16. Physiological Basis of the ECG Flashcards
how many electrodes are used in an ECG & how many readings does it produce
9 electrodes
12 readings
what does the ECG record
each electrode records a voltage difference btn itself and another
difference shown as a deflections (shows depol and repol)
when is a deflection produced in an ECG
part of the cardiac excitable tissue is at a different mem potential than the rest of the heart
-current flow can occur btn those regions
phase 0 & 3
electrodes report voltage differences in regions of —-
either the ventricles or atria
NOT btn the two
what are the steps of ECG deflection
- depol starts - there is diff btn diff parts –> deflection +
- once all cells are depol –> no diff
- repol starts - difference again – > - deflection
- once all repol –> no diff
*rmr measuring extracell potential*
where does current flow -negatively & -positively in the heart
average current flow occurs w/ negativity toward the base
& positivity towards the apex
what is the path of depolarization
start at SA node –> depol transmit to next contractile myocyte –> spread R to L thru atria = P wave
-at same time send signal to AV node –> then bundle of His –> bundle brances –> purkinje –> ventricles = beginning of QRS
which phase is represented by baseline on the ECG
phase 2
what is the PR interval
AV node delay
what direction does ventricular depolarization occur
R –> L
base–> apex
interior –> exterior
=QRS complex
what is represented at the ST segment
AP phase 2 delaying repol of ventricles
what direction does ventricular repol occur
L –> R
apex –> base
= T wave
What is Ca permeability btn the end of P wave & beginning of QRS in the atria
high Ca permeability
what is the state of all cells at the end of a P wave
all atrial cells depolarized
what is the state of the cells in the ST segment
all ventricular cells depolarized
-no deflection bc no difference
what is the Na conductance in cells causing QRS complex
increase conductance
= phase 0
what phase is represented in the T wave
phase 3 (repolarization)
what is the difference btn a segment and interval
segment = duration of single event
interval = duration of 2 or more events
how is 1 sec measured in the ECG
horizontally 5 large boxes
what is the average length of the PR interval
0.16 ms
what is the average length of the QT interval
0.35 ms
what mV do each parts of the ECG form when on arms or one leg and one arm
P wave = 0.1-0.3 mV
QRS complex = 1-1.5 mV
T wave = 0.2-0.3 mV
what is the time represented at each light & dark line in horizontal boxes
light line = 0.04 s
dark line = 0.2 s
what do Leads 1, 2 & 3 present
1 = RA to LA
2 = RA to LL
3 = LA to LL
what is represented by aVF
augmented vector, foot
(RA + LA) to LL
what is represented by aVR
augemented vector, right
(LL + LA) to RA
what does aVL represent
augemented vector, left
(LL + RA) to LA
where are chest/precordial leads placed
assigned to regions of heart
each lead acts as a positive electrode
which leads look at the inferior heart
II, III and aVF
which leads represent the septum of the heart
V1-V2
which leads are represent the anterior heart
V1-4
which leads represent the lateral heart
V5, V6, lead I and aVL
what leads are represent the RV
V3 & V4
