Physiology Lab (Phsl 3063, Phsl 3701)

Fall 2012

UNIVERSITY OF MINNESOTA

 

 

 

 

 

 

 

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Schedule

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Atlas of Human Cardiac Anatomy

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Lesson 5 - ECG I Tutorial

CG animation

Animation from www.freewebs.com/amithimani/index.htm

This lesson will introduce you to the Electrocardiogram (ECG). The heart has a mechanical and an electrical component. The ECG is clinically useful because by placing electrodes at various points on the body surface, it can show the electrical activity within the heart. This enables clinicians to determine the state of the conducting system and of the myocardium itself, as damage to the myocardium can alter the way the impulses travel through it.

CG trace

Image modified from www.cvphysiology.com/Arrhythmias/A009.htm

The P wave represents atrial depolarization- there is little muscle in the atrium so the deflection is small.
The QRS wave represents the depolarization stage of the ventricles and the repolarization stage of the atrium, however since there is more muscle in the ventricles you can only see the electrical activity of the ventricles, not the atrias.
The T wave represents repolarization of the myocardium after systole is complete. This is a relatively slow process as shown by the smooth curved deflection.

One of the main measurements you will be conducting during this module is Beats Per Minute (BPM), otherwise known as heart rate. You will look at your BPM while lying down, sitting up, sitting and deep breathing, and after exercise.

So big question is how do you find your BPM from an ECG??? Let's look at that ECG trace again.

CG trace with grid

Each little box corresponds to 0.04 seconds. To figure out heart rate you want to go from one waveform to the next and count the boxes in between. Here I counted from P-wave to P-wave, but you could go from Q-wave to Q-wave, etc. The most common way to measure BPM from an ECG recording is to go from R-wave to R-wave because that wave is the most prominent. So you can see that there are 15 boxes in between the P-waves, so 15 X 0.04sec = 0.6sec. For 1 heartbeat, this ECG reads that it takes 0.6sec, so for beats per minute we want to convert this (0.6sec)/(1 beat) factor into beats/minute. If there are 60 seconds in one minute then we figure beats/minute by the following:

(60seconds/minute)/(0.6seconds/beat) = 100 beats/minute

Here the seconds cancel out leaving beats per minute.

Below are some interesting ECG traces from patients with various heart conditions.

Ventricular Fibrillation - Notice the chaotic rhythm, patient is not getting good blood flow and therefore this is a serious medical emergency.

CG Ventricular Fibrillation

Prior Myocardial Infarction - Notice the relatively large Q-wave and the shorter R-wave. This is due to the dead tissue from the previous MI. This causes an unbalancing of forces because one wall is dead and cannot contribute to the ECG reading, while the opposite wall is fine, so the Q-wave appears larger than from a healthy heart. The R-wave is shortened due to the loss of ventricular muscle, therefore there is less electrical activity to read.

G Infarction

Preventricular Contraction - Notice the downward wave followed by a large T-wave, you will notice there is no P-wave because this beat did not originate from the sinoatrial node like normal heart beats. PVCs occur when there is an ectopic beat, which is a beat originating from a site other than the sinoatrial node.

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This page was last updated on August 11, 2008