Cardiovascular Systems 10 - Electrocardiography Flashcards
What is a lead in an ECG?
Leads are a view of the electrical activity of the heart, they are not a cable/wire
What is a vector?
A quantity that has both magnitude and direction
Describe what can be deduced from the lines of the ECG, including steepness, direction and width of the deflection.
- The isoelectric line is where there is no net change in voltage, as vectors are perpendicular to the lead
- The steepness of the line shows the velocity of an action potential
- The width of the deflection shows the duration
- Upward deflections are towards the cathode (+) and downward are towards the anode (-)
- Every wave has both up and downstrokes
What is the P wave?
- The electrical signal that stimulates contraction of the atria
- Generated by the sinoatrial node
What is the QRS complex?
- The electrical signal that stimulates contraction of the ventricles
- Q is where the electrical signal is in bundle branches
- R is where the signal is in purkinje fibres (early ventricular depolarisation)
- S is also where the signal is in purkinje fibres (late ventricular depolarisation)
What is the T wave?
The electrical signal tat signifies relaxation of the ventricles
Where is the electrical signal in the heart between the P wave and QRS complex?
- It is in the atrioventricular node
- It is isoelectric
- AV node conducts slowly
What causes the line between the QRS complex and the T wave?
The signal is in the fully depolarised ventricles, it is isoelectric.
Describe the placement of electrodes on a patient?
- One on the right arm, one the left arm, right leg and left leg.
- The other 6 (V1-V6) are on the ribs
Where is the V1 electrode?
Right sternal border in the 4th intercostal space
Where is the V2 electrode?
Left sternal border in the 4th intercostal space
Where is the V3 electrode?
Halfway between V2 and V4
Where is the V4 electrode?
Mid-clavicular line in the 5th intercostal space
Where is the V5 electrode?
Anterior axillary line at the level of V4
Where is the V6 electrode?
Mid-axilliary line at the level of V4
What is lead 1?
Right arm to left arm
What is lead II?
Right arm to left leg
What is lead III?
Left arm to left leg
How is heart rate calculated form an ECG?
300/number of big squares
Which is bigger, T wave or P wave?
T wave
How is the cardiac axis calculated?
- Lead II and aVL are used, as they are 90 degrees apart
- Amplitude fo LII is calculated, as it that of aVL. This is done by taking away the number of squares of the S from that of the R downward stroke of the QRS complex.
- These are used as lines of a triangle
- Trigonometry is used to find the hypothesis
- 60-answer to find the angle of the heart (as the LII line is at an angle of 60 degrees)
Which leads have a virtual anode?
aVR, aVL, aVF, V1-V6
Which leads have a fixed anode?
LI, LII, LIII
Which lead is the most commonly used single lead?
Lead II
What is the normal range of the cardiac axis?
-30 to +90 degrees
What is the approach used to take and analyse an ECG?
- Ensure it is correct patient
- Look at rate and rhythm
- P-wave and PR interval
- QRS duration
- QRS axis
- ST segment
- QT interval
- T wave
Describe the ECG appearance in sinus bradycardia.
- Each P wave is followed by QRS
- Rate is regular but slow
What is the cause of sinus bradycardia?
- Healthy
- Caused by medication
- Caused by vagal stimulation
Describe the ECG appearance in sinus tachycardia.
- P wave followed by QRS
- Regular rate but fast
What causes sinus tachycardia?
Physiological response (secondary)
Describe the ECG appearance in sinus arrhythmia.
- Each P wave is followed by a QRS wave
- Rate is irregular, but normal.
- The R-R interval varies with breathing
Describe the ECG appearance in atrial fibrillation.
- Oscillating baseline, the atria contract asynchronously
- Rhythm can be irregular, rate may be slow
- Increased clot risk due to turbulent blood flow
Describe the ECG appearance of atrial flutter.
- Saw tooth pattern in the baseline of II, III, aVF
- The atrial to ventricular beats at 2:1, 3:1 or higher ratio
- The saw tooth is not always visible in all leads
Describe the ECG appearance in first degree heart block.
- Prolonged PR segment/interval due to slower AV conduction
- Regular rhythm, P wave before every QRS
Describe the ECG appearance of second degree heart block (Mobitz 1)
- Gradual prolongation of the PR interval until a beat is skipped
- Most P waves followed by QRS, some are not
- Regularly irregular (each component is regular, but the regularities are different)
Describe the ECG appearance of second degree heart block (Mobitz II)
- P waves are regular, but only some are followed by QRS
- There is no PR prolongation
- Regularly irregular, there is a ratio of successes to failures
Describe the ECG appearance of third degree heart block.
- P waves are regular, as are QRS, but there is no relationship
- P waves can be hidden within QRS
- There is non-sinus rhythm
Which type of heart block is the most progressed?
- Third degree heart block is complete
- First degree is the most benign, it is a disease of aging
- Second degree (Mobitz II) is more serious than Mobitz I, as it can rapidly deteriorate into third degree
Describe the ECG appearance of ventricular tachycardia.
- P waves are hidden
- The rate is irregular and fast (100-200bpm)
- High risk of fibrillation
- This is a shockable rhythm
Describe the ECG appearance of ventricular fibrillation.
- Heart rate is irregular, 250bpm+
- The heart cannot generate an output- just lots of squiggly lines!
Describe the ECG appearance of ST elevation
- P waves are visible and always followed by QRS
- Rhythm is regular and the rate is normal (85bpm)
- The ST segment is elevated 2mm or more over the isoelectric line
What causes ST elevation?
Infarction
Describe the ECG appearance of ST depression.
- P waves are visible, always followed by QRS
- Rhythm is regular and heart rate is normal
- The ST segment is depressed 2mm or more below the isoelectric line
What causes ST depression?
Myocardial ischaemia (coronary insufficiency)
What is the clinical relevance of ECG?
- Cheap, reliable and fast way to assess heart function
- Can evaluate the conduction system and structure
