DR.NOHA ELSAYED

Cardiovascular Monitoring

Electrocardiogram

, Cardiovascular System

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Electrocardiography

Electrical activity is recorded by electrocardiogram (ECG)

P wave corresponds to depolarization of SA node

QRS complex corresponds to ventricular depolarization

T wave corresponds to ventricular repolarization

Atrial repolarization record is masked by the larger QRS complex

Cardiovascular System: Intrinsic Conduction System

Chapter 18, Cardiovascular System

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Electrocardiography

Figure 18.16

Isoelectric line

No electrical activity baseline

EKG Records

1. Amount of voltage generated by ht – vertical scale

2. Time required for voltage to travel thru ht – horizontal scale

ECG Interpretation

What is your approach to reading an ECG?1. Rate 2. Rhythm3. Axis4. P wave5. PR interval6. QRS complex7. QT interval 8. ST segment – T wave

EKG Interpretation: 8 criteria

1. What is the rate? Quick estimate:

Count # R waves in a 6 second strip x 10 6 second strip = 3 “tic” marks

Square Counting: 300-150-100-75-60-50-42A

Role of 300

Take the number of “big boxes”between neighboring QRS complexes, and divide this into 300. The result will be approximately equal to the rate

Although fast, this method only works for regular rhythms.

10 Second Rule

As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the EKG and multiply by 6 to get the number of beats per 60 seconds.

This method works well for irregular rhythms.

Rythm

2. Is rhythm regular or irregular? Measure distance between RR interval Measure throughout full 6 second strip

Axis

3.Left or right axis deviation?Look at limb leads I and aVF.Normal: I +, aVF + LAD: I +, aVF – RAD: I -, aVF +

Axis Deviation

QRS-axis

4-P WaveFirst component of a normal ECGRepresents the spread of electrical activity over

the atrium, atrial depolarization. The normal depolarization begins at the sinoatrial (SA) node near the top of the atrium. Because of the top-to-bottom, right-to-left path, the P wave is normally largest in Lead II

Configuration: usually rounded and upright in all leads except R

Amplitude is usually 2-3 mv in any lead

Are P waves present? Should be 1:1

Abnormal rhythms with P waves

5. What is the PR Interval? PRI

PR Interval

Measured from the beginning of the P wave (atrial depolarization) to the beginning of the QRS complex (ventricular depolarization)

Represents the time it takes for an impulse to travel from the SA node through the atria and the AV node

Normally between 0.12 to 0.20 seconds in durationIf prolonged, > 0.20 seconds

indicates conduction delay through the AV node and is called a 1st degree AV block

6. What is the width of QRS complex?

Normal QRS duration is 0.06 to 0.12 seconds

QRS Complex

Represents activation of the ventricles, ventricular depolarization Special conducting bundles spread the wave of depolarization

rapidly over the bundles May have one, two or all three components : Q R S Q wave is the first negative wave after the P wave and before the

R wave. The Q wave represents activation of the ventricular septum

R wave is the first positive wave after the P wave. Most of the ventricle is depolarized during the R wave. S wave is the negative wave after the R wave. Lengthening of the QRS indicates some blockage of the electrical

conduction system either due to ischemia, necrosis of the conducting tissue, electrolyte imbalance or hypothermia

Sinus Rhythm Impulse originates in SA node (normal pacemaker site)

Normal Sinus Rhythm - NSR Regularity – regular Rate - 60 – 100 / minute P waves = 1:1 PRI = .12 - .20 QRS = .12 or <

7-Q-T Interval

QT interval represents total ventricular activity. It is the summation of ventricular depolarization to repolarization

Can vary with heart rateMeasured from the beginning of the Q wave to the end

of the T waveCorrected QT interval (QTc) takes heart rate into

account and provides various normal values based on the rates

Rule of thumb – QT interval should be less than half the preceeding R-R interval

The U wave represents repolarization of the Purkinje fibers but isn’t always seen on paper. A prominent U wave may be caused by hypercalcemia or hypokalemia

8.ST segment & T wave

Normal Values

PR interval: 0.12-0.21 sQRS complex: 0.06-0.1

sQT interval: < 0.44 s- HR dependent, cave:

long QT syndrome!ST segment changes:- significant if > 1mm

in limb leads, > 2mm in precordial leads

Q waves: significant if > 1/3 of

total QRS complex