Electrical%20Activity%20of%20the%20Heart

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Electrical Activity of the Heart
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Introduction

• Where does the electro in electrocardiography
  come from?

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Under this condition, the heart cell is said to
be polarized
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Polarization

• Imagine two micro-electrodes one outside the
  cell, one inside the cell
• Difference between the two equals -90 mV inside
• The cell is said to be polarized

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Action Potential
Depolarization
Repolarization
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closedgates
opened gates
Action Potential in Skeletal Muscle Fiber
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Action Potential
Skeletal
Cardiac
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Myocyte Action Potentials

• Fast and Slow
• Fast non-pacemaker cells
• Slow pacemaker cells (SA and AV node)

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Ions
Ion Extra- Intra-
Na 140 10
K 4 135
Ca 2 0.1
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Action Potential

• Ion influx
• Na channels (fast and slow)
• K channels
• Ca channels

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Inside
Outside
thevirtualheart.org/CAPindex.html
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Action Potential

• Phase 0
• Stimulation of the myocardial cell
• Influx of sodium
• The cell becomes depolarize
• Inside the cell 20 mV

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Action Potential

• Phase 1
• Ions
• Influx of sodium
• Efflux of potassium
• Partial repolarization
• Phase 2
• Ions
• Sodium
• Efflux of potassium
• Influx of calcium
• Plateau

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Action Potential

• Phase 3
• Ions
• Efflux of potassium
• Influx of calcium
• Repolarization (slower process than
  depolarization)
• Phase 4
• Interval between repolarization to the next
  action potential
• Pumps restore ionic concentrations

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Ion 0 1 2 3 4
Na influx influx pump
K efflux efflux efflux pump
Ca influx influx pump
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Refractory Periods

• Absolute refractory period - phase 1 - midway
  through phase 3
• Relative refractory period - midway through phase
  3 - end of phase 3

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SA Node Action Potential

• Funny currents (phase 4) slow Na channels that
  initiate spontaneous depolarization
• No fast sodium channels
• Calcium channels (slow)
• Long-lasting, L-type
• Transient, T-type
• Potassium channels

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Action Potentials

• Fast and Slow

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Action Potentials
It is important to note that non-pacemaker action
potentials can change into pacemaker cells under
certain conditions. For example, if a cell
becomes hypoxic, the membrane depolarizes, which
closes fast Na channels. At a membrane potential
of about 50 mV, all the fast Na channels are
inactivated. When this occurs, action potentials
can still be elicited however, the inward
current are carried by Ca (slow inward
channels) exclusively. These action potentials
resemble those found in pacemaker cells located
in the SA node,and can sometimes display
spontaneous depolarization and automaticity. This
mechanism may serve as the electrophysiological
mechanism behind certain types of ectopic
beats and arrhythmias, particularly in ischemic
heart disease and following myocardial infarction.
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• Conduction speed varies throughout the heart
• Slow - AV node
• Fast - Purkinje fibers

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Action Potential

• ECG records depolarization and repolarization
• Atrial depolarization
• Ventricular depolarization
• Atrial repolarization
• Ventricular repolarization

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The Body as a Conductor
This is a graphical representation of the
geometry and electrical current flow in a model
of the human thorax. The model was created from
MRI images taken of an actual patient. Shown are
segments of the body surface, the heart, and
lungs. The colored loops represent the flow of
electric current through the thorax for a single
instant of time, computed from voltages recorded
from the surface of the heart during open chest
surgery.
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Assignment

• Read Non-pacemaker Action Potentials
• Read SA node action potentials

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Basic ECG Waves

• Chapter 2

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ECG Complexes
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ECG Complexes
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Action Potential Mechanical Contraction
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ECG Paper

• Small boxes 1 mm
• Large boxes 5 mm
• Small boxes 0.04 seconds
• Large boxes 0.20 seconds
• 5 large boxes 1.0 second
• Paper speed 25 mm / sec

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ECG Paper

• Horizontal measurements in seconds
• Example, PR interval .14 seconds (3.5 small
  boxes)

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ECG Paper

• Standardization mark
• 10 mm vertical deflection 1 mVolt

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ECG Paper

• Standardization marks
• Double if ECG is too small
• Half is ECG is too large

Top Low amplitude complexes in an obese women
with hypothyroidism Bottom High amplitude
complexes in a hypertensive man
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ECG Description

• ECG amplitude (voltage)
• recorded in mm
• positive or negative or biphasic

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ECG Waves

• Upward wave is described as positive
• Downward wave is described as negative
• A flat wave is said to be isoelectric
• Isoelectric as describes the baseline
• A deflection that is partially positive and
  negative is referred to as biphasic

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ECG Waves

• P wave
• atrial depolarization
• 2.5 mm in amplitude
• lt 0.12 sec in width
• PR interval (0.12 - 0.20 sec.)
• time of stimulus through atria and AV node
• e.g. prolonged interval first-degree heart block

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ECG Waves

• QRS wave
• Ventricle depolarization
• Q wave when initial deflection is negative
• R wave first positive deflection
• S wave negative deflection after the R wave

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ECG Waves

• QRS
• May contain R wave only
• May contain QS wave only
• Small waves indicated with small letters (q, r,
  s)
• Repeated waves are indicated as prime

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ECG Waves

• QRS
• width usually 0.10 second or less

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ECG Waves

• RR interval
• interval between two consecutive QRS complexes

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ECG Waves

• J point
• end of QRS wave and...
• ...beginning of ST segment
• ST segment
• beginning of ventricular repolarization
• normally isoelectric (flat)
• changes-elevation or depression-may indicate a
  pathological condition

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ECG Waves
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ECG Waves

• T wave
• part of ventricular repolarization
• asymmetrical shape
• usually not measured

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ECG Waves

• QT interval
• from beginning of QRS to the end of the T wave
• ventricular depolarization repolarization
• length varies with heart rate (table 2.1)
• long QT intervals occur with ischemia,
  infarction, and hemorrhage
• short QT intervals occur with certain medications
  and hypercalcemia

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ECG Waves

• QT interval should be less than half the R-R
  interval
• If not, use Rate Corrected QT Interval
• normal 0.44 sec.

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ECG Waves
FYI

• Long QT interval
• certain drugs
• electrolyte distrubances
• hypothermia
• ischemia
• infarction
• subarachnoid hemorrhage
• Short QT interval
• drugs or hypercalcemia

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ECG Waves

• U Wave
• last phase of repolarization
• small wave after the T wave
• not always seen
• significance is not known
• prominent U waves are seen with hypokalemia

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Heart Rate Calculation
1. 1500 divided by the number of small boxes between two R waves most accurate take time to calculate only use with regular rhythms
2. 300 divided by the number of large boxes between two R waves quick not too accurate only use with regular rhythm
3. Number of large squares w/i RR interval 3. Number of large squares w/i RR interval
1 lg sq 300 bpm 2 lg sq 150 bpm 3 lg sq 100 bpm 4 lg sq 75 bpm 5 lg sq 60 bpm 6 lg sq 50 bpm
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Heart Rate Calculation

• For regular rhythm...
• Count the number of large boxes between two
  consecutive QRS complexes. Divide 300 by that
  number
• 300 4 75
• Count the small boxes. Divide 1500 by that number
• 1500 20 75

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Heart Rate Calculation

• For irregular rhythms
• Count the number of cardiac cycles in 6 seconds
  and multiple this by 10. (Figure 2.15)

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The ECG as a Combination of Atrial and
Ventricular Parts

• Atrial ECG P wave
• Ventricular ECG QRS-T waves
• Normally, sinus node paces the heart and P wave
  precedes QRS
• P-QRS-T
• Sometimes, atria and ventricles paced separately
  (e.g. complete heart block)

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ECG in Perspective

1. ECG recording of electrical activity not the
   mechanical function
2. ECG is not a direct depiction of abnormalities
3. ECG does not record all the hearts electrical
   activity

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Questions

• End of chapter 2, questions 1-5 and 7.