The output of an ECG recorder is a graph.
A dedicated ECG machine would usually print onto graph paper which has a background pattern of 1mm squares (often in red or green), with bold divisions every 5 mm in both vertical and horizontal directions.
Each ECG is divided by large boxes and small boxes to help measure times and distances.
Time is represented on x-scale in seconds and current in millivolts(mV) on y-axis.
Each large box is 0.20 seconds. There are five small boxes in each large box, thus each small box is 0.04 seconds.
If the entire ECG is 10 seconds, then there must be 50 large boxes (10/0.20=50 boxes). Each small box is also exactly 1 mm in length and so one large box is 5 mm. In general, when measuring amplitudes of waves/complexes, the units are expressed in millimeters (mm) and when measuring lengths for intervals, the units are expressed in time (seconds or milliseconds).
If each small box is equal to 0.04 seconds or 1 mm, then the standard ECG speed is 1 mm per 0.04 seconds or 25 mm per second.
The standard approach to reading an ECG is in this order:
Examining the rate
2) Examining the rhythm
3) Examining the axis, intervals and segments
4) Examining everything else.
There are two different rates that can be determined on ECGs. The atrial rate is indicated by the frequency of the P waves and the ventricular rate is indicated by the frequency of the QRS complexes. Normally, the artial rate should be the same as the ventricular rate in the absence of disease, however certain conditions, such as third degree AV nodal block, can alter this normal relationship.
- One quick and easy way to determine the ventricular rate is to examine the R-R interval and use a standard scale to find the rate. If two consecutive R waves are seperated by only one large box, then the rate is 300 beats per minute. If the R waves are separated by two large blocks, then the ventricular rate is 150 beats per minute. The scale continues down to show that if two consecutive R waves are separated by 8 large boxes, then the rate is 37 beats per minute. The pictoral explaination of this method is below:
- Another quick way to calculate the rate is based on the fact that the entire ECG is 10 seconds. So by counting the number of QRS complexes and multiplying by 6, the number per minute can be calculated. This is a better method when the QRS complexes are irregular making the first method less accurate, since the R to R intervals may vary from beat to beat.
- Another way can be used. If the interval between two beats is 1 second, the heart rate is 60 beats per minute. The normal interval between two successive QRS complexes in an adult person is 0.83 seconds. This is a heart rate of 60/0.83 times per minute, or 72 beats per minute.
The rhythm is either sinus or not sinus. If there is a P wave before every QRS complex, the P wave is upright in lead II, and the P wave has a normal morphology, then normal sinus rhythm is said to be present. If there is sinus rhythm and the heart rate is greater than 100, then sinus tachycardia is present. If the there is sinus rhythm and the heart rate is less than 60, then sinus bradycardia is present.
If there are no P waves present or the P wave morphology is not normal, then the exact rhythm must be determined.
The axis of the ECG is the major direction of the overall electrical activity of the heart. It can be normal, leftward, rightward, or indeterminate. Usually, the QRS axis is determined; however the P wave or T wave axis can also be calculated.
Note that lead I is at zero degrees, lead II is at +60 degrees, and lead III is at +120 degrees. Lead aVL (L for left arm) is at -30 degrees, lead aVF (F for “floor”) is at +90 degrees, and the negative of lead aVR (R for right arm) is at +30 degrees. The positive of lead aVR is actually at -150 degrees. Memorizing the above picture is crucial to accurately determining axis.
LAD = Left Axis Deviation
RAD = Right Axis Deviation
NW = northwest axis or indeterminate axis
- One quick, non-specific method can be used to determine the QRS axis. If the QRS complex is upright (positive) in both lead I and lead aVF, then the axis is normal (see the above picture). If the QRS is upright in lead I and downward (negative) in lead aVF, then the axis is leftward. If the QRS is predominately downward in both leads I and aVF, then the axis is rightward. If the QRS is downward (negative) in lead I and upward (positive) in lead aVF, then the axis is northwest or indeterminate.
- Another way to determine QRS axis, again using the first picture above, is to find which lead is the “isoelectric lead”, that is the lead in which the upward deflection of the QRS is about the same as the downward deflection. The axis will then be exactly perpendicular to that isoelectric lead.
It is the best way to measure and diagnose abnormal rhythms of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances.