Risk estimation for scuba diving
It is known that, although regular exercise adds to a beneficial cardiovascular risk profile, exercise by itself may trigger cardiovascular events, such as myocardial infarction, sudden cardiac death (SCD) and arrhythmias. By definition, these events occur during exercise or shortly after (usually within one hour) the exercise. The risk of such events depends on the intensity of the exercise and the degree of training of the individual. For instance, the risk of SCD in marathon runners is about 2 per 100,000 runners or one death per 215,000 hours of marathon running (26). The incidence of exercise (jogging) related SCD is about 7-13 death per 100,000 joggers per year (27). Of note is, that these figures are valid for a mainly male population, over 30 years of age. The risk of SCD during exercise in a young population is notably low (in young high school and college age athletes 1: 200,000 to 1: 300,000 per year (28). Of note is also, that these figures hold for previously asymptomatic individuals. The risk of exercise performed by an individual with known cardiovascular disease depends on several factors, especially the underlying cardiac disease. For coronary artery disease, a risk stratification has been given with respect to death and myocardial infarction ( 17,18,19.). For the low risk group ( <1 % mortality/year), exercise does not seem to have an increased additional risk. However, for the moderate (1-3 % annual mortality/year) or high (> 3 % mortality /year) categories, the risk of exercise has not been well established, although it seems low. This could mean, that the additional risk of the effort of scuba diving might not add much to the risk of an adverse event.
Another approach to risk assessment lies in the “Risk of Harm “ formula, derived from the Canadian Cardiovascular Society Consensus conference in 1992. This formula has been developed for the estimation of risk in drivers with an ICD. This formula has the following equitation:
RH =TD x V x SCI x Ac,
in with RH is the Risk of Harm, TD is the proportion of time spent on driving or distance driven in a given time period, V is the type of vehicle driven, SCI is the yearly risk of sudden incapacitation, and Ac is the probability that such an event will result in a fatal or injury producing accident. An acceptable risk of death or injury to others appears to be in Canada 5 in 100.000 persons/year (29,30).
For scuba diving, the formula can be rewritten as follows.
RH: Risk of Harm: this is 15/100,000 divers per year (see introduction)
TD: diving time per year: for the average diver, who performs 20-30 dives a year, TD can be put on 0.003. (averaging the duration of the dive to one hour, the 30 hours per year means 30/8760 hours exposure , equals about to 0.003).
Ac: the probability that an event will result in a fatal or injury producing accident: this can be set to 1, since it is likely that every sudden incapacitation under water will lead to a fatality.
SCI: yearly sudden risk of incapacitation: this can be derived from epidemiological and clinical data.
V: variable: this has to be calculated for normal dives (which is open water diving with compressed air, no decompression dives).
Most diving accidents occur apparently in divers with a low cardiovascular risk profile (SCI maximum 0.01). Divers Alert network (DAN) reports as known medical history in diving fatalities 12% hypertension, 5 % cardiovascular disease and 4 % diabetes mellitus. However, in the majority of fatalities the medical history was unknown. (35) Diving accidents occur due to all kinds of causes like technical problems, barotrauma of ascent and decompression illness. For a diver with a low cardiovascular risk profile, V can be calculated as follows:
V= RH/TDxSCIxAc= 0.00015/0.003x0.01x1=5.
However, this is an all cause fatality calculation. Assuming that about 20 % of the diving casualties is caused by cardiovascular problems, the RH attributable to cardiac problems is (0.2 x 0.00015=) 0.00003. Assuming that cardiac problems under water result in an Ac of 1, V can be calculated as:
So, the chance that an asymptomatic person, with an annual chance of a cardiovascular event of 1 % will have a fatal diving accident due to cardiovascular problems can be estimated to be 3 per 100,000 divers. The simplified formula is thus:
RH = 0.003x SCI.
An SCI of 2% per year renders an RH of 0.00006 (6 casualties due to cardiovascular problems /100,000 divers /year). This is close to the level of acceptance of 5 casualties/ 100,000 drivers /year set by the Canadian Cardiovascular Society, and will be used as a cut off point in the following chapters.
For an asymptomatic person with elevated risk of cardiovascular events, for instance with a risk of acute incapacitation of 5 % per year after a myocardial infarction with severely reduced left ventricular function, the resulting figure is 0.00015 or 15 per 100,000 divers. The relative risk is 2,5 (assuming the level of RH = 0.00006 as the “normal” limit).
Some remarks can be made with respect to the adapted RH formula.
1) The assumption has been made, that the RH attributable to cardiovascular problems is 20 % of all casualties. However, there is a high number of fatalities due to unknown causes or diagnosed as drowning, to which cardiovascular problems may have contributed to some extent. (35) Assuming that a higher percentage, for instance 40 %, of fatalities is caused by cardiovascular problems, would result in a cardiovascular RH of 0,00006. This would result in a V value of 2. The formula then should be rewritten as follows:
RH = 0.006 x SCI.
In order to remain close to the level of acceptance of 0.005, the SCI should be maximal 0.01, or 1 % per year.
2) the average time of diving per year has been put on 30 hours per year. For an enthusiastic diver, who performs 100 dives a year, the formula has to be rewritten as
RH = 0.01 SCI
In this case, even a (low) SCI of 0.01 results in an RH of 0.00010, so beyond the level of acceptance of 0.00006. In other words, the more one dives, the higher the risk of a fatal accident. This can only be “offset” by a reduction in SCI. The threshold for SCI would be 0.006 (RH/0.01 =0.00006/ 0.01), so approximately 0,5 % per year. This is typically the case in young divers (< 35 years), with a SCI of about 0.00001, which renders a RH of 0.00000009, well below the level of acceptance of 0.00006. Otherwise stated: the more you dive, the better your cardiovascular health should be to meet the same level of acceptance.
This might have implications for a “mandatory” diving medical examination by a diving physician for scuba diving. Since the risk increases with exposure time and SCI, one could argue, that a diving medical examination is only useful for the older diver who dives frequently, and that there is no benefit for the older casual diver who dives for instance maximum 10 times a year. (from a cardiovascular risk perspective, there may be other considerations too of course).
3) the V (for all diving fatalities) has been set on 5. This has been done with the presumption that most divers are open water divers using compressed air. Although the common opinion is that for instance diving with enriched air