Basic Electrophysiology
-Types of cardiac cells
-Myocardial cells- working or mechanical cells
-Pacemaker cells- responsible for spontaneous generation and
conduction of impulses
-Electrical impulses are the result of brief but rapid flow of ions back and
forth across the cell membrane
-The major electrolytes that that affect cardiac function are sodium,
potassium, and calcium
-Membrane potential- difference in electrical charge across the cell
membrane
-Threshold- membrane potential at which cell membrane will depolarize and
generate an action potential
-Action potential- reflection of the difference in the concentration of ions
across a cell membrane at any given time
-Types of action potentials
-Fast-response- cells of the atria, ventricles, and purkinje fibers,
occurs because of the presence of many voltage-sensitive
sodium channels that allow a rapid influx of sodium when these
channels are open and prevent influx when they are closed
-slow-response- cells of the SA and AV node have slow calcium and
sodium channels, can also abnormally occur anywhere in the
heart (Usually secondary to ischemia, injury, or an electrolyte
imbalance)
-Polarization- resting state during which no electrical activity occurs, the cell
is negatively charged
-Depolarization- when the cardiac cell is stimulated, the cell is positively
charged
-Repolarization- the returning back to a resting state, the cell is returned
back to a negative charge
Phases of the cardiac action potential
Phase 0- rapid depolarization; sodium moves rapidly into the cells,
potassium leaves the cells, calcium slowly moves into the cells,
the cell depolarizes and cardiac contraction begins
Phase 1- early repolarization; the flow of sodium into the cell is
slowed down
Phase 2- repolarization (plateau phase); there is still a slow inward
movement of calcium into the cells and a continued outward
movement of potassium from the cells
Phase 3- rapid repolarization; the flow of calcium, potassium, and
sodium has stopped
Phase 4- resting membrane potential; the sodium-potassium pump
actively transports sodium out of the cell and potassium back in
the cell
-Refractoriness- extent to which a cell is able to respond to a stimulus
-Absolute refractory period- cardiac cells cannot be stimulated to conduct an
electrical impulse; corresponds with the onset of the QRS to the mid
point of the T wave
-Relative refractory period- some cardiac cells have repolarized to their
threshold potential and can be stimulated to respond to a stronger than
normal stimulus; corresponds with the mid point of the T wave to the
end of the T wave
-Properties of cardiac cells
-Automaticity- the ability for a cardiac cell to spontaneously create an
impulse
-Excitability- the ability for a cardiac cell to respond to an external
stimulus
-Conductivity- the ability for a cardiac cell to receive an impulse and
conduct that impulse to an adjacent cardiac cell
-Contractility- the ability for cardiac cells to contract to electrical
stimulus
-Conduction System
-Sinoatrial node (SA node) - is the normally the primary pacemaker of
the heart, initiating impulse at a regular rate of 60-100 beats a
minute; it is located in the upper posterior portion of the right
atrium at the junction of the superior vena cava and the right
atrium and it receives it blood supply from the right coronary
artery
-Atrioventricular junction
-AV node- is located in the posterior septal wall of the right
atrium and it receives it blood supply from the left
circumflex artery
-Bundle of His- it is located in the upper potion of the
interventricular septum and connects the AV node with
the two bundle branches; it receives it blood supply from
left anterior and posterior descending coronary arteries;
has the capability of discharging an electrical impulse of
40-60 beats a minute
-Right and left bundle branches- conduct the impulse to the purkinje fibers
in the ventricles
-Purkinje Fibers- an elaborate web of fibers distributed throughout the
ventricular myocardium
Causes of dysrhythmias
-Enhanced automaticity- an abnormal condition in which cardiac cells
not normally associated with the property of automaticity begin
to depolarize spontaneously or when escape pacemaker sites
increase their firing rate beyond that which is considered
normal
-Reentry- Propagation of an impulse through tissue already activated
by the same impulse
-Escape beats or rhythms- when the sinus node slows down or fails to
initiate depolarization and a lower pacemaker site
spontaneously produces electrical impulses, assuming
responsibility for pacing the heart
-Conduction disturbances-can result from trauma, drug toxicity,
electrolyte disturbances, myocardial ischemia, or infarction
-Electrodes- are used for surface (skin) electrocardiography
-Leads- is a record of electrical activity between two electrodes
-Frontal Plane leads- view the heart from the front of the body;
there are six of these leads (I, II, III, aVR, aVL, and aVF)
-Horizontal plane leads- view the heart as if the body were
sliced in half horizontally at the heart; there are six of
these leads (V1, V2, V3, V4, V5, and V6)
-each lead has a negative, positive, and a pole
-if the electrical impulse moves toward the positive electrode, the
waveform would be upright, if the electrical impulse moves
away from the positive electrode, the waveform would be
inverted
-there are three types of leads
-Standard limb leads- if an electrode is placed on the right arm,
left arm, and left leg, three leads are formed. An imaginary line
joining the positive and negative electrodes of a lead is called
the axis of the lead. The axes of these three limb leads form an
equilateral triangle with the hear at the center (Einthovens’s
triangle)
-Lead I- records the electrical potential between the left
arm (+) and the right arm (-)
-Lead II- records the electrical potential between the left
leg (+) and the right arm (-)
-Lead III- records the electrical potential between the left
leg (+) and the left arm (-)
Augmented limb leads- are augmented limb leads that are
unipolar consisting of only one electrode
-Lead aVR- views the heart from the right shoulder
-Lead aVL- views the heart form the left shoulder
-Lead aVF- views the heart from the left foot
Precordial (chest) leads- are unipolar leads that view the heart
in the horizontal plane
-V1- located in the fourth intercostals space, just to the
right of the sternum
-V2- located in the fourth intercostals space, just to the
left of the sternum
-V3- located on a line midway between V2 and V4
-V4- located in the left midclavicular line in the fifth
intercostals space
-V5- located in the anterior midaxillary line in the fifth
intercostals space
-V6- located in the midaxillary line in the fifth
intercostals space
Right precordial leads
-V1R- V2
-V2R- V1
-V3R- midway between V2r and V4R
-V4R- located in the right midclavicular line in the
fifth intercostals space
-V5R- located in the anterior midaxillary line in
the fifth intercostals space
-V6R- located in the midaxillary line in the fifth
intercostals space
-ECG paper -the horizontal axis on the ecg paper corresponds with time,
each 1mm box equals 0.04 seconds; the lines between every 5
small boxes equal 0.20 seconds; every 5 large boxes equal 1
second; 15 large boxes equal 3 seconds; 30 large boxes equal 6
seconds
-the vertical axis represents voltage or amplitude, the vertical
axis of one small box represents 1 mm
-Waveforms
-P wave- represents depolarization and the spread of the electrical
impulse throughout the right left atria; a normal P wave appear
smooth, rounded, and upright in lead II
-PR segment- represents the conduction of the impulse through the
AV node, bundle of His, right and left bundle branches, and the
purkinje fibers
-PR interval- represents the depolarization and the spread of the
electrical impulse throughout the right left atria and the spread
of the impulse through the AV node, bundle of His, right and
left bundle branches, and the purkinje fibers
-QRS complex- represents depolarization of the ventricles; it consists
of the Q wave, R wave, and the S wave
-Q wave- begins as a downward deflection; the Q wave is
always a negative waveform
-R waves- the QRS complex continues as a large, upright,
triangular waveform known as the R wave; the R wave is
always positive
-S wave- the S wave is the negative waveform following the R
wave; the S wave is always negative
Variations of the QRS complex- although the term QRS
complex is used, not every QRS complex contains Q
wave, R wave, and S wave
-ST segment- represents the early part of repolarization of the
ventricles. The term is regardless weather the QRS ends in a Q,
R, or S wave.
-normal characteristics are for the ST segment to be within 1
mm above or below the PR segment
-the point where the QRS complex and the ST segment meet is
called the junction or J point
-T wave- represents ventricular repolarization. The Lead II the T
wave is positive and bigger than the P wave
-QT interval- represents total ventricular activity
-U wave- small waveform when seen follows the T wave, the
mechanism is unknown
-Artifact- distortion of an ECG tracing by electrical activity that is
noncardiac in origin
-Loose electrode
-Patient movement/muscle activity
-60-cycle (AC) interference
Rate measurement
-Six second count- count the number of QRS complexes within a 6
second period, then multiply that number by 10
-Large boxes- count the number of large boxes between two
consecutive QRS complexes, and divide that number by 300
-Small boxes- count the number of small boxes between two
consecutive QRS complexes, and divide that number by 1500
-Sequence method- select a QRS complex that falls on a dark vertical
line. Number the next consecutive dark vertical lines as
follows: 300, 150, 100, 75, 60, and 50. Note were the next
QRS complex falls in relation to the 6 dark vertical lines
already marked.
-Rhythm/ regularity- are used to describe the regularity or irregularity of a
waveform. Primarily we are concerned with the regularity of the QRS
complexes “Ventricular rhythm”. If the rhythm is regular, the R-R
intervals are the same. The atrial rhythm can be checked for
regularity also.
Analyzing a rhythm strip
-Assess rate
-Assess rhythm/regularity
-Identify and examine P waves
-Assess intervals
-PR interval
-Qt interval
-Evaluate overall appearance of the rhythm
-Interpret the rhythm and evaluate its clinical significance
-noting wide QRS complexes, abnormal appearing Q waves, ST
depression or elevation, abnormal appearing P waves,
and premature beats