Abstract
INTRODUCTION
Exposure to air pollution is a well-established risk factor for cardiovascular morbidity and mortality. Most of the evidence supporting an association between air pollution and adverse cardiovascular effects involves exposure to particulate matter (PM). To date, little attention has been paid to acute cardiovascular responses to ozone, in part due to the notion that ozone causes primarily local effects on lung function, which are the basis for the current ozone National Ambient Air Quality Standards (NAAQS). There is evidence from a few epidemiological studies of adverse health effects of chronic exposure to ambient ozone, including increased risk of mortality from cardiovascular disease. However, in contrast to the well-established association between ambient ozone and various nonfatal adverse respiratory effects, the observational evidence for impacts of acute (previous few days) increases in ambient ozone levels on total cardiovascular mortality and morbidity is mixed.
Ozone is a prototypic oxidant gas that reacts with constituents of the respiratory tract lining fluid to generate reactive oxygen species (ROS) that can overwhelm antioxidant defenses and cause local oxidative stress. Pathways by which ozone could cause cardiovascular dysfunction include alterations in autonomic balance, systemic inflammation, and oxidative stress. These initial responses could lead ultimately to arrhythmias, endothelial dysfunction, acute arterial vasoconstriction, and procoagulant activity. Individuals with impaired antioxidant defenses, such as those with the null variant of glutathione S-transferase mu 1 (GSTM1), may be at increased risk for acute health effects.
The Multicenter Ozone Study in oldEr Subjects (MOSES) was a controlled human exposure study designed to evaluate whether short-term exposure of older, healthy individuals to ambient levels of ozone induces acute cardiovascular responses. The study was designed to test the a priori hypothesis that short-term exposure to ambient levels of ozone would induce acute cardiovascular responses through the following mechanisms: autonomic imbalance, systemic inflammation, and development of a prothrombotic vascular state. We also postulated a priori the confirmatory hypothesis that exposure to ozone would induce airway inflammation, lung injury, and lung function decrements. Finally, we postulated the secondary hypotheses that ozone-induced acute cardiovascular responses would be associated with: (a) increased systemic oxidative stress and lung effects, and (b) the GSTM1-null genotype.
METHODS
The study was conducted at three clinical centers with a separate Data Coordinating and Analysis Center (DCAC) using a common protocol. All procedures were approved by the institutional review boards (IRBs) of the participating centers. Healthy volunteers 55 to 70 years of age were recruited. Consented participants who successfully completed the screening and training sessions were enrolled in the study. All three clinical centers adhered to common standard operating procedures (SOPs) and used common tracking and data forms. Each subject was scheduled to participate in a total of 11 visits: screening visit, training visit, and three sets of exposure visits, each consisting of the pre-exposure day, the exposure day, and the post-exposure day. The subjects spent the night in a nearby hotel the night of the pre-exposure day.
On exposure days, the subjects were exposed for three hours in random order to 0 ppb ozone (clean air), 70 ppb ozone, and 120 ppm ozone, alternating 15 minutes of moderate exercise with 15 minutes of rest. A suite of cardiovascular and pulmonary endpoints was measured on the day before, the day of, and up to 22 hours after, each exposure. The endpoints included: (1) electrocardiographic changes (continuous Holter monitoring: heart rate variability [HRV], repolarization, and arrhythmia); (2) markers of inflammation and oxidative stress (C-reactive protein [CRP], interleukin-6 [IL-6], 8-isoprostane, nitrotyrosine, and P-selectin); (3) vascular function measures (blood pressure [BP], flow-mediated dilatation [FMD] of the brachial artery, and endothelin-1 [ET-1]; (4) venous blood markers of platelet activation, thrombosis, and microparticle-associated tissue factor activity (MP-TFA); (5) pulmonary function (spirometry); (6) markers of airway epithelial cell injury (increases in plasma club cell protein 16 [CC16] and sputum total protein); and (7) markers of lung inflammation in sputum (polymorphonuclear leukocytes [PMN], IL-6, interleukin-8 [IL-8], and tumor necrosis factor-alpha [TNF-α]). Sputum was collected only at 22 hours after exposure.
The analyses of the continuous electrocardiographic monitoring, the brachial artery ultrasound (BAU) images, and the blood and sputum samples were carried out by core laboratories. The results of all analyses were submitted directly to the DCAC.
The variables analyzed in the statistical models were represented as changes from pre-exposure to post-exposure (post-exposure minus pre-exposure). Mixed-effect linear models were used to evaluate the impact of exposure to ozone on the prespecified primary and secondary continuous outcomes. Site and time (when multiple measurements were taken) were controlled for in the models. Three separate interaction models were constructed for each outcome: ozone concentration by subject sex; ozone concentration by subject age; and ozone concentration by subject GSTM1 status (null or sufficient). Because of the issue of multiple comparisons, the statistical significance threshold was set a priori at P < 0.01.
RESULTS
Subject recruitment started in June 2012, and the first subject was randomized on July 25, 2012. Subject recruitment ended on December 31, 2014, and testing of all subjects was completed by April 30, 2015. A total of 87 subjects completed all three exposures. The mean age was 59.9 ± 4.5 years, 60% of the subjects were female, 88% were white, and 57% were GSTM1 null. Mean baseline body mass index (BMI), BP, cholesterol (total and low-density lipoprotein), and lung function were all within the normal range.
We found no significant effects of ozone exposure on any of the primary or secondary endpoints for autonomic function, repolarization, ST segment change, or arrhythmia. Ozone exposure also did not cause significant changes in the primary endpoints for systemic inflammation (CRP) and vascular function (systolic blood pressure [SBP] and FMD) or secondary endpoints for systemic inflammation and oxidative stress (IL-6, P-selectin, and 8-isoprostane). Ozone did cause changes in two secondary endpoints: a significant increase in plasma ET-1 (P = 0.008) and a marginally significant decrease in nitrotyrosine (P = 0.017). Lastly, ozone exposure did not affect the primary prothrombotic endpoints (MP-TFA and monocyte-platelet conjugate count) or any secondary markers of prothrombotic vascular status (platelet activation, circulating microparticles [MPs], von Willebrand factor [vWF], or fibrinogen.).
Although our hypothesis focused on possible acute cardiovascular effects of exposure to low levels of ozone, we recognized that the initial effects of inhaled ozone involve the lower airways. Therefore, we looked for: (a) changes in lung function, which are known to occur during exposure to ozone and are maximal at the end of exposure; and (b) markers of airway injury and inflammation. We found an increase in forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV₁) after exposure to 0 ppb ozone, likely due to the effects of exercise. The FEV₁ increased significantly 15 minutes after 0 ppb exposure (85 mL; 95% confidence interval [CI], 64 to 106; P < 0.001), and remained significantly increased from pre-exposure at 22 hours (45 mL; 95% CI, 26 to 64; P < 0.001). The increase in FVC followed a similar pattern. The increase in FEV₁ and FVC were attenuated in a dose-response manner by exposure to 70 and 120 ppb ozone. We also observed a significant ozone-induced increase in the percentage of sputum PMN 22 hours after exposure at 120 ppb compared to 0 ppb exposure (P = 0.003). Plasma CC16 also increased significantly after exposure to 120 ppb (P < 0.001). Sputum IL-6, IL-8, and TNF-α concentrations were not significantly different after ozone exposure. We found no significant interactions with sex, age, or GSTM1 status regarding the effect of ozone on lung function, percentage of sputum PMN, or plasma CC16.
CONCLUSIONS
In this multicenter clinical study of older healthy subjects, ozone exposure caused concentration-related reductions in lung function and presented evidence for airway inflammation and injury. However, there was no convincing evidence for effects on cardiovascular function. Blood levels of the potent vasoconstrictor, ET-1, increased with ozone exposure (with marginal statistical significance), but there were no effects on BP, FMD, or other markers of vascular function. Blood levels of nitrotyrosine decreased with ozone exposure, the opposite of our hypothesis. Our study does not support acute cardiovascular effects of low-level ozone exposure in healthy older subjects. Inclusion of only healthy older individuals is a major limitation, which may affect the generalizability of our findings. We cannot exclude the possibility of effects with higher ozone exposure concentrations or more prolonged exposure, or the possibility that subjects with underlying vascular disease, such as hypertension or diabetes, would show effects under these conditions.
Collapse