What makes it possible to record the EKG?
Synchronous nature and the large mass of the heart
What is being measured in an EKG?
Electrical potentials generated by the current flow that spreads to the surface of the body can be recorded at the skin with special electrodes
What are some applications of an EKG?
- Determination of heart rate
- Appraisal of normal and abnormal cardiac rhythms
- Analysis of intervals, duration of electrical complexes and segments
- Determination of the mean electrical axis of the heart (e.g. R or L hypertrophy)
- Detection of acute and chronic ischemic heart diseases
The heart is a dipole that creates a polarized environment by generating an electric field (some cells depolarizing, others resting). How does this influence the measurement of an EKG?
- The farther from the source of the dipole, the smaller the measured voltage difference
- The more in line with the direction of the dipole (0˚ and 180˚), the larger the voltage difference measured
- The electrodes placed orthogonally with respect to the dipole (90˚ or 270˚) will read null because they are sensing as many positive as negative charges
Explain the recording seen in this diagram
A. At the beginning of depolarization
a. Postive deflection
B. As it depolarizes, there is no difference seen between the two electrodes
C. One segment starts to repolarize, the other segment is still active
b. Negative deflection
D. Wave has passed, back to resting potential
a. Hit 0 between the two electrodes now
What does the P wave indicate on an EKG?
PR interval: 160 msec
What does the QRS complex indicate on an EKG?
What does the T wave indicate on an EKG?
What does the QT interval indicate on an EKG?
It is a good index of action potential duration
What portions of the heart are recorded during the flat part of the EKG, and why don't they deflect?
These structures are activated, but they are so small that they don't deflect
- AV node
- His bundle
- Bundle branches
- Purkinje network
What is the relationship between the EKG and the venticular action potential?
QT is the interval between ventricular depolarization and repolarization.
Describe the arrangement of the limb leads on an EKG and their axes.
- Lead I is fromR arm to L arm: 0˚ angle
- Lead II is from R arm to L foot: 60˚
- Lead III is from L arm to L foot: 120˚
Augmented limb leads: unipolar leads with resistor
- aVR: L arm to L foot: -210˚ (negative deflection)
- aVL: R arm to L foot: -30˚ (negative deflection, small QRS)
- aVF: R arm to L arm: 90˚
What part of the ventricles depolarizes first?
At rest, we measure a positive charge in the heart. Why?
The inside is really negative, but we are recording extracellular potential
At rest, all cells are positive outside and negative inside = flat line
Why is the repolarizing T wave in the same direction as the depolarizing R wave?
Repolarization wave is in the same direction, but it is opposite charge
Which is activated first: endocardium or epicardium?
Endocardium is activated first (positive deflection)
Over time the epicardium is activated, but it is delayed
Endocardium action potential is longer than it is in the epicardium
What is the benefit of the positioning of the precordial chest leads?
They allow you to have a representation of electropotential development in 3 dimensions
On an EKG, what does a positive deflection indicate?
A positive deflection means the impulse is moving toward the lead
A negative deflection means the impulse is moving away from the lead
The squares on an EKG measure time. How much time is respresneted by a small square? By a large square?
Small square: 40 ms
Large square: 200 ms
How do you determine the heart rate from an EKG?
Measure the space between two R peaks
1 square: 300 bpm
2: 150 bpm
3: 100 bpm
4: 75 bpm
5: 60 bpm
6: 50 bpm
Is it possible to have a normal heart rate with an abnormal rhythm?
Could be due to SA node problem
What is atrial flutter?
Flutter: more than one P wave per QRS complex
AV node is filtering this impulse
What is atrial fibrillation?
Fibrillation: escape pacemakers fire on their own and generate a pulse
Multiple foci of excitation orignate in atria
What are the characteristics of a 1st degree AV block?
Prolonged PR interval
What are the characteristics of a type I 2nd degree AV block (Wenckebach)?
PR interval changes
Gets longer until skipped beat
PR after skipped beat is shorter than the last
What are the characteristics of a type II 2nd degree AV blcok?
Normal PR interval
2 P waves for 1 QRS complex
What are the characteristics of a 3rd degree AV block?
P wave occurs, so SA node is firing, but impulse is not going through AV node
Complete heart block
Ventricles fire on their own from escape pacemakers
What are the characteristics of vetricular tachycardia?
Pattern is constant
High frequency, large waves
What are the characteristics of ventricular fibrillation?
Similar to atrial flutter
All over the place, pumping ability is not very good
What is the method for correcting ventricular fibrillation?
Defibrillation of the ventricles by electroshock
Hope that SA or AV node will kick in an start
What is the consequence of an extremely short PR interval?
Less ventricular filling time
What is the consequence of a long PR interval?
Conduction delay and AV node fires intrinsically -- Heart block