ECG

24.5.12

How ECG is done?

• The electrical impulses originating from the heart can be transmitted to the body surface because the body contains fluids and chemicals that can conduct electricity

• These impulses can be recorded by placing pairs of electrodes to the different areas of the body connected via a galvanometer for measuring potential difference between the two points. One electrode is connected to the positive end of the galvanometer and other to the negative end

3 lead ECG: Bipolar leads of ECG

• An imaginary line connecting any two electrodes is called the “ECG lead”

• A lead is bipolar when both positive and negative electrodes contribute to the deflection in ECG

• The positive and negative electrodes are placed at an equal distance away from the heart and the resulting ECG deflection is the sum of the electrical forces going in opposite directions

Bipolar leads I, II & III

Surface ECG & Transmembrane Action Potential

• An ECG is a recording of the part of the electrical activity present in body fluids from the cardiac impulse that reaches the body surface, not a direct recording of the actual electrical activity of the heart

ECG

• It is not a recording of a single action potential in a single cell at a single point in time. The record at any time represents the sum of electrical activity in all the cardiac muscle cells, some of which may be undergoing action potentials while others may not yet be activated

• The recording represents comparison in voltage detected by electrodes at two points on the body surface, not the actual potential.

ECG

• Whether an upward or downward deflection is recorded is determined by the way the electrodes are oriented with respect to the current flow in the heart

• Even though the same electrical events are occurring in the heart, different waveforms representing the same electrical activity result when the activity is recorded by electrodes at different points in the body (e.g. left arm, right arm or left leg etc)

Important Segments in ECG

P-R segment

• Time between the end of P and the onset of QRS. Named PR rather than PQ because Q deflection is small and sometimes absent

• Normal duration = 0.12-2.0 sec

• Represents atria to ventricular conduction time (through His bundle)

• Prolonged PR interval may indicate a 1st degree heart block

S-T Segment

• Connects the QRS complex and T wave• Represents the plateau phase of the action

potential of the ventricular myocardial cells and the time when all cells have just been depolarized and the muscle cells are in a state of sustained contraction. The ventricular muscles are completely refractory during this period

T-P Segment

• Time after the T wave and before the next P wave

• Represents the time when the heart muscle is completely repolarized and at rest and ventricular filling is taking place

Cardiac Cycle - Filling of Heart Chambers

• Heart is two pumps that work together, right and left half

• Repetitive contraction (systole) and relaxation (diastole) of heart chambers

• Blood moves through circulatory system from areas of higher to lower pressure.

– Contraction of heart produces the pressure

Cardiac Cycle - Filling of Heart Chambers

Heart rate

• SA node which has the fastest rate of autorhytmicity, at 70 to 80 action potentials per minute drives the rest of the heart at this rate and is thus known as the “pacemaker of the heart”

• Distance between two consecutive QRS complexes

• A rapid heart rate of more than 100 beats/min is called tachycardia

• A slow heart rate of fewer than 60 beats per minute is called bradycardia

Information Gained from ECG

Non-invasive technique• Heart Rate• Signal conduction/ Heart tissue• Abnormalities in heart rate, arythmias and damage

of cardiac muscle