Flashcards in The electrocardiogram theory and practice Deck (18)
how does the heart generate surface potentials upon the skin?
electrical activity within and between (via gap junctions) cardiac muscle cells causes
- current flow within the heart
- current flow within the surrounding tissues
why does current flow within the surrounding tissues?
to complete the circuit. this is what is being recorded in ECG
what does an ecg do?
measures the potential differences in the oder of a millivolt between distant sites on the body surface measurable by electrodes placed on the skin
what creates the net electrical dipole?
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
what does the dipole then create?
The dipole generates an electrical field (lines of isopotential) that are measurable with two electrodes. The imaginary line connecting these is the lead axis
when is the measured potential different greatest ?
when the lead axis is parallel to the direction of the dipole - an electrical vector. It is zero when they are perpendicular
what 2 components does the electrical vector have?
magnitude and direction
why is the vector clinically important?
it allows the electrical axis of the heart to be estimated
what is the magnitude determined by?
the mass of cardiac muscle that is involved in the generation of the signal
what is the direction determined by?
the overall activity of the heart any instant in time varies during the cardiac cycle
what are the standard limp leads (bipolar)
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
what does a depolarisation movement towards the recording (+ve) electrode show on the ECG?
upward deflection on the ECG
what does a depolarisation movement away from the recording (+ve) electrode show on the ECG?
what does P correspond to and what is the normal time for it?
action potential upstroke throughout atrial muscle (0.08 – 0.10 s)
what does QRS correspond to and what is the normal time for it?
: action potential upstroke throughout ventricular muscle (
what does t correspond to?
which points on the ECG are isoelectric and why?
during the PR (atria completely depolarized) and ST (ventricles completely depolarized) segments and TP interval (all regions repolarized)