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Background Ventricular tachyarrhythmias (VTA) with low and varying signal amplitudes and morphologies may not be successfully identified utilizing traditional implantable cardioverter defibrillator algorithms. Objective Develop and validate a novel algorithm (VF Therapy Assurance, VFTA) to improve detection and timely delivery of high voltage therapy (HVT) for these arrhythmias. Method Arrhythmia detection was simulated on recorded VTA electrograms (EGMs) utilizing Abbott's Merlin.net database. EGMs where an HVT occurred only when VFTA was enabled, or where VFTA provided an HVT >30 seconds earlier than without VFTA were re-adjudicated with physician review. As VFTA never prevents detection or therapy, EGMs where VFTA didn't activate or alter HVT were not adjudicated. Results Among 564,353 recorded VTA EGMs from 20,000 devices, VFTA altered HVT in 105 EGMs from 67 devices. Physician adjudication determined that 81.9% (86/105) of these EGMs were true undertreated VTA episodes and would have received appropriate HVT with VFTA enabled. Furthermore, 65% (56/86) of the episodes were ventricular fibrillation, polymorphic, did not self-terminate during the recording window, or not amenable anti-tachycardia pacing. 87.5% (49/56) would not have elicited HVT without VFTA. Overall, VFTA provided new or earlier appropriate HVT in 0.27% (53/20,000) of devices with an increase in inappropriate HVT in 0.07% (14/20,000) devices. Conclusion The VFTA algorithm successfully identifies VTA missed by traditional detection algorithms, due to undersensed ventricular signals resulting in rate falling below the programmed detection rate. The use of VFTA increases the likelihood of delivering life-saving HVT.