Whenever a patient presents with chest pain, shortness of breath, dizziness, fainting, or irregular heartbeats, an electrocardiogram (ECG) will be ordered to determine any heart abnormalities. Many of these abnormalities may include coronary artery disease, unstable angina, acute myocardial infarction, arrhythmias, and cardiomyopathies, to name a few. Coronary artery disease is the buildup of plaque in the arteries surrounding the heart, restricting blood flow and resulting in chest pain, which is also known as unstable angina. A myocardial infarction (heart attack) is whenever this plaque builds up so much that it completely blocks the vessels, causing the heart to lose blood supply and killing heart tissue. Some symptoms can include: chest pain, shortness of breath, dizziness, nausea, vomiting, and heartburn. Arrhythmias are abnormally fast, slow or irregular heart beats. Cardiomyopathies are conditions affecting heart tissue, causing the tissue to be dilated, hypertrophic, or restrictive. Symptoms include shortness of breath, fatigue, swelling of lower extremities, coughing when lying down, dizziness, chest pain, and arrhythmias.
An ECG is performed by placing 10 electrodes on a patient in a supine and resting position. The electrodes are named as following: V1, V2, V3, V4, V5, V6, RL, RA, LL, and LA. These electrodes make up 12 leads consisting of: lead I, lead II, lead II, augmented vector right (aVR), augmented vector left (aVL), augmented vector foot (aVF), and V1 to V6. V1 is placed on the fourth intercostal space to the right of the sternum. V2 is adjacent, in the fourth intercostal space to the left of the sternum. V4 is placed in the fifth intercostal space on the left-hand side at the midclavicular line. V3 is placed between V2 and V4. V5 is placed at the anterior axillary line. V6 is placed at the midaxillary line. The limb leads RL and LL are placed between the ankle and hip (right and left), and RA and LA are placed between the shoulder and elbow (right and left). Incorrect placement of electrodes can result in incorrect tracings appearing in the ECG. Electrodes must also be in direct contact with skin, so any oils or hair on the skin must be removed. Electrodes should not be placed over bones or where a lot of muscle movement occurs.
How is an ECG used in the detection of cardiac abnormalities? To begin with, a heart rate is determined by counting the number of large squares between the highest peaks of the ECG (also known as R-R intervals). Then, 300 is divided by the number of squares to calculate the heart rate. However, this calculation only works for regular heart rates.
If a heartbeat is irregular, the number of complexes per rhythm strip (rhythm strip=10 secs) is counted and multiplied by six, averaging the number of complexes in one minute. A normal heart rate falls between 60-100 bpm. A patient is considered tachycardic if their heart rate is greater than 100 bpm, or bradycardic if their heart rate is below 60 bpm. Heartbeat regularity and irregularity is determined based on the cardiac axis, by looking at leads I, II and II of the ECG. In a normal cardiac axis, lead II should be the highest peak (most positive deflection) than the other leads.
In certain conditions, such as right ventricular hypertrophy, right axis deviation occurs in which lead II has the highest positive deflection and lead I is negative (when normally positive).
In heart conduction defects, left axis deviation occurs, and lead I has the most positive deflection, while the others are negative.
After a normal or abnormal heart rate is determined, the morphology of the ECG is reviewed to determine if any abnormalities are present. The P wave is first seen in an ECG and indicates the contraction of blood from the atria to the ventricles. If the P-waves are absent and an irregular rhythm is present, this may suggest atrial fibrillation.
After the P wave, the P-R interval is reviewed. A normal P-R interval should be 120-200 ms long (3-5 squares). A prolonged PR interval suggests atrioventricular delay, such as in a heart block. A first degree heart block (between the sinoatrial node and atrioventricular node) consists of a PR interval greater than 200 ms.
A second degree heart block, also known as Mobitz type 1, occurs when the P-R interval is increased along with a dropped QRS complex. This type of heart block occurs in the atrioventricular node.
Likewise, a Mobitz type 2 heart block occurs when the PR interval remains the same with dropped beats. This type of heart block occurs after the atrioventricular node in the bundle of His or Purkinje fibres.
A third degree heart block, also known as a complete heart block, occurs whenever the P waves are not associated with the QRS complexes. This block occurs anywhere in the atrioventricular node.
The QRS complex occurs after the P wave and consists of a downward deflection (Q), a high peak (R) and a final drop (S). This complex represents the depolarization and contraction of the ventricles.
The QRS complex can either be narrow (less than 0.12 seconds), indicating normal ventricular depolarisation, or broad (greater than 0.12 seconds), indicating abnormal depolarisation. A tall QRS complex also indicates ventricular hypertrophy. A delta wave (slurred upstroke of QRS complex) can also be present if the ventricles were activated earlier than normal, and is associated with Wolff Parkinson White Syndrome.
Q waves are seen with the necrosis of heart muscle, and are the result of an absence of electrical activity. They are pathological when they are 25% the depth of the QRS complex, more than 2 mm in height, or greater than 40 ms in width. An isolated Q wave is normal, however, several Q waves are a sign of a previous myocardial infarction.
The ST segment is then examined, which is the space between the end of the S wave and the beginning of the T wave. In healthy patients, it should be neither elevated nor depressed. An ST elevation occurs whenever the segment is greater than 1 mm in 2 or more limb leads or greater than 2 mm in 2 or more chest leads and is seen in myocardial infarction. ST depression occurs in myocardial ischemia, with a depression of 0.5 mm or greater in more than 2 contiguous leads.
The T wave represents the repolarisation of ventricles. T waves greater than 5 mm in limb leads and greater than 10 mm in the chest leads indicate hyperkalemia. If inverted T-waves are present, they can indicate ischemia, bundle branch blocks, pulmonary embolism, left ventricular hypertrophy, hypertrophic cardiomyopathy, and general illness.
Biphasic T waves have two peaks which indicates either ischemia or hypokalemia. Flattened T waves indicate ischemia or electrolyte imbalance.
Finally, U waves are not a common finding, but can be seen after the T wave in V2 or V3. These waves indicate electrolyte imbalances, hypothermia, or antiarrhythmic therapy.
There is a lot of valuable information that can be acquired in an ECG for the diagnosis of many heart conditions and diseases. However valuable this information may be, all of these findings must be interpreted by a physician aware of the patient’s condition and that has all clinical information available to make a diagnosis.