Inclusion Criteria:  Have been suspected of a circulatory arrest for any cause. Have had HeartStart HS1 Defibrillator with electrodes:  Adult SMART Pads Cartridge [REF: M5071A], Infant/Child SMART Pads Cartridge (REF:M5072A) applied to their body and powered up with HS1 battery (Model M5070A), regardless of whether a defibrillation shock was delivered  Exclusion Criteria:  Subjects will be excluded if any of the following are present: AED or pad use other than the HeartStart HS1 Defibrillator with above referenced Philips electrode pads. Note: Use of a device other than the Philips HeartStart HS1 AED does not preclude enrollment if the AED or an Advanced Life Support defibrillator was used after the HeartStart HS1 AED. AED used for training purposes.