...though, it is called 12 lead ECG (6 chest leads I,II,III.aVL,aVR,aVF and 6 limb leads V1-6), only 10 electrodes are used to get the ECG strip - 4 over the 4 limbs and 6 over the chest. Each lead rhythm is taken for 10 seconds, therefore, total time taken for all 12 leads is 10x12 = 120 seconds = 2 minutes strip.
...the ECG paper speed is 25 mm/second, therefore in 60 seconds, it runs 25x60=1500 mm. That is why 1500 is divided by the number of small squares between RR complexes to calculate heart rate. The large square is of 5 small squares, therefore, 300 is divided by the number of large squares between RR complexes.
...each small square is of 0.04 second duration because the paper speed is 25 mm/second. That is to say 100 mm or 100 small squares = 4 seconds, therefore, 4/100 = 0.04 second = each small square duration.
...amplitude of the P, QRS or T shows the voltage: 10 mm = 1 mV. It is said to be small QRS complex when height is <5 mm in limb and <10 mm in chest leads. Tall complexes imply left ventricular hypertrophy = S in V1 + R in V5 or V6 = >35 mm (7 large squares). There are other criteria as well.
...normal QRS axis is between minus 30 degrees and plus 90 degrees. Generally speaking, QRS amplitude in lead I and aVF if positive, it is normal. If negative in I but positive in aVF, it is right axis deviation. If positive in I but negative in aVF, it is left axis deviation.
...broadly speaking, sodium changes do not show up on ECG. Hyperkalemia shows as tall peaked T waves and hypokalemia as flat or inverted T or U wave after T. Hypocalcemia shows as prolonged and hypercalcemia as shortened QT interval (normal <0.42 seconds).
...grossly, ST depression (J point below isoelectric line - J point is junction between S and ST segment) and T flattening or inversion occur in myocardial ischaemia. ST elevation in myocardial infarction. Prominent Q waves imply previous MI.
HOW TO READ AN ECG:
https://geekymedics.com/how-to-read-an-ecg/
Left Ventricular Hypertrophy (LVH) ECG Review (ECG Reviews and Criteria):
https://www.healio.com/cardiology/learn-the-heart/ecg-review/ecg-topic-reviews-and-criteria/left-ventricular-hypertrophy-review
Left ventricular hypertrophy:
https://en.m.wikipedia.org/wiki/Left_ventricular_hypertrophy
Determining Axis:
https://www.healio.com/cardiology/learn-the-heart/ecg-review/ecg-interpretation-tutorial/determining-axis
ECG & Electrolytes:
https://cdemcurriculum.com/electrolyte-abnormalities-and-the-ecg/
ECG changes due to electrolyte imbalance (electrolyte disorder):
https://ecgwaves.com/ecg-electrolyte-imbalance-electrolyte-disorder-calcium-potassium-magnesium/
Myocardial Infarction:
https://en.ecgpedia.org/wiki/Myocardial_Infarction
...the ECG paper speed is 25 mm/second, therefore in 60 seconds, it runs 25x60=1500 mm. That is why 1500 is divided by the number of small squares between RR complexes to calculate heart rate. The large square is of 5 small squares, therefore, 300 is divided by the number of large squares between RR complexes.
...each small square is of 0.04 second duration because the paper speed is 25 mm/second. That is to say 100 mm or 100 small squares = 4 seconds, therefore, 4/100 = 0.04 second = each small square duration.
...amplitude of the P, QRS or T shows the voltage: 10 mm = 1 mV. It is said to be small QRS complex when height is <5 mm in limb and <10 mm in chest leads. Tall complexes imply left ventricular hypertrophy = S in V1 + R in V5 or V6 = >35 mm (7 large squares). There are other criteria as well.
...normal QRS axis is between minus 30 degrees and plus 90 degrees. Generally speaking, QRS amplitude in lead I and aVF if positive, it is normal. If negative in I but positive in aVF, it is right axis deviation. If positive in I but negative in aVF, it is left axis deviation.
...broadly speaking, sodium changes do not show up on ECG. Hyperkalemia shows as tall peaked T waves and hypokalemia as flat or inverted T or U wave after T. Hypocalcemia shows as prolonged and hypercalcemia as shortened QT interval (normal <0.42 seconds).
...grossly, ST depression (J point below isoelectric line - J point is junction between S and ST segment) and T flattening or inversion occur in myocardial ischaemia. ST elevation in myocardial infarction. Prominent Q waves imply previous MI.
HOW TO READ AN ECG:
https://geekymedics.com/how-to-read-an-ecg/
Left Ventricular Hypertrophy (LVH) ECG Review (ECG Reviews and Criteria):
https://www.healio.com/cardiology/learn-the-heart/ecg-review/ecg-topic-reviews-and-criteria/left-ventricular-hypertrophy-review
Left ventricular hypertrophy:
https://en.m.wikipedia.org/wiki/Left_ventricular_hypertrophy
Determining Axis:
https://www.healio.com/cardiology/learn-the-heart/ecg-review/ecg-interpretation-tutorial/determining-axis
ECG & Electrolytes:
https://cdemcurriculum.com/electrolyte-abnormalities-and-the-ecg/
ECG changes due to electrolyte imbalance (electrolyte disorder):
https://ecgwaves.com/ecg-electrolyte-imbalance-electrolyte-disorder-calcium-potassium-magnesium/
Myocardial Infarction:
https://en.ecgpedia.org/wiki/Myocardial_Infarction
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