Lecture 16: Physiological Basis of the ECG Flashcards
When do you have a deflection on the ECG?
When there is a voltage difference in extracellular potential at Phase 0 or Phase 3
What are the conditions of deflection?
Excitable tissue has a different potential compared to rest of heart and currents can travel between those two regions
Why can ECGs only report voltage differences within one type of chamber but not in between (e.g. between atria and ventricles?
A and V are isolated electrically so AP s cannot travel between (has to go through the nodes)
What do leads do?
Measures an average summation vector of all the APs in the heart from different vantage points (that’s why some leads see upright while others see downward waves)
Explain how deflection works in an ECG:
Whenever there is a deflection away from baseline, one end has different potential from the other. If volts returns to baseline, the AP has reached the other end and they now both have the same potential
What do these waves mean physiologically (include directionality and phases if applicable)? P wave PR interval QRS complex ST segment T wave
- atrial depolarization (R to L), phase 0
- delayed AP at AV Node
- V depolarization (R to L, apex to base), phase 0
- delay for V repolarization
- ventricle repolarization followed by relaxation (L to R, base to apex), phase 3
At the end of the P wave and QRS, describe the status of Ca2+ permeability
End of p wave - all the atria/ventricle is depolarized. Ca2+ voltage are open = high permeability
What is a U wave? Is there a clear cause?
Wave after T wave. No.
In EKG …
X and y axes represent?
5 boxes horizontally equals?
2 boxes vertically equals?
- time and voltage
- 1 second
- 1 mV
What is the difference between a segment and interval?
Segment: single event
Interval: combination of multiple events
What does the PR interval represent? What does the QT interval represent? What are their normal values
- P wave ending and beginning of QRS (0.16 ms)
- QRS beginning to end of T wave (0.35 ms)
What does the PR segment represent? What does the ST segment represent?
- end of atrial depolarization to beginning of QRS
- end of QRS to beginning of T wave
Normal values for…
QRS complex
P wave
T wave
What can affect these values?
- 1.0 - 1.5 mV
- 0.1 - 0.3 mV
- 0.2 - 0.3 mV
-Electrode placement
How much time does between 2 dark lines represent?
How much time does between 2 light lines represent?
- 0.2 s
- 0.04 s
Why do 12 lead ECGs do?
Uses 9 electrodes (perspectives) to average the axes to get the average AP direction in the heart, and determine which parts can conduct electricity (alive) and which cannot (infarct)
