GENEVA -- Repeat scuba dives appear to harm the endothelium, particularly when breathing the mix of gases popular with recreational divers, according to a small study.
With the low-nitrogen air mix called nitrox, endothelial function as measured by flow mediated dilation dropped significantly after each dive -- and didn't fully recover between dives, Emeline Van Craenenbroeck, MD, PhD, of Antwerp University Hospital in Antwerp, Belgium, and colleagues reported.
Endothelial repair mechanisms appeared activated as well in their study of 10 divers, presented here at the European Society of Cardiology's EuroPRevent meeting.
Based on the results of this small study, Van Craenenbroeck told MedPage Today that in his opinion the use of regular compressed air was safer for vessels.
Nitrox is popular because its lower nitrogen content allows divers to stay down longer and return to the surface faster with less decompression time needed, she explained.
But in the study, each dive with standard air reduced endothelial function by roughly 2 percentage points from around 5% flow mediated dilation at baseline (P=NS), whereas it dropped roughly 4 percentage points on the initial dive with nitrox and then, without returning to baseline, fell more than 2 percentage points on a subsequent dive two days later (both P<0.05).
"We already knew diving was bad for the endothelium," Van Craenenbroeck told MedPage Today, noting that one prior study suggested that endothelial function didn't return to baseline more than six days after just a single dive.
What effect these repeated dings to blood vessels function and repair mechanisms for frequent divers, such as military and professional divers, have on cardiovascular outcomes isn't known.
"There's no data on it," Van Craenenbroeck cautioned.
But she noted that it might be expected to lead to more atherosclerosis and coronary artery disease.
Still, divers have a choice in the gas mix they use and could attempt to protect their vessels by a preconditioning jog before going down into the water, since this method boosts oxygenation in soccer players and stimulates endothelial progenitor cells in healthy individuals, she suggested.
Her group studied a group of 10 experienced military divers who dove in the open sea with regular compressed air once a day for three days with measurements immediately before and after the dives on day one and three, and then repeated the experiment with nitrox after 15 days of rest.
All the dives were to a depth of 18 m (59 feet) with 47 minutes spent at the bottom. No cases of decompression sickness occurred.
The acute effect of a single dive pooled across the 36 dives was a decrease in endothelial progenitor cells (which differentiate into endothelial cells) circulating in the peripheral blood and an increase in circulating angiogenic cells (which contribute to re-endothelialization by stimulating mature endothelial cells to proliferate).
Together, the two factors seem to indicate an uptick in endothelial repair, the researchers suggested.
But the effects were significant only with nitrox use, they noted.
"Diving with nitrox generates less bubbles during decompression, but elicits more hyperoxia," she told attendees. "We know that the hyperoxia during diving elicits oxidative stress and subsequent endothelial dysfunction."
Primary source: European Association for Cardiovascular Prevention & Rehabilitation
Source reference: Van Craenenbroeck E, et al "Effect of successive dives on circulating cells with vasculogenic potential" EuroPRevent 2011; Abstract 375.