Spiky burst of high frequency heart rate variability: A prodromal sign of syncope accompanying heatstroke

Heart rate variability in Holter electrocardiogram (ECG) that incidentally recorded the onset process of heatstroke during golf play was analyzed in comparison to control ECGs in the same subject and in a healthy control group. During playing golf normally, the amplitude of high-frequency (HF) component, known as an index of cardiac parasympathetic function, was suppressed (4.3 ±2.2 ms). Whereas, 135 min before the onset of syncope, spiky HF bursts (amplitude ~10 ms and width <60 s) appeared repeatedly at an interval of 10-15 min and the burst amplitude increased to 20-30 ms from 51 min before to the onset of syncope. No such phenomenon was observed in control ECGs even during walking exercise. Although spiky HF burst was observed 30% of subjects in control group, none of them showed repetitive burst during short period <15 min. Repetitive spiky HF burst may be a prodromal sign for predicting heatstroke from heartbeat information.