An external defibrillator includes patient electrodes (20) for obtaining the patient's electrocardiogram (ECG) and for applying a shock to the patient. A microprocessor (24) analyses the patient's ECG using a diagnostic algorithm to detect if the patient's heart is in a shockable rhythm, and shock delivery circuitry (10) is enabled when a shockable rhythm is detected by the diagnostic algorithm. The patient electrodes also allow obtaining a signal (Z) which is a measure of the patient's transthoracic impedance and the microprocessor is responsive to Z to detect conditions likely to cause the diagnostic algorithm to generate a false detection of a shockable rhythm. If such detection is made, the microprocessor prevents detection of a shockable rhythm by the diagnostic algorithm, at least for a period of time.