Scuba diving is an increasingly popular activity. The Professional Association of Diving instructors (PADI) which covers 60–70% of the global scuba diving market, has issued more than 28 million diver certifications globally since 1976 [1]. Because of this increased popularity, there has also been an increase in people with cardiovascular disease who wish to start diving, including patients with (repaired) congenital heart disease (CHD). This topic has become more relevant as the number of adults with CHD (ACHD) is expanding, with more than 90% of children born with CHD reaching adulthood nowadays ^[2][3][4][2,3,4]. Most of these patients experience a good quality of life, and want to live a normal life, including sports participation, and a growing number of ACHD patients consider scuba diving ^[5][6][5,6]. As such, cardiologists are frequently confronted with ACHD patients seeking advice whether they are fit to dive.

Recently, the Task Force on Sports Cardiology and exercise in patients with cardiovascular disease of the European Society of Cardiology (ESC) published updated guidelines regarding the participation in competitive sports of patients with cardiovascular disease [7]. Although some general contra-indications for diving are included in this guideline, eligibility for scuba diving is only mentioned briefly. More specific recommendations regarding participation in competitive sports in ACHD patients were recently published in a position paper by the Working Group on Adult Congenital Heart Disease and on Sports Cardiology [8]. In this document, scuba diving has been classified as a “skill sports discipline”, due to its presumed limited effects on heart rate and blood pressure, but with an intrinsic risk of serious harm or death for the athlete in the event of syncope.

The classification of scuba diving as a skill sport has profound implications for the evaluation of fitness-to-dive in patients with ACHD. The spectrum of ACHD and its associated complications range from conditions conventionally considered as incompatible with scuba diving (e.g., pulmonary hypertension, cyanosis and ventricular hypertrophy) to other conditions which may be compatible with recreational scuba diving. Severity of ACHD differs substantially amongst and within different diagnosis groups.

2. Diving Physiology: Cardiovascular and General Aspects

Immersion and submersion have profound effects on the cardiovascular, respiratory, endocrinal, renal and central nervous systems. In healthy individuals, immersion leads to an increase in hydrostatic pressure, resulting in a fluid-shift towards the central circulation, consequently increasing the preload of the left ventricle augmenting cardiac output via the Frank–Starling mechanism. This volume loading of the ventricles leads to right atrial and ventricular dilatation, with a compensatory increase in the secretion of natriuretic peptide. Throughout the dive, this will gradually increase diuresis (“diver’s pee”), resulting in a relatively hypovolemic state at the end of the dive [9]. Secondly, even in tropical waters, immersion and submersion will induce peripheral vasoconstriction, resulting in a rise in systolic blood pressure and ventricular afterload. Finally, exposure of the trigeminus nerve to cold water will stimulate the diving reflex. This will induce inhibition of the cardio-respiratory center in the medulla oblongata, resulting in bradycardia (decrease in the heart rate of up to 60%), QT interval prolongation and vasoconstriction [10]. The combination of sympathetic and parasympathetic stimulation may in rare cases lead to an “autonomic conflict”, which has been associated with the onset of ventricular arrhythmias [11]. This might lead to life-threatening arrhythmias in vulnerable hearts, for instance with hypertrophy, ischemia, pre-existing arrhythmias and channelopathies [11]. In patients with cardiovascular disease, diving-associated hemodynamic alterations potentially disturb a previously well-tolerated cardiac condition. In particular in patients with diminished right or left ventricular systolic function, restrictive diastolic function or patients with moderate to severe valvular disease, these volume shifts might lead to cardiac decompensation, and pulmonary oedema may develop (immersion pulmonary oedema (IPE)) [12]. The development of a bradycardia by the diving reflex might be especially relevant in patients with a depressed systolic left or right ventricular function, resulting in a decrease in cardiac output. In addition, peripheral vasoconstriction will result in an increase in systemic afterload and may challenge divers with a left ventricular outflow tract (LVOT) obstruction or depressed left ventricular function. In addition to these cardiovascular effects, several general aspects are important in all divers. Water is in principle an unforgiving environment. To survive, one needs an air supply, whether from the surface (with an “umbilical”), or from a carried tank filled with air (or another gas mixture). In the water there are hyperbaric conditions. At every 10 meters of increasing depth, the ambient pressure increases by 1 atmosphere. As a result, inhalation gases become denser, increasing the respiratory work [13]. Additionally, the solubility of the gases increases. This results in storage of especially nitrogen from the air in the blood and tissues. The main factors for this build-up are the nitrogen content of the inhaled gas, depth of the dive and duration of the dive. One can calculate how much nitrogen is being stored in the tissues during the dive, and how long and how deep one can dive without extra stops to wash-out the nitrogen from the tissues. These calculations are available as “no decompression dive tables” (no-deco dives). When exceeding the limits as to depth and time indicated by these tables, chances of decompression sickness (DCS) increase. All patients are advised to remain within the limits of these decompression tables. After the dive, when ambient pressure has been reduced to normal (1 atmosphere), this nitrogen is released from the tissues. This may cause local bubble formation, resulting in local DCS (for instance in the joints, “the bends”). The nitrogen may also be transported through the blood to the lungs, where it is exhaled. When there is too much nitrogen in the blood, bubble formation may result. These bubbles cause a temporary increase in pulmonary artery pressure [14]. When there is a connection between the pulmonic and systemic circulation, for instance by means of a patent foramen ovale (PFO), these bubbles may enter the systemic circulation and cause DCS. The symptoms of shunt-related DCS are dependent on the organ involved, for instance, neurological syndromes, unconsciousness, vertigo or cutis marmorata. DCS should be differentiated from Arterial Gas Embolism (AGE), which occurs when expanding gas stretches and ruptures alveolar capillaries (pulmonary barotrauma) allowing alveolar gas to enter the arterial circulation [15].

3. Exercise Capacity and Diving

Diving exercise intensity was recently reported to be 5 ± 2 METs in 139 experienced recreational divers (age 42 ± 10 years, total 959 dives), leading to the suggestion that maintaining an exercise capacity of >7 METs (peak VO2 24.5 mL/kg/min in men and 22.4 mL/kg/min in women) would generally be adequate for uncomplicated recreational diving [16]. However, in some circumstances, a higher energy expenditure is required, for instance in case of a difficult entry or exit from the diving place (long walk with the equipment that may weight up to 20–30 kg), waves or currents. Of particular importance is that a diver should be able to rescue his or her buddy diver, which may require a substantially higher exercise capacity. Professional diving organizations (military, police, fire brigade) usually require an exercise capacity of 13 METs (peak VO2 40 mL/kg/min) [17]. Patients (whether with ACHD) who do not meet an exercise capacity of 25 mL/min/kg are conventionally advised not to dive, while patients with a VO2 max between 25 and 40 (men) or 25 and 35 (women) are advised to engage in non-strenuous diving or fitness optimization prior to commencing diving [18].

4. Evaluating Fitness to Dive in Patients with ACHD

Many patients with ACHD will be under regular follow-up by their own congenital cardiologist. Medical Examiners of Divers (MED) should obtain information from the treating physician during the evaluation process. Topics that should be discussed are the medical history, current cardiovascular status (including exercise capacity and echocardiography), medication use and any cardiovascular complications disqualifying the candidate for diving. The focus of this information should be on how diving physiology potentially may lead to a deterioration in cardiac function and thus threaten the patient or his/her diving-buddy. Patients with ACHD who are approved fit-to-dive after thorough evaluation should have an annual reassessment by their MED, to rule out any onset of new symptoms or signs of cardiac deterioration. Comparable to a general cardiovascular diving evaluation, a detailed history-taking and physical examination should be performed. Chest pain, dyspnea, dizziness, palpitations or loss of consciousness, especially during exercise, disqualify candidate-divers pending further evaluation. Many ACHD patients have undergone one or more surgical procedures involving a sternal and/or lateral thoracotomy. If the surgery involved the pleura, the risk of persisting pleural adhesions remains. Patients after thoracotomy should undergo pulmonary function testing and when abnormal should also have a CT thorax, and in case of abnormalities (for instance adhesions) be referred to a pulmonologist. Patients who have undergone a lateral thoracotomy are advised to have a CT thorax in any case, because of the perceived risk of pleural adhesions. Patients with any pleural adhesions should be advised against diving ^[19][20][19,20]. Physical examination should include at least office blood-pressure and cardiac examination, focusing on signs of heart failure and the presence of (new) cardiac murmurs. As hypertension is associated with IPE, patients with a blood pressure exceeding 160/100 mmHg at rest should be advised against diving until adequate treatment of blood pressure has been achieved ^[21][22][21,22]. Any signs of heart failure or the presence of previously unknown cardiac murmurs should prompt further evaluation before certification. A recent 12-lead electrocardiogram is mandatory. New onset supraventricular arrhythmia temporally disqualifies a patient from diving pending further analysis and adequate treatment. If the ECG shows new signs of left or right ventricular hypertrophy, especially in patients known to have abnormal loading conditions, echocardiographic evaluation is indicated to rule out any progression in valvular abnormalities or the development of hypertrophic cardiomyopathy. In addition, new onset AV block or bundle branch block warrants further analysis with advanced AV block being a contra-indication for diving [23]. Specific recommendations in patients with cardiac arrhythmias are provided in the section below covering specific conditions. Transthoracic echocardiography is essential in the evaluation of candidate-divers with ACHD, and a recent (<12 months) echo study should be available in all patients. Basic chamber quantification with assessment of the left and right ventricular dimensions and function, aortic dimensions, valvular function and estimation of the pulmonary pressures should be performed ^[24][25][26][24,25,26]. Echocardiography of the right ventricle in ACHD patients can be challenging. The right ventricle’s unique crescent shape complicates quantification of its size and function by echocardiography. By using multiple acoustic windows, both qualitative and quantitative parameters could be examined [24]. Specific considerations with respect to echocardiographic assessment of several common forms of congenital heart disease are discussed separately in the section on specific conditions. Echocardiographic aspects and cut-offs that are important in all ACHD patients are summarized in Table 1. In this table, results compatible with diving are marked green while results that are incompatible with diving are marked red. Results marked orange are conditionally compatible with diving.