Every ECG interpretation must begin with these three basic observations: rate, rhythm, and axis. If the interpreter is not disciplined, it is easy to jump to an obvious diagnosis such as left ventricular hypertrophy and miss other important and more specific information provided by the ECG.
Measuring intervals is basic to ECG interpretation. The ECG has a background divided into one-millimeter blocks with every fifth block being a heavier line. Each one-millimeter block represents 1/25th of a second or 0.04 seconds and every large block (5mm) represents 1/5th of a second or 0.20 seconds. This information is vital when measuring conduction through nodes and chambers as well as determining the rate.
The most common interval that will be measured is the R to R interval to determine the ventricular rate. For a rough estimate of rate, the following guide can be helpful (number of 5mm boxes or every dark line on ECG with the matching rate): 1 = 300, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50, 7 = 43, 8 = 37. The same intervals can be used when measuring the atrial rate when it is different from the ventricular rate (when block or VT occurs). Another way to determine rate is to count the number of complexes in 5, 6 or 10 seconds and multiply by 12, 10 or 6 respectively. This will provide a more accurate rate when the rate is over 100 or if the rhythm is irregular as in atrial fibrillation.
After becoming familiar with interpreting ECGs, it will be easier to identify regular from irregular rhythms but calipers always provide this information accurately. Taking the time to march out the rhythm can provide subtle information and prevent obvious omissions. The irregular rhythms are commonly atrial fibrillation, Mobitz type I second degree AV block, multifocal atrial tachycardia, and atrial flutter with variable block. These are all be covered in other chapters.
Normal sinus rhythm is the most common rhythm that will be seen. The criteria include regular p waves followed by associated QRS complexes. The P waves must be upright in leads II, III and aVF. The P waves are upright because the direction of the impulse is going toward the inferior wall away from the SA node. Observing these simple points of sinus rhythm can help identify other supraventricular rhythms.
Determining the axis is generally straightforward but is probably the most commonly skipped step in ECG interpretation. Knowing the frontal plane and identifying the equiphasic lead is all that is needed in most ECGs to determine the axis of the heart (Table 1). The normal axis is between –30 degrees and +90 degrees. If the axis is more than +90 degrees then there is right axis deviation and if the axis is less than –30 degrees it is considered left axis deviation. The common causes of right axis deviation and left axis deviation are listed in the links. Sometimes, there are multiple equiphasic leads and the axis is indeterminable from the surface ECG.
Leads I and aVF can be used to quickly determine the axis. If both I and aVF are predominantly upright then the axis can be considered normal. If lead I is predominantly negative and aVF is upright, then the axis is greater than 90 degrees and rightward. If lead I is positive but lead aVF is negative, then lead II is used to determine axis. If lead II is negative, then the axis is more negative than –30 degrees and is leftward. If lead II is positive, then the axis is between 0 and –30 degrees and is normal.