ECG

BACKGROUND:

• Sum of the electrical signals from the cardiac muscle as recorded on the surface of the body.
• Pattern of the electrical activity depends on the orientation of the electrodes and the electrical activity of the cardiac cells.

EINTHOVEN TRIANGLE:

Willem Einthoven (1860-1927) attempted to explain the principles of the ECG in
scientific terms. In Einthoven's triangle, the heart may be considered to lie at the
centre of an equilateral triangle and the corners of the triangles are the effective
sensing points - the right arm, left arm and left leg electrodes.

CONVENTIONAL ECG ELECTRODE DERIVATIONS:

· 12 standard leads

· ECG is recorded:

• Bipolar recording: between two points of the body (= bipolar recording)
• Unipolar recording: between one point of the body (different electrode) and ground (indifferent electrode)
• 6 limb leads: I, II, III (bipolar) and aVR, aVL, aVF (unipolar)
• 6 precordial leads: V1 - V6 (unipolar)

LEADS CONNECTION:

Standard Limb Leads: I, II, III; bipolar, form a set of axes 60° apart

• Lead I: Composed of negative electrode on the right arm and positive electrode on the left arm.
• Lead II: Composed of negative electrode on the right arm and positive electrode on the left leg.
• Lead III: Composed of negative electrode on the left arm and positive electrode on the left leg.

Augmented Voltage Leads:

aVR, aVL aVF; unipolar ; form a set of axes 60° apart but are rotated 30° from the axes of the standard limb leads.

· aVR: Exploring electrode located at the right shoulder.
· aVL: Exploring electrode located at the left shoulder.
· aVF: Exploring electrode located at the left foot.

Reference Point for Augmented Leads: The opposing standard limb lead; i.e., that
standard limb lead whose axis is perpendicular to the particular augmented lead.

Chest Leads: Vl, V2, V3, V4, V5, V6, explore the electrical activity of the heart in
the horizontal plane; i.e., as if looking down on a cross section of the body at the level
of the heart. These are exploring leads.

ECG curve:

• P wave: Atrial depolarization (Small, rounded and upright)
• QRS complex: Ventricular depolarization (Spiked with one or more deflections from the baseline)
• T wave: Ventricular repolarization (Broad, rounded. if .QRS. then must be a .T. wave)
• PR segment: AV nodal delay
• ST segment: Ventricles are contracting and emptying the action potential of ventricular muscle cells in plateau phase
• TP interval: Ventricle cells at rest, ventricular filling.

NORMAL VALUES FOR AMPLITUDES AND DURATIONS OF
IMPORTANT ECG PARAMETERS:

Amplitude:

P – wave 0.25 mV
R – wave 1.60 mV
Q – wave 25% of R wave
T – wave 0.1 to 0.5 mV

Duration:

P – R interval - 0.12 to 0.22 s
Q – T interval - 0.35 to 0.44 s
S – T interval - 0.05 to 0.15 s
P – wave interval - 0.11 s
QRS interval - 0.09 to 0.10 s

Links:

http://library.med.utah.edu/kw/ecg/
http://www.anaesthetist.com/icu/organs/heart/ecg/Findex.htm
http://nobelprize.org/educational/medicine/ecg/