Endothelium is a single cell layer lining the inner surface of blood vessels. It plays an important role in the regulation of circulation and reaction to various kinds of stress. Endothelial dysfunction is associated with the progression of atherosclerosis of blood vessels and heart disease.
Recently it has been shown that underwater diving may transiently decrease the post-ischemic flow mediated dilation (FMD), which is an indicator of endothelial function. This has caused some divers concern and raised questions in the media about the safety of various aspects of diving.
The FMD is a test that uses the ultrasound Doppler method to measure the diameter of brachial artery (the main artery in the upper arm) before and after a five-minute forearm ischemia caused by external compression with a blood pressure measurement cuff. The average diameter of the brachial artery in adults is about four to five millimeters. After the five minutes of circulation occlusion, the flow through the artery increases and the artery diameter increases for about seven to 10 percent of its initial value. This is called the flow mediated dilatation (FMD).
The extent of FMD may be decreased by various factors like exercise, obesity, hormonal status, diurnal changes, fatty meals, acute/chronic moderate to severe alcohol consumption, etc. On the other hand, dietary factors such as light alcohol consumption, chocolate, and various medications like nitroglycerine and statins may transiently increase FMD.
It has been also shown that FMD is transiently reduced after a single compressed air dive, breathing 60% oxygen at surface, a breath-hold dive and a nitrox dive. Both the increased partial pressure of oxygen and circulating gas bubbles have been suspected as possible causes. Oxygen does affect the metabolism and availability of nitric oxide (NO), which plays an important role in the relaxation of arteries, but the effects of it on the FMD are controversial.
Implications in Diving
In general, reported transient changes in FMD are not specific for diving. They appear to be a common endothelial response to a variety of stressors as a part of normal defense mechanisms including self-repair. If the stressors become excessive and exposure chronic, the endothelium may be harmed beyond repair. In extreme cases, dysfunctional endothelium in coronary artery disease may paradoxically cause narrowing of arteries in response to exercise and provoke angina or myocardial infarction. Nothing indicates that diving could reach significance of such an excessive and chronic stressor that it could acutely or in the long-term affect the health of divers through reduced FMD.
So far, only transient decreases of the brachial artery FMD after diving have been reported in asymptomatic divers. FMD is equally affected by repeated breath-hold diving and a single scuba dive on air. Nitrox diving reportedly causes a larger decrease of FMD than air diving. This would be consistent with the dose-dependent effects of hyperoxia; however, hyperbaric oxygen treatment which exposes patients to a much higher partial pressure of oxygen than diving does not appear to affect the FMD. The possible dose-dependent effect of circulating gas bubbles has not been studied, but the greater decrease of FMD observed in nitrox, which is supposed to generate less bubbles, versus air diving is counterintuitive. Most importantly, there is no obvious acute or long-term harm (diseases, increased mortality) associated with reduced FMD after diving.
Having a bite of chocolate or large dose of vitamin C one hour before a dive may prevent the FMD decrease. However, the transient decrease of FMD described so far is not an injury and prevention is not necessary, regardless of how tempting the chocolate is.