Even with the development of more advanced technology of ECG, there are still problems on interference to ECG signals. Many attempts have been made to detect and eliminate the source of noises and artifacts from the original ECG signals. Several studies have been done to observe and study the EMI effect, however, most of them only focus on the EMI effect of mobile phone during ECG acquisition. Thus, this study is emphasized on the interference problem when other medical devices were being used together with the ECG device. The R-R peak distance of the ECG signal was detected by using QRS detection algorithm invented by J. Pan and W. J. Tompkins. The data from the experiment showed that even the EMI from the medical devices did not affect the physical shape of ECG, but it does affect the R-R peak distance of the ECG signal