Dobutamine stress echocardiography is a common test to provoke myocardial ischemia in patients unable to undergo routine exercise stress testing. Heart rate elevation, achieved by staged increases in dobutamine doses, acts as a surrogate for exercise. The physicians monitor the ECG of the patient and echocardiographic images to evaluate for myocardial ischemia. However, the actual mechanical stress on the heart is not readily available to the physician. The motivation for the present preliminary study is to both investigate the feasibility of producing such information for clinicians as well as to investigate the variation between different patients as the heart rate varies. Echocardiograms were obtained from three patients undergoing dobutamine stress tests. Using standard equations of motion, the surface shear stress at the surface of the left ventricle was calculated. The average shear stress around the left ventricle is shown, as well as the peak stresses at selected locations as a function of time. It was found that generally the surface shear stress increased with heart rate around most of the left ventricle. While the time averaged shear stress may be important for diagnosis, the maximum shear stress is probably the limiting factor in terminating testing.
There has been considerable research and speculation that if the brain is under stress then it could affect the heart and lead to heart disease. The purpose of the present research was to examine the inverse problem of whether the heart could potentially cause undesirable reactions in the brain. One method to evaluate for underlying coronary artery disease is to perform stress testing. Often, myocardial stress is achieved by the patient walking on a treadmill while being monitored. In patients who are unable to exercise, pharmacologic stress testing is performed, either with vasodilatory agents (e.g. adenosine) or dobutamine, which is a pro-inotropic and chronotropic drug. During dobutamine infusion, the heart rate increases, but there is a negligible increase in blood pressure. Five patients who were undergoing dobutamine stress testing were instrumented with the standard 19 electrode EEG sensors to record brain activity. It was found that all patients showed resonance in the brain activity at frequencies around 10Hz. The signal strengths and the electrode locations where a resonance varied between patients. The one location where all of the patients showed resonance was at T5-O1; towards the back of the head and for this location, all patients showed an EEG resonance frequency at approximately 10Hz. Further analysis of the EEG data is needed to appreciate the consequences of this neurocardilogical phenomenon.
Often in the treatment of patients echocardiograms are performed to analyse cardiac function. As part of the calculation the shear and normal stress on the endocardial surface of the left ventricle can be calculated. This was undertaken for five elderly women patients undergoing stress testing. It was found that when the stress was plotted as a function of strain rate the expected variation would be a linear increase in stress. In some patients there was initially a sharp increase in stress with strain rate then an abrupt change in the stress at the apex or at the mitral valve annulus. With further increase in strain rate the rate of increase in stress decreased. This would suggest that there was some defect in the wall structure. The variation of ventricular volume with strain rate can be obtained. In this case the volume should decrease initially and as the strain rate increases the volume change becomes less. In two of the five patients the expected initial rise in stress did not occur. This suggests a lack of strength of the ventricle. As expected these were not the same patients where the abrupt increase in stress occurred. Such information may be helpful to a clinician in reaching a diagnosis.
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