Electrocardiogram (ECG) false alarms are common in electrically-hostile peri-operative environments. Newer integrated monitoring, with sophisticated hardware and software, has the potential to minimise artefacts. However, monitoring issues continue to occur, with the potential for critical incidents and unnecessary and harmful interventions. We describe the root cause analysis of a series of apparent ECG flatline asystolic events that appeared in the operating room shortly after the introduction of new intra-operative monitoring systems. Clinical events and biomedical laboratory testing revealed complete loss of ECG signal with increasing resistance. The new ECG systems had incorporated both software and hardware changes to improve the fidelity of signal acquisition and display, but had become much more sensitive to impedance changes. After we alerted the manufacturer, they added software and hardware updates that resulted in resolution of all incidents of ECG loss-of-signal.