The cardiovascular system can be macro-modelled as a collection of coupled oscillators. Recently, the use of mutual couplings of these oscillators to characterize the state of the system during anaesthesia has been proposed. Assuming that the synchronization status between three systems, namely cardiac, respiratory and cortical oscillators changes with respect to the depth of anaesthesia, we analyzed the synchronization between these three oscillators using a measure for synchronization, the S estimator. Furthermore, we applied a statistical assessment to detect precisely the deep - light change of anaesthesia.