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Biological Sciences
Stephen Reid

Lecture 2 Study Notes Bipolar ECG limb leads record the voltage b/n electrodes placed on the wrists and the arms • This forms an equilateral triangle around the heart, referred to as Einhoven’s Triangle • There are 3 standard bipolar limb leads: slide 3 o Lead I:Right arm (always negative) to left arm o Lead II: Right arm to left leg (always positive) o Lead III: Left arm (in this case -) to left leg • Applying these leads to the axial reference system: o Lead I (RA – LA): corresponds to 0 degrees o Lead II (RA – LL): corresponds to 60 degrees o Lead III (LA – LL): corresponds to 120 degrees • So if you were to apply these bipolar limb leads to an ECG trace, it would be straightforward o Depolarisation Any depolarisation in the direction of the Left Leg (always +) will give a positive inflection on the trace Any depolarisation in direction of Right Arm (always -) will give negative inflection on the trace Apply it this way o Repolarisation Opposite of depolarisation patterns ECG waves • P wave: Atrial depolarisation (small cause of less muscle mass than ventricles) • QRS complex: Depolarisation of ventricles and repolarisation of atrium (silent atrium repolarisation). Blip sound you hear • T wave: ventricular repolarisation Important ECG Wave and Conduction co-ordination pathway: slide 8 – 15 Augmented Unipolar limb leads: use exactly the same type of leads as bipolar limb leads, with exception of all destinations being positive. This changes the reference degrees: • aVL: Left arm positive lead -30 degrees from Lead I (RA to LA) of bipolar limb lead • aVR: Right arm positive lead -150 degrees from Lead I • aVF: Left leg positive +90 degrees from Lead I • if going in opposite directions e.g. Lead II and aVR, the inflections on ECG will be opposite to each other Unipolar chest leads: • Just a series of six leads V1 – V6 placed all over upper body ECG segments • R – R segment: represents the distance/time interval b/n two heart beats • P – Q/P – R segment: time of conduction from AV node down through the purkinje fibres • T – Q segment: represents time period ventricles are relaxing (diastole period) • Q – T segment: represents ventricular contraction (systole period) Arrhythmias: Irregular heart rhythms • Sinus Arrhythmia: slide 23 o QRS complex not evenly spaced, total irregularity of beat (R-R) o P-R distance, however, remains the same o Disappears with holding breath (irregular R-R distance) • Wandering Atrial pacemaker: slide 24 o Pacemaker activity originates from differ
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