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WEEK 19:
Electrocardiogram (ECG) Principles:
| Question | Answer |
|---|---|
| what is an ECG | extracellular recording of electrical activity of the heart |
| where do you put ECG electrodes | placed on arms (wrists), legs (ankles) and chest (V1-6) to detect electrical activity |
| ECG graph axis | voltage vs time |
| ECG electrodes | sticky thing you place |
| ECG leads | graphical recordings of the data collected from all electrodes |
| ECG cables | line that connects electrodes to ECG machine |
| on a standard 12 lead ECG, how many electrodes are placed | 10 |
| V1 + V2 | 4th intercostal space L/R |
| V5 | placed horizontally from V4 at the anterior axillary line |
| V3 | midpoint between the diagonal line ( \ ) of V2 and V4 |
| V4 | 5th intercostal space |
| V6 | placed horizontally from V4 at the midaxillary line |
| how is voltage (difference in electrical potential) recorded | potential of one electrode compared to another (or combination of other electrodes) |
| right leg electrode | earth electrode (not needed, there for reference) |
| how many active limb electrodes | 3 |
| how many chest electrodes | 6 |
| limb leads and electrodes | 6 leads (I, II, III, aVR, aVL, aVF) from only 3 electrodes |
| chest leads and electrodes | 6 leads from 6 electrodes |
| aVL (augmented voltage left arm) | limb lead measuring heart voltage from left arm |
| aVR (augmented voltage right arm) | limb lead measuring heart voltage from right arm |
| aVF (augmented voltage left foot/ leg) | limb lead measuring heart voltage from left leg/ foot |
| bipolar leads | Lead I, II,III |
| lead I | compares PD in left arm (positive electrode) and right arm.- horizontal line |
| lead II | compares PD in right arm and left leg (positive electrode) - diagonal line down |
| lead III | compares PD |
| standard ECG speed (x axis) | 25mm/sec |
| 1 small square on ECG | 1mm |
| 1 big square on ECG | 5mm |
| 1 sec on ECG is equivalent to how many squares | 5 large squares |
| each large square is how many ms | 200ms |
| each small square is how many ms | 40ms |
| 10mm per how many volts | 10mv |
| how do ECGs use a galvanometer | measure direction of small electric currents |
| how do leads measure current | through dipole (different in positive and negative electrode pole) |
| when is there no dipole (isoelectric) | when muscle is polarised (at rest)/ all muscle is depolarised (contracting) |
| when is there dipole (positive deflection) | when part of the muscle is depolarised |
| depolarisation moving towards unipolar electrode/ + electrode of bipolar lead leads to | positive deflection upwards (de) |
| depolarisation moving away from unipolar electrode/ + electrode of bipolar lead leads to | negative deflection downwards (de) |
| repolarisation moving towards unipolar electrode/ + pole of bipolar lead | negative deflection downwards (re) |
| repolarisation moving away unipolar electrode/ + pole of bipolar lead | positive deflection upwards (re) |
| P wave | atrial depolarisation |
| QRS complex | ventricular depolarisation |
| Q wave | septal depolarisation |
| T wave | ventricular repolarisation |
| why is T wave (ventricular repolarisation) positive in normal patients | layers in myocardium (sub endocardial + sub epicardium) depolarise + repolarise differently (sub endo depolarises first then sub epi but sub endo takes longer to repolarise) meaning ventricular repolarisation is opposite to ventricular depolarisation |
| where is SAN | RA |
| describe P wave (atrial depolarisation) in normal patients | begins in SAN in RA, there is little muscle so P wave has a small amplitude with a slight notch (bifid) |
| bifid | slight notch in P wave in normal patients |
| PR interval | measures time from onset of atrial depolarisation to ventricular depolarisation (120-200ms) |
| how long is PR interval usually | 120-200ms |
| describe QRS complex | ventricular depolarisation where large muscle mass of LV mean QRS represents LV signal |
| Q wave shape | any initial negative deflection |
| R wave shape and height | variable height and any positive deflection |
| S wave shape | any negative deflection after R |
| normal QRS duration | <120ms |
| S wave depth | <30mm |
| large R wavs indicate | left ventricular hypertrophy |
| Q waves size | <2mm in depth (2 small squares) |
| normal Q waves found where | leads facing LV (I,II, aVL, V5, V6) |
| Q wave duration | <40ms (1 small square) |
| ST segment | end of ventricular depolarisation to start of repolarisation |
| J point | where QRS meets isoelectric line |
| size of ST segment | +/- 1mm from isoelectric baseline |
| QT interval | total time for depolarisation and repolarisation to occur in ventricles |
| describe T wave | ventricular repolarisation where shape is asymmetrical and rarely exceed 10mm (10 small squares) |
| describe U wave (dont need)** | small deflection after T wave (not common) |
| how many classes of anti-arrhythmic drugs | 4 |
| AF ECG pattern | no identifiable P waves and irregular pattern across ECG |
| tachycardia | above 100bpm |
| bradycardia | below 60bpm |
| ventricular fibrillation on ECG pattern | amplitude signal and deflections are big (no identifiable P, QRS or T waves) with wavy baseline |
| ventricular ectopic on ECG pattern | no P wave before it with early/wide (prolonged) QRS followed by a pause |
| heart block on ECG pattern | no P wave before it with early/wide QRS followed by a pause |
| class I anti-arrhythmic drugs | blocks sodium channels to reduce maximum rate of depolarisation to treat ventricular dysrhythmias eg lidocaine |
| class II anti-arrhythmic drugs | B-adrenoceptor antagonists used to treat tachyarrhythmias and decrease mortality post MI eg atenolol |
| class III anti-arrhythmic drugs | block potassium channels to slow repolarisation and prolong cardiac AP thus increase refractory period eg amiodarone |
| class IV anti-arrhythmic drugs | calcium channel antagonists which block L type calcium channels to slow conduction in SAN and AVN to treat supraventricular tachycardias (SVT- atria contract too fast) eg verapamil |
| other types of anti-arrhythmic drugs (2) | adenosine (slow AV conduction in SVTs) and digoxin (increase vagal tone via CNS to slow AV conduction in SVTs) |
Created by:
kablooey
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