Apnoea and diving induce autonomic cardiovascular responses of bradycardia and blood flow redistribution toward vital organs that are aimed at improving subject survival in hypoxic conditions. Among factors that influence autonomic nervous activity, and thus affect cardiovascular responses, cigarette smoking is known to reduce vagal cardiac-nerve activity and to increase sympathetic nervous activity. In this study we have assessed the bradycardia response to apnoea and to diving in human smokers. By recording electrocardiograms, heart rate (HR) was monitored on ten healthy habitual smokers (SM) and ten non-smokers (non-SM), in eupnoea, in air, and in simulated diving (facial immersion in water, 22 °C). The latter two conditions included apnoea and snorkelling sessions lasting 30 s each. Apnoea in air induced a 3% and a 4% HR reduction in SM and non-SM, respectively. Only in the latter, however, instantaneous HR decreased throughout the session, thus showing the occurrence of a weaker response in SM. During apnoea in simulated diving, a delayed and lower-amplitude bradycardia occurred in SM compared to non-SM, HR decreasing by 13% and 22%, respectively. Analogously, the cardiac response to snorkelling in simulated diving was smaller in SM with respect to non-SM, HR decreasing by 7% and 14%, respectively. These response patterns suggest that cardiac homeostatic adjustments to apnoea and diving are impaired in smokers. Besides causing a number of pathologies, cigarette smoking represents a risk factor for subjects performing these activities at an even non-competitive level.
