Birefringence-based eye fixation monitor with no moving parts.

For the purpose of vision screening, we develop an eye fixation monitor that detects the fovea by its unique radial orientation of birefringent Henle fibers. Polarized near-infrared light is reflected from the foveal area in a bow-tie pattern of polarization states, similar to the Haidinger brush phenomenon. In contrast to previous devices that used scanning systems, this instrument uses no moving parts. It rather utilizes four spots of linearly polarized light-two aligned with the "bright" arms and two aligned with the "dark" arms-of the bow-tie pattern surrounding the fovea. The light reflected from the fundus is imaged onto a quadrant photodetector, whereby the circular polarization component of the polarization state of each reflected patch of light is measured. The signals from the four photodetectors are amplified, digitized, and analyzed. A normalized differential signal is computed to detect central fixation. The algorithm is tested on a computer model, and the apparatus is tested on human subjects. This work demonstrates the feasibility of a fixation monitor with no moving parts.

Effects of anodal vs. cathodal pacing on the mechanical performance of the isolated rabbit heart.

Previous studies have suggested that anodal pacing enhances electrical conduction in the heart near the pacing site. It was hypothesized that enhanced conduction by anodal pacing would also enhance ventricular pressure in the heart. Left ventricular pressure measurements were made in isolated, Langendorff-perfused rabbit hearts by means of a Millar pressure transducer with the use of a balloon catheter fixed in the left ventricle. The pressure wave was analyzed for maximum pressure (Pmax) generated in the left ventricle and the work done by the left ventricle (Parea). Eight hearts were paced with monophasic square-wave pulses of varying amplitudes (2, 4, 6, and 8 V) with 100 pulses of each waveform delivered to the epicardium. Anodal stimulation pulses showed statistically significant improvement in mechanical response at 2, 4, and 8 V. Relative to unipolar cathodal pacing, unipolar anodal pacing improved Pmax by 4.4 +/- 2.3 (SD), 5.3 +/- 3.1, 3.5 +/- 4.9, and 4.8 +/- 1.9% at 2, 4, 6, and 8 V, respectively. Unipolar anodal stimulation also improved Parea by 9.0 +/- 3.0, 12.0 +/- 6.0, 10.1 +/- 7.7, and 11.9 +/- 6.0% at 2, 4, 6, and 8 V, respectively. Improvements in Pmax and Parea indicate that an anodally paced heart has a stronger mechanical response than does a cathodally paced heart. Anodal pacing might be useful as a novel therapeutic technology to treat mechanically impaired or failed hearts.

Wavelet analysis and time-frequency distributions of the body surface ECG before and after angioplasty.

In a pilot study, electrocardiographic (ECG) recordings of patients with left and right coronary stenosis taken before and after angioplasty were analyzed using the continuous wavelet transform. Time-frequency distributions were obtained for different leads in order to examine the dynamics of the QRS-spectrum and establish features specific of ischemia in the time-frequency domain. We found relevant changes in the mid-frequency range, reflecting the ECG's response to percutaneous transluminal coronary angioplasty (PTCA). The changes appeared in ECG leads close to ischemic zones of the myocardium. Time-frequency distributions of the ECG during the QRS may thus become another electrocardiographic indicator of ischemia, alternative to ST-level in standard ECG or body surface mapping. The paper demonstrates the ability of the continuous wavelet transform to detect short lasting events of low amplitude superimposed on large signal deflections.

Variability maps of body surface ECG in normal subjects.

A method for quantifying the fluctuations of the QRS electrocardiogram over the flat (unrolled) thoracic surface in normal subjects is presented. Serial comparisons were performed on 32-lead ECG recordings and body surface maps from seven healthy men, using as similarity measures correlation and RMS-difference in both time-signal and 2D map domain. Recordings were made on one and the same day, and on different days. The time-signal correlation and RMS-differences were plotted as 2D distributions. Correlation was higher on the front of the torso and RMS-differences were largest in the precordial area. The average time-signal correlation calculated over the QRS in comparisons of signals taken on different days was 0.9717, whereas the corresponding RMS-difference was 57.8 microV. Additionally, 2D comparisons between reconstructed map frames at different time instants in the QRS were carried out, using the 2D versions of the correlation and RMS-difference. The averaged results of the 2D comparisons were close to the time-signal comparison values (respectively 0.9696 and 68.9 microV). Finally, 2D comparisons between isointegral maps (mapping the QRS signal integral) were performed. They yielded even higher correlations (0.98, 3.453 microV s). Although the number of subjects studied was not large, the investigation brought to light important variability ranges that may serve as a basis when detecting pathologies. In addition, the topology of normal ECG variability on the body surface was revealed. This study confirmed the pertinence of correlation and RMS-difference as measures of time-signal and map similarity.

Multiresolution wavelet analysis of the body surface ECG before and after angioplasty.

Electrocardiographic recordings of patients with coronary artery stenosis, made before and after angioplasty, were analyzed by the multiresolution wavelet transform (MRWT) technique. The MRWT decomposes the signal of interest into its coarse and detail components at successively finer scales. MRWT was carried out on different leads in order to compare the P-QRS-T complex from recordings made before with those made after percutaneous transluminal coronary angioplasty (PTCA). ECG signals before and after successful PTCA procedures show distinctive changes at certain scales, thus helping to identify whether the procedure has been successful. In six patients who underwent right coronary artery PTCA, varying levels of reperfusion were achieved, and the changes in the detail components of ECG were shown to correlate with the successful reperfusion. The detail components at scales 5 and 6, corresponding approximately to the frequencies in the range of 2.3-8.3 Hz, are shown to be the most sensitive to ischemia-reperfusion changes (p < 0.05). The same conclusion was reached by synthesizing the post-PTCA signals from pre-PTCA signals with the help of these detail components. For on-line monitoring a vector plot, analogous to vector cardiogram, of the two most sensitive MRWT detail components is proposed. Thus, multiresolution analysis of ECG may be useful as a monitoring and diagnostic tool during angioplasty procedures.

Wavelets as alternative to short-time Fourier transform in signal-averaged electrocardiography.

The paper reports experience of using the wavelet transform to build time-frequency distributions of the terminal portion of the QRS-complex. We used wavelets of Morlet at 12 scales, grouped in three sets, to analyse the frequency range 33-404 Hz. On the same patient data we applied the short-time Fourier transform and compared the results. Both representations reflected the time-frequency contents and detected irregular structures in the terminal portion of the QRS complex. The wavelet transform revealed more adequately QRS prolongations characteristic of patients prone to ventricular tachycardia. We may conclude that the wavelet transform can be a flexible alternative to short-time Fourier transform.

Detection of late potentials in the signal-averaged ECG combining time and frequency domain analysis.

The paper reports experience of flexible, computer-based instrumentation for high-resolution ECG, designed for laboratory research and clinical applications. It is an attempt to improve methods of detection of late potentials when identifying patients at increased risk of ventricular tachycardia (VT) and sudden cardiac death. Several factors influencing the efficiency of a high-resolution ECG system are examined. Of primary importance are signal, gain, type of filter, length and position of the time interval taken for spectral analysis, type of window used etc. Some possible error sources are analysed. Time-domain analysis is illustrated by representative XYZ-vector magnitude tracings of a non-VT and a VT patient. In an effort to further refine the diagnostic strategy, an approach to data analysis is presented, combining time-domain with frequency-domain measurements. A spectral ratio is proposed that helps recognise patients prone to VT. Two-dimensional plots of this ratio against time-domain parameters reveal clearly distinguishable normal and pathologic groups. It can be concluded that an appropriate frequency analysis can add significant diagnostic power to existing instrumentation for signal-averaged ECG to predict VT risk.