Abstract 066: Adaptive and physiological cerebrovascular changes at high altitudes: a systematic review and meta‐analysis
Bibliographic record
Abstract
Introduction moderate altitude, considered over 1,500 meters (about 4921 ft) above sea level (m.a.s.l), is a particular environment due to the lower oxygen pressure that conditions the development of adaptive mechanisms such as cerebrovascular changes expressed in cerebral blood flow (CBF). Several studies have explored changes in CBF in small populations, however, changes at different altitudes, populations and units of measurement have not been described. These characteristics have been associated with cerebrovascular adaptation Methods We systematically searched databases (PubMed‐Medline, Embase, Web of Science, Scopus and Google Scholar) until January 2022 for studies evaluating cerebrovascular changes resulting from exposures at altitudes greater and lower than 1500 m.a.s.l. The Newcastle‐Ottawa scale and RoB‐2 tools was used to assess the risk of bias from observational studies and randomized clinical trials, respectively. A meta‐analysis using a random‐effects model to estimate pooled effects was realized for each outcome and a narrative synthesis when this was not possible. GRADE criteria were used to identify the certainty of evidence for each result. Results We included 58 studies (2 randomized clinical trials and 56 non‐randomized clinical trials) with 1959 patients. Most studies were at low risk of bias. We found that in the middle meningeal artery, altitude higher than 1500 m.a.s.l may reduce autoregulatory index (MD −0.42, 95% CI −0.67 to −0.18, I2 84%, 2 studies, low certainty), cerebral oxygen saturation (MD −5.65, 95% CI −8.63 to −2.67, I2 91%), cerebral oxygen saturation (MD −5.65, 95% CI ‐8.63 to ‐2.67, I2 91%) and cerebral oxygen saturation (MD ‐5.65, 95% CI ‐8.63 to ‐2.67, I2 91%): ‐8.63 to ‐2.67, I2: 91%, 3 studies, low certainty), cerebrovascular resistance (MD: ‐25.04, 95% CI: ‐75.61 to 25.54, I2: 100%, 2 studies, low certainty) and Cerebrovascular Conductance (MD: ‐0.01, 95% CI: ‐0.05 to 0.03, I2: 0%, 2 studies, low certainty). Similarly, in the internal carotid and vertebral artery, altitude may increase global cerebral blood flow (MD 200.82, 95% CI 47.17 to 354.46, I2 70%, 2 studies, low certainty). Conclusion Exposure to altitudes above 1500 m.a.s.l reduces autoregulation and cerebral oxygen saturation but increases overall cerebral blood flow. These changes could indicate cerebrovascular adaptations to lower oxygen pressures, although the evidence is of low certainty due to variability between studies.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.010 | 0.025 |
| Meta-epidemiology (narrow) | 0.002 | 0.001 |
| Meta-epidemiology (broad) | 0.015 | 0.023 |
| Bibliometrics | 0.006 | 0.007 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.003 | 0.002 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.002 | 0.002 |
| Insufficient payload (model declined to judge) | 0.005 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".