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P Lander

Publications and source records attributed to P Lander.

At least 37 records · Page 2Linked to original sources
PubMed1995

Optimal filtering and quality control of the signal-averaged ECG. High-fidelity 1-minute recordings.

BACKGROUND: The clinical performance of the signal-averaged ECG (SAECG) for prediction of ventricular tachycardia (VT) depends on its quality, or final noise level. However, signal averaging is a statistical estimation procedure that is time-consuming and vulnerable to noise-induced error. The optimally filtered SAECG is proposed as a simple, quality-assured procedure requiring only 1 minute of data. METHODS AND RESULTS: The optimally filtered SAECG is based on measures of signal variance and time-frequency representations. Forty subjects were studied to compare a 0.3-microV root-mean-square (RMS) noise endpoint SAECG with an optimally filtered 64-beat ensemble. Eight SAECGs were computed with noise endpoints of 1.0-through 0.3-microV RMS. Noise measurements were also made directly from the filtered SAECG. From these and previously published data, sensitivity was predicted as a function of noise endpoint. Measured QRS parameters and final noise were highly similar between the optimally filtered SAECG and the 0.3-microV RMS noise endpoint SAECG. CONCLUSIONS: The optimally filtered 64-beat SAECG achieves a performance (equivalent noise reduction, signal definition, and mathematically predicted sensitivity for VT) similar to a 0.3-microV RMS noise endpoint average. Testing in a large clinical database is required to validate the method for routine clinical use. SAECGs terminated by use of different noise measurement techniques are not directly comparable because of measurement technique dependence. However, a formula is presented for comparison of statistics between studies that have used the most popular noise measurement techniques.

Electrocardiography↗
PubMed1995

Analysis of high-resolution ECG changes during percutaneous transluminal coronary angioplasty.

The authors have hypothesized that low-level, electrocardiographic changes may accompany transient ischemia induced by percutaneous transluminal coronary angioplasty. Altered repolarization may manifest as subclinical changes in ST-T morphology. Changes in depolarization may manifest as low-amplitude notches and slurs, a phenomenon the authors term abnormal intra-QRS potentials. The initial aim of this study was to characterize changes in high-resolution electrocardiograph signals that might be linked to ischemic involvement of the ventricular myocardium.

Angioplasty, Balloon, Coronary↗
PubMed1995

Improved time-frequency filtering of signal-averaged ECGs.

A recently proposed time-frequency filtering technique has shown promising results for the enhancement of signal-averaged electrocardiograms. This method weights the short-time Fourier transform of the ensemble-averaged signal, analogous to the spectral domain Wiener filtering of stationary signals. In effect, it is a self-designing, time-varying Wiener filter applied to the high-resolution electrocardiogram (HRECG). In this study, the authors empirically show that the performance of the proposed technique is about 2-3 dB lower over the critical late-potential portion of the HRECG than the optimal fixed-window, time-frequency filter based on ideal a priori knowledge of statistics. Although this ideal knowledge and performance is unattainable in practice, these results suggest that there remains potential for modest improvement. To narrow this gap in performance, improvements based on alternative structures for the time-frequency filter, including time-varying short-time Fourier transform windows, are proposed. Simulation results show that an improved fixed-window technique can potentially yield an improvement of about 1-1.5 dB. By using properly chosen time-varying windows, the performance could potentially be improved of about 1-1.5 dB. By using properly chosen time-varying windows, the performance could potentially be improved even further. Thus, the improved techniques could produce an HRECG using fewer averages than the existing method, or that could tolerate a lower initial signal-to-noise ratio.

Algorithms↗
PubMed1995

Analysis of alternans in late potentials. Correlations between epicardial and body surface recordings.

The methodical performance of the signal-averaged electrocardiogram is strongly influenced by the beat-to-beat reproducibility of late potentials (LPs). Especially at higher heart rates, epicardial recordings from infarct regions show progressive beat-to-beat prolongations with alternating conduction block. To analyze the influence of beat-to-beat-alternans of LPs on the signal-averaging process, epicardial and body surface recordings were studied at different heart rates and extrastimulation. Epicardial and body surface recordings were obtained from dogs with 4-day postligation of the left anterior descending coronary artery (Harris model). Body surface potentials were averaged in different modes to a final noise level of 0.3 microV (rms) and digitally bandpass filtered (40-250 Hz). Modulation of the heart rate was performed by atrial or His-bundle pacing and by atrial premature extrastimulation. Pacing up to heart rates close to 180 beats/min produced no significant changes in the duration of LPs in epicardial and averaged body surface recordings; however, at higher pacing rates, considerable prolongation of LPs with different patterns in the epicardial leads was observed. In these cases, averaging of all beats revealed only a slight prolongation of LPs, as seen from the body surface. Selective averaging of beats with prolonged epicardial LPs showed the prolongation or absence of LPs, as seen in the epicardial recordings. Similar observations were made using an atrial extrastimulation technique, whereby the average of the premature beats was compared to the average of all normal sinus beats. Selective beat averaging of body surface potentials can unmask the prolongation of LPs due to atrial pacing or extrastimulation, as seen in recordings from the infarcted epicardium. The evidence of this modulation of LPs may improve the positive predictive value of the signal-averaged electrocardiogram.

Action Potentials↗
PubMed1995

Appropriate imaging studies and their interpretation.

Imaging studies are essential for the diagnosis and management of many of the hip disease entities dealt with in this symposium. This paper briefly touches on salient aspects of standard radiography, "special" views, arthrography, radioisotope imaging, computed tomography and magnetic resonance imaging. Orthopedic surgeons must be familiar with the correct applications and interpretation of these modalities so as to use them efficiently and economically.

Arthrography↗
PubMed1994

Percutaneous transpedicular biopsy of the spine.

STUDY DESIGN: Despite increasing information regarding pedicular instrumentation, the transpedicular biopsy of the vertebral body has not been popularized. The precise technique for percutaneous transpedicular biopsy and the volume of the vertebral body accessible through the pedicle has not been reported. OBJECTIVE: To describe the percutaneous transpedicular biopsy, its diagnostic yield, to determine the amount of bone retrievable through the pedicle. SUMMARY OF BACKGROUND DATA: Experience gained from open biopsy facilitates the development of the percutaneous technique. There were no complications with the open or closed procedures. METHODS: Transpedicular biopsy was performed on 18 patients (nine open and nine percutaneous). Unilateral transpedicular curetments were performed on the cadaveric lumbar spines. The volume of the vertebral body accessible through the pedicle was determined using roentgenograms, computed tomography, and image analysis. RESULTS: Adequate specimens for a correct diagnosis were obtained in all biopsies. Laboratory volumetric analysis demonstrated that more than 50% of the volume of a lumbar vertebral body is accessible through a unilateral transpedicular approach, without compromising the anatomic structure of the pedicle. CONCLUSIONS: The vertebral pedicle provides a safe conduit for biopsy of most vertebral body lesions. Percutaneous transpedicular biopsy can be performed with minimal morbidity and high diagnostic yield.

Adult↗
PubMed1994

Time-frequency structure of the high-resolution ECG.

This study considers the problem of representing high-resolution ECG (HRECG) signals in the time-frequency plane using spectrotemporal mapping (STM). High-resolution ECG signal components overlap in both time and frequency. The central issue with STM techniques is whether sufficient time-frequency resolution exists to discriminate normal and abnormal QRS signals. The trade-off between signal resolution in time and in frequency must be made without a priori knowledge of the HRECG's time-frequency structure. A simulation experiment was performed to examine the time-frequency distribution of normal, abnormal low-level (late potentials), and bundle branch block components of the QRS. Results suggest that discrimination of these signals with STM is problematic. Signals and noise within the HRECG ensemble can, however, be easily distinguished. This observation forms the basis of a new optimally filtered ensemble averaging technique for signal-to-noise ratio enhancement.

Electrocardiography↗
PubMed1994

Identifying the end of ventricular activation: body surface late potentials versus electrogram measurements in a canine infarction model.

INTRODUCTION: Identification of the end of the QRS is perhaps the single most important feature obtained from the high resolution signal-averaged electrocardiogram (SAECG). This point relies on computer algorithms to select a point above the noise levels. Prior studies to substantiate this approach using electrograms for comparison have demonstrated many examples of the body surface recordings failing to detect the full extent of the late potentials. METHODS AND RESULTS: An animal model that generates late potentials was used in conjunction with epicardial cardiac mapping system to systematically examine the reasons for these failures. In 11 of 13 dogs we found a concordance between the signal-averaged recordings and the epicardial recordings within 5 msec. The two discordant studies were attributed to a failure of epicardial mapping to record all late potential sources. Also, a means of accurately comparing measurements from the two recording technologies was required in this study as well as a new definition for identifying the end of activation currents in epicardial electrograms. CONCLUSION: To achieve these results required approaches different from those used in the clinical setting to record the SAECG. These include: (1) the analysis of individual XYZ leads as opposed to the vector magnitude derived from these leads; (2) visual identification of very low level signals, as automatic algorithms often fail to detect low level signals; and (3) the use of finite impulse response digital filters instead of the bidirectional Butterworth filter.

Action Potentials↗
PubMed1994

An algorithmic approach to the treatment of Paget's disease of the spine.

An algorithmic approach to the treatment of Paget's disease of the spine is based on the authors' collective experience in managing 70 patients and a review of the literature. For patients with active disease, even if asymptomatic, cyclical treatment with etidronate is recommended until normalization of biochemical parameters is achieved. For patients with back pain and/or neurologic dysfunction, cyclical etidronate treatment in the blastic and mixed phase is advocated. Those who fail to respond to calcitonin in the lytic phase should be given the new generation of bisphosphonates and gallium nitrate. No antipagetic drug therapy is indicated for the sclerotic phase. Surgery for complications of Paget's disease should be preceded by intravenous therapy to minimize intraoperative bleeding. For cases of spinal stenosis with impending paraplegia, the new bisphosphonates, mithramycin (plicamycin), or gallium nitrate should be used. If conservative treatment fails, surgery should not be delayed.

Algorithms↗
PubMed1993

[Chemonucleolysis. Its effectiveness in the treatment of lumbar diskopathies caused by compressive or rotational injuries].

One hundred patients who meet McCulloch's indicators for treatment of lumbar disc herniation by chemonucleolysis were entered in this study. Chemonucleolysis in the patients suffering from sciatica secondary to a compression injury, especially with clinical and radiographic evidence of nerve root compression at a single level, can be expected to attain good to excellent results. Chemonucleolysis is far less efficacious in patients suffering from either torsional injury or multiple level abnormality, especially in the absence of neurological deficit. Meticulous attention to patient selection will yield optimal results. The result was excellent or very good in 60% of patients, while in 40% the procedure failed. The rate of success at the L.5/S.1 level was 70%, at the L.4/5 level it was 63% and 37% were successful when multiple levels were treated.

Adult↗
PubMed1993

Critical analysis of the signal-averaged electrocardiogram. Improved identification of late potentials.

BACKGROUND: This study performed a critical analysis of signal-averaging methods. The objective was to optimize detection of late potentials. METHODS AND RESULTS: We studied two patient populations: a low-arrhythmia-risk group with no evidence of heart disease and a group with clinically documented ventricular tachycardia (VT). Filtered QRS duration (QRSD) and terminal QRS amplitude (RMS40) were measured from the vector magnitude. A QRS duration based on the latest detectable ventricular activity in any of the three individual XYZ leads was also measured. Because of improved signal-to-noise ratio, both individual lead analysis and extended (600-versus 200-beat) averaging yielded significant changes in signal-averaged ECG parameters. Both approaches gave an increased sensitivity for VT identification. Sensitivity, specificity, and accuracy were evaluated as functions of critical values of QRSD and RMS40. RMS measurements in the terminal QRS, ranging from 20 to 100 msec and including RMS40, did not contribute to maximizing sensitivity and were highly correlated with QRSD. Our results from the low-arrhythmia-risk group suggest that age and sex should be considered in the definition of late potentials. CONCLUSIONS: We propose a VT risk stratification scheme using signal-averaged ECG parameters obtained from both individual lead and vector magnitude analysis. This allows definition of four categories of VT risk derived statistically from the study data. This definition is based on combined measures of sensitivity, specificity, and negative and positive predictive value.

Aged↗
PubMed1992

Malignant transformation in Paget disease of bone.

BACKGROUND: The previously reported incidence of malignant transformation of Paget disease up to 5.5% and its dismal prognosis have prompted the clinical investigation of a large population with Paget disease. METHODS: A chart review of symptomatic and asymptomatic patients with a diagnosis of Paget disease between 1970 and 1985 at four large Montreal Hospitals revealed 1078 patients. Eight patients with malignant transformation were studied in detail. RESULTS: The incidence of malignant transformation was 0.7%, and the most frequent histologic type was osteogenic sarcoma. The most prevalent site was the femur, and pathologic fracture with focal osteolysis was present in 50% of patients at the initial appearance of the tumor. Healing at the fracture site was demonstrated in one patient. Another patient died of uncontrollable hemorrhage. A case of malignant lymphoma in Paget disease is recorded. CONCLUSIONS: Surgery or biopsy should be preceded by preoperative scintigraphic bone blood flow evaluation and, if necessary, administration of a preoperative course of mithramycin and selective embolization to minimize bleeding. The prognosis of malignant transformation in Paget disease is poor, although one patient with malignant lymphoma survived after aggressive treatment. Early biopsy and aggressive treatment should not be delayed.

Adult↗
PubMed1992

Advanced time-frequency methods for signal-averaged ECG analysis.

Frequency-domain techniques have been extensively investigated for the analysis of high-resolution electrocardiograms (ECGs), although the merit of frequency-domain analysis is still subject to controversy. Time-frequency analysis methods, which estimate the frequency content of a signal as a function of time, potentially provide even more information for improved ECG analysis. Some researchers report impressive results in predicting the outcome of electrophysiologic studies using the short-time Fourier transform (spectrogram). Other time-frequency representations, such as the Wigner distribution, short-time spectral estimators, and the wavelet transform, have also been investigated. The authors present a unified overview of time-frequency representations, showing that only four classes characterize most time-frequency representations. The authors describe the advantages and drawbacks of the various approaches and speculate on their promise for ECG analysis. Very preliminary experiments in applying some of these techniques to the prediction of the outcome of electrophysiologic studies have suggested some possible new research directions.

Electrocardiography↗