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

Abstract

Background: Intrathoracic impedance-derived OptiVol fluid index calculated using implanted devices has been shown to predict mortality; direct measurements of impedance have not been examined. We hypothesized that baseline measured impedance predicts all-cause mortality; changes in measured impedance result in a change in the predicted mortality; and the prognostic value of measured impedance is additive to the calculated OptiVol fluid index.

Methods and results: A retrospective analysis of 146,238 patients within the Medtronic CareLink database with implanted devices was performed. Baseline measured impedance was determined using daily values averaged from month 6 to 9 after implant and were used to divide patients into tertiles: group L = low impedance, ≤ 65 ohms; group M = medium impedance, 66 to 72 ohms; group H = high impedance, ≥ 73 ohms. Change in measured impedance was determined from values averaged from month 9 to 12 post implant compared with the 6- to 9-month values. OptiVol fluid index was calculated using published methods. All-cause mortality was assessed beginning 9 months post implant; changes in mortality was assessed beginning 12 months post implant. Baseline measured impedance predicted all-cause mortality; 5-year mortality for group L was 41%, M was 29%, and H was 25%, P < 0.001 among all groups. Changes in measured impedance resulted in a change in the predicted mortality; the prognostic value of measured impedance was additive to the OptiVol fluid index.

Conclusions: Direct measurements of intrathoracic impedance using an implanted device can be used to stratify patients at varying mortality risk.

Keywords: all-cause mortality; heart failure; impedance; risk.

Figure 1

Patient sample size disposition for primary and secondary survival analysis. ICD = implantable cardiac defibrillator, CRT-D = cardiac resynchronization with an cardiac defibrillator.

Figure 2

Panel A- Experimental design for the primary survival analysis, see text for details. Panel B- Experimental design for the secondary survival analysis, see text for details.

Figure 3

Kaplan-Meier estimate of all-cause mortality in the primary mortality analysis. Patients were divided into tertiles based on average baseline measured impedance from 6–9 months after implantation: Group L measured impedance ≤ 65 ohms, group M 66–72 ohms, group H ≥ 73 ohms.

Figure 4

Incremental value of measured impedance to threshold crossings: Panel A-. There was predictive discrimination for all-cause mortality provided by the baseline (6–9 month) measured impedance in patients that did not have an OptiVol fluid index threshold crossing. Panel B- There was predictive discrimination for all-cause mortality provided by the baseline (6–9 month) measured impedance in patients that did have an OptiVol fluid index threshold crossing. However, 5 year mortality was higher in patients with a threshold crossing in L, M, and H groups.

Figure 4

Incremental value of measured impedance to threshold crossings: Panel A-. There was predictive discrimination for all-cause mortality provided by the baseline (6–9 month) measured impedance in patients that did not have an OptiVol fluid index threshold crossing. Panel B- There was predictive discrimination for all-cause mortality provided by the baseline (6–9 month) measured impedance in patients that did have an OptiVol fluid index threshold crossing. However, 5 year mortality was higher in patients with a threshold crossing in L, M, and H groups.

Figure 5

Panel A- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes in baseline measured impedance of ≥ 73 ohms. Decreased measured impedance resulted in increased mortality (group H.L vs H.M and H.H). Panel B- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes in baseline measured impedance 66–72 ohms. Increased measured impedance resulted in decreased mortality (group M.L vs M.M); decreased measured impedance resulted in increased mortality (group M.L vs M.H). Panel C- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes from baseline measured impedance of ≤ 65 ohms. Increased measured impedance resulted in decreased mortality (group L.L vs L.M and L.H).

Figure 5

Panel A- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes in baseline measured impedance of ≥ 73 ohms. Decreased measured impedance resulted in increased mortality (group H.L vs H.M and H.H). Panel B- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes in baseline measured impedance 66–72 ohms. Increased measured impedance resulted in decreased mortality (group M.L vs M.M); decreased measured impedance resulted in increased mortality (group M.L vs M.H). Panel C- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes from baseline measured impedance of ≤ 65 ohms. Increased measured impedance resulted in decreased mortality (group L.L vs L.M and L.H).

Figure 5

Panel A- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes in baseline measured impedance of ≥ 73 ohms. Decreased measured impedance resulted in increased mortality (group H.L vs H.M and H.H). Panel B- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes in baseline measured impedance 66–72 ohms. Increased measured impedance resulted in decreased mortality (group M.L vs M.M); decreased measured impedance resulted in increased mortality (group M.L vs M.H). Panel C- Kaplan-Meier estimate of all-cause mortality in the secondary mortality analysis for patients with changes from baseline measured impedance of ≤ 65 ohms. Increased measured impedance resulted in decreased mortality (group L.L vs L.M and L.H).