Dose-Response Relationship between the Risk of Vasovagal Syncope and Body Mass Index or Systolic Blood Pressure in Young Adults Undergoing Blood Tests

pared with the median SBP (120 mm Hg), lower SBP showed a linear association with a higher risk of VVS (90 mm Hg: 3.18, 1.55–6.54; p for non-linearity = 0.09; Fig. 1 b) after adjustment for age and BMI. There were no significant differences of other baseline characteristics between the VVS and non-VVS groups (hemoglobin: 15.2 ± 1.1 vs. 14.9 ± 1.3g/dL, p = 0.1; age: 18.4 ± 0.8 vs. 18.5 ± 1.3 years, p = 0.72; time from last meal to blood test: 7.6 ± 2.8 vs. 7.9 ± 2.8 h, p = 0.46; and creatinine: 0.79 ± 0.13 vs. 0.78 ± 0.13 mg/dL, p = 0.85). Analysis stratified by gender showed similar results in males, but not in females. In males, VVS risk increased at a BMI of 18.5 kg/m 2 (2.35, 1.41–3.92) and SBP of 90 mm Hg (4.41, 1.91–10.2), but there was no significant relation between VVS and BMI or SBP in females. The gender difference was possibly due to the relatively small female population (females n = 3,522, males n = 15,366), leading to insufficient statistical power. VVS occurs because autonomic “hypersensitivity” to triggering stimuli (dehydration, pain, and stress) results in simultaneous enhancement of parasympathetic tone and suppression of sympathetic tone that slows the heart rate and reduces the blood pressure, leading to cerebral ischemia and syncope [1] . In contrast to blood donation, only 10 mL of blood is collected for health screening. One possibility is that persons with low SBP and low BMI might show an impaired muscle contractile response to a sharp decline of blood pressure. Limitations of this study include its single-center design and no information about psychological factors related to VVS [1] . Although unknown confounders may have influenced the results, we demonstrated a dose-response relation between VVS and BMI or SBP. Lower BMI and SBP were positively associated with the occurrence of VVS. Advice on preventive methods such as the counter-pressure maneuver [4] and water ingestion [5] might be beneficial for high-risk persons, especially those who are “underweight” or have “hypotension”.