Lecture 8 Flashcards Preview

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Flashcards in Lecture 8 Deck (40)
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1
Q

Describe conductive system

A

-SA Node -AV Node - Slow conduction is caused mainly by diminished numbers of gap junctions along pathway resulting in an increase in the resistance to conduction.

2
Q

Describe SA node

A

• Composed of special cardiac muscle fibers. • SA fibers connect directly to atrial fibers.

3
Q

Describe AV node

A
  • Receives signal from SA node .03 sec. after origin.
  • Signal is delayed in the AV node for .09 sec.
  • Due to small size of cells, low amplitude of action potential, and slow rate of depolarization during excitation.
  • A final delay of .04 sec. occurs in the penetrating bundles.
  • Therefore, there is a .16 sec delay from the initial origin of the signal until onset of ventricular contraction. Slow conduction is caused mainly by diminished numbers of gap junctions along pathway resulting in an increase in the resistance to conduction.
4
Q

Draw organization of AV node

A
5
Q

Draw cardiac impulse through heart

A
6
Q

Describe membrane potentials of SA node

A
  • Resting membrane potential of SA node fiber:
  • -55 to -60 mV (Threshold ≈ -40 mv)
  • Fast sodium channels are already inactivated (blocked).
  • Inactivation gates close when membrane potential is less negative than -55 mV.
7
Q

Describe gates of SA node

A

Only slow sodium-calcium channels can open.

  • Therefore, atrial nodal action potential is slower to develop.
  • Therefore, repolarization is also slower.
  • There is a slow leak of sodium ions back into the cells.
  • Membrane potential becomes more positive.
  • At -40 mV, sodium-calcium channels become activated.
  • Sodium-calcium channels are inactivated within 100-150 msec after opening.
  • Large numbers of potassium channels open at the time the sodium calcium channels become inactivated. • Nodal cells become repolarized.
  • Potassium channels remain open for a few tenths of a second.
8
Q

What is sinus rhythm

A

Action potentials originating in the SA node

9
Q

What is an ectopic focus

A

Action potentials originating anywhere other than SA node

10
Q

Ventricular fiber resting potential

A

• -85 to -90 mV

11
Q

Describe vagus nerve

A
  • Distributed mainly to the SA and AV nodes
  • Neurotransmitter = acetylcholine
  • Involves muscarinic receptors
  • Decreases rate of rhythm of SA node (negative chronotropic effect)
  • Decreases excitability of the AV junctional fibers, slowing transmission of the cardiac impulse into the ventricles.
  • Increases permeability of fiber membranes to potassium ions
  • Hyperpolarization: -65 to -70 mV rather than -55 to -60 mV
12
Q

Describe sympathetic innervation

A
  • Distributed to all parts of the heart, mainly the ventricles
  • Neurotransmitter = norepinephrine
  • Stimulates beta-1 adrenergic receptors
  • Increases depolarization rate (positive chronotropic effect)
  • May increase permeability of fiber membranes to sodium and calcium ions.
13
Q

Describe characteristics of an ECG

A

• An ECG measures extracellular potential; it is not the same as the potential recorded from an axon when recording the transmembrane potential. •

Deflection from 0 occurs only when there is current flow between regions of the heart (when there is variation in the membrane potential in different regions of the heart). Current flows between regions of different membrane potentials.

• Because the atrial musculature and ventricular musculature are electrically isolated, current flow does not occur when only the atria and the ventricles have different potentials.

14
Q

The P-Q (P-R) time interval

A

0.16 seconds

15
Q

The Q-T time interval

A

0.35 seconds

16
Q

What does the P wave represent

A

The P wave represents phase 0 of the AP’s spreading through the atrial muscle

17
Q

What does the QRS wave represent

A

The QRS complex represents phase 0 of the action potential spreading throughout the ventricles

18
Q

What does the T wave represent

A

The T wave represents phase 3 or repolarization of ventricular muscle fibers spreading through the ventricles

19
Q

Describe recording limb lead I

A
  • Negative terminal is connected to the right arm.
  • Positive terminal is connected to the left arm.
  • Looks at the heart from right to left. - When the point where the right arm connects to the chest is negative with respect to the point where the left arm connects, the ECG records positive.
20
Q

Describe recording limb lead II

A
  • Negative terminal of ECG is connected to the right arm.
  • Positive terminal of ECG is connected to the left leg.
  • Looks at heart from upper right to lower left. - When the right arm is negative with respect to the left leg, the ECG records positive.
21
Q

Describe recording limb lead III

A
  • Negative terminal is connected to the left arm.
  • Positive terminal is connected to the left leg.
  • Looks at heart from upper left to lower left. - When the left arm is negative with respect to the left leg, the ECG records positive.
22
Q

What is Einthoven’s triangle

A

• Two arms and left leg form apices of triangle.

23
Q

Describe Einthoven’s law

A

• If the electrical potentials of any two of the three bipolar limb ECG leads are known at any given instant, the third can be determined by summing the first two.

24
Q

Define vector

A
  • A vector is an arrow that points in the direction of the electrical potential generated by the current flow, with the arrowhead in the positive direction.
  • Length of the arrow is drawn proportional to the voltage of the potential.
25
Q

Axis for lead I

A
  • Two electrodes on two arms:
  • R = negative
  • L = positive
  • Direction of the lead = 0°
26
Q

Axis for lead II

A
  • Electrodes on right arm and left leg:
  • A = negative
  • L = positive
  • Direction of the lead = 60
27
Q

Axis for lead III

A
  • Electrodes on left arm and left leg:
  • A = negative
  • L = positive
  • Direction of the lead = 120°
28
Q

Describe reading when vector of heart is perpendicular to axis of lead

A
  • Voltage recorded in this ECG is very low
29
Q

Describe reading when vector has the same axis as axis of the lead

A

The entire voltage of the vector will be recorded

30
Q

What is the instantaneous mean vector

A
  • The summated vector of the generated potential at a particular instant
31
Q

Describe the reference vector

A
  • The reference vector is horizontal and extends towards a person’s left side, this referenc direction = 0 degrees
32
Q

What is the mean electrical axis of the heart

A
  • 59 degrees
33
Q

Describe T wave in vectorial analysis

A
  • Greatest portion of ventricular mass to repolarize first is the entire outer surface of the ventricles, especially near the apex
  • Endocardial areas repolarize last
  • Ventricular vector during repolarization is toward the apex of the heart.
  • Normal T-wave in all three bipolar leads is positive
34
Q

Describe vectorial P-wave

A
  • Atrial depolarization begins in the sinus node
  • Vector direction is generally in the direction of the axes of the three standard bipolar limb leads
  • ECG is normally positive in all three leads
35
Q

Describe vectorial atrial T-wave

A
  • Spread of depolarization through atrial muscle is slower than in the ventricles
  • Musculature around the SA node becomes depolarized a long time before musculature in the distal parts of the atria
  • Sinus node area is the first to become repolarized, at beginning of repolarization, SA node is positive with respect to rest of atria
  • Atrial T-wave obscured by QRS wave
36
Q

Describe ECG leads

A
  • Typical ECG may utilize 12 leads
  • In addition to three conventional leads;
  • Six standard leads (V1-V6)
  • Three augmented leads (aVR, aVL, aVF)
37
Q

What are abnormal ventricular conditions that cause axis deviation

A
  • Change in position of heart in the chest
  • Hypertrophy of one ventricle
  • Bundle branch block
  • Fluid in pericardium
  • Pulmonary emphysema
38
Q

Describe increased voltages in bipolar leads

A
  • High voltage ECG = when sum of QRS complexes of all leads is > 4mV
  • Most common cause = hypertrophy of one ventricle
39
Q

Describe decreased voltage

A
  • Cardiac myopathies
  • Caused by conditions surrounding the heart
40
Q
A