The electrocardiogram - ECG (CVS6) Flashcards
how does the heart generate surface potentials upon the skin
electrical activity within the cardiac muscle causes current flow within the heart and within surrounding tissues distant to the heart
what is caused by electrical activity within cardiac muscle cells
- current flow within the heart
- current flow within the surrounding tissues
- potential differences between distant sites on the body surface measurable by electrodes placed on the skin coupled to a sensitive recording device, the electrocardiograph
what is an electrocardiogram
recording of potential differences in current flow between distant sites on the body surface measurable by electrodes placed on the skin coupled to a sensitive recording device, the electrocardiograph
3 standard limb leads used when recording an ECG
- lead I: RA/right arm(-ve) - LA/left arm(+ve)
- lead II:RA/right arm(-ve) - LL/left leg (+ve)
- lead III: LA/left arm(-ve) - LL/left leg (+ve)
einthoven’s triangle
imaginary equilateral triangle with the heart at its center, formed by the axes of the three bipolar limb leads
why is lead II (between RA - LL) used when recording ECG
-lead II goes along the direction of the heart of the heart and so picks up electrical potentials better
why are the limb leads described as bipolar
- the heart is like a big generator
- so bipolar limb leads have a positive and negative end to them so that the maximum deflection in the voltage can be detected (depolarization moving towards the electrode gives an upward deflection)
why is right leg not included in ECG
- right leg is the earth leg
- but you don’t need to see that area of the heart so its irrelevant
how does the ECG work (calum sreenans explanation)
- there are no actual leads, the leads are vectors created between the two points that the pads are put on the person
- so as the electricity passes across the heart and then back up it, it causes deflections in the voltage across different directions, and you can work out where it is from the direction the deflection on the graph goes
- and you can work out where heart problems are occurring by which leads are affected
P wave on ECG
atrial depolarization (0.8-0.10seconds)
QRS complex on ECG
-ventricular depolarization (
T wave in ECG
ventricular repolarization (why an upward deflection?)
why does ventricular repolarization cause an upward deflection (T wave on ECG)
?
PR interval on ECG
largely due to AV node delay (0.12-0.20 seconds)
ST segment on ECG
ventricular systole
TP interval on ECG
diastole
unipolar limb leads
- The unipolar leads
- augmented leads and their respective limb electrodes are:
- aVR lead: right arm
- aVL lead: left arm
- aVF lead: left leg
- one +ve electrode, two others linked as -ve eg. if looking at aVR, one lead will be +ve and the other 2 leads will be negative
- > augmented limb leads are used to look at the heard in other directions/look at the heart in 3D
aVR
augmented voltage right
limb lead I
RA (-ve) to LA (+ve)
limb lead II
RA (-ve) to LL (+ve)
limb lead III
LA (-ve) - LL (+ve)
P wave
atrial
aVR
- augmented voltage right
- RA (+ve) to [LA and LF] (-ve)
aVL
- augmented voltage left
- LA (+ve) to [RA and LF] (-ve)
