ECGs Flashcards
What are the intervals on an ECG?
How long are they?
PR Interval: 120-200ms
QRS Interval: 80-120ms
ST Interval: 320ms
QT Interval: 350-450ms
What are the two segments of an ECG and how long are they?
PR Segment: 50-120ms
ST Segment: 80-120ms
What are the five waves of an ECG?
How long are they?
P Wave: 80ms
Q Wave: \
R Wave: QRS Complex: 80-120ms
S Wave: /
T Wave: 160ms
Label the Waves, Segments and Intervals on this ECG:
What do upward and downward deflections represent?
Upward deflection: wave of depolarisation towards the cathode (+) or repolarisation away from the cathode (+)
Downward deflection: wave of depolarisation towards the anode (-) or repolarisation away from the anode (-)
What does the gradient on an ECG represent?
The velocity of the action potential
What causes the P-wave?
P wave: SAN autorhythmic myocytes depolarise, causing the P-wave and atrial depolarisation, with the wave moving across from the right to left atrium via internodal fibres, and slightly towards the cathode
What causes the PR segment?
AVN depolarises in the PR segment and isoelectric to delay impulse and allow for ventricular filling
What causes the Q wave?
Bundle of His rapidly conducts wave of depolarisation down septum
What causes the R wave?
Ventricular depolarisation due to the purkinje fibres, with wave spreading towards the cathode
What causes the S wave?
Purkinje fibres carry wave up the myocardium for late ventricular depolarisation, moving away from the cathode
What causes the ST segment?
Depolarised ventricles produce an isoelectric ECG
What causes the T wave?
Ventricular repolarisation moving towards the cathode
What causes the U wave?
Purkinje fibre repolarisation
Where would you place the chest leads?
- V1: 4th intercostal, right sternal margin
- V2: 4th intercostal, left sternal margin
- V3: in-between V2 and V4 (on top of 5th rib)
- V4: 5th intercostal, mid-clavicular line
- V5: 5th intercostal, anterior axillary line (usually half-way between V4/V6)
- V6: 5th intercostal, mid-axillary line
Where are the limb leads placed?
Red: Right Arm
Yellow: Left Arm
Green: Left Leg
Black: Right Leg
What do the small squares on an ECG represent?
0.04s width, 0.1mV height
What do the big squares on an ECG represent?
0.2s width, 0.5mV height
What is the run-speed of an ECG?
25mm/sec
What is Einthoven’s Triangle?
Formed by leads I, II and III running RA-LA, RA-LL, LA-LL; using a coronal plane to measure movement of electricity away from heart
What are the Bipolar leads?
I, II, III - measuring potential difference between the limbs; for a given complex the net deflection is equal to the sum of the net deflections in the other two leads;
What are the Unipolar leads?
aVR, aVF, aVL - measuring potential difference between null point with relative 0 potential at centre of triangle
Measure flow of electricity from centre of heart towards right arm (aVR), left arm (aVL) and left foot (aVF)
What is the difference between Bipolar and Unipolar leads?
Bipolar uses two physical leads, unipolar uses one lead and a combination of other leads
What is the Rule of L’s?
Lead I (1 L):
Right Arm to Left arm
Lead II (2 L’s)
Right arm to Left leg
Lead III (3 L’s)
Left arm to Left leg
Also remembered by reading left to right, up to down
Describe the Polarity, Plane, Cath/anode, and View of Leads I-III
Describe the Polarity, Plane, Cath/anode, and View of Leads aVR, aVF, aVL
Describe the Polarity, Plane, Cath/anode, and View of Leads V1 and V2
Describe the Polarity, Plane, Cath/anode, and View of Leads V3 and V4
Describe the Polarity, Plane, Cath/anode, and View of Leads V5 and V6
Which leads would you use to look at the Left Circumflex artery?
- 1
- aVL
- V5
- V6
What leads would you use to look at the Right Coronary?
- 2
- 3
- aVF
- V3
- V4
What leads would you use to look at the Left Anterior Descending artery?
- V1
- V2
What is the cardiac axis?
The axis of a wave/complex is the direction the wave of depolarisation is moving in on the coronal plane (does not use precordial leads)
What is the normal range for the Cardiac Axis?
-30o to +120o General axis of depolarisation
What are the axes of Leads I-III?
I: 0o
II: 60o
III: 120o
What are the axes of leads aVL, aVF, aVR?
aVL: -30o
aVF: 90o
aVR: -150o
Which combinations of leads might you select to calculate the axis?
I/aVF
II/aVL
III/aVR
How do you calculate the cardiac axis?
- Select two leads at 90o
- Work out net QRS depolarisation on each lead
- Form a triangle by drawing x mm along the numerical lead e.g. II, and y mm perpendicular from the lead (where x is the numerical lead net depolarisation and y is the augmented lead net depolarisation)
- Theta = tan-1(aug depol/num depol)
- Cardiac Axis = Numerical Lead Axis - theta
What is the systematic approach to ECG interpretation?
- Correct recording?
- Signal quality and leads? (movement can affect)
- Voltage and paper speed?
- Patient background? CVS/Resp disease? (axis moves on COPD)
- Rate: 300/no. big squares = heart rate
- P Wave: should be 80ms; PR: should be 50-120ms
- QRS duration: should be 80-100ms
- QRS axis: should be -30O to 120O
- ST Segment: should be 80-120ms and not isoelectric
- QT interval: should be 420ms
- T Wave should be 160ms and will change shape in electrolyte disturbances
What does this ECG show?
Sinus:
Ps followed by QRS 1:1
Regular rate and normal
Otherwise unremarkable
What does this ECG show?
Sinus Brady:
- Ps followed by QRS 1:1
- Regular rate and slow
- Can be healthy/medication/vagal stimulation
What does this ECG show?
Sinus Tachy:
- Ps followed by QRS 1:1
- Regular rate and fast
- Often physiological or could be decreased venous return
What does this ECG show?
Sinus Arrhythmia
- Ps followed by QRS 1:1
- Irregular rate and normal-ish rate
- R-R varies with breathing
What does this ECG show?
Atrial Fibrillation:
- Oscillating baseline so asynchronous atria contraction
- Irregular and slow rhythm
- Turbulent flow pattern increases clot risk
What does this ECG show?
Atrial Flutter:
- Regular saw-tooth pattern in the baseline (II, III, aVF)
- Atrial:ventricular beats 2:1/3:1 or higher
- Not visible in all leads; some flutters hidden in QRS complexes
What does this ECG show?
1st deg heart block
- Prolonged PR due to slower AV conduction
- Regular rhythm of P:QRS
- Mostly benign
What does this ECG show?
2nd deg heart block (Mobitz 1)
- Increasing PR until missed QRS
- Regularly irregular due to diseased AV node
- AKA Wenkebach
What does this ECG show?
2nd deg heart block (Mobitz 2)
- No PR elongation and some QRS complexes are missed
- Regularly irregular with fixed ratio of successes to failures e.g. 2:1
- Can rapidly deteriorate to 3rddegree
What does this ECG show?
3rd deg heart block
- Regular P waves and QRS complexes but no relationship
- AVN/myocardium can be auto-rhythmic
- Non-sinus with backup pacemakers
- P waves regular and fast, QRS regular and slow
What does this ECG show?
VTach
- P waves hidden in dissociated atrial rhythm
- Ventricles beating rapidly and very hard
- Rate regular and fast
- Risk of deterioration to Vfib
- Shockable rhythm
- All vectors look same
What does this ECG show?
VFib:
- Heart rate irregular and >250bpm
- No output
- Shockable
- Vectors will change and do not all look same
What does this ECG show?
ST Elevation
- P waves visible and followed by QRS
- Regular normal rate
- ST-segment elevated >2mm above isoelectric
- Caused by infarction
What does this ECG show?
ST Depression:
- P waves visible and followed by QRS
- Regular normal rate
- ST-segment depressed >2mm below isoelectric
- Caused by ischaemia
What are the Shockable rhythms?
VTach & VFib
