Impedance Trends Research Articles

Automatic Identification of Clinical Lead Dysfunctions

Implantable cardioverter defibrillators (ICD) lead dysfunctions can cause inappropriate shocks. Current ICDs store lead diagnostics and detected episodes. This stored information with intracardiac electrograms (EGM) and sensed RR interval patterns may characterize the ICD lead performance. The aim of this analysis was to determine the sensitivity and positive predictive value (PPV) of an automatic lead dysfunction identification algorithm. This algorithm uses RR and EGM data to distinguish noncardiac oversensing (OS), for example, due to conductor fracture, and cardiac OS, for example, T-wave OS, from detected episodes. The algorithm also uses lead diagnostics: sensing integrity counter trends (e.g., RR intervals <140 ms), nonsustained tachyarrhythmias episodes with a mean RR <200 ms and impedance trends to identify lead fractures. The PPV was determined using the stored memory from 1,756 ICD patients enrolled in a 13-center long-term lead study with an average follow-up of 18.3 patient-months. Sensitivity was determined in 35 patients who presented with OS or lead fracture-related adverse events confirmed by stored ICD diagnostics. The algorithm sensitivity was 97.1% (34/35). There were 43 additional patients identified by the algorithm without an adverse event. Stored ICD diagnostics confirmed lead dysfunctions in 32 of 43 patients corresponding with an 85.7% PPV (66/77). ICD memory diagnostics and episodes with intracardiac EGM may be used to identify ICD lead dysfunctions with high sensitivity and PPV. This algorithm may be implemented in postprocessing ICD environments (e.g., remote server, programmer) to rapidly identify lead dysfunction prior its clinical manifestation.

An algorithm to predict implantable cardioverter-defibrillator lead failure

The goal of this analysis was to test an algorithm that identifies implantable cardioverter-defibrillator (ICD) lead problems before clinical failure and/or inappropriate therapy. The ICD lead failures typically present as inappropriate shock therapy. Identifying lead failures before their clinical presentation may prevent patient discomfort, improve device longevity, and avoid device-induced proarrhythmia. We tested an algorithm that uses two measures of oversensing and one measure of abnormal impedance to detect a lead failure. The oversensing measures consisted of a counter for RR intervals <140 ms and nonsustained ventricular tachycardia episodes with mean RR interval <200 ms. The impedance measure tracked lead impedances every day and each week. Abnormal impedance was defined as a decrease in impedances or an outlier value compared with baseline. Lead failures were identified when both oversensing measures were met or abnormal impedance and one oversensing measure occurred. The stored data from 696 patients with an ICD were analyzed to determine the sensitivity and specificity of the algorithm to detect lead failures. Twenty-nine patients demonstrated clinical lead failures with an average of 6 +/- 9 inappropriate shocks per patient. The two oversensing measures used in the algorithm predicted 72% (21 of 29) of the lead failures. Fulfilling at least two of the three impedance and oversensing measures, the sensitivity of our algorithm was 83% (24 of 29) with a 100% (667 of 667) specificity. Oversensing combined with abnormal impedance trends may be used to identify ICD lead failures with high sensitivity and very high specificity.

A constrained global inversion method using an overparameterized scheme: Application to poststack seismic data

A global optimization algorithm using simulated annealing has advantages over local optimization approaches in that it can escape from being trapped in local minima and it does not require a good initial model and function derivatives to find a global minimum. It is therefore more attractive and suitable for seismic waveform inversion. I adopt an improved version of a simulated annealing algorithm to invert simultaneously for acoustic impedance and layer interfaces from poststack seismic data. The earth’s subsurface is overparameterized by a series of microlayers with constant thickness in two‐way traveltime. The algorithm is constrained using the low‐frequency impedance trend and has been made computationally more efficient using this a priori information as an initial model. A search bound of each parameter, derived directly from the a priori information, reduces the nonuniqueness problem. Application of this technique to synthetic and field data examples helps one recover the true model parameters and reveals good continuity of estimated impedance across a seismic section. This approach has the capability of revealing the high‐resolution detail needed for reservoir characterization when a reliable migrated image is available with good well ties.

Automatic Pace-Sense Polarity Switch as an Indicator of Early Lead Corrosion: The Usefulness of Impedance Trend Graphing

A rate responsive dual chamber pacemaker system (Medtronic Inc.) was implanted without complications. At 6-week postimplantation a routine pacemaker check showed a spontaneous switch from programmed bipolar pace-sense to unipolar pace-sense on the atrial and ventricular leads. Pacing and sensing thresholds were not significantly changed from implantation. The atrial and ventricular lead impedances increased from 680 and 720 ohms at implantation to 1,290 and 2,400 ohms, respectively. The device was reprogrammed to bipolar pace-sense and the continuous lead telemetry trend option was programmed On. Evaluation of the system 1 month later revealed a decrease in atrial and ventricular lead impedances, 680 and 2,100 ohms, without a change in pace-sense polarity. One month later, the lead polarity had again switched from programmed bipolar to unipolar pace-sense. The lead trend data revealed stable atrial impedances with sporadic increases in the ventricular lead impedance to values > 3,000 ohms. The pacemaker lead system was invasively investigated and visible gross corrosion of the ventricular lead distal connector pin was discovered.

Using converted shear waves to image reservoirs with low-impedance contrast

Imaging sand/shale sequences is problematic when the impedance contrast between them is very small, a problem commonly encountered in basins with Tertiary clastics. Impedance trends for sand and shale in these basins display higher impedances for shales than for sands at shallow depths; however, sands exhibit higher impedances than shales at greater depths. That implies a crossover between the trends at some depth and that it is going to be difficult to image sands near the crossover depth with traditional seismic surveys. Sometimes stacking far-offset data is a solution because a strong AVO gradient can generate significant reflections at high angles of incidence, even if the normal-incidence reflection coefficient is close to zero. Converted waves can, in some cases, image the reservoir. The question is when because there is no generally accepted insight regarding what conditions favor converted-wave imaging. In other words, why does converted-wave imaging work very well in some reservoirs and not so well in others? Three conditions must exist for converted shear-wave imaging to be preferable to conventional P-P imaging: 1. A very small acoustic impedance contrast results in very weak reflections and thereby poor images. 2. The offset dependence of P-P amplitudes is not useful because (a) a small contrast in Poisson's ratio across the contact between seal and reservoir makes the P-P AVO gradient small, resulting in weak amplitudes at all offsets, or (b) velocity distribution in the overburden makes it impossible to achieve large angles of incidence at the reservoir level. 3. A large shear-wave impedance contrast generates strong converted waves. The first point simply states the obvious—converted-wave imaging is cost effective only when traditional P-P imaging fails, and P-P imaging fails when the acoustic-impedance contrast is close to zero. Point …

The Bragg condition limitation on inversion of normal incidence reflection data

A simple analytic expression is derived for the impulse response of a continuously stratified sediment. The expression, which neglects multiple scattering, allows a straightforward calculation of both the forward and inverse problem, but more importantly, it clearly establishes the ‘‘Bragg condition’’ as a fundamental limitation on the inversion of acoustic data dominated by single scattering. (For normal incidence backscatter, the Bragg condition states that an acoustic probe signal of wavelength λ senses or ‘‘filters out’’ only the Fourier component of the impedance profile having wavelength λ/2, i.e., λmedium=λacoustic/2.) The validity of the analytic expression is demonstrated by comparing it with exact numerical calculations for both the forward and inverse problem. In particular, it is shown that to obtain trends in impedance that occur over meters, requires a probe signal with wavelengths of approximately twice the desired trend distance. Many high‐frequency inversions reported in the literature show trends with wavelengths that are orders of magnitude larger than any wavelength in the probe signal. Consequently, it is worthwhile to ask whether the long wavelength trends, which cannot be acoustically sensed by the short wavelength probe signals, are physically meaningful, or whether they are artifacts of arbitrary underlying assumptions in the signal processing method. [Work supported by NRL and ONR.]

ANNUAL TRENDS IN FROST HARDINESS AND ELECTRICAL IMPEDANCE FOR SEVEN CONIFEROUS SPECIES

Potted 3–4-yr-old seedlings of white, red, and jack pine (Pinus strobus L., P. resinosa Ait., and P. banksiana Lamb.), white, black, and Norway spruce (Picea glauca (Moench) Voss, P. mariana (Mill.) B.S.P., and P. abies (L.) Karst.), and Eastern larch or tamarack (Larix laricina (Du Roi) K. Koch) were grown outdoors and subsequently subjected to freezing temperatures in a freezing chamber during the period from September 1968 to November 1969. Each freezing test consisted of five trees per species. Electrical impedance readings were taken on each tree. The frost hardiness was determined by visually assessing damage and measuring subsequent growth. Differences in frost hardiness between species were small. The minimum frost hardiness in the summer was between −3 and −5 C. The maximum frost hardiness in the winter was below −40 C, with the exception of Norway spruce, which had its maximum around −40 C. It is suggested that the processes of hardening and dehardening consist of two stages. Electrical impedance trends appear to correspond with frost hardiness trends, especially in the spruces and larch.