Personalising device therapy by redefining the sensing mechanism of the subcutaneous implantable cardioverter defibrillator

Abstract

Ventricular tachyarrhythmias (VTA) are rapid abnormal heart rhythms which can result in haemodynamic compromise, collapse and sudden cardiac death (SCD). The annual global mortality burden attributed to VTA is approximately 6 million. Fortunately, in populations at high risk of arrhythmic death, the implantable cardioverter defibrillator (ICD) significantly reduces mortality and is superior to medical therapy in both the primary and secondary prevention of SCD.

The subcutaneous ICD (S‐ICD) represents a new approach in defibrillator therapy. Utilising an entirely avascular location, the S‐ICD can diagnose and treat VTA, whilst avoiding the significant complications that have traditionally been associated with transvenous defibrillator leads. Accurate rhythm detection remains vital and increasingly sophisticated diagnostic algorithms are utilised. Life‐saving therapy must never be incorrectly withheld, but inappropriate shocks, which are themselves associated with increased mortality and psychological morbidity, must also be minimised.

The S‐ICD senses electrocardiogram (ECG) signals from a standardised subcutaneous location at which effective defibrillation has been consistently demonstrated. Three different sensing vectors are available of which one is selected for clinical use. Rhythm detection requires certain morphological ECG characteristics to be present in the selected vector and pre‐implant ECG screening is therefore a mandatory requirement.
The commonest cause for vector screening failure is the presence of a low R:T ratio, as this prevents the S‐ICD from easily distinguishing R wave signal (ventricular depolarisation) from T wave signal (ventricular repolarisation). The overall axes of ventricular depolarisation and repolarisation are unique to an individual. R and T wave amplitudes are therefore determined, in part, by the angle from which they are observed.
Mathematical vector rotation is a novel strategy which can manipulate the angle of observation of an individual’s ECG, using data recorded from the current S‐ICD location.
This can produce personalised vectors; unique individualised vectors with a recipient’s maximal R:T ratio.

In this thesis, I will describe how personalised vector generation can be achieved, before applying the technique to a cohort of S‐ICD ineligible patients. Significant improvements in R:T ratio and device eligibility will be demonstrated. I will then explore the broader impact of vector rotation on the current rhythm discrimination properties of the S‐ICD system. I will demonstrate that both ventricular fibrillation detection and supraventricular
tachycardia discrimination are not impaired by vector rotation. These are key principles of S‐ICD sensing which must be maintained by any future sensing strategy.

Finally, I shall consider the phenomena of T wave over‐sensing (TWOS), which despite the current screening process, remains the commonest cause of inappropriate shock therapy in the S‐ICD population. I will describe a new concept, ‘eligible vector time’, and demonstrate experimentally that patients experience chronological fluctuations in their device eligibility. This preliminary work will redefine our current understanding of device eligibility and justify future research into the role of vector rotation in reducing inappropriate shock therapies.

In summary, I believe that clinicians and patients should not be restricted by the inherent limitations of standardised vector selection. Personalised vector generation can be achieved from the current S‐ICD location, whilst maintaining the excellent rhythm detection qualities of the S‐ICD system. Increased S‐ICD eligibility can be achieved and the potential to reduce TWOS in the future cannot be ignored.

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