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Understanding EKGs: A Practical Approach

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Kristin Smith

on 27 December 2013

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Transcript of Understanding EKGs: A Practical Approach

Understanding EKGs
A Practical Approach

Chapter 5 and 6
Define the term electrocardiogram
Describe the basis of EKG monitoring
List the types of EKG leads
Discuss the relevance of Einthoven’s
Identify and explain the grids and markings on a representative strip of EKG graph paper
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

The Electrocardiogram
Graphic representation of the electrical activity of the heart

Machine used to record the electrocardiogram, or EKG machine

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

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

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

EKG Leads

An adhesive pad that contains conductive gel and attaches to patient’s skin


How electrodes are connected to the cardiac monitor

3 leads must have a positive, a negative, and a ground

Bipolar leads
Have one positive and one negative

Limb leads

Lead I
Left arm (+) and right arm (-)

Lead II
Left leg (+) and right arm (-)
Lead II is most common due to ability to visualize P wave

Lead III
Left leg (+) and left arm (-)

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” or view of the heart

EKG Graph Paper
Leaves the machine at constant speed of 25 millimeters per second (mm/sec)

Measured on horizontal line

Amplitude or voltage
Measured on vertical line on graph paper


Paper divided into small squares

1 millimeter (mm) in width

Time interval of 0.04 seconds

Electrocardiographic paper

Darker lines divide paper every fifth square

Vertically and horizontally

Large squares measure 5 millimeters in height and width

Represents time interval of 0.20 seconds

Each small square represents time interval of 0.04 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

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

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 and T Wave
ST Segment is the 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

Interpretation and Introduction to Normal Sinus Rhythm
The Five-Step Approach
This five-step approach, in order of application, includes analysis of the following

Heart Rate

Step 1: Heart Rate
Number of electrical impulses as represented by PQRST complexes conducted through the myocardium in 60 seconds (1 minute)

Atrial rate
Counting the number of P waves noted

Ventricular rate
Counting the number of QRS complexes noted

Heart Rate Determination
Two methods of determination of the heart rate

1. The 6-second method
Denotes a 6-second interval on EKG strip
Strip is marked by 3 or 6 second tick marks on the top or bottom of graph paper
Count the number of QRS complexes occurring within the 6-second interval, and then multiply that number by 10

How many P waves?
8 P Waves= 80 for the Atrial Rate
How many QRS complexes?
8 QRS=80 for the Vent. Rate
2) Second method is the R-R interval method
Most accurate if heart rate is regular
An estimation of the heart rate
Look at QRS complex that falls on a heavy line on the strip, count number of large boxes between this R wave and the next R wave
Divide this number into 300

How many big boxes are from R to R?
2 Big Boxes + 2 0.5 Boxes
Heart Rhythm
STEP 2: Heart Rhythm
Sequential beating of the heart as a result of the generation of electrical impulses

Classified as

Regular pattern
Interval between R waves are regular

Irregular pattern
Interval between R waves are not regular

Regular Rhythm
Measure the intervals between P to P waves or R to R waves

If the intervals vary by less than 0.06 seconds or 1.5 small boxes, we can consider the rhythm to be regular

What is the difference from P to P?
What is the difference from R to R?
Irregular Rhythm
If the intervals between the P to P waves or R to R waves are variable by greater than 0.06 seconds, rhythm is considered irregular

Regularly irregular
Irregular rhythms that occur in a pattern
Occasionally irregular
Intervals of only one or two R to R are uneven
Irregularly irregular
R to R intervals exhibit no similarity

What is the Rate? Atrial and Ventricle
What is the Rhythm?
What is the Rate? Atrial and Ventricle?
What is the Rhythm?
The P Wave
STEP 3: The P Wave
P wave is produced when the right and left atria depolarize
First deviation from the isoelectric line
Should be rounded and upright
P wave is SA node pacing or firing at regular intervals
This pattern is referred to as a sinus rhythm

P Wave: Five Questions to Ask
1. Are P Waves present?
2. Are P Waves occurring regularly?
3. Is there one P wave present for each QRS complex present and/or is there
a QRS for each P wave present?

4. Are the P Waves smooth, rounded, and upright in appearance, or are they inverted?
5. Do all P Waves look similar?
P Wave Practice Strip
The PR Interval
STEP 4: The PR Interval
The PR Interval
Measures the time interval from the onset of atrial contraction to onset of ventricular contraction

Measured from onset of P wave to the onset of the QRS complex

**Normal interval is 0.12-0.20 seconds (3-5 small squares)**

PR Interval: 3 Questions to Ask
1. Are PR Intervals
than 0.20 seconds?
2. Are PR Intervals
than 0.12 seconds?
3. Are the PR Intervals constant across the EKG strip?
What is the PR Interval?
The QRS Complex

STEP 5: The QRS Complex

STEP 5: The QRS Complex
Represents depolarization or contraction of the ventricles

Q wave
First negative or downward deflection of this large complex

R wave

First upward or positive deflection following the P wave (tallest waveform)

S wave
The sharp, negative, or downward deflection that follows the R wave

QRS Complex: 3 Questions to Ask
1. Are QRS intervals
than 0.12 seconds (wide)?

2. Are QRS intervals
than 0.12 seconds (narrow)?

3. Are the QRS complexes similar in appearance across the EKG strip?

ST Segment
Begins with the end of the QRS complex and ends with the onset of the T wave.
If the ST segment is ELEVATED or DEPRESSED myocardial ischemia or injury may be indicated.
T Wave
Produced by ventricular repolarization or relaxation
Commonly seen as the First upward or positive deflection following the QRS Complex
EKG waveforms from sources outside of the heart
Interference seen on a monitor or EKG Strip
4 Causes
1. Patient movement
2. Loose or defective electrodes- fuzzy baseline
3. Improper grounding (60 Cycle interference)
4. Faulty EKG apparatus
Chapter 7
The Sinus Rhythms
Full transcript