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Background Panel 8: Respiratory Exchange Ratio (RER) vs Time, of CPET output has been predominantly used as an objective index to evaluate exercise effort and validate a maximal test (RER ≥1.1). However, a transient increase (overshoot) in respiratory gas analyses, depicted as a rise in RER, has been observed during exercise recovery and may prove to have clinical significance in patients not clearly understood. The study aimed to analyze the clinical relevance of RER behavior after a maximal effort and to understand if this could correlate with other CPET parameters in patients with Heart Failure with preserved ejection fraction (HFpEF).Methods This cross-sectional study included 13 patients of HFpEF identified via the European Society of Cardiology guidelines who underwent a maximal CPET with over 2 minutes of recovery phase at the Cardiac Investigative Unit, Ulster Hospital, Belfast. VO2peak and VE/VCO2 slope were recorded. RER behavior (Panel 8) during recovery was evaluated by assessing maximal value (RER max), the magnitude of RER overshoot (RER mag) which is the percentual value of the difference between RER max and RER peak. Data represented a mean ± S.D with correlation and regression applied. P value <0.05 was considered significant.Results Thirteen (13) patients with HFpEF were included (61.7 ± 5.6 years; 84% male). VO2peak and VE/CO2 were recorded to be 16.3 ± 4.6 and 35.8 ± 10.9, respectively. The mean RER recovery overshoot was recorded at 15.8 ± 7.0% which is even lower than results in heart failure with reduced ejection fraction (Vecchiato et al, 2024). RER overshoot magnitude showed a significant positive correlation with VO2peak (r = 0.648, p =0.017). Correlation between RER magnitude and VE/VCO2 was found to have a negative weak/non-significant (r = -0.167, p=0.631). One patient showed negligible RER overshoot, showing worse metabolic (VO2peak: 14.9 ml/kg/min) and ventilatory efficiency (VE/VCO2: 39.43) when compared to others.Conclusion RER overshoot may prove to be a meaningful cardiorespiratory index to monitor using CPET testing in HF patients having the potential to support clinicians in interpreting patients’ functional impairment and prognosis comprehensively. Thus, analysis of CPET recovery parameters is recommended in clinical decision-making.