Acute smoking also affected cardiac autonomic modulation, as reflected by the significant reduction in the SDNN
and SD2 indexes, which represent overall variability. This
is in agreement with findings described in previous studies
in which changes in heart rate variability indexes were
found in the time and frequency domains after cigarette
smoking.55 Acute changes in cardiac autonomic control
induced by smoking are mostly attributed to nicotine, which
stimulates the release of catecholamines (norepinephrine
and epinephrine) through activation of nicotinic receptors
located on sympathetic nerve endings and the adrenal
glands, causing widespread adrenergic stimulation at the
peripheral level.56