Relationship between baroreflex sensitivity and orthostatic blood pressure increases in humans
Authors: Hamaoka Mami , Leuenberger Urs , Blaha Cheryl , Luck Jonathan , Hamaoka Takuto , Dalton Paul , Sinoway Lawrence 
Published: May 2024

Background: Orthostatic hypertension (OHT) is characterized by an increase in blood pressure (BP) upon standing. Although not a common condition, it is not considered rare. Autonomic dysfunction has been speculated to be a potential cause of OHT as the baroreflex is essential for maintaining homeostasis. However, the exact mechanism responsible for OHT remains unclear. Additionally, the definition of OHT varied among prior reports, and diagnostic criteria have not been firmly established. Thus, this study aims to assess differences in autonomic function in the supine and standing positions in individuals with and without elevated BP upon standing. Hypothesis: Individuals with elevated BP upon standing will have lower cardiovagal baroreflex sensitivity (CBRS) than individuals without elevated BP upon standing. Methods: This study included 90 healthy volunteers (39 ± 14 years; 21 men and 69 women). Brachial BP was measured with a cuff on an upper arm. Beat-by-beat BP with a Finometer and heart rate (HR) with ECG were measured during 10 minutes of supine rest and 10 minutes of standing. Spontaneous CBRS was calculated from the relationship between systolic BP (SBP) and R-R intervals using the sequence method. Heart rate variability (HRV) indices were also calculated. Results: Subjects were sorted into four groups based on the change in brachial SBP upon standing: Control (-5 < ΔSBP < 10 mmHg, n = 53), Mild (10 ≤ ΔSBP < 15 mmHg, n = 22), Moderate (15 ≤ ΔSBP < 20 mmHg, n = 8), and Severe (ΔSBP ≥ 20 mmHg, n = 7). There were no significant differences in age, body mass index, or sex ratio among groups (all P > 0.05). Supine CBRS was significantly lower in the Severe group than in the Control group (9.8 ± 4.1 vs. 16.9 ± 9.0 ms/mmHg, P = 0.003) and the other two groups (all P < 0.05). Additionally, ΔSBP was negatively correlated with supine CBRS in subjects with ΔSBP ≥ 10 mmHg (R = -0.39, P = 0.02, n = 37). Supine high-frequency (HF) power of HRV, an index of vagal activity, was also significantly lower in the Severe group than in the Control group (493 ± 503 vs. 1561 ± 2277 ms2, P = 0.006). Standing induced significant decreases in CBRS and HF power in all groups (all P < 0.05). During standing, CBRS remained significantly lower in the Severe group than in the Control group (P = 0.045), but no other significant differences were observed in CBRS or HRV indices between the groups. Discussion and Conclusion: OHT is associated with adverse cardiovascular events. Our data show that supine baroreflex sensitivity and vagal tone are lower in individuals with higher (ΔSBP ≥ 20 mmHg) elevated BP upon standing. Additionally, a negative relationship was observed between CBRS and elevated SBP (ΔSBP ≥ 10 mmHg) upon standing. These findings suggest that altered or impaired autonomic function may contribute to OHT. In addition, the data support the recent consensus definition of clinical OHT (ΔSBP ≥ 20 mmHg). Further studies are warranted to examine the mechanisms in more detail. This study was supported by NIH R01 HL144781 and R01 HL141198. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.