Bartoli et al. (Table 2). We observed the strongest associa- tions with particle number concentration. Discussion The goal of this study was to evaluate the effects of short-term exposure to CAPs on myocardial blood flow. Our results show that acute exposure to CAPs decreases myocardial blood flow during subsequent coronary artery occlusion in chronically instrumented dogs. The decrease in myocardial blood flow was most pronounced in tissue within or near the ischemic zone and was accompanied by increases in coronary vascular resistance. Altered myocardial metabolic demand did not account for changes in myocardial blood flow. Myocardial ischemia results from an imbalance between myocardial oxygen sup- ply and demand. Several lines of evidence suggest that short-term exposure to ambient PM may lead to such an imbalance even in the absence of an acute myocardial infarction. We have previously shown in this animal model that occlusion-induced ST-segment changes (an electrocardiographic marker of myocardial ischemia) are more pronounced after CAPs exposure than after filtered air exposure (Godleski et al. 2000; Wellenius et al. 2003). Pekkanen et al. (2002) found an association between ambient PM levels 2 days earlier and risk of ST-segment depression dur- ing repeated submaximal exercise tests in a panel of subjects with stable coronary heart disease. Gold et al. (2005) found an associa- tion between ambient levels of black carbon, a marker of traffic-related PM, and ST-segment depression during rest and after brief periods of exercise. Mills et al. (2007) recently showed that a 1-hr controlled exposure to dilute diesel exhaust versus filtered air acutely increased exercise-induced ST-segment depression in males with stable coronary heart disease. The results of the present study extend this existing literature by providing direct evidence that the exacerbation of myocardial ischemia observed in previous studies is due at least in part to impaired myocardial blood flow. Our observation that CAPs exposure did not alter cardiac metabolic demand as measured by the rate–pressure product is in agreement with the findings of Mills et al. (2007), as well as our previous study (Wellenius et al. 2003), and suggests that reduced myocardial blood flow rather than altered myocardial energetics may be the primary mechanism of PM-associated exacerbation of myocardial ischemia. Our results do not directly identify the pathophysiologic mechanisms responsible for the observed effects of CAPs on myocardial blood flow, but may provide some insights. Our observation that CAPs exposure increases coronary vascular resistance during ischemia suggests an important role for PM-induced coronary vasoconstriction. Such a role for PM would be consistent with the findings by Brook et al. (2002) of decreased brachial artery diameter (but not endothelium-dependent vas- cular reactivity) after a 2-hr exposure to CAPs and ozone in healthy volunteers. PM exposure might instead affect coronary vascular reactiv- ity, as suggested by findings of increased con- tractile response to endothelin-1 and decreased dilatory response to the nitric oxide donor sodium nitroprusside in the coronary arter- ies of apolipoprotein E knockout (ApoE–/–) mice exposed to diesel exhaust (Campen et al. 2005). In the setting of complete coronary artery occlusion, oxygen supply to the region at risk of ischemia depends primarily on the extent of recruitment of collateral vessels in the hypoperfused region. Conditions that reduce endothelium-derived nitric oxide have been shown to limit the dilator reserve of collat- eral vessels (Klassen et al. 1999; Traverse et al. 1998). Thus, it is plausible that PM-induced alterations in coronary collateral vasoactive responses could exacerbate myocardial isch- emia by impairing collateral vessel recruitment in the setting of ischemia. Although our study did not test this hypothesis, the observation that the effects of CAPs on myocardial blood flow were more pronounced in tissue within or near the ischemic zone compared with more remote myocardium is consistent with this hypothesis. In these experiments, we assessed myo- cardial blood flow during the second of two sequential coronary artery occlusions 20 min apart. The first coronary artery occlusion each day represents ischemia-reperfusion injury that is expected to result in the initiation of a series of short-term adaptive responses collectively referred to as preconditioning (Vinten-Johansen et al. 2007). Thus, an alternative explanation for the results observed in the present study is that exposure to CAPs limited or delayed the Table 2. Change in myocardial blood flow and coronary vascular resistance associated with an interquartile range increase in concentrations of CAPs mass, particle number, and black carbon. Effecta –0.07 –0.34 –0.11 Myocardial blood flow 95% CI –0.09 to –0.05 –0.38 to –0.30 –0.15 to –0.07 p-Value < 0.001 < 0.001 < 0.001 Effectb 10.5% 17.3% 11.9% Coronary vascular resistance 95% CI 8.7 to 12.4% 12.9 to 22.0% 9.4 to 14.5% p-Value < 0.001 < 0.001 < 0.001 aRepresents change in myocardial blood flow associated with an interquartile range increase in each CAPs metric com- pared with filtered air. bRepresents percent increase in coronary vascular resistance associated with an interquartile range increase in each CAPs metric compared with filtered air. Interquartile ranges were 231.6 μg/m3 for mass concentra- tion, 19,650 particles/cm3 for particle number concentration, and 6.5 μg/m3 for black carbon concentration. protective effects of ischemia-reperfusion injury, which includes recruitment of coronary collat- eral vessels (Billinger et al. 1999). The results of the present study do not directly implicate the PM sources or chemical constituents of ambient PM responsible for the observed effects. We found statistically significant inverse associations between myo- cardial blood flow and concentrations of CAPs mass, particle number, and black carbon, with the strongest association observed with par- ticle number concentration. Given the loca- tion of the HAPC adjacent to a busy urban roadway, this finding may implicate traffic- related pollution as an important source. This notion is consistent with the study of Gold et al. (2005), which found that ambient black carbon, but not fine PM, was associated with enhanced ST-segment changes in a panel of Boston-area elderly subjects. Similarly, the Exposure and Risk Assessment for Fine and Ultrafine Particles in Ambient Air (ULTRA) study by Pekkanen et al. (2002) found that exercise-induced ST-segment changes in a Helsinki, Finland, population were associated with ultrafine PM, carbon monoxide, and nitrogen dioxide (in addition to fine PM), again suggesting a role for traffic-related pol- lution. Because of the relatively small sample size, we could not explore this hypothesis fur- ther in the context of the present study. Strengths and limitations. An important limitation of this study is the small number of animals used in the primary analysis. The surgical preparation and maintenance of the animals used in this study were technically challenging and costly. The unexpected high failure rate of animals prevented us from com- pleting these experiments in a larger cohort as originally planned. When relatively few animals are studied, there is a risk that the observed effects could be driven by chance findings in just one animal. Our secondary analysis using the end point of number of spheres rather than flow allowed us to evaluate the effects of CAPs on myocardial blood flow in five animals. Encouragingly, the results of this analysis were qualitatively similar to those of the primary analysis based on four animals. Still, because of the relatively small sample size, we may have failed to detect important effects (large type 2 error). This study has other potential limitations. First, dogs were exposed to CAPs via a per- manent tracheostomy, which bypasses the nasopharynx. Irritant receptors located in the nostrils, nasopharynx, and upper trachea may participate in respiratory and cardiovascular responses (Coleridge and Coleridge 1984). Bypassing upper-airway receptors implicated in eliciting physiologic responses may exclude important pathophysiologic pathways, espe- cially in dogs, which rely heavily on olfactory senses. Second, although dogs are an excellent ovlume 117 | number 3 | March 2009 • Environmental Health Perspectives CAP metric Mass Particle number Black carbon 336