Download Phys chapter 11

Phys Ch 11
The Normal Electrocardiogram
 When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the
adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the
body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the
current can be recorded; the recording is known as an electrocardiogram
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Normal electrocardiogram
Consists of P wave, QRS complex, and T wave – QRS complex can be described as Q wave, R wave, and S wave
o P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction
begins (depolarization wave)
o QRS complex is caused by potentials generated when the ventricles depolarize before contraction (as
the depolarization wave spreads through the ventricles) (depolarization wave)
o T wave is caused by potentials generated as the ventricles recover from the state of depolarization
(repolarization wave)
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In part A above, depolarization (positive charges inside and negative charges outside) is traveling from
left to right. The first half of the fiber has already depolarized, while the remaining half is still polarized.
Therefore, the left electrode on the outside of the fiber is in an area of negativity, and the right
electrode is in an area of positivity; this causes the meter to record positively
In part B, depolarization has extended over the entire muscle fiber, and the recording to the right has
returned to the zero baseline because both electrodes are now in areas of equal negativity
Part C shows halfway repolarization of the same muscle fiber, with positivity returning to the outside of
the fiber. At this point, the left electrode is in an area of positivity, and the right electrode is in an area
of negativity
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In part D, the muscle fiber has completely repolarized, and both electrodes are now in areas of positivity
so that no potential difference is recorded between them
o Above, Monophasic action potential from a single ventricular muscle fiber during normal cardiac
function, showing rapid depolarization and then repolarization occurring slowly during the plateau stage
but rapidly toward the end. Below, Electrocardiogram recorded simultaneously
o No potential is recorded in the electrocardiogram when the ventricular muscle is either completely
polarized or completely depolarized – only when muscle is partly polarized and partly depolarized does
current flow from one part of ventricles to another part and therefore current also flows to surface of
body to produce electrocardiogram
P wave occurs at beginning of contraction of atria, QRS complex occurs at beginning of contraction of ventricles,
and ventricles remain contracted until after repolarization (after end of T wave)
Atria repolarize 0.15-0.20 sec after termination of P wave (when QRS complex is being recorded), so atrial
repolarization cannot be seen well
Repolarization of ventricular muscle begins to repolarize about 0.20 sec after beginning of depolarization wave
(QRS complex), but can be as long as 0.35 sec – this is why T wave is a prolonged wave, but voltage is
considerably less than QRS complex
10 horizontal lines on the paper represent 1 mV, and about 25 mm = 1 sec in horizontal direction (dark lines
represent 0.20-second intervals, so each thin line is 0.04 sec)
Normal voltage spread of QRS complex
o 3-4 mV when one electrode placed directly over the ventricles and the second electrode placed
elsewhere (far from the heart)
o 110 mV if recorded directly at heart muscle membrane
o 1.0-1.5 mV if electrodes are placed on separate arms or an arm and a leg
Time between beginning of P wave and beginning of QRS complex is the P-Q interval – interval between
beginning of electrical excitation of atria and beginning of excitation of ventricles – normally about 0.16 sec –
can also be called the P-R interval because the Q wave is likely to be absent
Q-T interval – duration of contraction of ventricles, which last from beginning of Q wave (or R wave, if Q absent)
until end of T wave – usually about 0.35 sec
Heart rate can be determined by calculating 60/[time between 2 QRS complexes], so in an adult, it would be
normal to find time between QRS complexes to be 0.83 sec, so their heart rate would be 60/0.83 or 72 bpm
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Flow of current in chest around partially depolarized ventricles shown above
Average current flow occurs with negativity toward the base of the heart and positivity toward the apex
Immediately after depolarization has completed its course through the ventricles, the average direction of
current flow reverses for about 0.01 sec, flowing from the ventricular apex toward the base, because the last
part of the heart to become depolarized is the outer walls of the ventricles near the base of the heart
In normal heart ventricles, current flows from negative to positive primarily in direction from base of heart
toward apex, and if a meter is connected to electrodes on the surface of the body, the electrode nearer the base
will be negative, whereas the electrode nearer the apex will be positive, and the recording meter will show a
positive recording in the electrocardiogram
Three bipolar limb leads – shown below – standard bipolar limb leads – bipolar means that electrocardiogram is
recorded from 2 electrodes located on different sides of the heart (in this case, on the limbs) – lead readings are
always read (+) terminal connection minus (-) terminal connection
Einthoven’s triangle (shown on figure above) – drawn around the area of the heart and illustrates that the two
arms and left leg form apices of a triangle surrounding the heart, and the upper 2 apices are the points at which
the two arms connect electrically with the fluids around the heart, and the lower apex is the point at which the
left leg connects with the fluids
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Einthoven’s Law – if the electrical potentials of any two of the three bipolar limb electrocardiographic leads are
known at any given instant, the third one can be determined mathematically by simply summing the first two
Because recordings from all bipolar limb leads similar, it does not matter greatly which lead is recorded when
one wants to diagnose different cardiac arrhythmias
o However, when one wants to diagnose damage in the ventricular or atrial muscle or in Purkinje fibers, it
matters which leads are recorded because abnormalities of cardiac muscle contraction or cardiac
impulse conduction do change the patterns of the electrocardiograms markedly in some leads yet may
not affect other leads
Chest leads (precordial leads) – recorded with one electrode placed on anterior surface of chest directly over
heart at one of the points shown below (this is the positive electrode) and the negative electrode (indifferent
electrode) is connected through equal resistances to right arm, left arm, and left leg – usually 6 standard chest
leads are recorded, one from each of the points shown below, and recordings are labeled V1-V6 – normal
recordings for these leads are shown below as well – remember V1 and V2 show negatively because they are
closer to the base of the heart than the apex
Augmented unipolar limb leads – two of the limbs are connected through electrical resistances to the negative
terminal of the electrocardiograph, and the third limb is connected to the positive terminal – when the positive
terminal is on the right arm, the lead is known as aVR lead, when it’s on left arm, it’s the aVL lead, and when it’s
on the leg, it’s the aVF lead