Kenneth Khaw

Importance of posterior chest leads in patients with suspected myocardial infarction, but nondiagnostic, routine 12-lead electrocardiogram

Criteria for reperfusion therapy in acute myocardial infarction require the presence of ST elevation in 2 contiguous leads. However, many patients with myocardial infarction do not show these changes on a routine 12-lead electrocardiogram and hence are denied reperfusion therapy. Posterior chest leads (V7 to V9) were recorded in 58 patients with clinically suspected myocardial infarction, but nondiagnostic routine electrocardiogram. ST elevation >0.1 mV or Q waves in > or =2 posterior chest leads were considered to be diagnostic of posterior myocardial infarction. Eighteen patients had these changes of posterior myocardial infarction. All 18 patients were confirmed to have myocardial infarction by creatine phosphokinase criteria or cardiac catheterization. Of the 17 patients who had cardiac catheterization, 16 had left circumflex as the culprit vessel. We conclude that posterior chest leads should be routinely recorded in patients with suspected myocardial infarction and nondiagnostic, routine electrocardiogram. This simple bedside technique may help proper treatment of some of these patients now classified as having unstable angina or non-Q-wave myocardial infarction.

Improved detection of posterior myocardial wall ischemia with the 15-lead electrocardiogram.

BACKGROUND A routine 12-lead electrocardiogram is commonly obtained to evaluate for possible acute myocardial infarction during the initial screening of patients with chest discomfort. Posterior myocardial infarction is commonly missed because it is not usually visible in the standard leads. In this study, we compared the sensitivity and specificity of posterior chest leads (V(7), V(8), and V(9)) and 12-lead electrocardiography in detecting posterior injury pattern during single-vessel percutaneous transluminal coronary angioplasty. METHODS AND RESULTS Three posterior chest leads in addition to the routine 12-lead electrocardiogram were monitored simultaneously during single-vessel percutaneous transluminal coronary angioplasty of the right, circumflex, and left anterior descending coronary arteries in a total of 223 patients. Posterior injury patterns (95%) were detected mostly during circumflex coronary occlusion. Posterior leads were able to detect injury pattern in 49% (36 of 74) of patients, whereas the 12-lead electrocardiogram was able to detect only 32% (P <.04). When all 15 leads were used to detect all ST elevations, sensitivity increased to 57%, with a specificity of 98% for the circumflex coronary artery. If maximal ST depressions in leads V(2) to V(3) are considered to be from posterior myocardial injury, then the overall sensitivity is increased to 69%. CONCLUSIONS Posterior leads significantly increased the detection of posterior injury pattern compared with the standard 12-lead electrocardiogram. Using all 15 leads significantly further improved the detection of circumflex coronary-related injury pattern over the standard 12-lead electrocardiogram.

Vasoactive Stimulations on Ventricular and Vascular Performances

How the left ventricle (LV) interacts with the arterial system (AS) has been a subject of considerable interest. Methods for analysis of their interaction have primarily been experimental. In recent years, model-based analyses have emerged including one that utilizes a time-varying elastance LV model coupled to a windkessel model of the arterial system. The ratio of the derived effective arterial elastance (Eas) to the maximum elastance of the LV (Emax) has been used extensively to describe the interaction but has its limitations. We developed an interactive model of the LV and AS that affords the analysis of the individual component variations on the overall dynamic interaction of the LV and AS. This model is used to investigate how vasoactive states can affect the interaction in terms of pressure–volume relations and energetics. Results show that with methoxamine (MTX) induced vasoconstriction the overall energy generated is greater than in nitroprusside (NTP) induced vasodilation, but the reverse is true in terms of kinetic power. Calculated cardiac efficiencies were generally lower than those found by other investigators, attributable to lower stroke volumes. The cardiac efficiency did not alter substantially during vasoactive conditions. This might be because the primary effect of MTX was in increasing peripheral resistance and a reflex induced decrease in heart rate prolonged the diastolic filling period, hence maintaining the stroke volume.

Cardiac Parametric Variations in Post-Ischemic Myocardium

Mechanisms governing post-ischemic ventricular function after episodes of acute myocardial ischemia are still unclear. We investigated this stunned myocardial function with a computer model in conjunction with animal experiments. A modified lumped cardiac muscle model was subjected to parametric changes similar to regionally recorded ventricular parameters. The model was perturbed by alterations in contractility and rates of activation and deactivation. Results show that the cardiac muscle model can mimic many of the physiological changes observed in a stunned myocardium.

Myocardial stunning analyzed by a single muscle fiber model

Mechanisms underlying myocardial stunning are yet to be clarified. We investigated this with a computer model in conjunction with experimental data obtained from canine subjects. A modified Hills 3-element muscle model was subjected to parametric changes similar to regional ventricular measurements and perturbed by reductions in contractility and rates of activation and deactivation. Results show that many of the myocardial stunning features can be reproduced even in a single muscle fiber model.

Nonuniform geometric and elastic properties of arteries: local and global characterization

Nonuniformities in geometric and elastic properties arise from vascular structure, tapering and branching. Methods to quantify these are reviewed and new approaches are introduced that can be used to assess local and global nonuniformities.

Fractional flow reserve in coronary artery stenosis and hyperemia

Coronary angioplasty and pressure tracing records of patients with coronary artery stenosis and subsequent hyperemia conditions were obtained and analyzed. Fractional flow reserve (FFR) was calculated and plotted against percent of area stenosis. Under adenosine-induced hyperemia, FFR decreased. The extent of decrease is determined by the percent of stenosis and the responsiveness of intramural coronary artery.

Progression of left ventricular wall thickness during pressure-overload left ventricular hypertrophy

Pressure-overload in terms of decreased arterial compliance (C) and increased vascular resistance (Rs) was used in conjunction with a negative feedback control model to predict left ventricular (LV) wall thickness growth over a prescribed time period. The progression of left ventricular wall thickness growth was found to change proportionally with the peak systolic pressure and the extent of the combination of a reduction in C and an increase in Rs.

Ventricular relaxation in myocardial ischemia: a computer model-based analysis

A computer model-based approach is presented to characterize hemodynamic consequences of myocardial ischemia in terms of depressed ventricular elastance and delayed and reduced ischemic regional cardiac muscle shortening, as well as relaxation time constants. The model is shown to be versatile in quantifying the severity of myocardial ischemia.