Rats that rode in a truck on the New York State Thruway between Rochester and Buffalo and were exposed to the same highway pollution that motorists encounter, showed a drop in heart rate and effects on the autonomic nervous system, according to a study published this month in the journal Inhalation Toxicology.
The University of Rochester research helps to explain a disturbing trend noted by epidemiologists: that on smoggy days in urban areas, hospitals report a sharp rise in visits to emergency rooms for treatment of heart attacks.
In humans, disruption of the autonomic nervous system could lead to serious health risks, particularly for people who already have heart disease. Such disruptions are associated with arrhythmia, and sometimes forewarn an early death after a heart attack, said Alison Elder, Ph.D., lead author of the study and a research assistant professor in the Department of Environmental Medicine.
"The fact that exposure to air pollution can change the heart rate, independent of other factors, is a cause for concern," Elder said. "It's important to understand that these changes are taking place outside of the lung. Air pollution is either having a direct effect on the heart in rats or is altering something within the circulatory system."
The investigation began with the hypothesis that rats exposed to fresh vehicle emissions would show the same adverse outcomes that had been documented in a recent European study of humans. That study showed that people exposed to air pollution while riding in a bus or car or cycling to work were more likely to suffer a heart attack within one hour of their morning commute.
Rochester researchers placed aged hypertensive rats in a mobile laboratory for six hours on Interstate 90 in western New York for a total of 320 miles. The rats inhaled aerosols that any motorist would likely inhale on the same route. Researchers monitored the rats' blood pressure, heart rate and took electrocardiogram readings during the trip. Post-exposure recordings began 30 minutes after the trip and continued for five days.
Results showed a maximum drop in heart rate of 40 beats per minute, or a 10 percent decline. The effect persisted for up to 14 hours. In addition, researchers found a 70 percent decrease in the vagosympathetic balance, which is an index of heart rate variability and how the autonomic nervous system responds to change.
Studies such as this one are helping scientists understand why vehicle emissions, even at low levels, cause cardiovascular problems in addition to respiratory problems.
The biggest culprit seems to be the tiniest of air particles, known as ultrafines. They are 60,000 times more numerous than coarse particles found in air pollution. Ultrafines in ambient air are especially harmful because they mix with reactive gases and are taken deep into the lungs as aerosols. At that point, they may interact with the cells that line the blood vessels and enter the circulatory system.
Elder's group is the first to report a link between freshly generated engine emissions and changes in the rat autonomic nervous system, which controls the heart and other key organs.
The investigators are now looking at how on-road aerosols affect blood platelets, which could impact clotting and other essential circulatory functions, Elder said.
The University of Rochester Medical Center has a long history of conducting this type of investigation through a special research center funded by the U.S. Environmental Protection Agency. In 2005 the EPA renewed its commitment to the University by awarding $8 million for additional studies of the link between pollution and heart disease.