Reading an ECG is like juggling fire while riding a unicycle: performing the fundamentals systematically, the same way, every time, will prevent you from getting burned. Presented here is the authors' approach to reading an ECG. It is less important to follow 1 particular approach; rather, choose 1 validated approach that works for you and apply it the same way, every time, to every tracing.
Recall that each "little box" on the time axis of a tracing is 0.04 seconds in duration, with each "big box" comprising 5 little boxes and equal to 0.2 seconds. Thus, calculate the rate as 300 divided by the number of big boxes between complexes (300/1 = rate of 300; 300/2 = rate of 150; etc). Alternatively (more accurate and more difficult math), calculate the rate as 1500 divided by the number of little boxes between complexes (1500/5 = rate of 300; 1500/17 = rate of 88; etc).
The above methods are accurate only if the rhythm is regular. A second approach to calculate rate is to recall that the rhythm strip is 10 seconds in duration. Count the number of complexes present in the rhythm strip and multiply by 6, yielding the rate. This method is accurate whether the rhythm is regular or irregular.
Using your choice of these methods, calculate the atrial rate (P waves) and the ventricular rate (QRS complexes).
First, search for atrial activity. Are there P waves? The best place to find P waves is in the inferior leads (II, III, and aVF) and V1.
Second, are the P waves sinus P waves or nonsinus P waves? Sinus P waves should be upright in the inferior leads and biphasic in lead V1. If the atrial activity is not a sinus P wave, what is it? Atrial flutter? Atrial tachycardia? Is there no organized atrial activity suggesting atrial fibrillation?
Finally, what is the relationship between the atrial activity and the ventricular activity? Does the atrial activity precede the ventricular activity with a constant interval? Does the atrial activity follow the ventricular activity, suggesting retrograde conduction? Are the atrial and ventricular depolarizations independent of each other? Is A-V block present?
Consider first if the axis is normal or not. Recall that lead I is located at 0 degrees, lead II at +60 degrees, and lead aVF at +90 degrees:
If the QRS complex is more positive than negative in leads I, II, and aVF, the axis is normal, defined as axis between +100 and -30 degrees.
If the QRS complex is positive in aVF but predominantly negative in lead I, a rightward axis is present.