When looking at an ECG, think of the leads as views. Leads I, II and III are three different ‘views’ of the heart. When electrical activity is travelling towards a positive electrode, there is a positive deflection on ECG (a deflection above the baseline). If electrical activity is therefore travelling away; there is a negative deflection (a deflection below the baseline).
Lead I is looking from the Right Arm (RA) to the Left Arm (LA) electrode. This draws an almost horizontal line across the heart from (RA) to (LA). Left Arm (LA) is therefore the positive electrode. Consider the normal expected movement of electrical activity in the heart – is the Left Arm (LA) electrode therefore likely to see electrical activity coming towards it, or away from it?
Lead II is looking from the Right Arm (RA) to the Left Leg (LL) electrode. Left Leg (LL) is therefore the positive electrode. This draws a line that follows the expected cardiac axis (meaning the average movement of electrical activity) from the sino-atrial (SA) node, to the atrioventricular (AV) junction, then toward the comparatively more muscular mass of the Left Ventricle (LV). The normal cardiac axis is therefore from the ‘top right’ to the ‘bottom left’ of the heart (SA —> LV). This is often why Lead II has the greatest positive deflections on ECG, as it most closely follows the normal expected cardiac axis; it is therefore a fantastic ‘view’ for rhythm analysis.
Lead III is looking from the Left Arm (LA) to the Left Leg (LL) electrode. Left Leg (LL) is therefore the positive electrode. This draws a line that is slightly more rightward, relatively from the left atrium toward the right ventricle.
Thinking of these three views, notice that Lead I = (RA) to (LA), Lead II = (RA) to (LL) and Lead III = (LA) to (LL)…… if you forget which leads are involved…. count the L’s !
Additional: On a standard ECG monitor there is often a fourth electrode for 3-Lead monitoring, normally black in colour. This electrode plays no part in Einthoven’s Triangle, and does not form any lead/view of the heart. It therefore can, in theory, be placed anywhere on the body. This electrode works similar in nature to noise-cancelling headphones. It listens to and distinguishes artefact, uses algorithms to turn unwanted noise ‘upside down’ and then re-enters it into the final mix. We are then left with an improved electrocardiogram from which an interpretation can be made.
Full Image and Text HERE:
By Phil Poskitt @PhilDrummer64