Mitigating Initial Orthostatic Hypotension: Mechanistic Roles of Muscle Contraction Versus Sympathetic Activation

BACKGROUND: Initial orthostatic hypotension (IOH) is defined by a large drop in blood pressure (BP) within 15 s of standing. IOH often presents during an active stand, but not with a passive tilt, suggesting that a muscle activation reflex involving lower body muscles plays an important role. To our knowledge, there is no literature exploring how sympathetic activation affects IOH. We hypothesized involuntary muscle contractions before standing would significantly reduce the drop in BP seen in IOH while increasing sympathetic activity would not. METHODS: Study participants performed 4 sit-to-stand maneuvers including a mental stress test (serial 7 mental arithmetic stress test), cold pressor test, electrical stimulation, and no intervention. Continuous heart rate and beat-to-beat BP were measured. Cardiac output and systemic vascular resistance were estimated from these waveforms. Data are presented as mean +/- SD. RESULTS: A total of 23 female IOH participants (31 +/- 8 years) completed the study. The drops in systolic BP following the serial 7 mental arithmetic stress test (-26 +/- 12 mm Hg; P=0.004), cold pressor test (-20 +/- 15 mm Hg; F<0.001), and electrical stimulation (-28 +/- 12 mm Hg; P=0.01) were significantly reduced compared with no intervention (-34 +/- 11 mm Hg). The drops in systemic vascular resistance following the serial 7 mental arithmetic stress test (-391 +/- 206 dynexs/cm(5); P=0.006) and cold pressor test (-386 +/- 179 dynexs/cm(5); P=0.011) were significantly reduced compared with no intervention (-488 +/- 173 dynexs/cm(5)). Cardiac output was significantly increased upon standing (7 +/- 2 Umin) compared with during the sit (6 +/- 1 L/ min; P<0.001) for electrical stimulation. CONCLUSION: Sympathetic activation mitigates the BP response in IOH, while involuntary muscle contraction mitigates the BP response and reduces symptoms. Active muscle contractions may induce both of these mechanisms of action in their pretreatment of IOH.

Automated summary

Initial orthostatic hypotension (IOH) is defined as a large drop in blood pressure within 15 seconds of standing. This study investigated the effects of sympathetic activation and involuntary muscle contraction on IOH. The results showed that sympathetic activation mitigated the blood pressure response in IOH, while involuntary muscle contraction mitigated the blood pressure response and reduced symptoms.