Q waves that are smaller than these measurements are generally clinically insignificant. The ST segment represents the time from completion of ventricular depolarization to repolarization.
ST segments are measured in relation to the baseline or isoelectric line, which is between the end of the P wave to the beginning of the QRS complex. ST segment elevation may indicate acute myocardial injury, and ST segment depression may be an indication of myocardial ischemia. The ST segment and T wave changes are found in leads associated with ischemia and injury in a specific area of the heart. J points should be at least 1 to 2 mm above the baseline in at least two contiguous leads to be considered significant.
The T wave corresponds to ventricular repolarization and normally follows the direction positive or negative of the QRS complex in most leads. A T wave with high amplitude may be seen in hyperkalemia or very early MI.
However, variations in the ST segment and T-wave morphology can be seen in a variety of conditions, including ventricular hypertrophy, bundle branch block BBB , myocardial ischemia, or resolving MI. It is inversely related to heart rate, and is influenced by the patient's age and sex.
The "QTc" implies a QT interval that has been corrected for heart rate. There are tables available to determine appropriate QT intervals for given heart rates, but an easy rule may be applied: the QT interval is probably prolonged if it exceeds one-half of the R-R interval in heart rates from 60 to beats per minute.
A shortened QT interval may be related to the presence of hypercalcemia, digoxin toxicity, or thyrotoxicosis. Axis refers to the direction of the main vector in which depolarization occurs. A wave that travels toward a positive lead will result in an upward or positive deflection tracing on the ECG, and a wave traveling away from a positive lead will result in a downward or negative deflection.
Waves that travel at 90 degrees or at a right angle to a particular lead will be biphasic, appearing above and below the baseline or isoele ctric line. The mean axis represents the sum of the vectors, or direction of depolarization to produce as single vector. In a heart with normal axis, the summed vector should be point ing in a downward and leftward direction, as the conduction starts in the SA node near the right shoulder and travels downward and leftward to the ventricles.
Axis is actually expressed in degrees, but it is usually adequate in the clinical setting to know in general if an axis deviation is present. Only the first six leads are used to determine axis; the precordial leads are not utilized. An axis deviation suggests that the electrical forces do not follow the normal direction of depolarization from right shoulder to left foot. There are multiple ways to determine an accurate measurement of axis in degrees, but a simple method to assess the axis in general terms has been determined see Summary of Axis Determination.
Right axis deviation may include right bundle branch block RBBB ; left posterior hemiblock; right ventricular hypertrophy; and pulmonary disease such as chronic obstructive pulmonary disease, pulmonary hypertension, or pulmonary embolus.
Axis deviation as an isolated finding is not necessarily important. The ECG reader should note that it is present, and then correlate its significance in the clinical setting.
Remember that the first six leads should all be upright except aVR , and in the precordial leads the R wave progresses from small to tall across the precordium. The ECG should always be first evaluated for rate and rhythm and then examined for individual waves, intervals, and patterns. When a patient experiences chest pain, recognizing ECG changes consistent with myocardial ischemia, injury, and infarction are critical in facilitating rapid treatment to reestablish blood supply and salvage vulnerable heart muscle.
Ischemia and injury are often reversible with treatment. However, when the blood supply is interrupted for a prolonged period of time, infarction may occur, resulting in scarred or necrotic tissue. STEMI is usually caused by a thrombus, though a small percentage may be caused by vasospasm or embolus.
The left ventricle is referred as the site of infarction unless otherwise specified. The right ventricular infarction may occur in conjunction with a left ventricular inferior wall infarct because both areas are supplied by the right coronary artery. These groups have major relevance in ischemic heart disease.
In acute ST elevation infarction , electrocardiogram allows us to locate the occluded artery , and in patients with previous infarction, Q wave locates areas of myocardial infarction. We can distinguish three groups of leads, which are anatomically correlated with anterior, inferior and lateral walls of the left ventricle. There is also another group that provides information on the right ventricle. Alterations in the posterior wall are seen in V1-V3 as reciprocal image.
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The J point is elevated and, along with the T wave, and it looks like a tombstone. Do not confuse the ST segment elevation with the T wave. Look specifically where the ST segment is — waaaaay up from the baseline. Recall that the J point is where we need to measure the elevation from baseline, and the baseline is always the TP segment between the T wave and the P wave.
Below is another example of tombstoning with a slightly different shape. There is septal involvement lead V2 and a bit laterally, as well lead V5 and V6. The more examples you see, the better. There is no lateral involvement here. Although not quite a tombstone, there is still significant ST segment elevation here. The next example below is trying to tombstone — and maybe did in lead V4.
There is definite elevation of the J point in V2 to V6, at least, and minimal elevation in V1 and V6. This is a good example to quickly point out something else. If that were the case, a non-STEMI or unstable angina may be present, as the changes are indeed from myocardial ischemia, but not officially a STEMI — meaning a big time difference in regards to treatment.
However, as you can see, sometimes it is quite obvious that an anterior STEMI is present, and sometimes it is not. Below is an example where there is J point elevation, but it does not quite tombstone and does not really have eye-catching ST segment elevation.
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