Autonomic nervous system modulation during accidental syncope induced by heat and orthostatic stress

Heart rate variability (HRV) indices (LF, HF, LF/HF, RMSSD, and pNN50) under combined heat and orthostatic stress leading up to and during accidental syncope (EXP group: one man, two women; age: 23.7 +/- 2.9 yr) were compared with data collected from subjects who did not experience syncope (CON group: one man, two women; age: 22.3 +/- 1.5 yr).

Minute averages of HRV indices were collected during 5 min at baseline (Base), 5 min leading up to syncope (PRE), and 5 min during syncope (Syncope) (i.e., 2 min leading up to, 1 min during, and 2 min post-syncope). Data were individually analyzed as 1-min means during Syncope as well as 5-min means during Base, PRE, and Syncope.

Between-group results revealed that LF and LF/HF were significantly higher and HF was significantly lower in EXP compared to CON subjects at minutes 1, 2, and 3 during Syncope. Further, RMSSD (CON: 161.1 +/- 37.0 ms; EXP: 17.5 +/- 13.3 ms) and pNN50 (CON: 26.4 +/- 36.3%; EXP: 1.3 +/- 1.2%) were significantly lower in EXP compared to CON subjects at minute 3 during Syncope. During Syncope, 5-min averages of LF (CON: 46.1 +/- 13.9 nu; EXP: 77.5 +/- 6.6 nu) and LF/HF (CON: 1.0 0.5; EXP: 3.8 +/- 1.7) were significantly higher, and HF (CON: 53.9 +/- 13.9 nu; EXP: 22.5 +/ 1 6.6 nu) was significantly lower in EXP subjects compared to CON.

Our findings show that autonomic nervous system modulation leading up to and during accidental syncope induced by heat and orthostatic stress is characterized by an exaggerated suppression of parasympathetic and elevation of sympathetic activity. Thus, elevated LF and LF/HF, and lower HF, RMSSD, and pNN50 may represent risk factors for accidental syncope.