The ECG results provide the following:
- Determine whether the heart is performing normally or suffering from abnormalities (eg. extra or skipped heartbeats - Cardiac arrhythmia).
- May indicate coronary artery blockages (during or after a heart attack).
- Can be used for detecting calcium, magnesium and other electrolyte disturbances.
- Allows the detection of conduction abnormalities (heart blocks).
- Indicates the physical shape of a patient during stress tests.
- Can provide information on the physical condition of the heart (ie: left ventricular hypertrophy).
|The EKG complex. P=P wave, PR=PR segment, QRS=QRS complex, QT=QT interval, ST=ST segment, T=T wave.|
A typical ECG tracing of a normal heartbeat consists of a P wave, a QRS complex and a T wave. The P wave is the electrical signature of the current that causes atrial contraction. Both the left and right atria contract simultaneously. The QRS complex corresponds to the current that causes contraction of the left and right ventricles, which is much more forceful than that of the atria and involves more muscle area, thus resulting in a much greater ECG deflection. The QRS complex contains the atrial repolarization current. The T wave represents the repolarization of the ventricles. Electrically, the cardiac muscle cells are like loaded springs. A small impulse sets them off, they depolarize and contract. Setting the spring up again is repolarization (more at action potential).
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome and short QT syndrome. The QT interval varies based on the heart rate, and various correction factors have been developed to correct the QT interval for the heart rate.
The most commonly used method for correcting the QT interval for rate is the one formulated by Bazett. Bazett's formula is , where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds. However, this formula tends to not be accurate, and over-corrects at high heart rates and under-corrects at low heart rates.
A more accurate method to correct the QT interval for the rate was developed by Rautaharju et al., who developed the formula . This method is not widely used by clinicians.
EKG lead placement
A typical ECG report shows the cardiac cycle from 12 different vantage points (I, II, III, aVR, aVL, aVF, V1-V6), like viewing the event electrically from 12 different locations. Understanding the usual and abnormal directions, or vectors, of depolarization and repolarization yields important diagnostic information. The directions, or vectors, are known as leads. aVR is placed on the right arm (or shoulder), aVL is placed on the left arm (or shoulder), and aVF is placed on the left leg (or hip). Lead I represents activity that is going from the right arm to the left arm. Lead II represents activity that is going from the right arm to the left leg. Lead III represents activity that is going from the left arm to the left leg.
Electrical vectors and how they apply to the heart
The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior region (wall) of the heart. The lateral leads (I, aVL, V5 and V6) look at the electrical activity from the vantage point of the lateral wall of the heart. The anterior leads, V1 through V6, and represent the anterior wall of the heart. aVR is rarely used for diagnostic information, but indicates if the ECG leads were placed correctly on the patient.
The inferior leads record events from the apex of the left ventricle. The lateral and anterior leads record events from the left wall and front walls of the left ventricle, respectively. The right ventricle has very little muscle mass. It leaves only a small imprint on the ECG, making it more difficult to diagnose than changes in the right ventricle.
The leads measure the average electrical activity generated by the summation of the action potentials of the heart at a particular moment in time. For instance, during normal atrial systole, the summation of the electrical activity produces an electrical vector that is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium (since the SA node resides in the right atrium). This turns into the P wave on the EKG, which is upright in II, III, and aVF (since the general electrical activity is going towards those leads), and inverted in aVR (since it is going away from that lead).