Vasovagal syncope (VVS) is the most common type of syncope with associated fatigue, nausea, abdominal discomfort, and dizziness. VVS usually resolves uneventfully, but sometimes leads to head trauma or other injuries. Blood tests are the most commonly performed medical tests, and prevention of VVS associated with blood sample collection is important from a medical or public health perspective.
We investigated the association between body mass index (BMI), systolic blood pressure (SBP), estimated circulating blood volume (CBV), and the risk of VVS in healthy young adults undergoing blood tests, and attempted to quantify potential dose-response relations and identify high-risk populations.
We examined 18,888 students entering the University of Tokyo between 2011 and 2016, who had a normal electrocardiogram, no structural heart disease, and no previous syncopal loss of consciousness. VVS was defined as syncope occurring before, during, or immediately after blood collection, with symptoms including weakness, pallor, cold sweats, nausea, vomiting, a lower systolic blood pressure or pulse rate compared with baseline, and need for assistance from a physician without any sequelae. Occurrence of VVS was confirmed by medical staff. CBV was calculated according to Nadler’s method.
The subjects had a mean age of 19±1 years and 19% (n=3522) were women. Mean BMI, SBP, and CBV were 21.0±2.9 kg/m2, 121±12 mmHg, and 4.4±0.4 L for men, and were 19.8±2.3 kg/m2, 108±11 mmHg, and 3.3±0.3 L for women, respectively.
VVS was diagnosed in 0.63% of the subjects (n=119) (19% women). By dose-response analysis, lower BMI showed a non-linear association with a significantly higher risk of VVS in men (p for non-linearity<0.001). Compared with a BMI of 22 kg/m2, the risk of VVS increased markedly at a BMI of 20 (OR 1.52; 95%CI 1.02-2.27) and increased further at a BMI of 18.5 (2.35; 1.41-3.92). Similarly, compared with the median SBP (120 mmHg), lower SBP showed a non-linear association with a higher risk of VVS (110 mmHg: OR 1.46, 95%CI 1.11-1.91; 100 mmHg: 2.51, 1.57-4.02; and 90 mmHg: 4.41, 1.91-10.2) (p for non-linearity=0.04). There was a negative linear correlation between CBV and VVS (P for non-linearity=0.43) in men. Lower CBV was associated with a higher risk of VVS. However, there were no significant correlations of these parameters in women. Also, there were no significant association between VVS risk and other covariates.
We demonstrated that lower BMI and SBP showed a J-curve association with the risk of VVS in men. Low BMI, SBP, and CBV are useful predictors for primary prevention of VVS. Providing advice on preventive methods such as counter-pressure maneuvers might be beneficial for high-risk persons, especially those who are “underweight” or have “hypotension”.