The Electrocardiogram

Understanding EKGs: A Practical Approach, Third Edition Brenda M. Beasley

For the Dental Hygienist

CHAPTERCHAPTER

Understanding EKGsA Practical Approach

5The ElectrocardiogramThe Electrocardiogram

The Electrocardiogram

• Objectives– Define the term electrocardiogram– Describe the basics of EKG monitoring– List the types of EKG leads– Discuss the relevance of Einthoven’s triangle– Identify and explain the grids and markings

on a representative strip of EKG graph paper

• Objectives (continued)– Describe the relationship of the following EKG

waveforms to the electrical events in the heart P wave PR interval QRS complex ST segment T wave

• Electrocardiogram– Graphic representation of the electrical

activity of the heart

• Electrocardiograph– Machine used to record the

electrocardiogram, or EKG machine

• EKG– Graphic tracing of electrical activity of the

heart, not mechanical activity

The Electrical Basis of the EKG

• Electrical activity is sensed by electrodes placed on the skin surface

• Recorded in the form of an electrocardiogram

• Cardiac monitor depicts electrical impulses on monitor screen or oscilloscope

The Electrical Basis of the EKG

• Electrical impulses present on the skin surface are very low voltage; impulses are amplified by an EKG machine

• The printed record of the electrical activity of the heart is called a rhythm strip or an EKG strip

EKG Leads

• Electrode– An adhesive pad that contains conductive gel

and attaches to patient’s skin

• Leads– How electrodes are connected to the cardiac

monitor – 3 leads must have a positive, a negative,

and a ground

EKG Leads

• Einthoven’s triangle– Imaginary inverted triangle

formed around heart by proper placement of bipolar leads

– Top formed by lead I, right side formed by lead III, and left side formed by lead II

– Each lead represents a different look at, or view, of the heart

EKG Graph Paper

• Leaves the machine at constant speed of 25 mm/s

• Time– Measured on horizontal

• Amplitude or voltage – Measured on vertical line

on graph paper

EKG Graph Paper

• Electrocardiographic – Paper divided into

small squares– 1 mm in width– Time interval of 0.04

seconds

EKG Graph Paper

• Electrocardiographic paper– Darker lines divide paper

every fifth square– Vertically and horizontally– Large squares measure

5 mm in height and width– Represents time interval

of 0.20 seconds– Five small squares in

each large square

EKG Waveforms

• Wave or waveform refers to movement away from the baseline or isoelectric line (beginning and ending of all waves)

• Positive deflection– Above isoelectric line

• Negative deflection– Below isoelectric line

EKG Waveforms

• Electrical impulse leaves SA node, produces waveform on graph paper

• One complete cardiac cycle = P, Q, R, S, (QRS complex) and T wave

P Wave

• First wave produced by electrical impulse from SA node

• Smooth, rounded upward deflection

• Depolarization of left and right atria

• 0.10 seconds in length

PR Interval

• Time impulse travels from SA node through internodal pathways in atria toward ventricles

• Time interval from start of P wave to start of QRS

• 0.12-0.20 seconds in length

QRS Complex

• Consists of Q, R, and S waves

• Represents conduction of impulse from bundle of His through ventricular muscle

• Ventricular depolarization

QRS Complex

• Q wave– First downward deflection

• R wave– First upward deflection– Largest deflection seen in

lead I and II

• S wave– Downward deflection after

R wave

• Measures less than 0.12 seconds (3 small boxes)

ST Segment

• Time interval during which ventricles depolarized and repolarization of ventricles begin

• Isoelectric or consistent with baseline

T Wave

• Follows ST segment• Represents

ventricular repolarization

• Slightly rounded, positive deflection

• Resting phase of cardiac cycle

Summary of EKG Waveforms

• P wave– Atrial depolarization

• QRS complex– Ventricular

depolarization, atrial repolarization

• T wave– Ventricular

repolarization

The Electrocardiogram

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