The ECG What and why At its essence the ECG is a needle dipped in ink stood on paper which travels at a constant rate. The needle is deflected upwards (+ve) when the sum of electrical activity is toward the measuring electrode and down (-ve) when it moves away. This point is critical to the understanding of the cardiac axis. Because the heart must beat in a concerted and co-ordinated fashion in order to pump effectively, the typical sequence of deflections form a pattern, sinus rhythm. Any deviation from this pattern is pathological, more importantly since anatomy and physiology are so well entwined the site and type of pathology can be deduced without the need to resort to slower, more expensive imaging options. Time is muscle. Electrode placement Limbs Ride Your Green Bike (earth) RA, LA, LL, RL Chest leads Put on 1,2,4,6 then fill in the rest. L1 4th Intercostal space R sternal edge L2 As above, L sternal edge L4 Apex beat (5th Mid Clav) L6 Roughly in line with 4 Mid Axillary line Although this adds up to 9 electrodes, the classic method of diagnosis is the 12 lead ECG. The discrepancy is resolved when you consider that the ‘leads’ represented on the ECG are virtual, that is to say that they are derived from information received by one or more physical electrodes. One way to think about this is to consider each virtual lead as having a ‘view’ of the heart in a particular plane. This is why the classic PQRST pattern is only seen in some of the leads (e.g II, V4), it represents an easy way to marry the different parts of the contraction sequence to electrical activity and therefore is shown for the purposes of teaching, but is hardly a complete representation of the hearts functioning. ‘Funny’ current and (some) Anatomy Sinus rhythm is defined as the pattern of deflections which result from an impulse which begins in the SAN. Cardiac activity which starts at any other site affects both the rate (slower, by definition) but also the pattern seen in the ECG in ways that can be predicted by basic knowledge of the relevant anatomy Electrical activity spreads globally throughout the atria but is funnelled through the AVN and bundle of His by a fibrous disconnection between the atria and ventricles. The bundle of His can conduct in both directions, but exists to provide synchronous contraction of ventricles from the base. Only impulses which pass along the bundle of His provide narrow complex QRS, broad complex QRS patterns represent passage outside this pathway. The heart muscle acts as electrical touch paper. The cells are continuous with each other via gap junctions so depolarisation at any point leads to an explosive spread of synchronous firing form that point outward. The location is of that spark is determined by cells which depolarise spontaneously. These cells are able to do this because of the special ion channels within their cell membrane which mediate If , the ‘funny’ current. The funny current is important because it sets the pace of cardiac contraction, yet has a different gradient in the different cell groups which express it. These include SAN, AVN, Purkinje fibres and the ventricles. The gradient is steepest in the SAN and these cells depolarise most frequently (60-100bpm), setting the heart rate. When the SAN is compromised, perhaps due to an ischaemic insult, the cell group with the next steepest funny current gradient steps in and paces the heart. Since the anatomical location and subsequent spread of depolarisation is different, these changes are readily diagnosed on the ECG. Presenting ECGs – A Crib Sheet Patient Information o Name, age, do they have chest pain? Technical Information o Is the paper speed 25mm/sec? o Does 10mm correspond to 1mV? Rate o 300/number of large boxes between R waves or number of QRS complexes on rhythm strip x6 Rhythm o Is it regular with a p-wave before every QRS complex? Sinus Rhythm o If irregular, is it irregularly irregular or regularly irregular? Axis o o o +ve QRS in I and II - Normal –ve QRS in I and +ve in II – Reaching=Right Axis Deviation +ve QRS in I and –ve in II – Leaving=Left Axis Deviation Describe the waveform o P waves – Is there one before every QRS? Are they 80-100ms in length? o PR interval – It is longer than 200ms? o QRS complexes - <120ms o ST segment  If elevated, what is the distribution in the chest leads?  If depressed, is it flat or downward sloping? o T waves  Are they inverted? – Normal in aVR and III o QT Interval  Is it prolonged? What might be the cause? What is your diagnosis?