Electro -cardio Gram (ECG) Flashcards
what is an** ECG**?
- electrocardiography is the process of producing an electrocardiogram - a recording of the hearts electrical activity
- involves a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin
in what direction does a current flow?
from positive to negative
Describe the current flow and the electrode with the deflection
- current flowing towards an electrode gives a positive deflection
- currenr flowing away from an electrode gives a negative deflection
- current flowing at right angles to an electrode gives no deflection
Describe how the electrical activity in the heart spreads from the SA node
- the initiation of the heart beat begins with the pacemaker activity of the SA node
- it then spreads across the heart from the right atrium (where SA node is) to the left atrium before passing to the ventricles
- It passes to the ventricles via the AV node, bundle of His and its left and right branches.
what can the ECG be useful to discover?
- abnormal electrical activity of the heart & abnormal function
- example : cardiac arrhythmias (heartbeat is too fast or too slow)
How is an electrical potential generated?
ie what charges are involved
- Myocytes of one part of the heart must be at their normal resting potential while others are in a depolarised state
- since the myocytes are electrically connected and the heart muscle is a syncytium (acts in sync), current will flow between these 2 regions ( & this current can be detected by ECG)
what determines the deflection direction in an ECG recording?
- the nature of the chemical event - ie depolarisation or repolarisation
- its direction of travel - in relation to the positive electrode
what does each electrode placement of the ECG measure?
- detects the average current flowing towards or away from it at any time
- electrodes placed in different locations will have different views of the electrical current
What are the** waves** in a typical ECG recording? what do these waves correspond to?
- P wave
- Q wave
- R wave
- S wave
- T wave
These waves correspond to specific cardiac events
What cardiac event is represented by the P wave?
- As the atria depolarise, the extracellular current flows between the depolarised regions of the atria and the rest of the heart
- this wave is a small positive deflection
what cardiac event is represented by the Q wave?
- after a brief delay at the AV node, the septum depolarises and the current flows from left to right
- small negative deflection - current flows away from V5 & V6 (eg’s of the unipolar leads)
what cardiac event is represented by the** R wave**?
- depolarisation of the ventricular walls
- current flows towards V3-V6 and therefore a large positive wave is recorded
what cardiac event is represented by the S wave?
- as the depolarisation passes up the ventricular walls toward the atria, the current flows away from V2-V3 and therefore a ** large negative wave is recorded.**
- NB - by the end of the S wave, the ventricles are fully depolarised and there is no current flow
What cardiac event is represented by the T wave?
- the last myocytes to depolarise are also the first to repolarise
- hence the T wave is in an upward direction
How is the ECG actually recorded?
- It is recorded by placing electrodes at different points of the body and measuring voltage differences between them with an electronic amplifier (generates the graph)
what do the electrodes detect?
they detect a small portion of the current flow during depolarisation and repolarisation
What are ECG leads?
- information from a number of electrode placements
Describe the 2 types of ECG leads
1.** Bipolar -** record the voltage between electrodes placed on** both wrists and the left ankle **
2. Unipolar - record the voltage between a single electrode placed on the body surface and an electrode that is maintained at zero potential
What are the 3 standard bipolar leads?
- **limb leads **
- limb lead I, II & III
Describe limb lead I
- the** positive terminal of the amplifier is connected to the left arm and the negative terminal to the right arm**
- with this placement - the amplifer records the component of wave excitation that is moving along this axis between the left and right sides of heart
Describe the** limb lead II**
- the** right arm is the negative terminal and the left leg is the positive terminal **
- the component of the wave of excitation is moving from the **right upper portion of the heart to the tip of the ventricles **
Describe the limb lead III
- the** left leg is the positive terminal and the left arm is the negative terminal**
- this lead records the component of the wave of excitation spreading along an axis **between the left atrium and the tip of the ventricles **
What are the 2 types of unipolar leads used in electrocardiography?
- chest
- augmented limb leads
What is used when an ECG is recorded using the unpolar chest leads?
- a reference electrode is produced by joining the 3 limb leads while exploring electrode records along specific points on the chest
- the exploring electrode is placed in 1 in 6 positions - leads V1-V6
what do the chest leads provide information about?
- the current flow in the transverse plane
what are the 3 augmented unipolar leads?
- aVr
- aVl
- aVf
describe the difference between the frontal and transverse plane in terms of information activity of the heart
- the three limb leads and augmented limb leads each provide information about the activity of the heart viewed from a specific point on the frontal (vertical) plane
- each of the** 6 chest leads** (V1-6) provide information about the activity of the heart from a specific point in the transverse plane
