ECG Flashcards
how does electrical conduction occur in the heart?
- in the sinus rhythm, the SA node generates action potentials that automatically conduct
- rapidly through the atria - causing contraction
- slowly through the AV node
- rapidly through the bundle of his and left and right bundle branches
- rapidly through the purkinje fibres causing ventricular contraction
what is an ECG?
- recording of potential changes at the body surface which arise from currents that flow when the membrane potentials of myocardial tissue is changing
what is the purpose of an ECG?
- provides information about cardiac rate and rhythm, chamber size, the electrical axis of the heart
- main test to assess for myocardial ischaemia and infarction
what is an electrical dipole?
- a vector with components of magnitude and direction eg from atria to ventricles
- this allows the electrical axis of the heart to be estimated
what is magnitude determined by?
the mass of cardiac muscle that is involved in the generation of the signal - this atria and ventricles dominates
what is direction determined by?
the overall activity of the heart at any instant time and varies during the cardiac cycle
what is an ECG lead?
- the imaginary line, the . lead axis between two or more electrodes
- not the wire
what happens when depolarisation moves towards/away from the recording electrode (positive) ?
- towards = generates an upward deflection on the ECG
- away from = generates a downwards . deflection on the ECG
what is the 12 ECG composed of?
- 3 standard limb leads = bipolar
- 3 augmenteed voltage leads = unipolar
- 6 chest leads = precordial leads
where are the standard leads placed?
- RA - LA
- RA - LL
- LA - LL
(all negative to positive so there is an upwards deflection)
why is the P-wave upright?
- atrial depolarisation spread from SA to AV, inferiorly and to the left
- depolarisation is moving towards to recording electrode
- so an upward deflection is formed
what does the QRS look like it does?
Q = ventricular depolarisation starts in the interventricular septum and spreads from left to right causing the small and narrow Q wave R = the main free walls of the ventricles depolarise causing a tall and narrow R wave S = the ventricles at the base of the heart depolarise, producing a small and narrow S wave
what is the T wave an upright?
- it represents ventricular repolarisation but it is moving away from the recording electrode
- eg a negative signal moving away from a positive = positive signal
what is the isoelctric line?
- regions where there are no potential changes
- the straight line
what is the PR interval?
- dominated by the delay in conduction through the AV node
what is the QT interval?
- reflects the time for ventricular depolarisation and repolarisation, normall in males 0.44 and 0.46 in females
- prolongation predisposes to disturbances of cardiac rhythm
what do the augmented leads do?
- see heart from different angles
- one is positive, two are negative
what do the precordial leads do?
- look at the heart from horizontal directions
how do you place V1-V6?
1 = 4th intercostal space immediately right of sternum 2 = 4th intercostal space left of sternum 3 = midway between 2 and 4 4 = 5th space, mid clavicular line 5 = same horizontal line as 4, anterior axillary line 6 = same horizontal level as v4, mix axillary line
how fast must standard calibration paper move?
25mm/sec
what is ACG rhythm strip?
- prolonged reading of one lead
- usually 2 lead
- allows you to determine heart rate and identify the cardiac rhythm
why are 12 leads required?
allows you to
- determine the axis of the heart in the thorax
- to diagnose ischaemic HD
- to diagnose chamber hypertrophy
how do you calculate the rate of an ECG?
300 / number of large squares
what are the signs of atrial fibrillation?
- no P waves
- irregularly irregular QRS
