OSCEs ECG Flashcards
On an ECG, describe the P-R interval. Where does it start and end? How long is it meant to be? What does it show?
The P-R interval starts at the beginning of the P wave and ends just before the Q. It does not continue to the R becasue R is difficult to determine on abnormal ECG readouts. It is usually 120 to 200 ms long. On the usual 25 mm/s ECG tracing, this corresponds to 3 to 5 small boxes. The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node where it enters the ventricles. The PR interval is therefore a good estimate of AV node function.
On an ECG, describe the P-R segment. Where does it start and end? What does it show?
The P-R segment is the flat area between the end of the P wave to just before the Q. It does not continue to the R becasue R is difficult to determine on abnormal ECG readouts. The P-R segment represents the delay of the electrical impulse at the atrioventricular node.
On an ECG, describe the QRS interval. Why is it a complex signal? What does it show? How big and wide should it be?
The QRS complex appears as the ventricles depolarise. This is a stronger electrical signal than the P wave becasue the ventricles are larger. It is also a complex signal, largely because of the complex pathway that the spread of depolarisation takes through the ventricles. The limits for the sizes of R and S waves in normal subjects are usually said to be: 25mm for the R wave in lead V5 or V6 25mm for the S wave in V1 or V2 Sum of R wave in V5 or V6 plus S wave in lead V1 or V2 should be less than 35mm. The QRS complex should be less than 120msec in duration (less than 3 small squares) in all leads.
On an ECG, describe the ST segment, where does it start and end, what is it’s major clinical significance?
The ST segment is the part of the ECG between the end of the QRS and the beginning of the T wave. It should be horizontal and ‘isoelectric’ that is, at the same level as that between the end of the T wave and the start of the next P. However, in the chest leads the ST segment can slope upwards, and is not easy to define. Elevated ST segment is the hallmark of acute MI (not in leads V2-V5, this can be normal high take-off ST segment) and depression of more than 2mm of the ST segment can indicate ischemia.
On an ECG, describe the ST interval - where does it start, where does it end?
The ST interval is between the end of the QRS, and the end of the T wave.
