One minute of exposure to second-hand smoke (SHS) from marijuana diminishes blood vessel function to the same extent as tobacco, but the harmful cardiovascular effects last three times longer, according to a new study in rats led by UC San Francisco researchers.
In a healthy animal, increased blood flow prompts arteries to widen, a process known as flow-mediated dilation (FMD). When FMD is compromised, as happens during SHS exposure, blood flow is impeded, and the risks of heart attack, atherosclerosis and other heart problems increase, said UCSF’s Matthew Springer, PhD, professor of medicine and senior author of the new study.
“Your blood vessels can carry more blood if they sense that they need to pass more blood to the tissues,” Springer said. “They dilate to allow more blood through. But that’s inhibited by exposure to smoke.”
Previous work by Springer and others has shown that as little as one minute of exposure to tobacco SHS diminishes FMD, but the effects of marijuana SHS hadn’t been examined. In the new research, published online in the July 27, 2016, issue of the Journal of the American Heart Association, a team of scientists in Springer’s laboratory measured rats’ FMD, which works similarly to FMD in humans, before and after exposure to both tobacco SHS and marijuana SHS.
The researchers found that rats exposed to marijuana SHS experienced a more than 50 percent reduction in FMD, similar to the reduction in artery function seen in both rats and humans exposed to tobacco smoke in previous studies. As with tobacco, the reduction occurred after just one minute of exposure to SHS from marijuana. However, while rats exposed for one minute to tobacco SHS recover within 30 minutes – an observation that was reproduced in the new study – one minute of exposure to marijuana SHS still significantly affected FMD 90 minutes after the initial exposure.
The research group used equipment designed to mechanically “smoke” cigarettes and fill a reservoir with the resulting smoke. In a series of experiments using marijuana cigarettes, when the smoke in the collecting chamber was determined to be at a level roughly comparable to those found in restaurants that allow smoking, the rats were exposed to the marijuana smoke.
Using methodology that they developed for previous tobacco studies, the researchers temporarily blocked off blood flow to rats’ legs after they were exposed to SHS. They then let the blood rush back into the arteries and used ultrasound technology to measure the resulting widening of the femoral artery, a vessel similar to the human brachial artery of the arm, where FMD is typically measured in clinical studies.
The study fills a void in SHS research, as marijuana studies are difficult to undertake because of its illicit status and the numerous agencies, such as the Drug Enforcement Agency and the Food and Drug Administration, that must approve the use of the drug in experiments.
“The biggest reason that people believe marijuana second-hand smoke is harmless is because the public health community hasn’t had direct evidence of its harmful effects like it does with tobacco,” Springer said. “We hadn’t done the experiments, so I think there is definitely an underestimation of how harmful marijuana smoke is.”