Electrocardiography

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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

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Electrocardiogram (ECG/EKG)

• Is a recording of electrical activity of heart
  conducted thru ions in body to surface

Fig 13.22a
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• 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 -gt atrial muscle -gt AV node -gt bundle
of His -gt Left and Right Bundle Branches -gt
Ventricular muscle
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• Recording of the ECG
• Leads used
• Limb leads are I, II, II. 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
  Einthovens triangle.

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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
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Fig. 13.22b
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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

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• 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

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Fig. 13.24b
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Fig. 13.24c
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Fig. 13.24d
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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

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• 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

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• 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

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Fig. 13.24g
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• 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

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• 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

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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)

Click here to play Myocardial Infarction RealMedia
Movie
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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 lt60/min is bradycardia gt100/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
• Firstdegree AV node block is when conduction
  through AV node gt 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
• Firstdegree AV node block is when conduction
  thru AV node gt 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.