BACKGROUND: Severe bradycardia occurring in strabismus surgery is unpredictable and may lead to cardiac arrest. If we identify patients who have risk of severe bradycardia during strabismus surgery, it is helpful to provide a more meticulous anesthesia. In this study, we investigated the correlation between the degree of bradycardia and indices of power spectral analysis and nonlinear dynamic data analysis of preoperative ECG. METHODS: ECG was collected for twenty minutes from strabismus patients (n = 93). No premedicants were administered. After administration of anesthesia and traction of extraocular muscle, the lowest heart rate was recorded. We calculated the correlation coefficient between the power spectral density, approximate entropy and correlation dimension of preoperative ECG and the difference between preoperative and lowest HR. RESULTS: As the difference between the preoperative heart rate and the intraoperative bradycardia increased, the preoperative ECG showed a low total power (r = 0.27, P = 0.016), low low-frequency power (r = 0.31, P = 0.049), low high-frequency power (r = 0.30, P = 0.007) and high ratio of low to high-frequency power (r = 28, P = 0.03). There was no correlation between bradycardia and approximate entropy and correlation dimension, respectively. CONCLUSIONS: There was a correlation between indices of power spectral analysis of preoperative ECG and degree of bradycardia during strabismus surgery. Therefore we take into consideration preoperative ECG and its analytic indices in order to provide careful preparation for high risk patients who exhibit a tendency to severe bradycardia.