Overt and Latent Cardiac Effects of Ozone Inhalation in Rats: Evidence for Autonomic Modulation and Increased Myocardial Vulnerability

Background: Ozone (O(3)) is a well-documented respiratory oxidant, but increasing epidemiological evidence points to extrapulmonary effects, including positive associations between ambient O(3) concentrations and cardiovascular morbidity and mortality. Objective: With preliminary reports linking O(3) exposure with changes in heart rate (HR), we investigated the hypothesis that a single inhalation exposure to O(3) will cause concentration-dependent autonomic modulation of cardiac function in rats. Methods: Rats implanted with telemeters to monitor HR and cardiac electrophysiology [electrocardiography (ECG)] were exposed once by whole-body inhalation for 4 hr to 0.2 or 0.8 ppm O(3) or filtered air. A separate cohort was tested for vulnerability to aconitine-induced arrhythmia 24 hr after exposure. Results: Exposure to 0.8 ppm O(3) caused bradycardia, PR prolongation, ST depression, and substantial increases in atrial premature beats, sinoatrial block, and atrioventricular block, accompanied by concurrent increases in several HR variability parameters that were suggestive of increased parasympathetic tone. Low-O(3) exposure failed to elicit any overt changes in autonomic tone, heart rhythm, or ECG. However, both 0.2 and 0.8 ppm O(3) increased sensitivity to aconitine-induced arrhythmia formation, suggesting a latent O(3)-induced alteration in myocardial excitability. Conclusions: O(3) exposure causes several alterations in cardiac electrophysiology that are likely mediated by modulation of autonomic input to the heart. Moreover, exposure to low O(3) concentrations may cause subclinical effects that manifest only when triggered by a stressor, suggesting that the adverse health effects of ambient levels of air pollutants may be insidious and potentially underestimated.