To the Editor:

We have read with great interest the recently published research about shift work and vascular events by Vyas et al in BMJ (1). These authors have performed a systematic review and meta-analysis including 34 observational studies that reported risk ratios for vascular morbidity, vascular mortality, or all cause mortality in relation to shift work. The main finding of the study was that shift work was associated with vascular events, which may have implications for public policy and occupational medicine. However, we would like to make some comments regarding the role of the obstructive sleep apnea syndrome (OSAS) as an important cardiovascular risk factor in this population. The relationship between OSAS and cardiovascular events in the general population is well known (2). The question is if the main risk factor is OSAS or shift work.
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Tafil-Klawe et al (3) have evaluated daily variability in the cardiovascular response to arterial chemoreceptors activation during breath holding in shift workers who had obstructive sleep apnea The subjects were examined under two experimental protocols: after day-shift work and after night-shift work. During the part of the study conducted after day-shift work, repetitive apneas led to a significantly greater increase in blood pressure in the sleep apnea patients, as compared with the controls. Under the conditions of sleep deprivation (night-work shift), greater blood pressure responses to breath holding were observed in all subjects, but they were larger in the group of sleep apnea patients. In conclusion, their results strongly suggest that OSAS accompanied by arterial hypertension and obesity, should be considered a contraindication for night-shift work.

On the other hand, Paciorek et al (4) have shown that shift worker with OSAS have a significantly higher severity of OSAS during daytime sleep after night shift comparing with OSAS patients not working on shift and this may intensify unfavorable health effects of OSAS.

Laudencka et al (5) have study the direct effect of night-work on the occurrence of obstructive apneas during sleep after a night shift in fast-rotating shift workers with sleep-related breathing disorders. Eight obstructive sleep apnea patients were examined with the use of a polysomnograph during sleep under two conditions: after day-shift work and after night-shift work. Both sleep studies were conducted within 2 to 3 weeks of each other. In four of the 8 subjects, during sleep after a night-shift, an increase in apnea/hypopnea index was found. Night work significantly increased several breathing variables: total duration of obstructive apneas during REM sleep, mean duration of obstructive apneas during arousal, and apnea index during arousal. Authors conclusion was that in a subpopulation of OSAS patients, acute sleep deprivation may worsen obstructive sleep apnea index.

Therefore, there is some evidence that night-shift work can worsen the severity of sleep apnea syndrome in patients already diagnosed as having OSAS, can significantly increase the blood pressure as compared with non OSAS patients and all of this may intensify the importance of OSAS as a cardiovascular risk factor. In fact, some authors have said that this condition should be considered a contraindication for night-shift work. In modern society, the number of people working night shifts is increasing and OSAS should be investigated in shift workers. Further studies are necessary to clarify such an important issue.

References
1. Vyas MV, Garg AX, Lansavichus AV, Costella J, Donner A, Laughsand LE et al. Shift work and vascular events: systematic review and meta-analysis. BMJ 2012;345:e4800 doi: 10.1136/bmj.e4800 (Published 26 July 2012).
2. Jackson G. Obstructive sleep apnoea - a marker of increased cardiovascular risk. Int J Clin Pract. 2012 May;66(5):421-2.
3. Paciorek M, Korczyński P, Bielicki P, Byśkiniewicz K, Zieliński J, Chazan R. Obstructive sleep apnea in shift workers. Sleep Med. 2011 Mar;12(3):274-7.
4. Tafil-Klawe M, Klawe JJ, Złomańczuk P, Szczepańska B, Sikorski W, Smietanowski Daily changes in cardiac and vascular blood pressure components during breath holding episodes in obstructive sleep apnea patients after day-shift and night-shift work. Physiol Pharmacol. 2007 Nov;58 Suppl 5(Pt 2):685-90.
5. Laudencka A, Klawe JJ, Tafil-Klawe M, Złomańczuk P. Does night-shift work induce apnea events in obstructive sleep apnea patients? J Physiol Pharmacol. 2007 Nov;58 Suppl 5(Pt 1):345-7.

Competing interests: None declared