Lecture 8: Heart Rhythmicity And ECG

Question 1

What is the P-wave in an ECG?

Answer 1

• Caused by the Spreading depolarization throughout the atria, this is followed by an atrial contraction, which causes a rise in the atrial pressure curve as soon as the P-wave ends (That rise is the a-wave)

Question 2

Describe the QRS wave, including how long it occurs after the P wave.

Answer 2

• About 0.16 seconds after the P-wave first onsets, the QRS wave represents the depolarization of the Ventricles. Like the P-wave, this initiates a ventricular contraction and a rise in the ventricular pressure curve.
• Therefore, the QRS wave occurs slightly before the start of ventricular systole

Question 3

Describe the T-wave

Answer 3

• Represents the stage of repolarization of the ventricles when the ventricular muscle fiber first starts to relax. The T-wave occurs just before the End of Ventricular Systole.
• Reminder: Diastole: Chamber is “Filling”
  Systole: Chamber is “Emptying…or at least trying to via contraction”

Question 4

Describe the overall conduction system of the heart

Answer 4

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

Question 5

What is the resting membrane potential of an SA node fiber

Answer 5

• -55 to -60 mV. The threshold is around -40 mV. In comparison, the ventricular fiber has a resting membrane of around -85 mV.
• The fast sodium channels are pretty much always inactivated when the membrane is less than -55 mV. So only slow sodium-calcium channels can be used, action potential is slower to develop, and repolarization is also slower.
• The sodium ions still slowly leak back into the cell, which causes the membrane to slowly become more positive.
• The sodium-calcium channels will reclose around 100-150 msecs after opening.
• There are large amounts of potassium channels open by the time the sodium-calcium channels close, which allows the nodal cells to repolarize. Those potassium channels only remain open for a few tenths of a second.

Question 6

Compare a sinus rhythm to an ectopic focus

Answer 6

• Action potentials originating in the SA node generate a “sinus” rhythm.
• Action potentials that originate anywhere else are said to be from an ectopic focus or pacemaker.

Question 7

Describe the mechanism of the vagus nerve on the heart

Answer 7

• 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

Question 8

Describe the mechanism and effects of sympathetic innervation on the heart

Answer 8

Sympathetic innervation:

• 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.

Question 9

Where does depolarization and depolarization occur on the ECG?

Answer 9

Depolarization is represented by the:
- P and QRS waves
- P = atrial depolarization
- QRS = ventricular depolarization
Repolarization is represented by the:
- T wave
- T = ventricular repolarization

• Atrial repolarization occurs where the QRS wave is. And it’s blocked by the QRS wave.

Question 10

Describe the characteristics of an ECG

Answer 10

An ECG is a graphic representation of the electrical activity in cardiac muscle tissue produced by regions of depolarization and repolarization.

• 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.
• The voltage that is flowing in the extracellular potential around the heart. Not the same as an action potential voltage.

Question 11

Describe the monophasic action potential of a ventricular muscle

Answer 11

• Includes a depolarization and a repolarization
• QRS wave appears at the beginning of the potential.
• T wave appears at the end of the potential.
• No potential is recorded when the ventricle is:
• • Completely polarized or
• • Completely depolarized
• Only when the muscle is partly polarized or partly depolarized does current flow from one part of the ventricle to another.
• Monophasic means we get a depolarization and a repolarization.

Question 12

What are the expected components and times for a normal ECG?

Answer 12

• P wave occurs at the beginning of the contraction of the atria.
• QRS complex occurs at the beginning of the contraction of the ventricles.
• Repolarization of ventricles occurs after the end of the T wave.
• The P-Q (P-R) interval:
• • Beginning of P wave → beginning of QRS ≈ 0.16 sec.
• The Q-T interval: 22 • ≈ 0.35 sec.

Question 13

What is a noteworthy similarity between the P-wave and the QRS-wave?

Answer 13

• The P wave represents phase 0 of the action potential’s spreading through the atrial muscle
• The QRS complex represents phase 0 of the action potential Spreading throughout the ventricles
• Less related The T wave represents phase 3 or repolarization of ventricular muscle fibers spreading through the ventricles

Question 14

Describe Limb Lead 1

Answer 14

• 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.

Question 15

Describe Limb Lead II

Answer 15

• 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.

Question 16

Describe Limb Lead III

Answer 16

• 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.

Question 17

Describe Einthoven’s triangle and law

Answer 17

Einthoven’s triangle: Two arms and left leg form apices of triangle.
Einthoven’s law: 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.
- Normally, in the ventricles, current flows from negative to positive primarily in the direction from the base of the heart toward the apex for most of the heart cycle until the very end.
- Therefore, the electrode nearer the base will be negative and the electrode nearer the apex will be positive.

Question 18

Define a vector in regards to electrical potential

Answer 18

• 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.

Question 19

Describe the vector axis for lead I

Answer 19

Two electrodes on two arms:

• R = negative
• L = positive
• Direction of the lead = 0°

Question 20

Describe the vector axis for lead II

Answer 20

Electrodes on right arm and left leg:

• A = negative
• L = positive
• Direction of the lead = 60°

Question 21

Describe the vector axis for lead III

Answer 21

Electrodes on left arm and left leg:

• A = negative
• L = positive
• Direction of the lead = 120°

Question 22

What are the rules for an ECG vector?

Answer 22

• When the vector in the heart is in a direction almost perpendicular to the axis of the lead, the voltage recorded in the ECG of this lead is very low.
• When the heart vector has almost the same axis as the axis of the lead, the entire voltage of the vector will be recorded.
• The summated vector of the generated potential at a particular instant is called the instantaneous mean vector.
• Reference vector is horizontal and extends towards a person’s left side; this reference direction = 0 degrees.
• The average direction of the vector during spread of the depolarization wave through the ventricles results in the apex of the heart remaining positive with respect to the base of the heart.
• This vector (QRS vector) is about +59 degrees in relation to the zero reference point.
• • This is the mean electrical axis of the heart.

Question 23

Describe the T-Wave in regards to Vectorial analysis

Answer 23

• Greatest portion of ventricular mass to repolarize first is the entire outer surface of the ventricles, especially near the apex.
• Endocardial areas normally repolarize last.
• Ventricular vector during repolarization is toward the apex of the heart.
• Normal T-wave in all three bipolar leads is positive (as is most of the normal QRS complex).

Question 24

Describe the P-wave in regards to vectorial analysis

Answer 24

• 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.

Question 25

Describe the atrial T-Wave

Answer 25

• Spread of depolarization through atrial muscle is slower than in the ventricles.
• Musculature around the sinus node becomes depolarized a long time before the musculature in the distal parts of the atria.
• The sinus node area is the first part to become repolarized.
• At the beginning of repolarization, the sinus node is positive with respect to the rest of the atria.
• Atrial T-wave is almost always obscured by QRS wave.

Question 26

Take note of Figure 11-7

Answer 26

Do it. Not sure if it’s super-need-to-know, but should probably know.

Question 27

Describe the ECG leads used in ECGs

Answer 27

A typical ECG may utilize 12 leads:

• Each lead provides a different perspective on the voltage differences between areas of the heart.
• In addition to the three conventional leads already discussed, the additional leads include:
• • Six standard leads (V1 –V6)
• • Three augmented leads (aVR, aVL, aVF)

Question 28

Describe the Mean Electrical Axis

Answer 28

The mean electrical axis is the direction of the electrical potential (negative to positive) from the base of the ventricles toward the apex.
-59°
Determining the electrical axis from standard lead ECGs:
- See Figure 12-11. DO IT!

Question 29

What abnormal ventricular conditions can cause electrical axis deviations.

Answer 29

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

Question 30

What could cause abnormal voltages in the QRS Complex

Answer 30

• Increased voltage in the standard bipolar leads:
• High-voltage ECG is said to occur when the sum of the voltages of all the QRS complexes of the three standard leads is greater than 4 mV.
• Most common cause is hypertrophy of the ventricle.
• Decreased voltage:
• Caused by cardiac myopathies
• Caused by conditions surrounding the heart

Question 31

What could cause bizarre patterns in the QRS complex?

Answer 31

• Normal QRS lasts 0.06 to 0.08 seconds.
• In hypertrophy or dilation of the left or right ventricle, QRS may be prolonged to 0.09 to 0.12 seconds.
• Conditions causing bizarre complexes:
• • Destruction of cardiac muscle and replacement by scar tissue.
• • Multiple small local blocks in the conduction impulses at many points in the Purkinje system.

Question 32

What are Currents of Injury

Answer 32

• Different cardiac abnormalities cause part of the heart to remain partially or totally depolarized all the time.
• Current of injury flows between the pathologically depolarized and the normally polarized areas, even between heartbeats.
• Abnormalities causing current of injury:
• Mechanical trauma
• Infectious processes
• Ischemia (most common cause)
• Effect of current of injury on QRS complex: Refer to Figures 12-17 and 12-18.
• Abnormal negative current flows from infarcted area and spreads toward the rest of the ventricles.
• J point: = reference point for analyzing current of injury.
• J point occurs at the very end of the QRS wave and is the point at which all parts of the ventricle, including the injured areas, have become depolarized.