Download Transcripts/4_10 10-12 (pt.3) (McNicholas)

Cardio: 11:00 - 12:00 (3rd portion)
Scribe: Ryan O’Neill
Friday, April 10, 2009
Proof: Dr. McNicholas
Dr. McNicholas
ECG
Page 1 of 4
ECF – extracellular fluid
I. Introduction [S44]:
II. William Einthoven [S45]
a. Not testable material.
III. THE ELECTROCARDIOGRAM (ECG) [S46]
a. ECG is a standard non-invasive clinical tool to measure the electrical activity of the heart
b. The thorax acts as a volume conductor (a big bag of salt solution).
c. The movement of action potentials through cardiac myocytes cause minute voltages that transmits through the
bag of salt and can be detected on the surface of the thorax because of the electrical conduction.
IV. Info on ECGs [S47-48]
a. When one region of the heart is depolarized (the inside is positive; i.e. ECF –ve) and one is polarized (the inside
if negative; i.e. ECF +ve) a dipole (a separation of charge in space) is created and current will flow within the
volume conductor.
b. Thus there is a vector between two points which has both magnitude and direction.
i. The magnitude will change based upon the difference of the charges.
c. The ECG provides spatial information about the electrical activity of the heart.
d. The intensity of the signal detected by surface electrodes depends on the orientation of the electrodes placed on
the body surface with respect to the electrical field.
e. IMP: The amplitude of the signal will depend on the mass of tissue.
i. The greater the mass of tissue the greater the deflection from the isoelectric line you will observe.
f. The ECG shows the potential difference between two points - a difference (i.e. a deflection on the wave form of
the ECG) that can only be detected when electrical activity is changing.
V. Information that can be obtained from an ECG [S49]
a. Anatomical orientation of the heart
b. Relative size of the chambers of the heart
i. Remember: the mass of tissue will generate a greater deflection on the ECG
c. Variety of electrical disturbances of rhythm and conduction
i. The length of duration of certain portions of the ECG will tell you more about the action potentials that are
transmitting from one area of the heart to another and if there is a delay in this transmission.
d. The extent, location and progress of ischemic damage to the myocardium
e. Effects of altered electrolyte concentrations
f.
Influence of drugs on the electrical activity of the heart
g. NOTE: The ECG gives no direct information regarding the mechanical activity of the heart!
i. It tells us about the electrical activity of the heart.
h.
VI. Front vs. Horizontal Planes [S50]
a. With a standard ECG there are 3 electrodes placed on the extremities of the left arm, right arm and left leg (a
fourth on the right leg forms the ground electrode).
b. There are also 6 leads that are placed across the chest and the electrodes electrically form different leads that
are determined by the ECG machine.
c. Don’t worry about the configuration of the electrode combinations that forms these leads. Just know that the
ECG interprets the activity in the electrode verses another will give you an electrical view of the heart either in
the frontal plane or a view of the horizontal plane view.
d. Precordial leads – look through the ventricles of the heart in the horizontal plane.
e. IMP: Leads are combinations of the electrodes determined by the electronics of the ECG machine.
i. A 12-lead ECG (with only 3 recording limb electrodes and 6 chest electrodes placed on a person it typical),
and it is the combination of the electrodes that form the leads.
VII. Fluctuations [S51]
a. Memorize the electrical events occurring in the heart that are and observed in the ECG!!!!!
b. The fluctuations of the waveform above and below an isoelectric line (no changing electrical activity) in
extracellular voltage recorded by each lead are called waves. The shape and magnitude will differ in each lead
because each lead has its own unique view of the electrical activity of the heart.
c. If 2 electrodes are detecting electrical activity and there is no potential difference, then no activity will be
detected and the isoelectric line will be obtained.
d. P wave – atrial depolarization
i. Remember: tissue mass is very important in the detection of the amplitude of the waveform.
ii. Remember: electrical activity is occurring in the SA node, AV node and Purkinje fibers, but if you think of
the relative abundance of those tissues compared to the atrial or ventricular myocardium there are many
Cardio: 11:00 - 12:00 (3rd portion)
Scribe: Ryan O’Neill
Friday, April 10, 2009
Proof: Dr. McNicholas
Dr. McNicholas
ECG
Page 2 of 4
more myocardial cells than the pacemaker cells and so there is not sufficient numbers of those to generate
electrical activity that can be detected on a standard ECG.
e. No deflection in the waveform is seen that relates to the activity in the SA node.
f. Electrical activity is seen in the atria as the waveform of excitation moves down from the SA node down through
to the atria and that causes an upper deflection of the ECG.
g. The waveform appearance depends on which lead you are looking at that moment.
h. Note the diagram of lead # II (standard wave often used)
i. As the waveform moves down through the atria is converges again on the AV node, there is a slowing down at
the end of the P wave and the beginning of the Q wave. There is no electrical activity detected because there
isn’t sufficient activity in the AV node to cause a deflection that can be detected on the surface of the skin.
j. The time that occurs between the P wave and Q wave can tell you about the conduction time. It also tells you
about electrical activity and massive of tissue. The longer the duration, the longer it is taking for that activity to
move down through the AV node.
k. QRS complex – ventricular depolarization
i. The waveform moves down the septum and then up the ventricular myocardium.
ii. As the waveform moves straight at the combination of the electrodes the waveform causes an upward
deflection.
l. T wave – ventricular repolarization
VIII. Graph [S52]
a. The upstroke of the action potential in the ventricles is a deflection and repolarization of the ventricles.
b. Phase 3 – T wave – repolarization
c. Overall action potential duration can be estimated based on the length of time is takes to go through the QRS
complex and to the end of the T wave.
d. U wave is not always seen on a normal ECG.
IX. Einthoven’s triangle [S53] DO NOT WORRY ABOUT THIS- it is historical information to explain the principle of
the ECG. I used it to illustrate the difference in waveform appearance of the same electrical event viewed by
the different leads. This is the important 'take-home' message
a. Einthoven's triangle - An imaginary equilateral triangle with the heart at its centre, its equal sides representing
the three standard limb leads of the electrocardiogram.
b. Einthoven's law (electrical, mathematical relationship between the different electrodes) states that any
two of the three bipolar limb leads determine the third one with mathematical precision.
c. I + III = II
d. The waveform you look at will be different depending upon the lead that you evaluate.
X. Electrical axis of the heart [S54]
a. Electrical axis of the heart can also be determined based upon where the predominant vector of electrical
activity is occurring.
b. A normal electrical axis of the heart has the general vector moving down to about 40 degrees.
c. The different ECG lead configurations will give you an electrical view:
i. – 60 degrees = lead 3 and lead AVR
ii. 30 and 0 degrees = lead AVF or lead 1
iii. Don’t worry about these specific angles, just realize that each of these leads will look down on a circle of
axis and a mathematical interpretation will give you an overall general vector of electrical axis and if that
electrical axis is shifted in any way, then that will tell you about changes that have occurred within the
heart. Different leads look at different angles of the electrical activity of the heart. Again don't worry about
learning this, the 'take-home' is that a change in the electrical axis calculated from analyzing the ECG
recording can tell you about the underlying pathology
XI. Waves and Electrodes [S55]
a. A WAVE OF DEPOLARIZATION MOVING TOWARDS A ( + ) ELECTRODE = POSITIVE DEFLECTION
b. A WAVE OF DEPOLARIZATION MOVING AWAY FROM A ( + ) ELECTRODE = NEGATIVE DEFLECTION
c. A WAVE OF REPOLARIZATION MOVING TOWARDS A ( + ) ELECTRODE = NEGATIVE DEFLECTION
d. A WAVE OF REPOLARIZATION MOVING AWAY FROM A ( + ) ELECTRODE = POSITIVE DEFLECTION
e. A WAVE OF DEPOLARIZATION OR REPOLARIZATION TRAVELING PERPENDICULAR TO AN
ELECTRODE AXIS WILL RESULT IN NO DEFLECTION ON ECG.
XII. Example: The P wave [S56]
a. P wave – atrial depolarization
b. Pathway – the waveform moves down from the SA node in right atria through the right atria down through the
bundle branches and out of the Purkinje system.
c. The overall electrical vector is pointed directly at lead II.
d. Lead II will have the tallest P wave (depolarization) because the mean P wave vector is moving directly at it.
Cardio: 11:00 - 12:00 (3rd portion)
Scribe: Ryan O’Neill
Friday, April 10, 2009
Proof: Dr. McNicholas
Dr. McNicholas
ECG
Page 3 of 4
e. Lead aVR will have a negative P wave because the mean P wave vector is moving away from it.
f. Lead III or aVL will have the smallest P wave or no deflection at all observed because the mean P wave vector
is moving at approximately right angles to it.
XIII. Summary of cardiac electrical activity [S57]
a. skipped
XIV.
Cardiac Excitation Related to the ECG [S58]
a. Wave form initiates in the SA node, moves down through the atria, deflection on the ECG occurs relative to the
depolarization of the atria.
i. It is important to note that there is no waveform on a standard 12-lead ECG that represents repolarization
of the atria.
b. As the atria completely depolarizes and the waveform converges on the AV node, then there is no electrical
activity detected and the electrical activity returns to the isoelectric point
c. As the waveform moves down the bundle fibers and out into the ventricular myocytes the QRS complex occurs.
d. Memorize the next two slides!
XV. The Normal ECG Pattern [S59-60]
a. P wave: the sequential activation (depolarization) of the right and left atria
b. QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously due
to the fast conduction of the purkinje fibres transmitting the electrical waveform to all areas of the ventricular
myocardium)
c. T wave: ventricular repolarization
d. U wave: usually not present; origin for this wave is not clear - but probably represents "after depolarizations" in
the ventricles
e. PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS
complex)
f. Note: atrial repolarization not observed on the ECG – masked by QRS (ventricular depolarization of ventricular
myocytes)
g. QRS duration: duration of ventricular muscle depolarization
h. PR segment: The period of time between the P wave and the QRS complex.
i. Due to conduction through the AV node.
ii. If you slow the conduction, you increase the PR segment.
i. QT interval: duration of ventricular depolarization and repolarization
j. RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate)
k. PP interval: duration of atrial cycle (an indicator of atrial cycle/rate)
l. Remember: no deflection from the isoelectric line during segments; deflection on intervals.
m. If you memorize the waveform of the ECG and know whether it is called P, Q, R or S, you can actually figure
out if you were asked a question, “ What’s a QT interval?”
n. Intervals have to include an actual waveform.
o. AS AN EXAMPLE: If you evaluate the duration from the beginning of the QRS complex and until the end of the
T wave it gives an idea of how long a ventricular action potential takes - it gives an indication of alterations in the
underlying channel activity. If this is prolonged, for example, it tells you that there may be a problem with one of
the K channels involved in the repolarization phase of the action potential
XVI.
Interpretation of the ECG [S61]
a. Memorize this standard calibration of an ECG.
b. 5 mm (represents a particular amplitude (0.5 mV) on the Y-axis and time duration (0.2 seconds) on the X-axis.
c. When looking at an ECG:
i. Examine waves: height of the signal
ii. Examine intervals, durations and segments
iii. Determine the RATE (beats/min)
XVII. ECG rhythm interpretation [S62]
1. Is there a P wave associated with each QRS?
- A dissociation between the two it would indicate that the conduction pathway is not working properly
- No P waves present would indicate a malfunctioning SA node firing and atria excitation isn’t occuring
2. What is the PR interval. Is it constant?
3. Is the QRS duration normal?
- An increase in the height would indicate ventricular hypertrophy, a change in mass.
4. What is the ventricular rate (is the R-R interval constant) i.e. regular vs. irregular?
- Could indicate problems in the SA node (pacing/cycling activity)
Cardio: 11:00 - 12:00 (3rd portion)
Friday, April 10, 2009
Dr. McNicholas
XVIII.
ECG
Scribe: Ryan O’Neill
Proof: Dr. McNicholas
Page 4 of 4
Normal Sinus Rhythm [S63]
a. Calculating the rate of the heartbeat from an ECG.
b. The interval between the R waves = cycle length. LEARN THE EQUATION TO CONVERT R-R INTERVAL TO
RATE
XIX.
Arrhythmia (dysrhythmia) [S64]
a. DEFINITION: “Any change in rhythm from normal sinus rhythm”
b. RATE: too fast = tachyarrhythmia (>100 bpm)
c.
too slow = bradycardia (<60 bpm)
i. Some rates can be physiologically normal under particular circumstances (e.g. panic attacks)
d. REGULARITY: one or more beats occurring earlier or later than expected
e. SEQUENCE of cardiac activation
i. If you have a problem with the normal conduction pathway and sequence of activation
ii. Any change in this activation pathway can cause arrhythmias.
iii. SA node  atria  AV node  Purkinje fibers  ventricular muscle
XX. Why arrhythmias occur: [S65]
a. Hypoxia
b. Ischemia
c. Sympathetic stimulation
i. Overdrive of the sympathetic nervous system.
d. Drugs
e. Electrolyte disturbances
f.
Bradycardia
g. Stretch
h.
XXI.
Two basic problems give rise to cardiac arrhythmias [S66]
a. Impulse propagation (altered conduction)
b. Impulse formation (altered automaticity)
XXII. Four basic types of arrhythmia [S67]
a. Not covered because of time constraints. Might want to look over on your own.
[end 25:24 min]