Cerebrovascular reactivity and cerebral autoregulation are improved in the supine posture compared to upright in healthy men and women, 2020, Favre+

[SNT Gatchaman]

Cerebrovascular reactivity and cerebral autoregulation are improved in the supine posture compared to upright in healthy men and women
Michelle E. Favre; Valerie Lim; Michael J. Falvo; Jorge M. Serrador

Cerebrovascular reactivity and cerebral autoregulation are two major mechanisms that regulate cerebral blood flow. Both mechanisms are typically assessed in either supine or seated postures, but the effects of body position and sex differences remain unclear.

This study examined the effects of body posture (supine vs. seated vs. standing) on cerebrovascular reactivity during hyper and hypocapnia and on cerebral autoregulation during spontaneous and slow-paced breathing in healthy men and women using transcranial Doppler ultrasonography of the middle cerebral artery.

Results indicated significantly improved cerebrovascular reactivity in the supine compared with seated and standing postures (supine = 3.45±0.67, seated = 2.72±0.53, standing = 2.91±0.62%/mmHg, P<0.0167). Similarly, cerebral autoregulatory measures showed significant improvement in the supine posture during slow-paced breathing. Transfer function measures of gain significantly decreased and phase significantly increased in the supine posture compared with seated and standing postures (gain: supine = 1.98±0.56, seated = 2.37±0.53, standing = 2.36±0.71%/mmHg; phase: supine = 59.3±21.7, seated = 39.8±12.5, standing = 36.5±9.7°; all P<0.0167). In contrast, body posture had no effect on cerebral autoregulatory measures during spontaneous breathing.

Men and women had similar cerebrovascular reactivity and similar cerebral autoregulation during both spontaneous and slow-paced breathing. These data highlight the importance of making comparisons within the same body position to ensure there is not a confounding effect of posture.

Link | PDF (PLOS ONE)

 

[SNT Gatchaman]

Referenced in Cerebral Blood Flow in Orthostatic Intolerance (2025, Journal of the American Heart Association)

 

[Utsikt]

Similarly, cerebral autoregulatory measures showed significant improvement in the supine posture during slow-paced breathing.

Do they speculate on why this is?

 

[forestglip]

Do they speculate on why this is?

They give a few ideas, but give "however"s for all of them and ultimately say they aren't sure.

One explanation for improved autoregulation when supine could be because of reduced spectral power of mean arterial pressure and reduced pressure oscillations challenging the autoregulatory system. The cerebral vasculature may not have been challenged as much because there was a lower driving stimulus, i.e. pressure changes. However, there was also significantly lower mean amplitude of oscillations in middle cerebral artery velocity, and the magnitude of difference between supine vs. seated and standing were greater in cerebral flow velocity than mean arterial pressure. The greater difference between the postures in middle cerebral flow velocity compared to mean arterial pressure amplitude of oscillations is consistent with lower gain when supine.

We also must consider the possibility that the increased variability in end-tidal CO2 during slow breathing may have affected the autoregulatory measures. However, the variability (Table 4) was not significantly different across the three postures, so end-tidal CO2 changes across the slow breathing cycles were unlikely to affect the postural results.

Increased vasoconstriction improves autoregulation [29], and cerebrovascular resistance was significantly higher when supine, which could have caused the improved autoregulation. However, cerebrovascular resistance was also higher when supine during spontaneous breathing, but we did not find improved autoregulation in that condition, making it unlikely that higher vascular resistance was the sole driver of the improved autoregulation.

Another possible cause of improved autoregulation when supine that must be considered is the role of the vestibular system. However, vestibular effects on the cerebrovascular response would likely be related to changes in cerebrovascular resistance induced through vestibular pathways. As we discussed previously, differences in resistance do not appear to explain differences in autoregulation, suggesting vestibular inputs are unlikely to play a central role.

Therefore it remains unclear why autoregulation was improved during slow breathing while supine.

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Another study on breathing's influence on cerebral blood flow:

S4ME thread: Respiratory influence on cerebral blood flow and blood volume – A 4D flow MRI study, 2025, Söderström et al. (Journal of Cerebral Blood Flow & Metabolism)

[...] the arterial inflow was significantly larger during exhalation compared to inhalation, whereas the venous outflow was significantly larger during inhalation compared to exhalation. [...] Taken together, our results clearly demonstrate respiratory influences on cerebral blood flow.