Syncope or "blackout" (BO) in breath-hold diving (freediving) is generally considered to be caused by hypoxia. However, it has been suggested that cardiac arrhythmias affecting the pumping effectivity could contribute to BO. BO is fairly common in competitive freediving, where athletes aim for maximal performance. We recorded heart rate (HR) during a static apnea (STA) competition, to reveal if arrhythmias occur. Four male freedivers with STA personal best (PB) of 349±43s, volunteered during national championships, where they performed STA floating face down in a shallow indoor pool. A non-coded Polar T31 chest strap recorded R-R intervals and a water- and pressure proof pulse oximeter arterial oxygen saturation. Three divers produced STA near their PB without problems, while one diver ended with BO at 5min17s, which was 12s beyond his PB. He was immediately brought up by safety divers and resumed breathing within 10s. All divers attained similar lowest diving HR (47±4bpm), but HR recordings displayed a different pattern for the diver ending with BO. After a short tachycardia the three successful divers developed bradycardia which became more pronounced during the second half of the apnea. The fourth diver developed pronounced bradycardia earlier, and at 2.5min into the apnea HR started alternating between approximately 50 and 140 bpm, until the diver lost consciousness. At resumed breathing, HR returned to baseline. Nadir oxygen saturation was similar for all divers. We speculate that arrhythmia could have contributed to BO, by lowering stroke volume leading to a systolic blood pressure drop, affecting brain perfusion.