# The electrocardiogram theory and practice Flashcards

1
Q

how does the heart generate surface potentials upon the skin?

A

electrical activity within and between (via gap junctions) cardiac muscle cells causes

• current flow within the heart
• current flow within the surrounding tissues
2
Q

why does current flow within the surrounding tissues?

A

to complete the circuit. this is what is being recorded in ECG

3
Q

what does an ecg do?

A

measures the potential differences in the oder of a millivolt between distant sites on the body surface measurable by electrodes placed on the skin

4
Q

what creates the net electrical dipole?

A

wave of depolarisation as a separation of charge
During the cardiac cycle different regions of the heart are depolarized, or polarized, creating a charge separation, or dipole

5
Q

what does the dipole then create?

A

The dipole generates an electrical field (lines of isopotential) that are measurable with two electrodes. The imaginary line connecting these is the lead axis

6
Q

when is the measured potential different greatest ?

A

when the lead axis is parallel to the direction of the dipole - an electrical vector. It is zero when they are perpendicular

7
Q

what 2 components does the electrical vector have?

A

magnitude and direction

8
Q

why is the vector clinically important?

A

it allows the electrical axis of the heart to be estimated

9
Q

what is the magnitude determined by?

A

the mass of cardiac muscle that is involved in the generation of the signal

10
Q

what is the direction determined by?

A

the overall activity of the heart any instant in time varies during the cardiac cycle

11
Q

what are the standard limp leads (bipolar)

A

lead 1 - rRA -ve to LA +ve
lead 2 - RA -ve to LL +ve
lead 3 = LA -ve to LL +ve

these are connected together in Einthovens triangle

12
Q

what does a depolarisation movement towards the recording (+ve) electrode show on the ECG?

A

upward deflection on the ECG

13
Q

what does a depolarisation movement away from the recording (+ve) electrode show on the ECG?

A

downward deflection

14
Q

what does P correspond to and what is the normal time for it?

A

action potential upstroke throughout atrial muscle (0.08 – 0.10 s)

15
Q

what does QRS correspond to and what is the normal time for it?

A

: action potential upstroke throughout ventricular muscle (

16
Q

what does t correspond to?

A

ventricular repolarisation

17
Q

which points on the ECG are isoelectric and why?

A

during the PR (atria completely depolarized) and ST (ventricles completely depolarized) segments and TP interval (all regions repolarized)

18
Q

what is the purpose of the augmented limb leads (unipolar)?

A

to view the heart from a different angle