A wearable cardioverter defibrillator with a low false alarm rate

Abstract

Introduction: A wearable cardioverter defibrillator (WCD) is indicated in appropriate patients to reduce the risk for sudden cardiac death. Challenges for patients wearing a WCD have been frequent false shock alarms primarily due to electrocardiogram noise and wear discomfort. The objective of this study was to test a contemporary WCD designed for reduced false shock alarms and improved comfort.

Methods: One hundred and thirty patients with left ventricular ejection fraction ≤40% and an active implantable cardioverter defibrillator (ICD) were fitted with the ASSURE WCD (Kestra Medical Technologies) and followed for 30 days. WCD detection was enabled and shock alarm markers recorded, but shocks and shock alarms were disabled. All WCD episodes and ICD ventricular tachycardia/ventricular fibrillation (VT/VF) episodes were adjudicated. The primary endpoint was the false-positive shock alarm rate with a performance goal of 1 every 3.4 days (0.29 per patient-day).

Results: Of 163 WCD episodes, 4 were VT/VF and 159 non-VT/VF (121 rhythms with noise, 32 uncertain with noise, 6 atrial flutter without noise). Only three false-positive shock alarm markers were recorded; one false-positive shock alarm every 1333 patient-days (0.00075 per patient-day, 95% confidence interval: 0.00015-0.00361; p < .001). No ICD recorded VT/VF episodes meeting WCD detection criteria (≥170 bpm for ≥20 s) were missed by the WCD during 3501 patient-days of use. Median wear was 31.0 days (interquartile range [IQR] 2.0) and median daily use 23.0 h (IQR 1.7). Adverse events were mostly mild: skin irritation (19.4%) and musculoskeletal discomfort (8.5%).

Conclusion: The ASSURE WCD demonstrated a low false-positive shock alarm rate, low patient-reported discomfort, and no serious adverse events.

Keywords: defibrillator; sudden cardiac death; ventricular arrhythmia.

Figure 1

ASSURE® WCD System (A‐WCD) noise management. The A‐WCD employs three levels of protection to achieve a low false alarm rate due to noise. Level 1 (blue)—minimize noise; Level 2 (red)—detect and remove noise that does occur; and Level 3 (yellow)—allow time for remaining noise to subside before alarming

Figure 2

Electrocardiogram (ECG) sensing. (A) Five ECG electrodes are positioned circumferentially around the torso at the level of the subxiphoid process, labelled left front (LF), right front (RF), left back (LB), right back (RB), and right leg drive (RLD). Red arrows represent the four differential ECG vectors derived using RLD as a ground reference. (B) Garment interior depicting five embedded, cushioned ECG electrodes and defibrillation pads (two posterior and one anterior)

Figure 3

Matched filter training. The black line shows the filter kernel. The colored lines show detected complexes

Figure 4

ASSURE® WCD System (A‐WCD) segment with successful noise filtering. Example electrocardiogram segment from a 43‐year‐old female with dual‐chamber implantable cardioverter defibrillator (ICD) implanted for secondary prevention. ICD interrogation at the final 30‐day follow‐up revealed 3% atrial pacing and 99% ventricular pacing. (A) Signal contains baseline wander that is successfully removed by the algorithm filters. (B) Two channels were disqualified from segment analysis due to poor electrode contact (lead off). (C) This segment was categorized by the algorithm as normal sinus rhythm with a heart rate of 88 bpm and wide R‐wave. The episode was closed without a shock alarm event marker recorded

Figure 5

Initial detection and therapy timeline. The ASSURE® WCD System has two independently programmable therapy zones. Initial detection (gray) is the same for each zone (5/6 overlapping 4.8‐s segments of ventricular tachycardia [VT] or ventricular fibrillation [VF]). Confirmation (yellow) is dependent on zone; 2/2 segments for VF and 15/19 segments for VT. VT that is determined to be disorganized will be treated as slow VF requiring 2/2 segments for confirmation. Once VT or VF is confirmed, a shock alarm event marker is recorded, and the shock alarm sequence (red) is initiated. If 4/6 segments during the shock alarm period are VT or VF, a shock will be delivered. Four out of five non‐VT or VF segments will close an episode. After shock delivery, if an additional 3/5 VT or VF segments are detected, another shock will be delivered (up to five consecutive shocks) and the episode closed if 6/12 non‐VT or VF segments are detected. Electrocardiogram episode storage includes 2 min of onset before episode open, through confirmation and shock delivery and 1‐min post conversion. The shock alarm sequence consists of a triple‐sensory indicator: (1) a flashing red light and shock icon on the monitor, (2) an intense vibration from the alert button, and (3) siren and voice prompts. The rate thresholds are programmable down to 130 bpm for VT and 180 bpm for VF. For this study, the heart rate thresholds were set at nominal (VT at 170 bpm and VF at 200 bpm), and the shock alarm sequence was programmed off