It is estimated that there are about 3 million certified scuba divers in the United States. A large number of these individuals are middle-aged or older and at risk for coronary artery disease. Cardiovascular disease is the third most common cause of death while diving and remains the principal cause of death in the general population. The development of symptoms of angina, pulmonary edema, or sudden cardiac death underwater carries with it a much higher mortality than would the same event on land. This article will review the workloads related to scuba diving, ways to assess risk in those with or at risk of developing coronary artery disease, and make recommendations to make scuba diving safer.
Scope of the Problem
In 2008, Dr. Petar Denoble published a paper showing the annual death rate for scuba divers was 16.4 per 100,000 persons.1 This was similar to the rate of 13 jogger deaths per 100,000 participants each year2 or the risk of driving where motor-vehicle accidents result in 16 deaths per 100,000 persons per year.3 Thus, while the likelihood of dying when scuba diving is quite small, understanding how and why these deaths occur is imperative.
Unfortunately, the ultimate cause of death while scuba diving is drowning. This does not give us great insight into what led to the drowning. Denoble reported on the causative process of 947 fatalities in an attempt to better define scuba diving fatalities.4 He divided this into sequential components: trigger, disabling agent, disabling injury, and cause of death. Cardiac events constituted 26% of disabling injuries, and these events were frequently associated with a history of cardiovascular disease and age greater than 40 years. Thus, it looks like underlying cardiovascular disease is a major component in scuba diving deaths.
Workloads Associated with Scuba Diving
It is clear exercise itself is a cardiovascular stress, and the majority of nontraumatic deaths during exercise are cardiac in origin. In most situations, diving is not particularly physically stressful. However, there are times due to current, waves, wind, and other environmental stressors that demands during diving can reach 20 ml/kg/min (6-7 METS). Exercise capacity is reported in terms of estimated metabolic equivalents of task (METs). The MET unit reflects the resting volume of oxygen consumption per minute (VO2) for a 70-kg, 40-year-old man, with 1 MET equivalent to 3.5 mL/min/kg of body weight.
In the standard Bruce protocol, the starting point (i.e., Stage 1) is 1.7 mph at a 10% grade (5 METs). Stage 2 is 2.5mph at a 12% grade (7 METs). Stage 3 is 3.4 mph at a 14% grade (9 METs), and Stage 4 is 4.2 mph at 16% grade (12 METs). This protocol includes three-minute periods to allow achievement of a steady state before workload is increased.
Thus, a diver with a steady-state exercise capacity of 6-7 METS can expect to manage most diving contingencies without concern for cardiovascular complications. In most occupational exposures requiring increased physical activity, guidelines recommend maintaining workloads below 50% of maximal oxygen consumption. Based on this relationship, a diver who is expected to minimize safety concerns related to environmental contingencies should have a maximum oxygen consumption of 12-13 METS or about 12 minutes on a standard Bruce protocol exercise test. Divers with peak exercise capacity below that level could expect to dive safely in low-stress conditions such as warm water, minimal currents, and calm seas but could develop cardiovascular limitations under stressful diving conditions.
Who Is at Risk?
For divers older than 35 years, the dominant risk for sudden death is from coronary artery disease. Although the incidence of coronary artery disease death is falling, the rising incidence with age makes this diagnosis the most important consideration when clearing divers who are middle-aged or above.
One strategy to lower the risk of cardiovascular deaths would be to screen all adult participants prior to certification, as most exercise-related cardiac events in adults are due to atherosclerotic cardiovascular disease.
The Framingham Risk Score is one of a number of scoring systems used to determine an individual’s chances of developing cardiovascular disease. A number of these scoring systems are available online.5,6 Cardiovascular risk scoring systems give an estimate of the probability that a person will develop cardiovascular disease within a specified amount of time, usually ten to thirty years. Because they give an indication of the risk of developing cardiovascular disease, they also indicate who is most likely to benefit from prevention. For this reason, cardiovascular risk scores are used to determine who should be offered preventive medications such as drugs to lower blood pressure and drugs to lower cholesterol levels.
The population risk for divers could be predicted by using tools such as the Framingham Risk Score, and potential participants with a specific score could be identified and excluded. The problem with this approach is that atherosclerotic cardiovascular disease is prevalent among lower-risk subjects. Also, extremely high-risk subjects are only a small part of the total population. Consequently, the largest absolute number of acute events occurs not in the highest-risk subjects but in the moderate- and lower-risk groups. Excluding the highest-risk group likely has little effect on the total number of deaths.
A Framingham Risk Score lower than 10% ( less than 1% per year risk) is considered a low score. If a subject is assessed to be at low risk in general, that individual is not likely to have an acute coronary event while diving. On the other hand, high-risk individuals (Framingham score greater than 20%) could be at considerable risk and should have further evaluation to evaluate whether diving will be safe. Intermediate-risk individuals with a Framingham score between 10% and 20% should have further risk stratification to assess their risk for an acute coronary event while diving.
In all individuals, regardless of risk, we should practice primary prevention of coronary artery disease. The recommended performance measures for primary prevention are:
- Lifestyle/risk-factor screening
- Dietary-intake counseling
- Diabetes screening and management
- Physical-activity counseling
- Smoking/tobacco cessation
- Weight management
- Blood-pressure control
- Blood-lipid measurement and control
- Global risk estimation with tools such as Framingham Risk Score
- Aspirin use in selected individuals
Implementation of these measures requires performance of a careful history and physical examination, laboratory testing for lipids, and formal assessment of cardiovascular risk.
Performing stress testing in selected individuals, such as those with intermediate- or high-risk Framingham scores, is also an approach. In comparison to younger individuals, far less attention has been paid to designing screening programs for older, usually recreational, athletes. Few detailed preparticipation guidelines exist, and there is little reported experience in this age group. Instead, most authorities focus on strategies used in clinical medicine for the early detection of atherosclerotic diseases, as these are the most common cause of death in this age group.
Since most individuals are asymptomatic, the history is often more helpful in identifying risk factors rather than symptoms.Similarly, there may be few detectable abnormalities at rest or even with exercise, as events are often due to spontaneous rupture of nonobstructive plaque.
The American Heart Association issued recommendations for preparticipation screens in older athletes in 2007.7 This document recommends that older competitive athletes (over 35 to 40 years old) be “knowledgeable” regarding their personal history of coronary artery disease risk factors and family history of premature coronary artery disease. Further, stress testing should be performed selectively for individuals engaging in vigorous training and competitive sports and who meet the following criteria: men over 40 years or women over 55 years with diabetes mellitus, or at least two risk factors or one severe risk factor other than age. Finally, the document recommends education regarding prodromal cardiac symptoms, such as exertional chest pain.
What about Patients with Established Coronary Artery Disease?
Patients with known coronary disease often have been subject to revascularization either by coronary-artery bypass surgery or by percutaneous coronary intervention, usually with implantation of one or more coronary artery stents. The degree of revascularization can determine safety in diving. With complete revascularization, low-stress diving can be accomplished successfully, but diving in rough seas, fast currents, or cold water could be risky. Many divers have returned to diving after either coronary-artery bypass surgery or stenting. Success in return to diving is based on restored exercise capacity without ischemia after revascularization and choosing diving environments that do not produce excess stress on the cardiovascular system.
Patients with significant reduction in left ventricular systolic function (LVEF under 35%) are at risk for exacerbation of congestive heart failure while diving. Water immersion itself results in approximately 700 cc of fluid shift into the central circulation. This could provoke congestive heart failure in patients with impaired left ventricular systolic function. Additionally, most patients with LVEF under 30-35% will have impaired exercise tolerance when diving as outlined above. For these reasons, patients with significant left- ventricular systolic dysfunction should be advised against scuba diving.
1. All adults should be evaluated for their risk of coronary artery disease prior to scuba diving.
2. Selected individuals with intermediate to high-risk Framingham scores should be referred for additional evaluation, such as treadmill testing prior to scuba diving.
3. All individuals should practice primary prevention strategies to decrease their risk for the development of coronary artery disease.
a. Smoking cessation
b. Blood-pressure screening and management
c. Weight control
d. Physical-activity counseling
e. Cholesterol screening and management
4. Patients with coronary artery disease may begin (or return to) diving as long as they have beenre-vascularized with no ischemia on treadmill stress testing, have good exercise tolerance (defined as a maximum exercise capacity of 13 or an ability to sustain a workload of 6 METs), and have relatively preserved left ventricular systolic function.
- Denoble.PJ, Pollock NW, Vaithiyanathan P, Caruso JL, Dovenbarger JA, Vann RD. Scuba injury death rate among insured DAN members. Diving Hyperbaric Medicine 2008; 38:182-188.
- Thompson PD, Funk EJ, Carleton RA, Sturner WQ. Incidence of death during jogging in Rhode Island from 1975 through 1980. JAMA 1982; 247:2535-2538.
- Denoble P, Marroni A, Vann R. Annual fatality rates and associated risk factors for recreational scuba diving. In: Vann R, Lang M, eds. Recreational Diving Fatality Workshop. Durham, NC: Divers Alert Network; 2011.
- Denoble PJ, Caruso JL, Dear G de L, Pieper CF, Vann RD. Common causes of open-circuit recreational diving fatalities. Undersea Hyper Med 2008; 35:393-406.
- University of Edinburgh. Cardiovascular Risk Calculator and Chart. Cvrisk.mvm.ed.ac.uk. Last modified May 28, 2010. Accessed September 14, 2013.
- National Heart, Lung and Blood Institute. Framingham Risk Functions. https://www.framinghamheartstudy.org/risk-functions/index.php. Accessed May 7, 2013.
- Maron BJ, Thompson PD, Ackerman MJ, Balady G, Berger S, Cohen D, et al. Recommendations and considerations related to preparticipation screening for cardiovascular abnormalities in competitive athletes: 2007 update: a scientific statement from the American Heart Association Council on nutrition, physical activity, and metabolism: endorsed by the American College of Cardiology Foundation. Circulation 2007; 115:1643-1655.