An ECG is a ‘snapshot’ of the electrical activity of the heart presented on a graph. When interpreting ECG one can note the heart rate and rhythm, normal/abnormal conduction of both the atria or ventricles, structural changes within the heart such as atrial or ventricular enlargement, as well as an indication of a past Myocardial Infarction.
ECG Principles
The ECG’s value is magnified when recorded during a stress test eg. when the patient is running on a treadmill, or when recorded for a long period of time as in with a Holter.
The pumping action of the heart:
- DEPOLARISATION – is initiated by an electrical activation of the myocardium
- AUTOMATICITY – causes heart action
- EXCITABILITY – responds to the electrical impulse
- CONDUCTIVITY – conducts an electrical impulse
- CONTRACTILITY – initiates contraction
Repolarisation in an ECG acts as an indication for diagnosis of ischaemia, myocardial stretch, pharmacological effects, electrolyte imbalance, and congenital ionic diseases able to cause a sudden cardiac arrest and imminent death.
In both depolarisation and repolarisation, cardiac myocytes act like electric generators that cause electric currents to flow out into the body and back again into the heart. This produces various electrical potentials on the body’s surface, which are then recorded and represented on an ECG.
The ECG graph is usually set up at a speed of 25mm/s:
- 1 small square = 0.04 sec
- 1 large square = 0.2 sec
- 5 large squares = 1 sec
- 15 large squares = 3 sec
Each ECG lead used represents the heart from a different point of view on an ECG strip. The horizontal base line recorded is referred to as the iso-electric line, and a deflection from it signals electrical activity of the heart.
A normal ECG strip features the following:
- P Wave = electrical activity within the atrial chamber
- QRS Complex = ventricular depolarisation
- T Wave = ventricular repolarisation
The heart’s conductive system functions through the:
- SA Node (Sinus Node a.k.a. sino-atrial node) – The pacemaker of the heart, firing about 60-100 times per minute;
- AV Node (Atrio-Ventricular Junction) – Fires at a rate of 40-60 times per minute. The AV node takes charge whenever the SA node experiences impulse issues;
- AV Bundle (Bundle of His), Left Bundle and Right Bundle Branches, and the Purkinje Fibres – Fire at 20-40 times per minute if both the AV and the SA node experience impulse issues.
Interpreting ECG
Heart Rate
- The Rule of 300: when the rhythm is regular = 300 / (number of boxes between R to R wave)
- Six Second Method: when the rhythm is irregular = number of R waves per 6 seconds X 10
ECG Recording
Deflections
The direction of the electrical current determines the upward or downward deflection of an ECG waveform.
Major deflections include:
- P Wave – atrial depolarisation
- QRS Complex – ventricular depolarisation
- T Wave – ventricular repolarisation
P Wave should be small, rounded, and positive, visible through leads I, II, aVF, and V2-V6, with an amplitude of 0.5-2.5mm and duration of <120ms; there should be only 1 P Wave preceding the QRS Complex.
QRS Normal Interval should be less than 3 small squares on the ECG graph.
ST Segment is normally isoelectric and gently upsloping.
QT Prolonged could be indicating Hypokalaemia, Hypocalcaemia, Bradycardia, Drugs, issues with the CNS, Left Ventricular Hypertrophy and Pericarditis.
ST Elevation could be indicating MI or Myocardial Injury, Coronary Vasospasm or Pericarditis.
ST Depression could be indicating Ischaemia, Digitalis Glycocides use (eg. Digoxin), block in the left or right Bundle Branch, or left or right ventricular hypertrophy. ST Depression is a sign of a narrowed blood vessel.
NOTE: Some drugs such as antibiotics, anti-psychotic and anti-arrhythmic drugs, prolong depolarisation and repolarisation time.
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