Temporal Performance of Laplacian Eigenmaps and 3D Conduction Velocity in Detecting Ischemic Stress

Abstract

Background: Myocardial ischemia has a complex and time-varying electrocardiographic signature that is used to diagnose and stratify severity. Despite the ubiquitous clinical use of the ECG to detect ischemia, the sensitivity and specificity of ECG based detection of myocardial ischemia are still inadequate. Purpose: The purpose of this study was to compare, using animal models, the performance of several traditional ECG-based metrics for detecting acute ischemia against two novel metrics, the Laplacian Eigenmap (LE) parameters and a three-dimensional estimate of Conduction Velocity (CV). Methods: LE is a machine learning technique that reduces the dimensions of simultaneously recorded time signals using a non-linear embedding followed by an singular value decomposition to represent each multichannel recording as a single trajectory on a manifold. Perturbations in the trajectories suggest the presence of myocardial ischemia. CV was computed using a tetrahedral mesh created from the electrode locations of transmural plunge needles. To validate the results, we used electrograms collected over 95 episodes of acutely induced myocardial ischemia in 15 canine and 2 porcine subjects. The LE and CV metrics were compared against traditional metrics derived from the ST segment, the T wave, the QRS of the same electrograms. The response time and robustness of each metric was quantified using parameters we defined as time to threshold (TTT) and contrast ratio (CR). Results: The temporal performance of the metrics evaluated throughout the ischemic episodes showed a consistent relationship; the LE metrics changed earlier than those from the T wave, which were followed by those from the ST segment, and finally from the QRS. The CV results showed median drops in conduction velocity throughout the perfusion bed of more than 23 % in canines and over 12% during half of the induced ischemia episodes in swine. The other half of the episodes in swine produced a 76% drop. Conclusions: Our results suggest that the LE metric is more sensitive to acute ischemia than traditional single parameters used in previous studies, likely because it incorporates the entire QRST across multiple electrodes in a way that captures their most salient features in a low-dimensional space. The estimates of conduction velocity suggest substantial, in some cases dramatic slowing of the spread of activation, a finding that is not surprising but has not been documented in such three-dimensional detail before. The experiments and these new metrics provide a means to both explore details of the acute ischemic response not available from humans and suggest a path to translate this knowledge into improvements in clinical scoring of ischemia.

Novel Metric Using Laplacian Eigenmaps to Evaluate Ischemic Stress on the Torso Surface

“…For the experimental recordings, the manifold coordinates were learned using the beats recorded during the initial rest period before the first intervention was induced using a similar scheme as in previous studies. [3] Once the coordinate space was identified, it was populated with subsequent beats over repeated episodes of induced ischemia, as seen in Fig. 2B.…”

“…To compare the performance of the LE metric against the signal space metrics on the torso surface the quality metrics introduced in our previous studies were used. [3] These quality metrics, time to threshold (TTT) and contrast ratio (CR), were used to quantify the response time and robustness of that response. We have now extended these same measures to the torso surface recordings from our animal and human recordings.…”

“…Our previous results with LE showed that LE-based metrics could detect ischemia earlier in an ischemic episode than traditional signal space metrics. [3]…”

Validation of Intramural Wavefront Reconstruction and Estimation of 3D Conduction Velocity

“…The data used for this study was acquired during a series of large animal experiments carried out to study the electrophysiological response to acute myocardial ischemia and a number of other pathologies. [1,5,6]To provide measurements for this study, we conducted a series of experiments using 20-40 transmural plunge needles, each with 10 unipolar electrodes roughly uniformly distributed within the myocardial tissue of the perfusion bed in which ischemia was induced through restriction of coronary flow and elevated heart rates [5]. Figure 1 shows a schematic diagram of a typical intramural needle with 5-10 mm between each needle.…”

“…Regardless, measurement of CV is still challenging, and therefore rarely studied in three dimensions throughout the myocardial volume, nor at the level of resolution achieved in this study. [1,2].…”