Transcript AV node block

Electrocardiography
• A recording of the electrical activity of the heart over time
• Gold standard for diagnosis of cardiac arrhythmias
• Helps detect electrolyte disturbances (hyper- & hypokalemia)
• Allows for detection of conduction abnormalities
• Screening tool for ischemic heart disease during stress tests
• Helpful with non-cardiac diseases (e.g. pulmonary embolism or
hypothermia)
Electrocardiogram (ECG/EKG)
• Is a recording of electrical activity of heart conducted thru ions in
body to surface
ECG Graph Paper
• Runs at a paper speed of 25 mm/sec
• Each small block of ECG paper is 1 mm2
• 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)
• Hence, there are 5 large blocks per second
• Voltage: 1 mm = 0.1 mV between each individual block vertically
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Normal conduction pathway:
SA node -> atrial muscle -> AV node -> bundle of His -> Left and
Right Bundle Branches -> Ventricular muscle
Recording of the ECG:
Leads used:
• Limb leads are I, II, III. So called because at one time subjects
had to literally place arms and legs in buckets of salt water.
• Each of the leads are bipolar; i.e., it requires two sensors on the
skin to make a lead.
• If one connects a line between two sensors, one has a vector.
• There will be a positive end at one electrode and negative at the
other.
• The positioning for leads I, II, and III were first given by
Einthoven. Form the basis of Einthoven’s triangle.
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
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Fig. 13.22b
ECG
• 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
Fig 13.24
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Elements 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
• 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
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. 13.24c
Fig. 13.24d
Elements 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
•QRS complex:
• Represents 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
• 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
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
ST segment:
• Connects the QRS complex and T wave
• Duration of 0.08-0.12 sec (80-120 msec
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
Ischemic Heart Disease
• Is most commonly due to atherosclerosis in coronary
arteries
• Ischemia occurs when blood supply to tissue is
deficient
– Causes increased lactic acid from anaerobic metabolism
• Often accompanied by angina pectoris (chest pain)
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Ischemic Heart Disease
• Detectable by changes in S-T segment of ECG
• Myocardial infarction (MI) is a heart attack
– Diagnosed by high levels of creatine phosphate (CPK) & lactate
dehydrogenase (LDH)
Fig 13.34
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Arrhythmias Detected on ECG
• Arrhythmias are abnormal heart rhythms
• Heart rate <60/min is bradycardia; >100/min is
tachycardia
Fig 13.35
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Arrhythmias Detected on ECG continued
• In flutter contraction rates can be 200-300/min
• In fibrillation contraction of myocardial cells is
uncoordinated & pumping ineffective
– Ventricular fibrillation is life-threatening
• Electrical defibrillation resynchronizes heart by depolarizing all cells at same
time
Fig 13.35
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Arrhythmias Detected on ECG continued
• AV node block occur when node is damaged
• First–degree AV node block is when conduction through AV node >
0.2 sec
– Causes long P-R interval
• Second-degree AV node block is when only 1 out of 2-4 atrial APs can
pass to ventricles
– Causes P waves with no QRS
• In third-degree or complete AV node block no atrial activity passes to
ventricles
– Ventricles driven slowly by bundle of His or Purkinjes
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Arrhythmias Detected on ECG continued
• AV node block occurs when node is damaged
• First–degree AV node block is when conduction
thru AV node > 0.2 sec
– Causes long P-R interval
Fig 13.36
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Arrhythmias Detected on ECG continued
• Second-degree AV node block is when only 1 out of
2-4 atrial APs can pass to ventricles
– Causes P waves with no QRS
Fig 13.36
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Arrhythmias Detected on ECG continued
• In third-degree or complete AV node block, no atrial activity
passes to ventricles
– Ventricles are driven slowly by bundle of His or Purkinjes
Fig 13.36
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Representation in culture
• In TV medical dramas, an isoelectric ECG (no cardiac electrical
activity, aka, flatline, is used as a symbol of death or extreme medical
peril.
• Technically, this is known as asystole, a form of cardiac arrest,
with a partcularly bad prognosis.
• Defibrillation, which can be used to correct arrythmias such as
ventricular fibrillation and pulseless ventricular tachycardia, cannot
correct asystole.