electrocardiogram

Question 1

what is an ECG

Answer 1

recording of electrical activity of the heart (and potentially other muscles). electrodes on arms, legs and chest wall detect electrical activity. graph of voltage vs time. note that these are extracellular recordings

Question 2

normal path for conduction of electrical activity

Answer 2

• cells of SAN undergo spontaneous discharge
• electrical activity
• there is a slight pause in discharge of the AVN
• when the AVN discharges, this activates the bundle of His and Purkinje fibre system, ensuring near-simultaneous depolarisation of the muscle mass in both ventricles

Question 3

ECG leads

Answer 3

• in order to control a voltage, the potential at one electrode has to be compared to that at another electrode (or a combination of other electrodes)
• There are 3 active limb electrode (the right leg electrode is an earth electrode) and 6 chest electrodes, but 6 limb leads (I, II, III, aVR, aVL, aVF) and 6 chest leads (V1-V6)

Question 4

Bipolar leads -lead I

Answer 4

RA to LA, positive electrode at LA

Question 5

Bipolar leads- lead II

Answer 5

RA to LL, positive electrode at LL

Question 6

Bipolar leads- lead III

Answer 6

RL to LA, positive electrode at RL

Question 7

Unipolar (augmented) limb leads

Answer 7

RA (aVr), LA (aVl), LL (aVf), all electrodes positive

Question 8

leads and dipoles

Answer 8

• ECG machines use a galvanometer, an instrument used to detect, measure and determine the direction of small electric currents
• Each ECG lead has a positive and negative pole
• Leads record the electrical current associated with a dipole
• Thus, when all the muscle is polarized (at rest), there is no dipole
• When part of the muscle is depolarized, a dipole exists
• When all of the muscle is depolarized (contracting), there is no dipole