Sense of impending doom: inhibitory activity in waiting blood donors who subsequently experience vasovagal symptoms.

This study examined autonomic and hemodynamic processes associated with the development of naturally occurring vasovagal responses. Data from a study assessing the physiological correlates of an intervention to reduce vasovagal responses in blood donors were examined (Ditto et al., 2009). Ninety-eight participants were assigned randomly to groups that either practiced applied tension or not. Dependent variables included ratings of vasovagal symptoms, heart rate, blood pressure, and other parameters derived from ambulatory impedance cardiography. Participants who subsequently experienced vasovagal symptoms had a lower ratio between low and high frequency components (LF/HF HRV) of heart rate variability (HRV) before blood donation, suggesting lower sympathetic nervous system activity. They also showed sharper decreases in total peripheral resistance and lower respiration rates. The results suggest that vasovagal reactions that begin during anticipation of a medical procedure may be characterized by an inhibitory process from the outset and do not support the belief that reactions follow a diphasic pattern.