ECG Basics

What is an ECG?

ECG stands for electrocardiogram. It is a measure of the electrical activity of the heart, measured using 10 electrodes/ leads placed at various points on the body. You will hear the term ’12 lead ECG’, as some of the leads are virtual leads, which are produced using information from multiple leads.

The trace produced by an ECG shows the direction of electrical activity relative to the lead. An upwards movement on the trace indicates an electrical wave moving toward the lead whilst similarly a downward trace indicates an electrical wave moving away from the lead.

In a normal healthy heart, electrical activity starts at the pacemaker node (SAN/ Sinoatrial node) and moves through the atria to the atrioventricular node (AVN), indicating atrial contraction. It then moves along the bundles of his to the purkinje fibres, causing the ventricles to contract. We can work out the net direction of this electrical activity to give us an idea of the cardiac axis, as certain pathologies will result in an abnormal net direction of electrical activity and thus an abnormal cardiac axis..

ECG paper

An ECG is printed on paper with multiple small (1mm x 1mm) and big squares (5mm x 5mm). Each of the small squares typically represents 0.04 seconds, and thus each of the big squares represents 0.20 seconds. 5 big squares therefore is 1 second, and 300 is 1 minute. This is important to remember as it will allow us to work out the heart rate.

Image result for ecg paper

Shape of the ECG

Below is an example ECG waveform. First we see the P wave, the P wave demonstrates the wave of electrical activity as it moves from the SAN to the AVN, and thus the contraction of the atria. Typically the P wave should be around 0.12 seconds (3 small squares), and between 2/3 mm high. Changes in P wave morphology could indicate an underlying pathology.

Image result

Next we have the PR Interval, this represents conduction through the AVN, and thus the brief time between atrial contraction and ventricular contraction. In an normal heart, the PR interval should be between 0.12 and 0.20 seconds (3 to 5 small squares). Shorter than 3 small squares could indicate the presence of an accessory pathway between the atria and ventricles. Longer than 5 squares could be indicative of a number of types of heart block.

Following the PR interval we have the Q wave. The Q wave is the negative deflection that is just before the R wave. Usually small (septal) Q waves are seen in leads 1, AVL and V5-V6. They are however not usually seen in leads V1-V3, and there presence may be indicative of pathology such as a current or previous MI (Myocardial Infarction).

Next we see the R wave, the R wave represents ventricular depolarization/ contraction, and is usually demonstrable as an upwards movement on the trace after the P wave. High voltage R waves (tall QRS complexes) are often indicative of ventricular hypertrophy, a larger than normal wall thickness in the ventricle. The broadness of the QRS complexes allows us to work out whether the electrical activity of the ventricle has initiated in the atria as should normally happen (narrow complex), or in the ventricle (broad complex) as is the case in ventricular ectopics and VT for example.

Following on from the R wave is the S wave. The S wave is a small deflection shown after the R wave. As has been previously mentioned the Q, R and S waves together form the QRS complex.

The final part of the ECG wave form is the T wave, and this represents ventricular re-polarization, the return of the electrical activity of the cells to their resting potential. The interval between the S and T waves, the ST segment is an highly important in the measure of cardiac ischemia and thus important for diagnosing a Myocardial Infarction.

ECG Lead Placement Video (Geeky Medics)

000-017   000-080   000-089   000-104   000-105   000-106   070-461   100-101   100-105  , 100-105  , 101   101-400   102-400   1V0-601   1Y0-201   1Z0-051   1Z0-060   1Z0-061   1Z0-144   1z0-434   1Z0-803   1Z0-804   1z0-808   200-101   200-120   200-125  , 200-125  , 200-310   200-355   210-060   210-065   210-260   220-801   220-802   220-901   220-902   2V0-620   2V0-621   2V0-621D   300-070   300-075   300-101   300-115   300-135   3002   300-206   300-208   300-209   300-320   350-001   350-018   350-029   350-030   350-050   350-060   350-080   352-001   400-051   400-101   400-201   500-260   640-692   640-911   640-916   642-732   642-999   700-501   70-177   70-178   70-243   70-246   70-270   70-346   70-347   70-410   70-411   70-412   70-413   70-417   70-461   70-462   70-463   70-480   70-483   70-486   70-487   70-488   70-532   70-533   70-534   70-980   74-678   810-403   9A0-385   9L0-012   9L0-066   ADM-201   AWS-SYSOPS   C_TFIN52_66   c2010-652   c2010-657   CAP   CAS-002   CCA-500   CISM   CISSP   CRISC   EX200   EX300   HP0-S42   ICBB   ICGB   ITILFND   JK0-022   JN0-102   JN0-360   LX0-103   LX0-104   M70-101   MB2-704   MB2-707   MB5-705   MB6-703   N10-006   NS0-157   NSE4   OG0-091   OG0-093   PEGACPBA71V1   PMP   PR000041   SSCP   SY0-401   VCP550