ecg

Electrocardiogram :

Electrocardiogram Dr Humaira Kamal

Learning Objectives:

Learning Objectives Enumerate components of normal ECG Describe the significance of each component of ECG Describe the time calibration of ECG Draw normal ECG Determine Heart rate in normal ECG Describe the conduction of electric current during recording of normal ECG

ECG:

ECG The ECG records the electrical signal of the heart as the muscle cells depolarize (contract) and repolarize. Electrical currents generated by cardiac muscle during depolarization & repolarization spread into the tissues surrounding the heart and are conducted through the body fluids

ECG Signal:

ECG Signal Heart behaves as a syncytium: a propagating wave once initiated continues to propagate uniformly into the region that is still at rest. Signal will propagate along conduction paths – sinus node – AV node – bundle branches – Purkinjie fibers

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Depolarization Waves Versus Repolarization Waves

Slide 6:

Return of the potential to the baseline is caused by repolarization. No potential is recorded in the electrocardiogram when the ventricular muscle is either completely polarized or completely depolarized

Slide 7:

Only when the muscle is partly polarized and partly depolarized does current flow from one part of the ventricles to another part Current also flows to the surface of the body to produce the electrocardiogram

Flow of electrical currents in the chest around the heart:

Flow of electrical currents in the chest around the heart Before stimulation, all the exteriors of the muscle cells had been positive and the interiors negative. Heart is actually suspended in a conductive medium. When one portion of the ventricles depolarizes and becomes electronegative with respect to other ECG is rcorded

Flow of current in partially depolarized ventricles:

Flow of current in partially depolarized ventricles The cardiac impulse first arrives in the ventricles in the septum spreads to the inside surfaces of the remainder of the ventricles electrical current flows through the fluids surrounding the ventricles along elliptical paths Average current flow occurs with - tivity toward the base of the heart and with positivity toward the ap ex

Slide 10:

3 distinct waves are produced during cardiac cycle P wave caused by atrial depolarization QRS complex caused by ventricular depolarization T wave results from ventricular repolarization ECG Fig 13.24 13-63

Components of the ECG :

Components of the ECG P wave : Depolarization of both atria Relationship between P and QRS helps distinguish various cardiac arrhythmias Shape and duration of P may indicate atrial enlargement PR interval : from onset of P wave to onset of QRS Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes) Represents atria to ventricular conduction time (through His bundle) Prolonged PR interval may indicate a 1st degree heart block

Components of the ECG :

Components of the ECG QRS complex: Ventricular depolarization Larger than P wave because of greater muscle mass of ventricles Normal duration = 0.08-0.12 seconds Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc. Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

Components of the ECG:

Components of the ECG ST segment: Connects the QRS complex and T wave Duration of 0.08-0.12 sec (80-120 msec T wave: Represents repolarization or recovery of ventricles Interval from beginning of QRS to apex of T is referred to as the absolute refractory period QT Interval Measured from beginning of QRS to the end of the T wave Normal QT is usually about 0.40 sec QT interval varies based on heart rate

Fig. 13.24b:

Fig. 13.24b

Fig. 13.24c:

Fig. 13.24c

Fig. 13.24d:

Fig. 13.24d

Slide 20:

PR interval : From onset of P wave to onset of QRS Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes) Represents atria to ventricular conduction time (through His bundle) Prolonged PR interval may indicate a 1st degree heart block

Fig. 13.24g:

Fig. 13.24g

Slide 22:

P-R interval 0.16 sec P T R Q S Q-T interval 0.35 sec Atrial depolarization Ventricular depolarization Ventricular repolarization P R Q S T Normal EKG

Slide 23:

The septum and endocardial areas of the ventricular muscle depolarize first Greatest portion of ventricular muscle mass to repolarize first is the entire outer surface of the ventricles, especially near the apex of the heart Caused by the high blood pressure inside the ventricles during contraction

Electrocardiographic Leads limb leads:

Electrocardiographic Leads limb leads Three Bipolar Limb Leads= “bipolar” means that the ECG is recorded from two electrodes located on different sides of the heart i.e. on the limbs “lead” is not a single wire connecting from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph

Types of ECG Recordings:

Types of ECG Recordings Bipolar leads record voltage between electrodes placed on wrists & legs (right leg is ground) Lead I records between right arm & left arm Lead II : right arm & left leg Lead III : left arm & left leg Fig 13.23 13-61

Einthoven’s Triangle :

Einthoven’s Triangle is drawn around the area of the heart illustrates that the two arms and the left leg form apices of a triangle surrounding the heart

Chest Leads (Precordial Leads) :

Chest Leads ( Precordial Leads) electrocardiograms are recorded with one electrode placed on the anterior surface of the chest directly over the heart Electrode is connected to the positive terminal of the electrocardiograph, and the negative electrode, called the indifferent electrodelectrode, is connected through equal electrical resistances to the right arm,left arm, and left leg all at the same time

Precordial Leads:

Precordial Leads Adapted from: www.numed.co.uk/electrodepl.html

Precordial Leads:

Precordial Leads

Augmented Unipolar Limb Leads :

Augmented Unipolar Limb Leads Two of the limbs are connected through electrical resistances to the negative terminal of the electrocardiograph The third limb is connected to the positive terminal

ECG Graph Paper:

ECG Graph Paper Runs at a paper speed of 25 mm/sec Each small block of ECG paper is 1 mm 2 At a paper speed of 25 mm/s, one small block equals 0.04 s Five small blocks make up 1 large block which translates into 0.20 s (200 msec ) there are 5 large blocks per second Voltage: 1 mm = 0.1 mV between each individual block vertically

Slide 34:

Each inch in the horizontal direction is 1 second Each inch is usually broken into five segments by dark vertical lines The intervals between these dark lines represent 0.20 second 0.20 second intervals are then broken into five smaller intervals by thin lines, each of which represents 0.04 second

Normal ECG Signal:

Normal ECG Signal

ECG:

ECG

Determining the Heart Rate:

Determining the Heart Rate Rule of 300 10 Second Rule

Rule of 300:

Rule 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.

What is the heart rate?:

What is the heart rate? (300 / 6) = 50 bpm www.uptodate.com

What is the heart rate?:

What is the heart rate? (300 / ~ 4) = ~ 75 bpm www.uptodate.com

What is the heart rate?:

What is the heart rate? (300 / 1.5) = 200 bpm

Slide 42:

Well begun is work half done Thank you