Cardiac Electrical Activity & ECG Interpretation – Lecture 1

Three types of heart cells

Pacemaker cells; Electrical conducting cells; Myocardial cells

Pacemaker cells function

Generate electrical impulses automatically

Electrical conducting cells function

Transmit electrical impulses through the heart

Myocardial cells function

Contract to pump blood

EKG measures

Electrical activity of myocardial cells

EKG wave characteristics

Duration; Amplitude; Configuration

Duration (EKG)

Time in seconds

Amplitude (EKG)

Voltage in millivolts (mV)

Configuration (EKG)

Shape and appearance of wave

P wave represents

Atrial depolarization

QRS complex represents

Ventricular depolarization

T wave represents

Ventricular repolarization

Q wave definition

First downward deflection after P wave

R wave definition

First upward deflection after P wave

S wave definition

First downward deflection after R wave

QS complex

Entire QRS is one downward deflection

R prime (R')

Second upward deflection

PR interval definition

Start of atrial depolarization to start of ventricular depolarization

PR interval normal range

0.12 to 0.20 seconds

QRS interval definition

Duration of ventricular depolarization

QRS interval normal range

0.06 to 0.10 seconds

ST segment definition

End of ventricular depolarization to start of repolarization

J point definition

End of S wave (start of ST segment)

QT interval definition

Beginning of ventricular depolarization to end of repolarization

QT interval significance

Represents total ventricular electrical activity

QT interval and heart rate

Faster HR shortens QT; slower HR lengthens QT

QT interval proportion

About 40% of cardiac cycle

Small box on EKG equals

0.04 seconds

Large box on EKG equals

0.20 seconds

6 seconds equals

30 large boxes

Big box method formula

300 divided by number of large boxes between R waves

Big box method numbers

300; 150; 100; 75; 60; 50; 43; 38

Little box method formula

1500 divided by number of small boxes between R waves

When to use little box method

When heart rate is fast (>100 bpm)

6-second method formula

Count cycles in 6 seconds and multiply by 10

When to use 6-second method

When rhythm is irregular

ST segment ischemia indicator

Measured 0.08 seconds after J point

Electrical conduction system purpose

Coordinate heart contraction

Ventricular conduction system function

Conduct impulses through ventricles

Depolarization meaning

Electrical activation of cells

Repolarization meaning

Return to resting state

Sinus node role

Primary pacemaker of the heart