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39 Cards in this Set

  • Front
  • Back
What causes the P wave?
Depolarization of the atria (before contraction)
What is the QRS complex?
Potentials generated when ventricles depolarize before contraction.
What is the T wave caused by?
Ventricles repolarizing (.25-.35 seconds after depol)
What is the potential like during depolarization?
Positive inside the cell, negative outside. The negativity is what the ECG reads.
What causes the depolarization wave P to be positive?
When the cell is depolarizing the current is flowing depolarization at the halfway mark is positive. Meaning the negative electrode is receiving negative potential from the depolarized half, but the positive electrode is still receiving positive potential from the end of the cell.
When does the P wave return to zero?
When the depolarization wave extends over the entire muscle fiber and both electrodes are in areas of equal negativity.
How long does the monophasic action potential of ventricular muscle last?
.25-.35 seconds. The rapid upsweep = depolarization, slow return = depolarization
When is no potential recorded on an ECG?
When the muscle is either completely polarized or depolarized.
What waves start at the beginning of contraction (either atria or ventricles)?
The P wave = atrial
QRS complex = ventricluar
When do the atria repolarize?
.15-.20 seconds after termination of the P wave (when the QRS complex is being recorded) = atrial T wave is hidden by QRS
When does ventricular muscle begin to repolarize?
.20 seconds after QRS complex, may take as long as .35 seconds => extends over a period of .15 seconds (T wave is prolongued and less voltage)
What do 10 of the small line divisions represent vertically?
1 mV
What is the typical speed of an ECG?
25 mm/s, so 25 mm in the horizontal direction = 1 second
What do the dark vertical lines represent? What is the time interval they are broken into?
.2 second (large box) broken into .04 seconds (small boxes)
How large can the QRS complex get? What is it normally?
4mV = occurs when one electrode is over the heart and another is far away.

Normal is 1-1.5mV when using two arms, or an arm and a leg.
What is the monophasic action potential directly at the heart muscle membrane?
110mV
What is the voltage of the Pwave? The T wave?
P = .1-.3 mV

T = .2-.3 mV
What is the time of the P-Q interval?
.16s (P-R if Q is absent). This is from START of P wave to START of QRS --> atrial contraction.
What is the length of the Q-T interval?
Contraction of ventricle lasts from Q wave to the END of the T wave.

This is .35s
What is heart rate?
Rate = 1/time between heartbeats

60/.83 = 72bpm (for example)
Why is it essential for an ECG apparatus to record changes in potentials rapidly?
Because polarities on respective sides of the heart can change in less than .01s (thousandths of a second).
Before stimulation, what is the polarity of a syncytial mass of cardiac muscle?
All exteriors are positive and the interiors are negative. Depolarization spreads from the endocardial surface outward through the ventricular muscle mass.
What occurs as soon as an area of cardiac syncytium becomes depolarized?
Negative charges leak to the outsides of the depolarized muscle fibers, making this part of the surface electronegative.

The rest of the heart is still polarized, and its electropositive.
Where does cardiac impulse first arrive?
In the ventricles in the septum and shortly thereafter spreads to the inside surfaces of the ventricles.

Negative on inside of ventricles and positive on outer walls of ventricles with electrical current flowing through the fluids surrounding the ventricles along elliptical paths.
What happens when one averages all the lines of current flow (elliptical lines) coming from the conductive medium around the heart?
The average current flow occurs with negativity towards the base of the heart and positivity towards the apex.
What occurs immediately before depolarizaton ends in the heart?
The average direction of current flow reverses for .01 second, flowing from the apex to the base.

This is because the last part of the heart to become depolarized is the outer walls of the ventricles near the base of the heart.
What does an electrode near the base of the heart read? Near the apex?
Base = negative
Apex = positive
What two wires compose "Lead I"?
Negative terminal = right arm
Positive terminal = left arm

The ECG should read positive.
What two wires compose "Lead II"?
Negative terminal = right arm
Positive terminal = left leg

The ECG should record positive.
What two wires compose "Lead III"?
Negative terminal = left arm
Positive terminal = left leg

The ECG should record positive.
What is Einthoven's triangle?
Triangle drawn around the heart where the two arms form two apices at the upper part which represent the point at which the two arms connect electrically with fluids around the heart.

The apex of the triangle is inverted and is the point at which the left leg connects with the fluids.
What is Einthoven's law?
Any two of the three bipolar limb lead's potentials can be added together to determine the third one.

Holds true at any given instant while the three standard biopolar limb leads are being recorded.
Does the lead matter when diagnosing arrythmias?
No, this diagnosis depends on the time relations between different waves of the cardiac cycle.

The type of diagnosis that requires knowledge of the leads is heart damage or conduction issues (myopathies).
What are the precordial leads?
When the ECG is recorded using one electrode placed on the anterior surface of the chest (directly over heart) = positive

And the negative = "indifferent electrode" which is connected through equal resistances to the arms and the left leg.
What are the six standard chest leads that come from the anterior wall?
V1-V6

V1 = over right atrium = negative (near base of heart)
V2 = left atrium = negative (near base of heart)
V3 = apex = should be neutral
V4-6 = at apex but going across 5IC space. = all positive
What is the augmented unipolar limb lead?
Two of the limbs are connected through resistances to the negative terminal of the ECG, and the third limb is connected to the positive terminal.
What is a positive terminal on the right arm?
aVR, this recording will be inverted (essentially negative)
What is a positive terminal on the left arm?
aVL
What is a positive terminal on the leg?
aVF