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