Our study demonstrates that smoking is associated with microvascular dysfunction as measured by CFR. Both reduced hyperemic flow and increased resting flow were implicated in the lower CFR of smoking twins. The lower CFR in smokers compared to non-smokers remained even after adjusting for a comprehensive set of confounding factors. Since results were unchanged after adjusting for presence and severity of perfusion defects, and the relationship persisted after exclusion of individuals with perfusion defects, the lower CFR in smokers reflects lower microvascular function rather than epicardial stenoses. Importantly, we continued to observe the influence of smoking in a within pair analysis in both dizygotic and monozygotic smoking discordant twins. These results suggest a relationship between smoking and myocardial blood flow, while they rule out shared genetic or environmental confounding factors in this relationship.
Only a few studies have examined the relationship between smoking and CFR using PET 5–11
. Some studies failed to find a difference in CFR between long-term smokers and non-smokers 5–9
, while others found that the hyperemic response to the cold-pressor test was reduced in smokers compared with non-smokers 6, 11
. The different results may be due to variations in the age of the study populations, length of smoking, stress agents used (i.e., dipyridamole, adenosine), and ability to control for confounding factors. Using a co-twin control design, we were able to demonstrate that long-term smoking affects coronary flow reserve; since results were materially unaffected after adjusting for perfusion defects, differences in flow are likely due to differences in microvascular function.
Our results are in agreement with the acute effects of smoking on the coronary circulation in experimental studies. Acute smoking increases baseline myocardial flow, reduces hyperemic myocardial flow, and markedly reduces CFR 5
. Our findings show that similar vascular effects apply to chronic, habitual smoking, and that such effects are likely due to abnormalities of the coronary microcirculation, not to perfusion defects. A relationship between long-term smoking and CFR was also demonstrated by Kaufmann et al. (2000) in a study of the influence of vitamin C on CFR in habitual smokers. However, biomarkers for oxidative stress were not measured in the Kaufmann et al. study.
Inflammation and free-radical mediated oxidative stress are two potential mechanisms whereby smoking may be related to vascular dysfunction 23, 24
. Cigarette smoking can lead to vascular damage by impairing endothelial function 25
, and exacerbating proinflammatory and prothrombotic effects on the vasculature 26
. In addition, free radicals, found in the gas or tar phase of cigarette smoke, can result in the formation of highly reactive oxygen species and favor the formation of oxidized LDL 27
. This, in turn, can decrease microcirculatory function by producing the free radical superoxide anion, which decreases NO production 27
. In our study, consistent with previous reports 12, 25
plasma inflammatory and oxidative biomarkers tended to be higher in smokers than in non-smokers. However even after accounting for inflammatory and oxidative stress biomarkers, the negative relationship between smoking and CFR persisted. This suggests that although inflammation and oxidative stress may be involved in the microvascular effects of acute smoking, they do not fully account for the relationship of long-standing, chronic effects of smoking. In this study, the FORT test and the plasma level of reduced and oxidized glutathione were utilized as biomarkers of oxidative stress. The validity and reliability of these tests has been shown before 28–30,31, 32
. Although many biomarkers of oxidative stress have been proposed, none has yet emerged as the ideal marker 33
. We chose these tests because they provide an acceptable balance between validity, easiness of use and potential clinical utility.
Length of smoking behavior may moderate the effects of smoking on coronary vascular function. In our study, there was a negative relationship between CFR and cumulative smoking exposure, determined by pack years, in current and past smokers, but no relationship with time lapsed since quitting smoking in past smokers. Consistent with these findings, previous studies have found a lower CFR in smokers compared to non-smokers if subjects were middle-aged 10
but not young smokers with a briefer smoking history 5, 7, 9
. As a whole, these data suggest that the effects of smoking on the coronary circulation are cumulative, and long-standing smoking may be particularly deleterious.
CFR was determined by inducing hyperemia with adenosine, a potent coronary vasodilator. Adenosine causes vasodilation partly by endothelium-dependent mechanisms, but largely by endothelium-independent mechanisms through direct stimulation of adenosine A1 receptors. Nevertheless, the results presented here suggest that cigarette smoking is related to microcirculatory dysfunction in asymptomatic subjects. These effects are independent of traditional coronary risk factors and medications, as well as of cardiovascular activation during pharmacological stress as determined by RPP.
Our study results were similar in monozygotic and dizygotic twin pairs discordant for current smoking. This suggests that the effects of smoking on coronary microvascular function are independent of shared familial, genetic and early environmental factors. These results are consistent with studies that found a significant relationship between smoking and coronary heart disease mortality in monozygotic twins 34–36
Our study has limitations that should be acknowledged. This is a cross-sectional study, and thus unable to address the temporal relationship between smoking and coronary blood flow. In addition, the inclusion of only middle aged males limits the generalizability of the results to women or individuals in other age groups.
In conclusion, long-term smoking is associated with impaired coronary vascular function in asymptomatic individuals. Our results, using a quasi-experimental design of matched twin pairs, highlight the adverse effects of smoking in the early phases of cardiovascular disease. Since coronary microvascular dysfunction is, at least in part, reversible, our findings point to the importance of smoking cessation as an effective strategy to prevent cardiovascular disease.