Partial Pressure of Arterial Oxygen in Healthy Adults at High Altitudes: A Systematic Review and Meta-Analysis

Importance: With increasing altitude, the partial pressure of inspired oxygen decreases and, consequently, the Pao$_{2}$ decreases. Even though this phenomenon is well known, the extent of the reduction as a function of altitude remains unknown. Objective: To calculate an effect size estimate for the decrease in Pao$_{2}$ with each kilometer of vertical gain among healthy unacclimatized adults and to identify factors associated with Pao$_{2}$ at high altitude (HA). Data Sources: A systematic search of PubMed and Embase was performed from database inception to April 11, 2023. Search terms included arterial blood gases and altitude. Study Selection: A total of 53 peer-reviewed prospective studies in healthy adults providing results of arterial blood gas analysis at low altitude (<1500 m) and within the first 3 days at the target altitude (≥1500 m) were analyzed. Data Extraction and Synthesis: Primary and secondary outcomes as well as study characteristics were extracted from the included studies, and individual participant data (IPD) were requested. Estimates were pooled using a random-effects DerSimonian-Laird model for the meta-analysis. Main Outcomes and Measures: Mean effect size estimates and 95% CIs for reduction in Pao$_{2}$ at HA and factors associated with Pao$_{2}$ at HA in healthy adults. Results: All of the 53 studies involving 777 adults (mean [SD] age, 36.2 [10.5] years; 510 men [65.6%]) reporting 115 group ascents to altitudes between 1524 m and 8730 m were included in the aggregated data analysis; 13 of those studies involving 305 individuals (mean [SD] age, 39.8 [13.6] years; 185 men [60.7%]) reporting 29 ascents were included in the IPD analysis. The estimated effect size of Pao$_{2}$ was −1.60 kPa (95% CI, −1.73 to −1.47 kPa) for each 1000 m of altitude gain (τ$^{2}$ = 0.14; I$^{2}$ = 86%). The Pao$_{2}$ estimation model based on IPD data revealed that target altitude (−1.53 kPa per 1000 m; 95% CI, −1.63 to −1.42 kPa per 1000 m), age (−0.01 kPa per year; 95% CI, −0.02 to −0.003 kPa per year), and time spent at an altitude of 1500 m or higher (0.16 kPa per day; 95% CI, 0.11-0.21 kPa per day) were significantly associated with Pao$_{2}$. Conclusions and Relevance: In this systematic review and meta-analysis, the mean decrease in Pao$_{2}$ was 1.60 kPa per 1000 m of vertical ascent. This effect size estimate may improve the understanding of physiological mechanisms, assist in the clinical interpretation of acute altitude illness in healthy individuals, and serve as a reference for physicians counseling patients with cardiorespiratory disease who are traveling to HA regions

Quantification of intrathoracic fat adds prognostic value in women undergoing myocardial perfusion imaging

AIM Amongst patients with coronary artery disease (CAD), women experience relatively worse outcomes as compared to men. Evidence to date has failed to explore unique female imaging targets as major determinants of cardiovascular risk. We sought to assess the prognostic value of epicardial (EFV) and intrathoracic fat volume (IFV) quantification in women and men with suspected and known CAD. METHODS AND RESULTS Intrathoracic fat volume and EFV were calculated from non-contrast CT and analyzed in a propensity-matched cohort of 190 patients (95 women, mean age 62.5 ± 11.3 years) undergoing myocardial perfusion imaging (MPI) and coronary computed tomography angiography (CCTA) for evaluation of CAD. IFV and EFV were significantly lower in women as compared to men (198.2 ± 78.4 vs 293.2 ± 114.7 cm and 105.6 ± 48.9 vs 135.8 ± 60.9 cm, p < 0.001) and showed a strong association with coronary artery calcium score (CACS) and obstructive CAD in women (p < 0.05), but not in men. Fat volumes were not related to abnormal MPI in either population (p = NS). During a median follow-up of 2.8 years, high IFV was associated with reduced event free survival (log rank = 0.019 vs low IFV) in women, but not in men. Accordingly, a multivariate Cox regression model adjusted for cardiovascular risk factors, CACS, CCTA, and MPI findings selected IFV as a significant predictor of major adverse cardiovascular events (MACE) in women (HR 1.32, 95%CI 1.18-1.55, p = 0.001). CONCLUSION Quantification of IFV provides incremental prognostic value for MACE in women, beyond that provided by traditional risk factors and imaging findings