ECGs Revisited Flashcards
The ECG is a graph plotting… against …?
Voltage (y-axis)
Time (x-axis)
What speed and amplitude is the graph plotted at?
Speed = 25mm/s or 50mm/s Amplitude = 5mm/mV or 10mm/mV
What is shown on the ECG at PQRST?
P = atrial depolarisation QRS = Ventricular depolarisation T = Ventricular repolarisation
What is occurring when the ECG is at baseline?
Reflects that there is no movement of the action potential at that time.
Reflects the pause in conduction that occurs as the AP passes through the AV node
The ECG is generated by measurement of the … … … in the heart
Electrical potential difference
How is a potential difference generated in the heart?
- Movement of the cardiac AP cell-cell involves changes in the membrane potential
- Cells which are polarised have +ve charges and those that are depolarised have -ve charges
- While the AP is moving, a potential difference is generated
Where are the standard locations for placement of electrodes to measure the electrical activity of the heart?
- Right forelimb
- Left forelimb
- Left hindlimb
The placement of the leads generates 6 standard limb leads, what are they?
- The bipolar limb leads I, II, III
- Augmented unipolar limb leads: AVR, AVL, AVF
What do each of the bipolar limb leads connect?
I = RF to LF II = RF to LH III = LF to LH
Where is the origin of each of the augmented unipolar leads?
AVR = RF AVL = LF AVF = LH
How can information about the heart chamber size be derived from a graph?
By observing the direction and comparing the size of the wave deflections in the different leads to one another
What is created by summation of the electrical dipoles the cells create as they depolarise and repolarise?
The vector - the flow of depolarisation
What is an electrical dipole?
The difference between two ends of the cell as the ions move during an action potential
During an action potential, where in the cell becomes depolarised?
Trick question - it is outside the cell that becomes negatively charged and depolarised
What does a positive defection on an ECG show?
That the vector is pointing towards the positive electrode at that time
Comparing leads I, II and III, how can we estimate the direction of the vector?
- By comparing the size of the deflection
- The larger the deflection, the more parallel to the line created by that lead the vector must be
- Vectors have both direction and magnitude
- e.g. if lead III has the biggest deflection (i.e. biggest p wave) then the vector will be most parallel to III
When taking all of the vectors created during a PQSRT waveform, we can average them and get what?
The mean electrical axis, showing the overall vector for that cardiac cycle
What is needed to word out the mean electrical axis?
- All 6 leads so they can be examined
- A diagram called the Bailey’s Hexaxial Diagram
How can looking at the biggest QRS complex help estimate the mean electrical axis?
This lead is the most parallel to the MEA, and the MEA will be within 30 degrees of this lead.
If the trace for this lead is mostly above the baseline we read the positive end, and if it is mostly below the baseline we read the negative end.
What is the isoelectric lead?
The lead where the deviation above the baseline is equal to the deviation below the baseline
How can the isoelectric lead be used to estimate the mean electrical axis of the heart?
This lead will be approximately perpendicular to the MEA of the heart.
E.g. if the isoelectric lead for a given ECG is lead AVL, the mean electrical axis must be in approximately the same direction as lead II (perpendicular to AVL).
If the trace for lead II is largely positive (above the baseline) the MEA is approximately +600, which is normal – this tells us that the largest mass of tissue is lying to the left of midline near the apex.
How is the quadrant graphing method used to estimate the mean electrical axis?
- Uses the leads I and AVF to divide the circle into 1/4s
- Look at the traces for these leads and see if they have a positive or a negative QRS complex.
- Draw arrows on the hexaxial diagram to indicate the direction.
- This indicates the quadrant which contains the MEA, and you can be more accurate by comparing the size of the QRS in these two leads.
What is a normal MEA reading in:
- Dogs
- Cats
- +40 - +100
2. 0 - +160
What is an MEA outside normal ranged referred to as?
An axis deviation, can be right or left axis
What does an axis deviation show?
That the AP hasn’t followed the normal direction through the heart as perceived by the skin at the limbs
What are the potential reasons for an axial deviation?
- The heart isn’t sitting in the normal position in the chest so the position in relation to the limb leads is different and hence the voltage detected will be different
- The size of the chambers of the heart is abnormal
- The conduction pathway is abnormal so the AP is having to travel a different route
A normal rhythm generated by the sinoatrial node is called a…?
Sinus rhythm
Give the names of a fast and slow arrhythmias
Fast = tachyarrhythmias Slow = Bradyarrhythmias
Abnormal, unexpected waveforms may be seen on an ECG, either sporadically or in runs, what do these waveforms represent?
A portion of the heart depolarising outwith the normal cardiac cycle and my be generated by the atria, AV node/bundle or the ventricles
Because these are generated when the heart should not be depolarising, they are referred to as…?
Premature complexes
How can you identify a supraventricular premature complex on an ECG?
- Will look like a norm QRS just in the wrong place
- Because the AP follows the normal conduction pathway within the ventricles even though it came from the wrong place in the atria.
- It will not have a P wave because it did not depolarise the atria properly.
How will a ventricular premature complex (VPC) appear on an ECG?
- Usually a lot wider than the QRS complex
- Complexes do not have a P wave
- They always have a T wave in the opposite direction to the main direction of the QRS complex.
Runs of premature complexes are referred to as … ?
Supraventricular/ventricular tachycardia
Describe atrial fibrillation
- Associated with a large atria
- The atrial myocardium begins to generate random wavelets of APs all over the atria, which bombard the AV node much faster than it can conduct.
- Some are conducted but not all, giving a chaotic and very rapid rhythm.
How would an ECG for atrial fibrillation appear?
- Supraventricular tachycardia
- No p waves
- An irregular R-R interval
What is an important cardiac cause of Bradycardia?
Atrioventricular block - The conduction of the action potential is slowed or obstructed in the AV node/bundle.
What is 1st degree AV block?
Where the PR interval is prolonged, indicating slowing of conduction at this point
What is 2nd degree AV block?
Where the conduction of the AP through the AV node is partially blocked, meaning that intermittently the P wave will fail to generate a following QRS complex
Describe the 2 types of 2nd degree AV block
Mobitz I is where there is progressive lengthening of the PR interval until eventually there is a non-conducted P wave.
Mobitz II does not have the progressive lengthening of the PR interval seen in type I block. The P waves are at a constant rate, and intermittently there will be one or more non-conducted beats.
Describe 3rd degree AV block
Complete blockage at the AV node, although the P and QRST complexes are present, they are unrelated
The atria will depolarise at the rate generated by the sinoatrial node and the AV node takes over as the pacemaker for the ventricles
