ECG Flashcards
What is an ECG?
record of the pattern of electrical activity associated with concentration of cardiac muscle during heartbeat
**⇒ **voltage measured btw well defined points of the body as a function of time
- usually 5 peaks: P, Q, R, S, T, V
Explain the potential map of a single point charge.
in the center single point charge (= monopole)
- surrounded by straight electric-field lines
- density of electric-field lines indicates field strength
- head of arrow = positive charge
- circular lines = potential surface (= areas with same potential)
What is the dipole moment vector?
vector that characterizes the strength of the dipole → head of the arrow indicating positive pole
- changes periodically with changing electric field (action potential travelling through the heart)
What is the difference btw capillary micro- and macroscopic electrode?
- capillary microelectrode = membrane potential can be measured (one electrode intra-, one extracellular)
- macroscopic electrode = potential changes outside cell bundles can be measured (body surface electrodes)
In what way does the action potential of skeletal and cardiac (ventricular) muscle cells differ?
Explain the process w/r/t direction and potential.
skeletal muscle cells:
short action potential (1 - 2 ms) of the same duration unidirectional
- positive depolarization
- negative repolarization
cardiac muscle cells:
action potentials of shortening duration (endocardium ~400 ms → epicardium ~ 250 ms) bidirectional
- positive depolarization of endocardium
- positive depolarization of epicardium
⇒ propagating outwards
- repolarization of epicardium
- repolarization of endocardium
⇒ propagating inwards
BUT: both fronts positive
Explain the stepwise process of the spread of the cardiac action potential.
Refer to the different regions of the ECG and draw the graph.
- SA-node generates action potential
⇒ atrial muscle (P-wave) - to AV-node via internodal bundles
⇒ second action potential generated - bundle of HIS
- Tawara crura
- Purkinje-fibers → ventricular muscles
⇒ ventricles contract = depolarization (QRS-complex) - ventricles repolarize (T-wave)
- rest
Why does the P-wave itself not induce a further depolarization of the ventricular muscle?
cardiac skeleton = electrical insulator btw atria and ventricles
Why are in the ECG no peaks for SA-node, AV-node and bundle of HIS visible?
smaller than noise level → filtered out
What is a way other than the ECG to visualize the elementary dipole moment of the heart?
What are the axes?
vectocardiography
usage of an integral vector (= arrow) of continuously changing direction and length in 3D space
- x-axis: I
- y-axis: aVF
- z-axis: -V2
⇒ closed loops represent dipole moments
Differentiate btw different types of electrodes.
according to activity
- active (different) electrode: continuously changing potential during cardiac cycle
- inactive (indifferent) electrode: practically constant potential
- neutral electrode: reduction of noise
according to location
- limb electrodes: 3 + 1 neutral
- precordial electrodes: 6, V1 - V6
Differentiate btw different types of leads.
-
bipolar leads: record potential difference btw 2 active electrodes
⇒ 2 changing values -
unipolar leads: record potential difference btw 1 active and 1 inactive electrode
⇒ 1 changing value ⇒ real absolut change of potential
How is the inactive electrode constructed?
limb electrodes connected through equal resistances to one point → constant potential = central terminal (CT) = Wilson’s point
Explain Einthoven’s triangle.
Are the leads bipolar or unipolar?
roughly equilateral triangle of the 2 shoulders + hip
- Vertexes ~ limb electrodes (right foot = neutral el.)
- sides ~ standard limb leads (I - III)
- I: left arm - right arm
- II: left foot - right arm
- III: left foot - left arm
⇒ bipolar leads
How can the resultant integral vector in Einthoven’s triangle be constructed?
from any two of Einthoven’s standard leads (vectorially) by perpendicular projection
ex: UII = UI + UIII
- direction reproduces the direction of the cardiac dipole
- magnitude is proportional to that of cardiac dipole (amplitude btw peak and 0 mV-level)
What is the electrical axis of the heart?
integral vector constructed from the largest deflection of the R-wave
Which electrodes are used for Wilson’s chest leads?
Unipolar or bipolar?
6 unipolar precordial electrodes (V1 - V6)
V5 = inactive reference electrode at Wilson’s point
In what way does Goldberger’s leads differ from Einthoven’s standard leads?
What is the reason?
How is the measured region indicated?
Unipolar or bipolar?
modified Wilson’s central terminal (only the 2 electrodes used that aren’t measured) ⇒ “truncated” Wilson’s central terminal
because: only very low signal would be provided in case of Einthoven’s standard leads
⇒ a (= augmented) + V + R/L/F (= indicating active el.)
- aVR = right arm measured
- aVL = left arm measured
- aVF = left foot measured
⇒ Wilson’s CT not exactly inactive anymore → pseudo-unipolar
Explain the function of a differential amplifier.
amplifies the difference btw 2 input signals
- common mode: measures noise, right foot N electrode connected
- differential mode: 2 electrodes (unipolar/bipolar lead) connected
⇒ common mode noises are suppressed, would be measurable everywhere in the body, therefore:
ex: (UI + UN) - UN = UI
