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Myocardial cell resting state

Polarized (negative inside)

Depolarization

Electrical activation of the heart caused by ion movement

Repolarization

Return to resting state (negative)

Membrane pumps

Restore resting polarity by moving ions back across membrane

Role of calcium in myocardial cells

Triggers contraction after action potential

Pacemaker cells

Cells in SA and AV nodes that spontaneously depolarize

Conducting cells

Transmit electrical signals through the heart

Myocardial cells

Contractile cells responsible for pumping

Normal Sinus Rhythm (NSR)

Originates in SA node with regular rhythm and HR 60–100 bpm

Arrhythmia

Abnormal heart rhythm caused by ectopic beats or conduction issues

EKG measures

Electrical activity of myocardial cells

Waveform characteristics

Duration amplitude and configuration

Standard EKG paper speed

25 mm per second

One small box time

0.04 seconds

Vertical measurement

1 cm equals 1 mV

P wave

Atrial depolarization

Normal P wave duration

0.04 to 0.12 seconds

Normal P wave amplitude

Less than 0.25 mV

PR segment

AV node conduction pause

PR interval

Start of atrial depolarization to start of ventricular depolarization

Normal PR interval

0.12 to 0.20 seconds

Ventricular depolarization structures

Bundle of His bundle branches and Purkinje fibers

QRS complex

Ventricular depolarization

Normal QRS duration

0.06 to 0.12 seconds

Q wave

First downward deflection

R wave

First upward deflection

S wave

Downward deflection after R wave

T wave

Ventricular repolarization

U wave

Septal repolarization not always present

Segment

Flat line between waves

Interval

Wave plus a segment

ST segment

End of ventricular depolarization to start of repolarization

Normal ST segment

Flat or slightly upsloping

QT interval

Ventricular depolarization to repolarization

QT and heart rate relationship

Faster heart rate shortens QT interval

12 lead ECG electrodes

10 electrodes

12 lead ECG views

12 different angles of the heart

Lead I orientation

0 degrees

Lead II orientation

Positive 60 degrees

Lead III orientation

Positive 120 degrees

aVL orientation

Negative 30 degrees

aVR orientation

Negative 150 degrees

aVF orientation

Positive 90 degrees

Inferior leads

Leads II III and aVF

Left lateral leads

Leads I and aVL

Anterior leads

Leads V2 V3 and V4

Precordial leads view

Horizontal plane

V1 placement

4th intercostal space right of sternum

V2 placement

4th intercostal space left of sternum

V3 placement

Between V2 and V4

V4 placement

5th intercostal space midclavicular line

V5 placement

Anterior axillary line in line with V4

V6 placement

Midaxillary line in line with V4

Depolarization toward positive electrode

Upward deflection

Depolarization away from positive electrode

Downward deflection

Repolarization toward positive electrode

Downward deflection

Cardiac vector

Sum of all electrical forces in the heart approximately positive 60 degrees

First step EKG interpretation

Check if there is a P wave before every QRS

Additional EKG checks

Consistent waves normal intervals and regular rhythm

P wave meaning

Atrial depolarization representing electrical activity spreading through both atria

Normal P wave duration

Less than 0.12 seconds

Normal P wave amplitude

Less than 0.25 mV

P wave abnormal clue

Missing or irregular P waves suggest atrial arrhythmia

QRS complex meaning

Ventricular depolarization representing electrical activation of ventricles

Normal QRS duration

0.06 to 0.12 seconds

Wide QRS complex

May indicate ventricular conduction delay or bundle branch block

QRS absence after P wave

Suggests AV conduction problem

T wave meaning

Ventricular repolarization

Normal T wave shape

Small to moderate rounded positive deflection

T wave abnormality

Inversion or extreme size may indicate ischemia or electrolyte imbalance

U wave meaning

Represents septal repolarization and not always present

Prominent U wave

May indicate electrolyte imbalance

PR interval meaning

Time from start of atrial depolarization to start of ventricular depolarization

Normal PR interval

0.12 to 0.20 seconds

Prolonged PR interval

Suggests delayed AV node conduction first degree block

Short PR interval

May indicate abnormal conduction pathway

ST segment meaning

Period between ventricular depolarization and repolarization

Normal ST segment

Flat or slightly upsloping

ST elevation

May indicate myocardial injury or infarction

ST depression

May indicate ischemia

QT interval meaning

Total time for ventricular depolarization and repolarization

QT interval normal relationship

About 40 percent of RR interval

QT interval and heart rate

Faster heart rate shortens QT interval

Prolonged QT interval

Risk of dangerous arrhythmias

RR interval meaning

Time between two consecutive R waves representing one cardiac cycle

P wave typical pattern

Positive in inferior and lateral leads often biphasic in lead III and V1 most positive in lead II most negative in aVR

QRS pattern normal

Tall R waves in lateral and inferior leads with normal progression from V1 to V6

Poor R wave progression

May indicate anterior myocardial damage

T wave pattern

Usually positive in leads with tall R waves

Lead II importance

Often shows clearest P waves and is commonly used for rhythm analysis

Inferior leads

Leads II III and aVF view inferior surface of heart

Lateral leads

Leads I aVL V5 and V6 view left lateral wall

Anterior leads

Leads V2 V3 and V4 view anterior wall

Right sided leads

Leads aVR and V1 view right ventricle

12 lead ECG concept

Provides 12 different electrical perspectives of the heart using 10 electrodes

Cardiac vector meaning

Overall direction of electrical activity usually around positive 60 degrees

Depolarization toward electrode

Produces upward deflection

Depolarization away from electrode

Produces downward deflection

Repolarization toward electrode

Produces downward deflection

Repolarization away from electrode

Produces upward deflection