Introduction

All major diving organizations require medical screening before a person may participate in SCUBA (self-contained underwater breathing apparatus) diving. The screening intervals vary depending on the country and the purpose of the diving (recreational versus commercial). In 2017, there were 67 SCUBA diving fatalities reported to the Diver Alert Network (DAN), and of the cases with cause reported, 15% were related to an underlying cardiac disease.[1] Thus, all SCUBA divers should be assessed for cardiovascular comorbidities affecting their diving fitness.

Function

Several physiologic changes occur within the body during diving that can predispose a patient to cardiac events. Most evident, SCUBA diving causes an increase in metabolic oxygen consumption from the muscular contraction required to swim while carrying the necessary gear.[2] Swimming in a 1-knot current is estimated to correspond to about 8 METs (metabolic equivalents).[3] Decreased heart rate is also common during diving due to the increased vagal tone associated with the mammalian diving reflex and the increased partial pressure of hyperbaric oxygen inhaled from the SCUBA tank.[2] Water immersion also exerts increased pressure on the vasculature relative to air, causing an increase in intrathoracic blood volume.[4] This increase in blood volume is associated with increased central venous pressure and cardiac output.[4]

The other primary concern when assessing fitness to dive is the risk for decompression sickness (DCS). While resurfacing from diving, the ambient pressure exerted on the gases inhaled while underwater decreases. Consequently, the inert gasses (primarily nitrogen) can expand and form micro-bubbles in the blood. Typically, these bubbles can be exhaled through the gas exchange in the lungs. However, if the ascent is too rapid, the micro-bubbles can form in the body's other tissues. There are 2 recognized forms: minor and major. The minor type primarily affects the musculoskeletal system and is characterized by joint pain, itching, and skin changes. The major type is typified by neurological symptoms such as altered mental status, vertigo, confusion, and vision abnormalities.[3] The primary method for preventing DCS is to make appropriate safety stops to allow the gas to diffuse safely from the body; however, several cardiovascular factors may increase the risks of DCS while diving, which are discussed in this topic.

In general, it has been shown that the most effective tool for assessing cardiovascular risk is a well-designed questionnaire, as physical examination is unlikely to identify abnormalities that preclude diving.[5] Divers should be queried on their past medical history as well as for risk factors of cardiovascular disease, including obesity, hypertension, diabetes, smoking status, and hyperlipidemia.

Issues of Concern

Coronary Artery Disease and Congestive Heart Failure

As mentioned, SCUBA is a physically taxing sport that can increase myocardial oxygen demand and induce ischemia in patients with heart disease. Ninety percent of deaths while diving occur in the 50+ age range, and consequently, the Diver Alert Network recommends that all patients above the age of 40 be evaluated for coronary artery disease through an inventory of risk factors and electrocardiography.[3] In patients with findings concerning myocardial ischemia, exercise stress testing is indicated. Ideally, prospective divers should be able to tolerate 13 METs without ischemic findings during testing.[3]

In patients with a known history of myocardial infarction or therapeutic catheterization, it is recommended that divers abstain from SCUBA for at least 6 to 12 months. Beyond this observational period, fitness to dive should again be based on the patient’s exercise tolerance.[3]

Reduced ejection fraction is a relative but not an absolute contraindication to diving. Due to the hemodynamic changes from submersion, patients should know that diving can precipitate acute pulmonary edema. Consequently, it is recommended that patients with a history of congestive heart failure exacerbations refrain from diving. Of note, due to these hemodynamic changes, pulmonary edema has been reported in otherwise healthy divers; however, this finding does warrant further evaluation for heart failure with echocardiography.[4][6][4]

Diabetes Mellitus

In addition to being a cardiac risk factor, diabetes mellitus carries its own considerations for diving. As it would be difficult to manage neurological symptoms from blood glucose extremes underwater, current guidelines recommend that divers should not have had any episodes of hypoglycemia or hyperglycemia in the past 12 months. For patients dependent on insulin, their regimen should be stable before diving. Additionally, current guidelines recommend divers maintain a hemoglobin A1c below 9%.[7]

Patent Foramen Ovale and Septal Wall Defects

Patent foramen ovale (PFO), atrial, and ventricular septal defects create a right-to-left shunt in the heart. Thus, these findings theoretically increase the risk for DCS by creating a pathway for nitrogen bubbles to enter the systemic circulation while bypassing the lungs. In practice, epidemiological studies find the association to be somewhat rare. As a result, it is generally recommended not to screen specifically for PFO in the general diving population. In patients with a history of major decompression symptoms without provoking factors, transesophageal echocardiography may be performed to assess for the presence of a PFO or septal defect.[8][9][3] Patients with known PFO or septal defects may dive if these patients can otherwise tolerate exercise without symptoms. It is recommended that patients with known defects restrict their dives to 10 meters in maximum depth to reduce the extent to which nitrogen microbubbles form. Commercial divers with PFO should be offered surgical closure as there is some evidence that the procedure decreases DCS symptoms.[8][4]

Valvulopathy

Valvular disease is a relative contraindication; however, individuals with normal hemodynamics and asymptomatic exercise tolerance may be fit to dive. Despite this, patients with aortic or mitral stenosis and a valve opening < 1.5 cm2 should not dive.[6]

Cardiac Dysrhythmias and Pacing Devices

As in diabetes, the relative contraindication of cardiac dysrhythmias with diving stems from the difficulty in managing an incapacitated patient in the water. Patients with pre-excitation disorders, such as Wolf-Parkinson-White, may be permitted to dive if they have never experienced symptoms or cardiac arrhythmias stemming from their pre-excitation. Patients who have experienced paroxysmal supraventricular tachycardia are contraindicated in diving.[10] Chronic atrial fibrillation is another relative contraindication that may be permitted if the patient has been able to maintain good exercise tolerance.[10] As mentioned, diving can stimulate increased vagal tone, which may incite symptomatic bradycardia in susceptible individuals.[9]

Patients with pacing devices may be able to dive depending on the model; however, it is essential to check the barometric rating for the device.[10] It is recommended that patients with AICDs refrain from diving as the device can trigger while underwater, which could incapacitate the diver or otherwise provoke anxiety. Patients who have received AICD placement prophylactically for genetic susceptibility to dysrhythmia may be permitted to dive as long as they remain within the depth specifications provided by the manufacturer.[4]

Obesity

In addition to being a risk factor for coronary artery disease, obesity is associated with higher nitrogen microbubbles in tissues, as nitrogen has a higher solubility in fat.[6] Thus, there is theoretically a higher chance of DCS in obese patients; however, this has not yet been demonstrated in epidemiologic studies.

Hypertension and Medication Management

While breath-holding is discouraged, it may occur at times while diving and is associated with increased blood pressure. Current guidelines recommend that blood pressure be controlled for at least 3 months before clearance for diving. It should be noted that some agents should be avoided in divers, particularly beta-adrenergic blockers, as these can induce bradycardia when combined with the increase in vagal tone from the diving reflex.

Clinical Significance

SCUBA is a physiologically taxing exercise. Despite the many considerations, the risk factor screening questionnaires and the patient’s exercise tolerance are the most important tools in determining cardiovascular fitness to dive. While there are many relative contraindications, the absolute contraindications remain symptomatic and likely to incapacitate a diver, as explained above.

Other Issues

Other considerations must be accounted for when considering fitness to dive. In particular, respiratory history and otolaryngeal anomalies may cause difficulties with diving. These topics are discussed elsewhere and should be reviewed during the initial screening for diving clearance.

Enhancing Healthcare Team Outcomes

Coordination between a cardiologist and a hyperbaric or diving physician may be useful so that the cardiologist understands the stressors and physiology associated with diving and for the diving physician to be fully aware of the cardiac issues a particular patient is dealing with.