Threshold crossing of device-based intrathoracic impedance trends identifies relatively increased mortality risk

Abstract

AimsThreshold crossings of impedance trends detected by implanted devices have been associated with clinically relevant heart failure events, but long-term prognosis of such events has not been demonstrated. The aim of this study is to examine the relationship between alterations in intrathoracic impedance and mortality risk in patients with implantable devices.Methods and resultsWe reviewed remote monitoring data in the de-identified Medtronic CareLink® Discovery Link that captured intrathoracic impedance trends for >6 months. The initial 6 months of the cardiac and impedance trends were used as the observation period to create the patient groups and cross-referenced with the Social Security Death Index for mortality data. In our study cohort of 21 217 patients, 36% experienced impedance threshold crossing within the initial 6 months of monitoring (defined as the ‘early threshold crossing’ group). Patients with early threshold crossings demonstrated an increased risk of age- and gender-adjusted all-cause mortality [hazard ratio (HR) 2.15, 95% confidence interval (CI) 1.95–2.38, P< 0.0001]. Increased mortality risk remained significant when analysed in subgroups of patients without defibrillator shock (HR 2.10, 95% CI 1.90–2.34, P< 0.0001, n= 1621) or within those patients without device-detectable atrial fibrillation (AF) during the initial 6 months of monitoring (HR 2.09, 95% CI 1.86–2.34, P< 0.0001, n= 17 235). Both the number and the duration of early threshold crossings of impedance trends detectable by implanted devices were associated with increased mortality risk. Furthermore, the improvement of altered impedance trends portends more favourable prognosis.ConclusionsThreshold crossing of impedance trends detectable by implanted devices is associated with relatively increased mortality risk even after adjusted for demographic, device-detected AF, or defibrillator shocks.

Prediction of All-Cause Mortality Based on the Direct Measurement of Intrathoracic Impedance

“…Previous studies demonstrated that the presence of an OptiVol fluid index crossing was associated with increased mortality (2). In fact, patients with one or more crossings during the first six months after implant were shown to be at 2.15 times higher risk of a mortal event than patients with no OptiVol crossings.…”

“…In these previous studies, an impedance-derived fluid index (hereafter called “OptiVol fluid index”) was obtained using a cumulative sum mathematical model that calculates the differences between impedance measured in ohms (hereafter called directly “measured impedance”) and a reference impedance; the OptiVol fluid index was expressed in ohm-days. It was demonstrated that when the OptiVol fluid index value exceeded 60 ohm-days, the crossing of this threshold value predicted both increased morbidity and mortality (2–7). This calculated OptiVol fluid index was chosen instead of the directly measured impedance because of several anticipated confounding limitations of the direct impedance measurement.…”

Atrial Tachyarrhythmias Temporally Precede Fluid Accumulation in Implantable Device Patients

“…A fluid index value at or above the threshold (i.e., “fluid index threshold crossing” [FIC]) may be an indicator of thoracic fluid accumulation. Although individual threshold FIC events are not always associated with HF symptoms, FIC events have been shown to be strongly associated with increased risk of HF hospitalization and all‐cause mortality …”

Assessing Impedance in Heart Failure

“…3 Hence, recognizing alterations in impedance trends may uncover vulnerabilities that can be associated with adverse clinical events. [4][5][6][7] Meanwhile, accessing the information requires no special equipment or complex logistics, and there is no additional cost. In fact, the data are already incorporated into routine remote and in-person cardiac implantable electronic device checks.…”

“…Changes between the baseline and the subsequent measurements at 9 to 12 months post implant added predictive value. 9 In contrast to previous study, which have examined the calculated impedance trends and mortality relationship, 6 impedance measurements from the first 6 months after implant were censored in this study to allow time for the signal to stabilize and no algorithmic manipulation of the data was performed. Various measurement intervals were examined.…”