12. ECG Methods Flashcards
What is the difference between cables/wires and leads?
- Cables/wires - connects electrodes to the device
* Leads - digital representation of the changes of depolarisation in the heart
What is the deflection on an ECG is an impulse is moving towards an anode or a cathode?
- Anode - positive deflection
* Cathode - negative deflection
Why does the ECG look at many views?
- Sometimes the impulse is moving at a right angle to the electrodes
- This shows no deflection
- Looking at different views allows you to see these impulses
What does the steepness of a line and sharpness of turns show?
- Steepness - velocity of action potential
* Sharpness - denotes rapid changes in direction of action potential
What is the main lead and why?
- Lead II
- From Right Arm to Left Leg
- Angle of lead is roughly the same as the angle of the heart
- Deflections will be large
Why does the SAN only produce a small deflection?
- Small amount of muscle (autorhythmic myocytes)
* P wave
Why does the AVN lag the impulse?
- Prevents the impulse from immediately going to the ventricles
- Allows the atria to empty so the ventricles can fill as much as possible
- Limits the rate of contraction for ventricles too
- Isoelectric ECG (flat line)
What is the line on the ECG at the Bundle of His?
- Short isoelectric ECG just before QRS (PR segment)
- Rapid conduction
- Insulated
What happens at the bundle branches and how is this represented on the ECG?
- Common bundle branch splits into left and right bundle branches
- Heavily insulated
- Insulation on the LEFT bundle branch terminates - some impulses escape
- Ventricular septum is exposed to the impulse and depolarises
- This wave of depolarisation moves from the bottom up through the septum - towards the negative electrode
- Small amount of muscle
- Fast
- Short, sharp downwards spike on ECG - Q wave
What happens at the Purkinje fibres before the apex and how is this represented on the ECG?
- Ventricular depolarisation
- Quickl depolarisation - muscle can contract simultaneously for efficient contraction
- Towards positive electrode - positive deflection - R wave
- Large deflection due to large amount of muscle at the apex
What happens at the Purkinje fibres after the apex and how is this represented on the ECG?
- Late ventricular depolarisation
- Ventricles contract up the sides
- Towards negative electrode - negative deflection - S wave
- Less muscle - smaller deflection
What happens on the ECG when the ventricles are fully depolarised and why?
- Isoelectric ECG (ST segment)
* Muscle fibres are in tetany - contracted
What happens on the ECG during repolarisation and why?
- Ventricle repolarisation allows ventricles to relax
- Opposite direction to contraction - positive deflection
- T wave
Why do you not see atrial repolarisation on an ECG?
- Same time as the ventricle contraction
* More muscle mass in ventricles - hides the repolarisation on the ECG
Why does Lead II usually have a longer recording?
- Look for anomalies
* May not occur on every heartbeat
How many leads are there?
12
What does LCx, RCA and LAD stand for?
- Left Circumflex Artery
- Right Coronary Artery
- Left Anterior Descending Artery
What is Lead I, II and III?
I - right arm to Left arm (1 L)
2 - right arm to Left Leg (2 Ls)
3 - Left arm to Left Leg (3 Ls)
All form ‘Einthoven’s Triangle’ with the heart in the centre
Where are the chest leads attached (in order)?
- V1 - right sternal border, 4th intercostal space
- V2 - left sternal border, 4th intercostal space
- V4 - mid-clavicular line, 5th intercostal space
- V5 - anterior axillary line, level of V4
- V6 - mid-axillary line, level of V4
- V3 - halfway between V2 and V4
How are unipolar leads different from bipolar leads, and which ones are unipolar?
• Unipolar - one physical electrode compared to a virtual electrode
- augmented leads
- chest leads
• Bipolar - two physical electrodes compared
What is the PR, ST and QT interval?
- PR interval - start of the P wave to the last isoelectric part before Q deflection
- ST interval - start of isoelectric segment after S to the end of the T wave
- QT interval - start of Q deflection to the end of the T wave
If one square on an ECG is 0.04s, how long is one big square?
0.2s
How many milliseconds is 0.04s?
40ms
What do you need to work out the QRS axis?
- 2 leads
* 90 degrees apart
What are the 3 augmented leads and in which direction do they point from the heart?
- aVF - downwards from the heart
- aVR - 120° (to the top right of the heart) from aVF
- aVL - 120° (to the top left of the heart) from aVF
- All point at a coronal view
Which Lead is at 0°?
Lead I (horizontal from the left of the heart) - reference point
At what angle are aVL, Lead II, aVF and Lead III to the reference point?
- aVL: -30°
- Lead II: 60°
- aVF: 90°
- Lead III: 120°
How many degrees apart are Lead II and aVL?
90°
How do you measure net deflection on Lead II?
- Measure positive deflection (Q to the start of the S downward deflection)
- Take away the negative deflection (start of the negative S deflection down to S)
• e.g. positive = +6.5mm, negative = -2mm, so net = +4.5mm
How do you measure net deflection on Lead aVL?
- Measure positive deflection on left side of the complex
* Take away the negative deflection on the right side of what seems to be the S deflection
What is the QRS axis (cardiac axis) if the Lead II net deflection is +4.5mm and aVL net deflection is +2mm?
- +4.5 is on the Lead II heart vector (adjacent)
- +2 is towards aVL (opposite)
- Tan θ = 2 / 4.5
- θ = tan^−1
- θ = 36°
- So the QRS axis is 36°
What is the normal range for cardiac axis?
-30° to 90°
