ECG Simplified – Part 9

ECG Simplified – Part 9

Now we will have a look at ECG in coronary artery disease. Coronary artery disease is disease of blood vessels of the heart. Sudden blockage of a blood vessel of the heart usually produces a heart attack due to damage to a region of the heart muscle. It has certain important ECG changes which makes diagnosis possible in the emergency room or at patient side using a portable ECG machine.

Technical term for a heart attack is myocardial infarction. There are basically two types of myocardial infarction on ECG. One shown here is ST segment elevation myocardial infarction, popularly known as STEMI. Here the ST segment is seen elevated in leads II, III and aVF, marked by light blue arrows. In addition, there is second degree AV block with a non-conducted P wave marked by a pink arrow in lead II rhythm strip at the bottom of the tracing. Conducted P wave is marked by blue arrow.

This particular variety is called inferior wall STEMI because the ST elevation is seen in leads oriented to the lower part of the body, II, III, aVF. Inferior wall means the lower wall of the left ventricle. Atrioventricular conduction disturbances are common with inferior wall myocardial infarction. If there is a complete heart block, meaning that none of the electrical signals from the sinus node are conducted to the ventricles, the heart rate will be low. Some of them will require insertion of a temporary pacemaker to support the heart rate.

Another ECG showing gross ST segment elevation in leads V1 to V6, known as anterior wall myocardial infarction. Anterior wall is the front wall of the left ventricle. Maximum ST segment elevation of 7 mm is seen in V4. Extent of ST segment elevation and the number of leads showing ST segment elevation give a suggestion on the region of heart muscle affected as well as the severity. This one is more dangerous than the previous ECG showing inferior wall myocardial infarction.

ST segment elevation is seen soon after the blood vessel becomes blocked in a heart attack. When the person recovers from the heart attack, ST segment elevation subsides and becomes isoelectric as seen in several leads in this ECG. The T waves become inverted as seen in leads V2-V4. Abnormal Q waves appear, indicating loss of electrical activity of heart muscle in that region, often called an electrical window. Q waves are the reflection of unopposed electrical activity from the opposite wall of the ventricle. Q wave is an initial negative deflection of the QRS complex.

This a magnified X-ray image of a blocked blood vessel marked by yellow arrow. The test is known as coronary angiogram and is obtained by injecting radiocontrast medication directly into the blood vessel under continuous X-ray imaging. Small tubes known as catheters are introduced through the blood vessel of the wrist or groin and guided to the blood vessel of the heart under X-ray imaging in  a special procedure room known as cardiac catheterization laboratory. This test is usually done prior to mechanical removal of the block by angioplasty.

Another ECG showing old inferior wall myocardial infarction with pathological (abnormal) Q waves with T inversion in leads III and aVF. Additional findings marked are minimal ST segment depression in V6 and tall T waves in V4. These changes can occur when the blood supply to the left side of left ventricle is compromised. ST segment is isoelectric in the region of old myocardial infarction.

After a discussion on ECG changes in blocks of blood vessels of the heart, we will see what happens when there is a block in the blood vessels of the lungs. When a blood clot is carried in blood circulation to the lungs, it blocks blood vessels there. This is another life threatening condition called pulmonary embolism. Typical ECG pattern in pulmonary embolism is called S1,Q3,T3 pattern. This means that there is an S wave in lead I, Q wave in lead III and T inversion in lead III. Though this is the classical description, several other patterns are also possible in pulmonary embolism.

ECG can be abnormal when electrolyte levels in blood are abnormal. Important electrolytes in blood which can affect the ECG are potassium, magnesium, and calcium. Potassium and calcium are involved in the ionic currents which generate the electrical signals of the heart. Magnesium is important in regulating the levels of calcium and potassium by enzymes involved in transfer of these ions across the kidney. The ECG illustrated shows prolongation of ST segment with low calcium levels as calcium ions are involved in the ion currents during the inscription of the ST segment.