BACKGROUND: New-onset postoperative atrial fibrillation (POAF) is associated with several cardiovascular complications and higher mortality. Several pathophysiological processes such as hypoxia can trigger POAF, but these are sparsely elucidated, and POAF is often asymptomatic. In patients undergoing major gastrointestinal cancer surgery, we aimed to describe the frequency of POAF as automatically estimated and detected via wireless repeated sampling monitoring and secondarily to describe the association between preceding vital sign deviations and POAF. METHOD: Patients ≥60 years of age undergoing major gastrointestinal cancer surgery were continuously monitored for up to 4 days postoperatively. Electrocardiograms were obtained every minute throughout the monitoring period. Clinical staff were blinded to all measurements. As for the primary outcome, POAF was defined as 30 consecutive minutes or more detected by a purpose-built computerized algorithm and validated by cardiologists. The primary exposure variable was any episode of peripheral oxygen saturation (Spo₂) <85% for >5 consecutive minutes before POAF. RESULTS: A total of 30,145 hours of monitoring was performed in 398 patients, with a median of 92 hours per patient (interquartile range [IQR], 54-96). POAF was detected in 26 patients (6.5%; 95% confidence interval [CI], 4.5-9.4) compared with 14 (3.5%; 95% CI, 1.94-5.83) discovered by clinical staff in the monitoring period. POAF was followed by 9.4 days hospitalization (IQR, 6.5-16) versus 6.5 days (IQR, 2.5-11) in patients without POAF. Preceding episodes of Spo₂<85% for >5 minutes (OR, 1.02; 95% CI, 0.24-4.00; P =.98) or other vital sign deviations were not significantly associated with POAF. CONCLUSIONS: New-onset POAF occurred in 6.5% (95% CI, 4.5-9.4) of patients after major gastrointestinal cancer surgery, and 1 in 3 cases was not detected by the clinical staff (35%; 95% CI, 17-56). POAF was not preceded by vital sign deviations.