Smoking and hypertension: what is behind the mask?

In this analysis of the China Nationwide Ambulatory and Home Blood Pressure Registry, Zhang et al. document that cigarette smoking is associated with an increased odds of masked hypertension during the waking hours, as measured by ambulatory blood pressure monitoring (ABPM) and home blood pressure monitoring (HBPM) [1]. The risk is the highest in heavy smokers, individuals aged 55 years or older, and individuals with normal to high-normal office BP. The results corroborate previous evidence that smoking may increase the risk of masked hypertension [2], but also provide several new pieces of information that may help improve the screening and identification of individuals with this condition. Masked hypertension is known to enhance the chance of developing a fatal or nonfatal cardiac or cerebrovascular disease, with a risk comparable to that of sustained hypertension [2–4]. It is a complex phenomenon that recognizes several promoting factors that interplay to its determination (Table 1) [2,5–7]. As confirmed by the study of Zhang and coworkers [1], cigarette smoking is an important determinant of masked hypertension and particularly of daytime masked hypertension. People smoke when they are awake, and although the pressor and tachycardic effect of one single cigarette is relatively long-lasting (15--30 min), it is repeated over and over during the daytime hours in frequent smokers. According to an early study, smoking up to two cigarettes per hour contributes to an average day-time BP level increment of 5–6 mmHg (Fig. 1) [8].TABLE 1: Characteristics more often associated with an increased risk of masked hypertensionFIGURE 1: Systolic and diastolic blood pressure values immediately before (0900 h) and during an 8-h daytime ambulatory blood pressure monitoring (from 1000 to 1700 h) in absence of smoking (full line) and during smoking (dashed line, two cigarettes per hour, 16 cigarettes in total) in six normotensive healthy individuals. Data are shown as hourly means ± standard errors. ∗∗ P < 0.01 and ∗ P < 0.05 smoking vs. no smoking. Data with permission from [8].In their study, Zhang and coworkers determined masked hypertension using two different out-of-office BP-measuring methods in the same individuals [1], obtaining a prevalence of masked hypertension with daytime ABPM of 19% and with evening HBPM of 15%. This difference is expected since when both methodologies are applied to the same individuals, the ABPM has the advantage of a greater sensitivity compared with HBPM [2,9]. In smokers, HBPM may resemble more closely the BP measurement in the doctor's office. Individuals might refrain from smoking long before the self-measurement at home, and thus, the pressor effect of smoking may be attenuated or absent when taking the reading, this leading to underestimation of masked hypertension. On the other hand, HBPM may have the advantage of being less cumbersome for patients and of allowing the collection of BP values and the assessment of masked hypertension over more days, weeks, and even months. Thus, this study confirms that ABPM is the best method to assess masked hypertension, and it should be used to detect more precisely masked hypertension, whereas HBPM might be used to check for the persistence or disappearance of masked hypertension following the therapeutic intervention. The study by Zhang and coworkers [1] also confirms that morning BP self-measured at home may not be ideal for detecting masked hypertension in smokers, particularly when treated with antihypertensive medications. Although this measurement is usually taken in abstinence from cigarette smoking after a night-sleep, it is also contaminated by several confounding factors, including the attenuation of the hypotensive effect of the drug in the hours farthest from the last administration and the BP rising effect of surging from the bed. Another interesting finding of the study, in line with previous observations, is that the prevalence of masked hypertension is higher in individuals with normal to high-normal BP [2,10,11]. This result is particularly relevant in the case of smokers because office BP is usually taken far from cigarette smoking: the discrepancy with out-of-office BP might be larger, and the chance of detecting masked hypertension lower. In spite of the several strengths, the study by Zhang and coworkers presents a few critical issues. First, the authors did not record the exact time of smoking. Thus, we cannot be entirely sure that other factors than smoking per se may have played a role in the determination of masked hypertension, particularly in the case of HBPM, which is based on three consecutive measurements taken in only one session during the whole 24 h. Second, although the Registry also enrolled 1613 women, authors could not include female participants in the analysis because of the low prevalence of smoking in this category (approximately 1%). Thus, no inference can be made on the role of smoking in the genesis of masked hypertension in women, though previous studies have shown that masked hypertension is less likely to be observed in women. Whether this is due, as speculated, to a lower prevalence of smokers among women rather than to other factors, need to be elucidated in future gender studies. Third, as morning BP level in treated individuals may be influenced by the attenuation of the BP-lowering effect of the antihypertensive medications, whereas office BP is usually taken in the doctor's office at the peak of the effect, assessment of masked hypertension on morning BP may lead to overestimation of the phenomenon. As a matter of fact, in this study, the prevalence of masked hypertension was larger when evaluated by morning than evening BP (18 vs. 15%), particularly in treated individuals (19 vs. 15%). In summary, the study hints always to collect information on cigarette smoking in all individuals undergoing out-of-office BP measurement. Among smokers, heavy smokers should be considered as a potential target for the screening of masked hypertension, particularly if their office BP is in the normal or high-normal range and if they are middle-aged male individuals. In this context, future research is needed to develop optimal screening strategies and to understand population-level implications of using ABPM or HBPM for the detection of masked hypertension. ACKNOWLEDGEMENTS Conflicts of interest There are no conflicts of interest.