Med Equipment & Diagnosis
Page 20
TMS
Medical Diagnostics: ECG and Pericarditis
By Alberto Vasquez.
Learning Objective: Pericarditis
Distinguish between magnitude and direction of an electrocardiogram.
Distinguish between positive, negative and isoelectric deflection.
Distinguish between segments and intervals.
Measure an elevated S-T segment, and P-R interval as related to pathology: pericarditis.
Review the relevant pathology that can be diagnosed with a medical
device.
Physics of the Heart: Instead of starting with the electrical conduction
system of the heart, for the sake of the readers of this journal, an elaboration on how the electrical impulses are captured on an ECG will be discussed. In physiology textbooks we see a nice 2-D diagram showing the
conduction of the signal from top right (SA node) ending at bottom left
(apex) spreading throughout both ventricles. The reality is the electrical
impulse; much like the electromagnetic waveform of the heart travels in a
3-D orientation. The signal is travelling 1) parallel, 2) perpendicular, or 3)
some angle in reference to a particular lead. (fig. 1) Using ―Lead I‖ as an
example, being RA to LA electrodes, you can see in the figure the relationship between direction of conduction and its impact on the electrocardiograph. What’s important to focus on for this article is the ―isoelectric
points‖. These segments in the composite ECG can determine specific
myocardial Infarctions, acute pericarditis, or other heart abnormalities.
Fig 1: distinguishes between
positive, negative and
isoelectric deflection on ECG
lead 1.
Heart Pathology: Pericarditis on an ECG. Pericarditis has some indicators that show up on an ECG namely a depressed PR segment and
an elevated ST segment. There are many intervals, waves and segments
that physicians look at to help diagnose heart pathologies, however, we’ll
briefly review what is being represented by the ECG signal to understand
what is happening before, during and after the segments related to pericarditis. The P-R segment is the portion between the P-wave and the
QRS complex. The P-wave represents the depolarization of the atria. The
following QRS complex represents the depolarization of the ventricles.
The P-R segment represents the travelling of the signal from the AV node,
to the bundle of His, eventually to the Purkinje fibers. Contraction is not
occurring at this time therefore the ECG is flat between these waves. The
S-T segment is the portion of the signal between the QRS wave and T
wave. Remember that the QRS wave represents the rapid depolarization
ventricles depolarization.
Fig 2: shows an example of
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an exaggerated P-R
depression and S-T segment
elevation as seen on an ECG.