12-Lead ECG: The Art of Interpretation
Sinus Arrest
- SA node doesn't fire - notice absence of P-wave for a complete cycle (a missed cycle)
Resting myocyte electrical potential is_________.
-70 to -90 mV (millivolts)
negative deflection on ECG
-wave of depolarization moving away from positive electrode. -represented as a downward deflection below baseline
Time Equivalent to a "small box"
0.04 seconds
A wide QRS complex is considered > than:
0.12 seconds
A wide PRI is considered > than:
0.20 seconds
Time Equivalent to a "big box"
0.20 seconds (0.04 x 5)
How long is a regular 12 Lead ECG?
10 seconds long
Purkinje Fibers Pacemaker Setting (rate)
20-40 BPM
Bundle Branch Pacemaker Setting (rate)
40-45 BPM
His Bundle Pacemaker Setting (rate)
40-45 BPM
Junctional Rhythm
40-60 Regular! -impulse from AV node w/ retro/antegrade transmission - P wave often inverted/buried/follow QRS - slow rate - narrow QRS (not wide like ventricular)
AV Node Pacemaker Setting (rate)
45-50 BPM
Atrial Cells Pacemaker Setting (rate)
55-60 BPM
SA Node Blood Supply
59% of cases right coronary artery, 38% of cases left coronary artery and 3% of cases both
Bradycardia is considered any heart rate less than:
60 bpm
SA Node Pacemaker Setting (rate)
60-100 BPM
Electrical Potential
A difference in the concentration of charged particles between one point and another
The Bundle of His
AV node > right/left bundle branches. The only route of communication between the atria and the ventricles.
Coronal Plane
Divides the body into anterior and posterior, represented by the hexaxial system. (aVR, aVF, aVL, lead I, II, and III {with 30 degrees of separation})
Transverse Plane
Divides the body into superior and inferior, represented by the precordial leads (v 1-6)
Premature Atrial Contraction (PAC)
Heart Rate: Depends on underlying rhythm Regularity: Interrupts the regularity of underlying rhythm P-Wave: can be flattened, notched, or unusual. May be hidden within the T wave PRI: measures between .12-.20 seconds and can be prolonged; can be different from other complexes QRS: <.12 seconds
Lateral Focused Leads
I, aVL, V5, and V6
Phase 2
Influx of Calcium and Sodium maintains depolarized state. See physiology of Calcium ions.
Isoelectric Baseline of ECG
It is a line from one TP segment to the next
P wave
Represents electrical depolarization of BOTH atria, starts when the SA (sinoatrial) node fires, includes transmission through the internodal pathways, Bachmann Bundle, and the atrial myocytes. Standard duration is 0.08-0.11 seconds.
U wave represents
Represents repolarization of purkinje fibers
Tp wave
Represents repolarization of the atria, typically hidden by the qRs unless in the presence of a conduction abnormality in which you can appreciate a negative deflection.
Ventricular Escape Beat
Sinus block resulting in missed cycle and ventricular automaticity focus takes over. Typically produces enormous QRS complex.
The outside of a cardiac cell has a higher ______ concentration.
Sodium (Na+) and Calcium (Ca++)
Main Positively Charged Ions
Sodium (Na+), Potassium (K+), and Calcium (Ca+)
Natural Ionic Transfer Tendency
Sodium enters, Potassium exits
Phase 0
Sodium leaks into cells > threshold potential reached > fast sodium channels open > increases polarity = depolarization "spike"
The baseline is a straight line drawn between the ____________ of one complex to the ________________ of the succeeding complex.
TP Segment - TP Segment
What is the absolute refractory period?
The time period of about 1 msec after an action potential before another one can be initiated Best example is letting the trigger reset before firing a pistol a second time. If you don't allow for reset it won't fire.
Septal Focused Leads
V1, V2
Anterior Focused Leads
V3, V4
Multifocal Atrial Tachycardia
Varying PR interval with 3 or more morphologically distinct P waves in the same lead
Internodal Pathway
between SA and AV nodes, inlcudes the Bachmann Bundle
P wave represents
depolarization of the atria
QRS represents what?
depolarization of the ventricles
ST segment represents what?
depolarized state of the ventricles (isoelectric)
QRS Morphology
determining the differences or similarities between the QRS complexes within the ECG Strip
Bachmann Bundle
interatrial pathway that connects the right and left atria (transmits through the inter-atrial septum)
Atrial Flutter (Sawtooth)
irregular beating of the atria; often described as "a-flutter with 2 to 1 block or 3 to 1 block"
Interval
is the distance, measured in time, occurring between two cardiac events (time ONLY, not morphology)
Atrial Fibrillation
rapid, random, ineffective contractions of the atrium
T wave represents
repolarization of ventricles
Atrioventricular Node
slows conduction from the atria to the ventricles long enough for the atria to fill the ventricles and maintain cardiac output. The AV node is always supplied by the right coronary artery
What is a cardiac cycle?
the period between the start of one heartbeat and the beginning of the next
What is the relative refractory period?
the period shortly after the firing of a nerve fiber when partial repolarization has occurred and a greater than normal stimulus can stimulate a second response
QT interval represents what?
ventricular action potential duration (time for depolarization and repolarization)
positive deflection on ECG
~wave of depolarization going towards positive electrode ~wave of repolarization moving away from the positive electrode -represented as a wave form above baseline
Left Bundle Branch
Bundle of His > Left Anterior/Left Posterior Fascicle
Right Bundle Branch
Bundle of His > Purkinje fibers
Physiology of Calcium in Depolarization
Calcium activates the proteins troponin and tropomyosin to align actin and myosin heads allowing for contraction. More Ca++ = faster and longer held contraction.
Main Negatively Charged Ions
Chloride (Cl-)
Left Posterior Fascicle
Left Bundle Branch > Purkinje cells (which innervate the posterior and inferior aspects of the left ventricle), multi-stranded = hard to block
Left Anterior Fascicle
Left Bundle Branch > Purkinje fibers (which innervate the anterior and superior aspects of the left ventricle), single stranded (vs Left Posterior Fascicle)
PR segment
Occupies the waveform between the end of atrial depolarization and the beginning of ventricular depolarization/atrial repolarization. Typically consistent with the baseline, however can be appreciated at 0.08 mm depression under normal circumstances; if greater, this could indicate pericarditis or atrial infarct thus being pathologically depressed.
Phase 1
Peak positive charge > Chloride (Cl-) ions enter cell > slows Sodium (Na+) influx > fast sodium channels close > slow sodium channels/calcium channels open = "plateau"
The inside of a cardiac cell has a higher ________concentration.
Potassium (K+)
Phase 3
Potassium channels open > positive Potassium ions leave cell > relatively negative charge inside cell = repolarization/resting potential > sodium out and potassium in (Phase 0)
Accelerated Junctional
Rate: 60-100 Rhythm: Regular Pwave: before/after/absent - inverted PR: unknown QRS: Normal *** Fast junctional escape
Wandering Atrial Pacemaker (WAP)
Rate: Normal Rhythm: Reg/Irregular Pwave: Varies PR: Variable QRS: Normal *** conduction starts anywhere
Junctional Escape Beat
Rate: Regular Rhythm: NSR w/ ... Pwave: Before/after/absent - inverted PR: absent QRS: Normal *** AV AFTER expected
Sinoatrial Block (SA Block)
Rate: Varies Regularity: Irreg PWave: present unless area of dropped beat QRS: Normal Dropped Beats: Yes
S wave
The first negative deflection after an R wave
Q wave
The first negative deflection after the P wave
R wave
The first positive deflection produced by ventricular depolarization.
Treshold Potential
The point at which the channels (fast sodium channels) open
Sodium-Potassium ATPase Pump Physiology
The pump uses ATP to push out 3 Sodium ions and brings in 2 potassium ions = greater number of positive charges outside the cell.
Segment
a specific portion of the complex as it is represented on the ECG (displayed waveform)
The Purkinje System
a system of individual cells that directly innervate the myocardial cells and initiate ventricular depolarization cycle
premature ventricular contraction (PVC)
a ventricular contraction preceding the normal impulse initiated by the SA node (pacemaker)
Inferior Focused Leads
aVF, II, and III
PR interval represents?
atrial depolarization & transmission through AV junction; supraventricular