CVS session 7: electrocardiogram Flashcards
Why can electrodes on the surface of the body detect electrical changes in the myocardium?
The myocardium undergoes electrical changes at the same time, generating a large changing electrical field which can therefore be detected on the surface by EXTRACELLULAR RECORDING (so looks different to ventricular and pacemaker potentials as these are intracellular)
What do the electrodes “see”?
Two signals with each systole: one on depolarisation and one on repolarisation. Also detects the changing signal which the excitation produces.
These combine to give the ECG
Function of gap junctions?
Myocytes are connected by these to coordinate depolarisation, giving synchronous and rapid contraction
There is a pause between atrial and ventricular contraction
Describe the electrical pathway of the heart
SA node discharges depolarisation
Spreads via internodal tracts (specialised cells with gap junctions) across atria to AV node
Pause of 120ms at AV node before reaches bundle of His
Bundle of His divides the septum and spreads depolratisation as the atria are electrically isolated from the ventricles by AV valves
Reaches left bundle branch fractionally earlier than right bundle branch
Purkinje fibres depolarise the rest of the ventricles (more slowly)
In which direction does the heart repolarise?
From epicardium (outer) to endocardium (inner)
In which direction does the heart depolarise?
From endocardial to epicardial
What is an ECG lead?
An electrical picture of the heart, which inspects it in frontal (standard leads) and horizontal (chest leads). Each lead looks from a different direction, each with different electrical activity, therefore the ECG pattern in each lead is different. An imaginary line between two electrodes
10 electrodes are attached and computed into 12 leads
Which leads are unipolar and which are bipolar?
UNIPOLAR: read from the labelled positive electrode, and utilise several other electrodes as the negative. Chest leads (V1-V6) and augmented leads (AVR, AVL and AVF)
BIPOLAR: uses one positive-sensing electrode and one negative electrode from standard limb leads (I, II and III) in configuration of Einthoven’s triangle. Best readings from body aspects of limbs. Lead II looks from the apex of the heart so is best for looking at rhythm
How does the direction of depolarisation/repolarisation affect the ECG trace?
Depolarisation:
- moving TOWARDS electrode=POSITIVE deflection
- moving AWAY from electrode=NEGATIVE deflection
Repolarisation:
- moving TOWARDS electrode=NEGATIVE deflection
- moving AWAY from electrode=POSITIVE deflection
What determines amplitude of signal?
How much muscle is depolarising How directly towards the electrode this happens -directly towards/away=big -obliquely towards/away=smaller -at right angle=no signal
What might confound the ECG trace?
Lead confounders
Muscle contraction (need to avoid movement, shivering, talking, coughing)
Interference e.g. alternating current
Poor electrode contact (sweat, cable pull, hair)
How does each lead view the heart?
DRAW DIAGRAM! To properly understand
Where are the electrodes placed?
Chest leads:
V1 – 4th intercostal space – right sternal edge
V2 – 4th intercostal space – left sternal edge
V3 – midway between V2 & V4
V4 – 5th intercostal space – mid clavicular line
V5 – anterior axillary line – same horizontal level as V4
V6 – mid-axillary – same horizontal level as V4
Limb leads: Red-right arm (shoulder/wrist) Yellow-left arm (shoulder/wrist) Green-left leg (ASIS/ankle) Black-right leg (ASIS/ankle)
Draw a typical ECG trace from lead II. What does each landmark represent?
Draw and where to measure
P: atrial depolarisation
Q: depolarisation of interventricular septum (laterally)
QRS: depolarisation spread through ventricle. This masks atrial repolarisation
S: depolarisation from apex upwards from ventricles (So downwards as moving away from lead II, which views from the bottom)
T: ventricular repolarisation, from epicardial to endocardial surface
What are the normal values for the intervals measured on an ECG?
P-R interval: 3-5 small squares (120-220 ms)
QRS complex: 3 small squares (120 ms)
QT interval: 9-11 small squares (360-440 ms)
ECG machine must be calibrated: standard signal of 1 mV should move the graph 1cm vertically (2 large squares), so calibration signal included in every graph
What do the squares on an ECG mean?
ECG standard speed at 25 mm/s
1 large square= 5 mm = 0.2 s = 200 ms
1 small square= 1 mm = 0.04 s = 40 ms
There are 5 large squares per second, so 300 large squares per minute
How is heart rate measured using an ECG?
If rhythm is REGULAR: count the number of large squares in 1 R-R interval, and divide 300 by this number
If rhythm is IRREGULAR: count the number of R-R complexes in 30 large squares (6 seconds), then multiply by 10
Which lead is used to assess rhythm?
Lead II rhythm strip: a long (~10s) reading
Describe a simple method to check rhythm
Mark on paper a few R waves, then move paper alongn and see if they still line up (if they do=regular).
Irregular:
- pattern to irregularity=regularly irregular
- no pattern to irregularly irregular
Describe the factors that need to be looked at when assessing rhythm
- p waves
- present/normal/absent
- normally upright in leads I and II
- no p waves and irregular QRS: probably AF
- apparent absence of p waves and narrow QRS could be SVT
- saw-toothed shaped p wave: atrial flutter (SAN firing v. fast, AVN only lets through every 2nd/3rd/4th impulse so HR predictable (150, 100 or 75 bpm) - Cardiac axis-normal/left or right deviation
- PR interval
- normal/long/short
- estimates conduction in AV node and bundle of His - Relationship between p wave and QRS: is every p followed by a QRS, and every QRS preceded by a p?
- QRS complexes
- narrow means rhythm originating in atria (normal)
- broad means rhythm originates in ventricles or there is a bundle branch block so depolarisation has to spread down a back route - ST segment: elevated/depressed?
- T waves: present/inverted?
What heart rate do the following, regular R-R intervals correspond to?
Large squares vs. HR: 1-300 2-150 3-100 4-75 5-60 6-50
Describe the sinus rhythms
SINUS RHYTHM:
- normal
- depolarisation initiated by SA node
- rate 60-100 bpm
- normal p wave followed by QRS and each QRS preceded by a p
SINUS BRADYCARDIA
- sinus rhythm with a rate 100 bpm
- associated with exercise, fear, pain, haemorrhage or thyrotoxicosis
Atrial fibrillation
Multiple abnormal atrial pacemakers discharging randomly, so atrial depolarisation is chaotic and thus contraction is inefficient “quivering”. AV node continuously receives depolarisation waves of varying strength, so depolarisation spreads at irregular intervals down the bundle of His
ECG:
- no p waves
- irregularly irregular rhythm and HR
- normal QRS (as conduction into and out of ventricles is by the normal route)
First degree heart block
Each wave of depolarisation that originates in the SA node is conducted to the ventricles, but there is a delay somewhere in the pathway. This is not an issue in itself, but may be a sign of coronary heart disease or acute rheumatic carditis
ECG:
- normal p wave and QRS
- PR interval prolonged due to slow conduction in AV node and bundle of His due to ischaemia/degenerative change
