Lecture 6.1 - Basics of ECG PDF

Impulse conducting system of the heart: ◦Cardiac cycle relies on wave of excitation spreading across the myocardium in a sequential PETE process areas ◦The depolarising (+ve) current is generat...

Impulse conducting system of the heart: ◦Cardiac cycle relies on wave of excitation spreading across the myocardium in a sequential PETE process areas ◦The depolarising (+ve) current is generated in SA node ◦Spreads to atrium and then to the AV node ◦From the AV node, the +ve current moves to bundle branches depolarising the interventricular septum ◦The +ve current then moves to the purkinje fibres depolarising the ventricles ◦The depolarising wave is immediately followed by repolarising (+ve) current ◦Atria begin to repolarise when ventricles are depolarising ◦Ventricles repolarise before the next wave of depolarisation begins ECG: ◦Captures the electrical activity of the heart (movement of currents) ◦Graphical representation of electrical activity of the heart ◦The electrodes places in the body captures movement of ions and records it ◦+ve current moving towards +ve electrode will give a +ve deflection ◦+ve current moving away from +ve electrode will give a -ve deflection ◦+ve current moving perpendicular to the electrodes will give isoelectric line ◦-ve current moving towards -ve electrode will give a +ve deflection ◦-ve current moving away from -ve electrode will give a -ve deflection ◦-ve current moving perpendicular to the electrodes will give isoelectric line Normal ECG: Lead II: ◦Most commonly used to produce the sinus rhythm tracing - longer strip of 10s (rhythm strip) ◦-ve electrode placed in right arm and +ve electrode in left leg ‣ Picture (ECG) is formed by the positive electrode  P wave: ◦Formed by atrial depolarisation seedder P-R segment: ◦0.1s delay in conduction in AV node leading to P-R segment ◦+ve charge is not moving Q wave - septal depolarisation: ◦Left bundle depolarises the interventricular septum and right bundle branch R wave - ventricular (apex) depolarisation: ◦Depolarisation signal spread to apex ◦Atria begin to repolarise but this repolarising wave is 'hidden' within the R wave  sorceressressared S wave - ventricular wall depolarisation: ◦Purkinje fibres depolarise the lateral walls of the ventricles ◦QRS complex - ventricular depolarisation see threeervesced S-T segment: ◦Entire ventricular myocardium is depolarised and is not repolarised yet ◦No net movement of ions ◦Base of ventricles repolarise fast as they have a short duration of action potential. T wave - ventricular repolarisation: ◦Action potential durations are shorter in the anterobasal region ◦Negative charge moving towards the negative electrode giving a positive deflection after a short period of time ◦U wave may be present due to delayed repolarisation of papillary muscles and chordinae tendinae - not usually seen ECG waves:  ◦A change in height or amplitude means a pathological issue has arisen ◦PR interval - conduction of AV node ◦Conduction delay/block elongated - links to PR interval ◦Widening of QRS interval means ventricles are delaying depolarisation ◦ST segment - should always be isoelectric (conditions like MI could stop the straight line showing) ‣ J point - used to determine if the ST segment is isoelectric or elevated ◦QT interval - indicates the entire depolarisation and repolarisation of ventricles ECG and cardiac cycle: Electrodes: ◦ECG can be recorded by limb electrodes or including chest electrodes ‣ 12 lead ECH means 12 views of the heart ◦Chest leads - 6 electrodes giving us 6 tracings/views (horizontal plane) ‣ Unipolar leads - only have positive electrodes ◦Limb leads - 4 electrodes giving us. tracings/views (frontal plane) ‣ Among the 4 electrodes only 3 participate in reading ‣ The 4th one (placed in RL) acts as ground electrode The three bipolar limb leads: ◦Limb leads connected to voltmeter ◦Called bipolar leads or standard limb leads ◦Different combinations: ‣ Left arm (+) to right arm (-) = Lead I ‣ Left leg (+) to right arm (-) = Lead II  ‣ Left leg (+) to left arm (-) = Lead III ◦Leads I,II and III can be represented schematically in terms of a triangle, called Einthoven's triangle ‣ They will show similar ECGs, but the amplitude changes ◦For lead I - RA electrode acts as negative electrode and LA acts as positive electrode ◦For lead II - RA electrode acts as negative electrode and LL acts as positive electrode ◦For Lead III - LA (the same LA which acted as +ve electrode for Lead 1) now acts as the negative electrode and LL acts as the positive electrode seederosed Shee tracether ◦The bipolar leads are related by the equation -> Lead I + Lead III = Lead II ◦In other words, add the voltage in lead I to that in lead III and you will get the voltage in lead II ◦The area of heart covered by the leads: sea ‣ Lead I - high lateral wall of left ventricle ‣ Lead II and III - inferior portion of right and left ventricle and  The three unipolar limb leads: ◦The abbreviation 'a' refers to augmented; V to voltage/vector; and R, L and F to right arm, left arm and left foot (leg) respectively ◦The area of heart covered by the aVR: ‣ Right ventricle and basal septum ◦ECG tracing opposite of lead I, II and III ◦The area of heart covered by the aVL: ‣ High lateral wall of left ventricle ◦ECH tracing similar to lead I, II and III ◦The area of heart covered by the aVF: ‣ Inferior wall of the heart ◦ECH tracing similar to lead I, II and III The six precordial chest leads: ◦In horizontal/transverse plane ◦Placement: ‣ V1 - right 4th intercostal space, sternal border ‣ V2 - left 4th intercostal space, sternal border ‣ V4 - left 5th intercostal space, midclavicular line ‣ V5 - left 5th intercostal space, anterior axillary line ‣ V6 - left 5th intercostal space, mid axillary line ‣ V3 - between V2 and V4 ◦The area of heart covered by: ‣ V1-V2 - right ventricle ‣ V2-V3 - basal septum ‣ V2-V4 - anterior wall of the heart ‣ V5-V6 - lateral wall of the heart ◦The ECG waves: ‣ R wave increase in size from V1-V6 ‣ S wave decrease in size form from V1-V6  These happen due to location Summary: