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Aim: To investigate if tachypneic response to hypoxia is decreased in older patients., Methods: We included all patients ≥65 years of age attending 52 Spanish emergency departments (EDs) for whom peripheral arterial oxygen saturation (SatO 2 ) measured by pulsioxymetry and respiratory rate (RR) were registered at ED arrival. We assessed the relationship between SatO2 and RR in different models, and with the best-fitting model, we independently analyzed this relationship in four subgroups according to patient age (65-69, 70-79, 80-89, and ≥90 years). Five sensitivity analyses using different subsets of patients were carried out to check for the consistency of the results., Results: We included 7126 patients, with medians for SatO 2 and RR of 97% (interquartile range [IQR]: 94-98) and 15 bpm (IQR: 15-16), respectively. We found significant associations (P < 0.001) between SatO 2 and RR in every model tested (P < 0.001 for all), with the quadratic model obtaining the best fit (R 2 : 0.098) over those obtained with linear (R 2 : 0.096) and logarithmic (R 2 : 0.092) models. The same was observed in sensitivity analyses, with R 2 for quadratic models ranging from 0.069 in patients with low comorbidity and 0.102 in patients breathing room air. The mean RR for 100% SatO 2 was 15 bpm and increased as SatO 2 decreased, although with a progressive slowing of the slope, with a mean RR of 27 at 50% SatO 2 . We detected a decreased RR response to increasing hypoxemia according to age and, while the RR curve was higher and with a progressively steepening slope in the 972 patients aged 65-69 (mean RR of 42 bpm with 50% SatO 2 ), a progressive slowing of slope was observed in the 2693 patients aged 70-79 (mean RR of 28 with 50% SatO2), the 2582 aged 80-89 (mean RR of 25) and the 879 aged ≥90 (mean RR of 23). Sensitivity analyses provided very similar results., Conclusion: Tachypneic response to hypoxemia in older patients decreases as age advances, regardless of the reason leading to hypoxemia. Geriatr Gerontol Int 2024; 24: 1120-1129., (© 2024 Japan Geriatrics Society.)