The perioperative management in adult cardiac surgery patients remains highly variable across centres and regions. The APECx study (Anaesthesia, Perfusion and surgical practicEs in Cardiac surgery) aims to describe the incidence of key clinical patient outcomes, map global variation in anaesthesia, perfusion and surgical practices in cardiac surgery, identify potential modifiable factors associated with relevant clinical outcomes, and explore global, socioeconomic and sex-based differences within these practices. APECx is a prospective multicentre observational study scheduled to start in 2026 that will be conducted in cardiac surgery centres worldwide. Through a modular design, the research focus will evolve over time by going through multiple study phases during a 10-year period. Participating centres can join or leave each study phase and will be actively recruited over the course of the entire study period. Data collection will occur biannually, including all consecutive eligible cases during two prospectively pre-specified locally defined consecutive weeks within a 3-month window. Data will be collected using a web-based electronic case report form. Collected data will be limited to a clearly delineated minimised dataset. All data are routinely obtained as part of standard clinical care. Overall clinical outcomes include, but are not limited to, intensive care unit (ICU) and hospital length of stay, and in-hospital mortality with a maximum follow-up of 30 days. A pilot study will evaluate the feasibility and quantify the workload for participating sites. This study protocol was approved by the Medical Ethics Review Committee of Amsterdam UMC and will be conducted in accordance with local regulations at each participating centre. APECx is a modular, multiphase, large-scale, international, multicentre cohort study with the potential to contribute to standardised, evidence-based care worldwide. By minimising site burden, APECx will allow centres with varying amounts of resources from various economies to join. The modular design uniquely positions it to provide a broad overview of global perioperative practices in cardiac surgery, while enabling detailed investigation of specific evidence gaps.