Dr. Christoph Schukro ,
Shock delivery by implantable cardioverter defibrillator (ICD) is a life-saving measure in ventricular tachyarrhythmia. Nevertheless ICD shocks have been related to increased mortality in recent studies, when compared to ICD patients without documented shocks. This review takes a look at this "shock paradox".
Although shock delivery by implantable cardioverter-defibrillators (ICD) is a life-saving measure in cases of ventricular tachyarrhythmia, there have been a few notable effects since it was first discovered in the 80's:
Interestingly, intracardiac shocks during defibrillation threshold (DFT) testing lead to a significant reduction in cardiac index only in patients with a reduced LVEF <30% (6), thus suggesting a loss of systolic function especially in previously damaged myocardium. In 2008, a correlation was found between ICD shocks and mortality, and was labeled the shock paradox in ICD therapy.
Accelerated tachyarrhythmia was associated with significantly higher all-cause mortality: Our own single center analysis on a collective of 1,170 ICD patients over a mean follow-up of 5 years was the first to focus on accelerated ventricular tachyarrhythmia, defined as acceleration (or induction) of a ventricular arrhythmia with a decrease of 10% of the initial cycle length caused by ATP or shock delivery (11). Indeed, in previous publications, the incidence of accelerated ventricular arrhythmias ranged between 1% and 5% within follow-up periods of 2 years maximum. (12-16) Acceleration was documented in 8.5% of all patients and in 17% of patients with documented ATP during this longer follow-up period. Almost all ventricular accelerations were induced by predominantly appropriate ATP (97%) and were in the end, terminated with a shock (99%). Occurrence and recurrence of accelerated tachyarrhythmia were significantly correlated with reduced systolic left ventricular function (LVEF <40%). Most notably, accelerated tachyarrhythmia was associated with significantly higher all-cause mortality. In the multivariable analysis, accelerated ventricular tachyarrhythmia and LVEF <40% were independent predictors for all-cause mortality. In patients with documented ATP, acceleration was an independent mortality predictor as well. Therefore, we found once more a significant correlation between shock delivery and mortality.
However, progressive deterioration of systolic heart failure was likely to be the underlying cause for increased mortality in ICD patients (see next paragraph). The detailed interrelations we found in our collective are summarised in the following figure.Figure: Parameters with influence on all-cause mortality in patients with ICD (provided by author from reference 11).
Patients with stable heart failure NYHA-class I-III revealed local injury currents: A recent investigation on DFT testing in 310 patients with stable heart failure NYHA-class I-III revealed local injury currents (LIC) in the post-shock intracardiac ECG as a clue for pre-damaged myocardium. (17) These LIC are late-potential-like abnormalities of the right ventricular ECG. First, the presence of LIC during DFT testing resulted in a significantly higher adverse event rate (i.e. death, or hospitalisation for deterioration of heart failure). Remarkably, a significant correlation between a higher event rate and the occurrence of ICD shocks was only found in patients with documented LIC. On the other hand, a missing LIC leads to an identical adverse event rate in patients with or without ICD shock.
Thus, local injury currents reflecting a pre-damaged myocardium in patients with progression of systolic heart failure might be the missing link between ICD shock delivery and increased mortality.
The presented "shock paradox" is a chicken-and-egg question in cardiology: can ICD shocks be lethal in the long term, or, are patients with a higher risk of death supposed to undergo shocks?Intracardiac shocks are saving lives in cases of ventricular tachyarrhythmia, but a significant correlation between shocks and mortality was reported in several studies on ICD therapy. In conclusion, shock delivery is most likely a surrogate parameter, reflecting higher all-cause mortality based on progressive deterioration of the underlying chronic heart failure with reduced ejection fraction.
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Christoph Schukro, MD, PhD, FESC. Medizinische Universität WienUniv.Klinik für Innere Medizin IIAbteilung für KardiologieAuthor's disclosures: None declared. Other resourcesThe subcutaneous defibrillator: who stands to benefit. Remote monitoring, described.
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