Force 2 of the 36th Bethesda Conference defined criteria for each CHD, pertaining to the exclusively competitive practice of sports . The term “athlete”, which corresponds to intense training (i.e. exceeding the limit of breathlessness for ≥ 8 h per week over > 6 months) is beyond the scope of this discussion. In the same year, experts from the European Society of Cardiology published their recommendations pertaining to patients presenting with CHD, also discussing exclusively competitive sports activities at an amateur or professional level . With respect to specifically recreational physical activities, the first recommendations were published by experts from the European Society of Cardiology in 2006 . Underwater dives are listed as a contraindication in presence of residual interatrial shunt across an atrial septal defect or a patent foramen ovale, because of the risk of paradoxical embolism.

More recently, Takken et al. detailed the physical activities and sports recommended for each CHD. These authors maintain that, unlike competitive physical activities, those called recreational are characterized by exercises undertaken without personal or external pressure; therefore, the subject may discontinue or slow the physical activity as desired. As emphasized by the authors, this does not apply entirely to even recreational scuba diving, because of the risks related to the aquatic environment. The recommendations made are with respect to the CHD, without specific consideration of the type of physical activity, particularly in that environment.

Finally, all these recommendations formulated by sports medicine specialists and cardiologists are not based on randomized studies–rather on the opinions of a majority of experts. As a consequence, the writing is usually prudent, appearing at times excessive, when ultimately prohibiting scuba diving to all those with CHDs. Upon scrutinizing these articles, and taking into consideration possible concomitant extracardiac disorders, the anatomical–and hence clinical–diversity of CHD, even within so-called “homogeneous” groups of cardiac malformations, and the specific nature of scuba diving, we recommend an individual risk evaluation from an ensemble of clinical and paraclinical less-specialized criteria, allowing or disallowing this activity, regardless of the CHD.