Impact of inspiratory resistance on internal carotid arterial blood flow during orthostatic stress

Background: Orthostatic hypotension, a sustained decrease in systolic blood pressure (BP) of 20 mmHg or more. This condition causes dizziness or syncope and is associated with decreased cerebral blood flow. Thus, preventing the decrease in cerebral blood flow may reduce these symptoms of orthostatic hypotension. Impedance threshold device (ITD) has been shown to alleviate symptoms of orthostatic hypotension. ITD increases intrathoracic pressure and lowers intracranial pressure during inspiration, which may prevent reduced cerebral blood flow during orthostatic hypotension, but this hypothesis has not yet been tested. Moreover, despite significant individual variability in maximal inspiratory pressure (PImax), previous studies have utilized only an absolute threshold of -7 cmH 2 O for ITD, leaving the validity of using relative intensity unexplored. Purpose: This study aimed to clarify whether using an ITD would suppress the decrease in internal carotid arterial blood flow (ICA), an index of cerebral blood flow during orthostatic stress, and to identify the optimal intensity of ITD. Methods: Six healthy adults (five males and one female, 22 ± 2 years, BMI 20 ± 1 kg/m 2 , mean ± standard deviation) underwent four trials of orthostatic loading using lower body negative pressure (LBNP, -30 mmHg). After 4 minutes of controlled breathing in each trial, LBNP was performed for 4 minutes with or without ITD. ITD intensity was defined as -7 cmH 2 O, 13%PImax, and 15%PImax. During controlled breathing and LBNP, the breathing frequency was determined as 10 breaths/min. BP and stroke volume (SV) were measured using finger photoplethysmography. ICA blood velocity and diameter were measured using a duplex Doppler ultrasound. ICA blood flow was calculated from blood velocity and diameter. ΔSV and ΔICA blood flow were calculated by subtracting the values during LBNP alone from those during LBNP with ITD. Results: ΔSV varied depending on the intensity of the ITD, with higher values observed in the trial with 15%PImax ITD than in the trial with -7 cmH 2 O ITD (0.1 ± 2.5 ml vs. 4.2 ± 3.3 ml, P = 0.01). No significant difference in ΔICA blood flow was found between trials (without ITD, -5.6 ± 7.3 ml/min; -7 cmH 2 O, -6.2 ± 21.1 ml/min; 13%PImax, 9.1 ± 13.6 ml/min; 15%PImax, 4.0 ± 17.8 ml/min; P = 0.35). However, ITD prevented LBNP-induced reduction in ICA blood flow for 83% of participants (ΔICA blood flow, positive value) in the trial with 13%PImax ITD. Conclusion: Variations in the intensity of ITD may cause different effects on SV and ICA blood flow. This study was supported by Sapporo Medical University Grants for Programs promoting Academic advancements (2400230). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.