Biotronik Implantable Cardioverter Defibrillator for Measurement of Intrathoracic Impedance: A Report of Two Cases

ABSTRACT

Objectives

Two patients were received ICDs (Biotronic Iforia7 VR‐TDX). Holter analysis revealed pacing spikes, which occurred regularly at 100 ms intervals following QRS complexes, with consecutive 1024 test pulses at hourly intervals during a specific time, and the VP was 0%.

Methods

By analyzing the dynamic electrocardiogram and consulting relevant literature, it was found that this is a feature of the ICD that measures thoracic impedance.

Results

This phenomenon is a special function for the purpose of transthoracic impedance measurement, which can monitor the heart failure.

Conclusions

This phenomenon should not be regarded as an interference signal or an abnormal sign of pacemaker malfunction.

1. Case 1

A 53‐year‐old man was hospitalized for old myocardial infarction and heart failure. 1 month earlier, he had severe chest pain and was diagnosed with acute ST‐segment elevated myocardial infarction, and a PCI was performed successfully. An electrocardiogram showed a reduced LVEF of 30% in this admission, so an implantable cardioverter defibrillator (ICD), Biotronic Iforia7 VR‐TDX, was implanted on April 1, 2021. The patient was monitored for 24 h using a Holter ECG (Beneware CardioTrack CT‐08S Model). Pulse spikes were seen which occurred regularly at 100 ms intervals following QRS complexes, with consecutive 1024 test pulses at hourly intervals during 13:15–00:15 (Figure 1). Similar signals were obtained during the 01:55–07:55 period, with the pacemaker programming showing VP as 0%.

FIGURE 1.

Pacing spikes occurred regularly at 100 ms intervals following QRS complexes.

2. Case 2

A 61‐year‐old man was clinically diagnosed with dilated cardiomyopathy. The patient underwent Iforia7 VR‐TDX implantation on April 13, 2021, and then monitored for 24 h using Holter ECG. Signal analysis revealed pulse spikes, which occurred regularly at 100 ms intervals following QRS complexes, with consecutive 1024 test pulses at hourly intervals during 13:51–09:51 (Figure 2), and the VP was 0%.

FIGURE 2.

Pacing spikes occurred regularly at 100 ms intervals following QRS complexes.

3. Discussion

These unusual ventricular spikes are explained by the intrathoracic impedance measurement (ITI) of Biotronik ICDs. When the ITI is programmed “ON”, 1024 subthreshold test pulses will be sent per hour between the right ventricular coil and the ICD can. Though visible on the standard ECG leads, the test pulses have no adverse effect on patients. Because the test pulses are delivered 100 ms after Vs or Vp QRS complex meeting the refractory period of myocardium and their energy is too small to capture the heart (1 V @ 0.03 ms) (Rosa, Combes, and Boveda ²⁰¹⁴; Squara and Alonso ²⁰¹⁴; Ząbek et al. ²⁰¹⁹).

Notably, due to the low energy of the test pulses, the corresponding pacing spike signals on the ECG are often not easily discernible. In such cases, it is typically necessary to utilize the dedicated pacing detection mode of Holter recording devices. By employing specialized algorithms, these devices can identify and mark the pacing spikes, allowing for clear visualization of the impedance monitoring process despite the subtle nature of the pacing signals on the ECG trace.

Measurement of intrathoracic impedance has been shown to be useful in early detection of worsening heart failure (Yu et al. 2005; Heist et al. ²⁰¹⁴; Conraads et al. ²⁰¹¹; Bohm et al. ²⁰¹⁶; van Veldhuisen et al. ²⁰¹¹). As heart failure gets worse, accumulation of intrathoracic fluid decreases the impedance of electrical current passed across the lung (Yu et al. 2005). Therefore, fluid index derived from the changes of intrathoracic impedance has been supposed to guide the management of patients with heart failure. However, its reliability remained contradictory in further clinical trials, with wide‐ranging sensitivity (20.7%–76.9%) (Yu et al. 2005; Heist et al. ²⁰¹⁴; Conraads et al. ²⁰¹¹) and no improvement of outcomes in terms of cardiovascular hospitalization (Bohm et al. 2016; van Veldhuisen et al. ²⁰¹¹).

In view of the low specificity of intrathoracic impedance to pulmonary congestion, the intracardiac impedance measured between the RV and LV leads by cardiac resynchronization therapy (CRT) device has been investigated to predict heart failure‐related hospitalization (Stahl et al. 2009; Suzuki et al. ²⁰¹⁷; Delnoy et al. ²⁰²¹). After promising animal experiments (Stahl et al. 2009), BIO. Detect HF II study showed that intracardiac impedance correlated well with the stroke volume determined by echocardiography in a controlled clinical setting. Several studies have shown that the changes in intracardiac impedance correlate with LV volume changes (Delnoy et al. 2021). And without the influence of extracardiac tissues to the measurement, intracardiac impedance was proved to provide more accurate information to predicate worsening heart failure (Suzuki et al. 2017).

With further research, it was made known that the performance in predicting outcomes in a complex clinical syndrome by monitoring single parameter is limited (Maier et al. 2019; Whellan et al. ²⁰¹⁰). Therefore, intrathoracic impedance‐based multiparameter monitoring has been widely studied (Whellan et al. 2010; Boehmer et al. ²⁰¹⁷; Vamos et al. ²⁰¹⁸).

In PARTNERS HF study (Program to Access and Review Trending Information and Evaluate Correlation to Symptoms in Patients with Heart Failure study), the clinical outcomes of intrathoracic impedance monitoring could be improved by combining additional diagnostic parameters such as duration of atrial fibrillation, patient activity, nocturnal HR, heart rate variability, and biventricular pacing rate with CRT (Whellan et al. 2010). Vamos et al. refined the heart failure detection algorithm described in PARTNERS HF study and shown improved clinical reliability by increasing specificity from 37.5% to 86.5%, positive predictive value from 34.1% to 69.8% and area under the curve from 0.787 to 0.922 (p < 0.01), without a relevant loss in sensitivity (96.9% vs. 93.8%) (Vamos et al. 2018). Similarly, the HeartLogic, a multisensor algorithm combining intrathoracic impedance with heart sounds and respiration rate among other sensor data, showed high sensitivity of 70% at a false alert rate of 1.47 per patient‐year in the MultiSENSE (Multisensor Chronic Evaluation in Ambulatory Heart Failure Patients) study (Boehmer et al. 2017). And daily implant‐based multiparameter remote monitoring had a significantly beneficial effect on the composite clinical score and all‐cause mortality in heart failure patients treated with ICDs and CRT‐Ds (Hindricks et al. 2014). Different from the other trials that focus on combination of several predictive parameters, the SELENE HF (Selection of potential predictors of worsening heart failure) study developed a predicting algorithm based on temporal trends of remote monitoring data with sensitivity of 65.5% and only 0.69 false alerts per patient‐year (D'Onofrio et al. ²⁰²²).

Since intrathoracic impedance plays an important role in worsening heart failure prediction, Cardiologists need to be informed about its specific measurement in patients with Biotronik ICDs.

Author Contributions

Xuqing Ying: conceptualization, investigation, writing – original draft. Xinyue Yu: methodology, writing – review and editing. Weixun Cai: investigation, visualization. Xiaomin Chen: methodology. Lihua Wang: formal analysis. Qingcheng Wang: supervision, project administration. Jie Pang: supervision, funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data are available upon request from the authors. The data that support the findings of this study are available from the corresponding author upon reasonable request.