The next step in the orderly approach for 12-lead ECG interpretation is determining if the ECG standardization is normal.
Standardization
Before further interpretation, you must determine proper ECG standardization of voltage, as ECG voltages may vary with pathologic states such as hypertrophy. Standardization of paper speed is also important in order assess the correct heart rate and intervals; normal paper speed is 25mm/second. A calibration standard square wave is usually either at the beginning or the end of the ECG recording. The normal standardization deflection of 1 mV will be 10 mm in amplitude, and 0.2 seconds (one large box) in duration. Occasionally one will change the standardization or paper speed; for example halving the standardization (1 mV = 5 mm deflection) in a patient with increased voltage, or doubling the paper speed to 50 mm/second in a patient with a rapid tachycardia.