Electrocardiogram Flashcards
ECG
electrocardiogram
tool for evaluating electrical events within the heart
how do ECGs work?
when action potentials occur simultaneously in many individual myocardial cells, currents are conducted through body fluids around the heart and can be detected by recording electrodes at the skin surface
what different waves represent
P wave corresponds to current flow during atrial depolarisation
QRS complex (0.15s later) - ventricular depolarisation. currents generated in body fluids change direction.
T wave - ventricular repolarisation
atrial repolarisation not evident as it occurs at the same time as the QRS complex
lead 1 reference and recording electrode
reference (-) electrode - right arm
recording (+) electrode - left arm
lead 2 reference and recording electrode
reference (-) electrode - right arm
recording (+) electrode - left leg
lead 3 reference and recording electrode
reference (-) electrode - left arm
recording (+) electrode - left leg
augmented limb leads
aVR
aVL
aVF
aVR electrode placement
left arm and left leg (-) right arm (+)
aVL electrode placement
right arm and left leg (-) left arm (+)
aVF electrode placement
right arm and left arm (-) left leg (+)
precordial limb leads
V1, V2, V3, V4, V5, V6
precordial limb leads reference (-) electrode
combined limb leads
V1 recording (+) electrode
4th intercostal space, right of sternum
V2 recording (+) electrode
4th intercostal space, left of sternum
V3 recording (+) electrode
5th intercostal space, left of sternum
V4 recording (+) electrode
5th intercostal space, centred on clavicle
V5 recording (+) electrode
5th intercostal space, left of V4
V6 recording (+) electrode
5th intercostal space, under left arm
diagnostic value of ECG
provides info on electrical activity of the heart, not mechanical activity
summary of Ca2+ activity in the heart
small amount of Ca2+ enters through L-type channels during the plateau
this triggers the release of a large amount of Ca2+ from ryanodine receptors in the SR membrane
Ca2+ activation of thin filaments and cross-bridge cycling leads to generation of force
contraction ends when it’s returned to the SR and ECF by Ca2+ ATPase pumps and Na+/Ca2+ countertransporters
absolute refractory period of the heart
period during and following an action potential where an excitable membrane cannot be re-excited
heart cannot generate tetanic contractions
mechanisms of the refractory period of the heart
inactivation of Na+ channels
time of refractory period in skeletal muscles
absolute refractory period is much shorter (1-2 msec) than duration of contraction (20-100 msec), so a second action potential can be elicited while contraction from the first action potential is occuring
length of refractory period in cardiac muscle
due to the prolonged, depolarised plateau the refractory period lasts almost as long as the contraction (250 msec) and muscle can’t be re-excited multiple times during contraction
