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Background Cardiac resynchronisation therapy (CRT) is a well-established intervention for selected patients with heart failure, yet response rates remain highly variable. Cardiopulmonary exercise testing (CPET) provides objective functional assessments and may serve as a valuable tool in evaluating CRT efficacy. This systematic review and meta-analysis aim to determine the impact of CRT on key CPET parameters, including VO 2 max, anaerobic threshold, and ventilatory efficiency.Methods This study adhered to PRISMA guidelines and was registered in PROSPERO (CRD42024542614). A systematic search was conducted in MEDLINE, EMBASE, and Cochrane Central Register of Controlled Trials (May 2024) for randomised controlled trials (RCTs), non-randomized trials, and cohort studies evaluating CPET changes in patients undergoing CRT (See figure 1). The primary outcome was VO2 max, while secondary outcomes included anaerobic threshold and ventilatory efficiency (VE/VCO2 slope). The risk of bias was assessed using ROBINS-I and the Cochrane Risk-of-Bias Tool. Meta-analysis was performed using standardised mean differences (SMD), with heterogeneity evaluated via I² statistics.Abstract 5-030 Figure 1PRISMA flow diagram demonstrating study selection and exclusionResults A total of 14 studies met the inclusion criteria, 12 cohort studies and 2 RCTs, encompassing 1,009 patients. CRT was associated with a significant improvement in VO 2 max (SMD = 0.62, 95% CI 0.19–1.05, p = 0.00), indicating enhanced aerobic capacity (See figure 2). Anaerobic threshold and ventilatory efficiency also improved following CRT implantation. However, considerable heterogeneity was observed across studies, likely due to differences in CPET protocols, patient characteristics, and CRT programming. The variability in CRT response highlights the need for better predictive tools. Emerging evidence supports invasive CPET as a potential method for refining patient selection and improving therapeutic outcomes.Abstract 5-030 Figure 2Forest plot for the meta-analysis of the effect of CRT on VO2 max, 0 denotes null effect. (SMD = standardised mean difference)Conclusions CRT significantly enhances exercise capacity and ventilatory efficiency, confirming its physiological benefits beyond echocardiographic improvements. However, patient response remains inconsistent. Invasive CPET may improve the identification of patients who will benefit from CRT by providing direct haemodynamic and oxygen kinetic measurements. It may also guide and optimise post-implantation management. Future research should explore standardised invasive CPET parameters to refine CRT decision-making and enhance clinical outcomes.