Objectives Epidemiological studies of air pollution on cardiovascular health show associations of cardiac mortality and admissions with exposure to nitrogen dioxide (NO₂) at low concentrations. These associations could be causal or NO₂ could be acting as a surrogate measure for another air pollutant, most likely ultrafine particles. No studies of cardiac susceptibility to acute exposure to NO₂ have been undertaken.

Methods Randomised controlled exposures to NO₂ (400 ppb for 1 h) and air in subjects with coronary heart disease and impaired left ventricular systolic function not taking β adrenoceptor blocking drugs.

Results There were no significant changes in heart rate, blood pressure, leucocyte coping capacity or any heart rate variability measure following NO₂ exposure compared with air.

Conclusion These findings suggest that NO₂ does not affect heart rate variability at these concentrations (which are high for urban background levels) and in the absence of other pollutants. While a synergistic effect has not been ruled out, these data lend support to the idea that the epidemiological data associating cardiac outcomes with NO₂ are more likely due to an associated pollutant rather than NO₂ itself.