Cardio VI

Question 1

What is the difference between an electrocardiogram and an electrocardiograph?

Answer 1

The electrocardiogram refers to the recording, while the electrocardiograph refers to the instrument

Question 2

Describe the general set up of the ECG, including the locations of the leads.

Answer 2

Patient cable = 5 leads: chest, RA, LA, LL, RL
Lead-selector switch: takes the voltage and subtracts - from +

Question 3

Is the ECG an extracellular or an intracellular recording?

Answer 3

It is an extracellular recording that looks at the local circuit current that generates a voltage different in the skin when there’s depolarization or repolarization.

Question 4

In an ECG, which lead has a different function than the rest? Explain what that function is.

Answer 4

The right leg lead is a reference lead that is always connected.

Question 5

How long does the full ECG wave last at rest?

Answer 5

Around 1 second

Question 6

Name the major parts of an ECG wave.

Answer 6

P-wave, QRS complex, T-wave

Question 7

What is the amplitude of an ECG wave? How does it compare to an intracellular recording?

Answer 7

Amplitude is 1 mV (vs 100 mV for an intracellular recording)

Question 8

The ECG is generated by […]

Answer 8

local circuit currents

Question 9

Say there’s a local circuit current move from cell A, which is depolarized, to cell B. If you place the + lead near cell B and the - lead near cell A, how does the voltmeter compute the voltage? How would this appear on the ECG?

Answer 9

You would get a positive voltage reading, since it would be + voltage - - (+) voltage. This would appear as a positive-going wave on the ECG.

Question 10

Say there’s a local circuit current move from cell A, which is depolarized, to cell B. if you place the - lead near cell B and the + lead near cell A, how does the voltmeter compute the voltage? How would this appear on the ECG?

Answer 10

You would get a negative voltage reading of the same magnitude as (a), since it would be - voltage - + (-) voltage. This would appear as a negative-going wave on the ECG.

Question 11

Say there’s a wave of repolarization moving from cell A to cell B, which is depolarized. If you place the + lead near cell B and the - lead near cell A, how does the voltmeter compute the voltage? How would this appear on the ECG?

Answer 11

You would get a negative voltage reading of the same magnitude as (a), since it would be - voltage - + (-) voltage. This would appear as a negative-going wave on the ECG.

Question 12

If the propagation of a depolarization is towards the […] electrode and away from the […] electrode, you will see a positive-going wave on the ECG.

Answer 12

positive, negative

Question 13

If there is a wave of repolarization going towards the […] electrode and away from the […] electrode, you will see a negative-going wave on the ECG.

Answer 13

positive, negative

Question 14

What is happening in terms of depolarization in the time before the P-wave?

Answer 14

The SA node is depolarized. This doesn’t show up on the ECG because it’s a very small area of local circuit current.

Question 15

What is happening in terms of depolarization during the P-wave?

Answer 15

Atrial excitation is now complete. The action potential generated in SA node spreads to the left atrium, then stops. The P wave is caused by the activation of the atrial muscle. Left atrium = second half of P-wave. Right atrium = first half of P-wave. At this time, there is also activation of AV node (halfway), but it’s too small to see on the ECG.

Question 16

Explain what is happening in terms of depolarization during the Q portion of the QRS complex.

Answer 16

The AP then moves through bundle of his very quickly and goes into bundle branches and the purkinje fibers. While this happens, you still see nothing on the ECG because these are small structures.

The first part of the ventricular muscle to be depolarized/activated is the septum because of leakage from the left BB. This gives the upward deflection of the Q wave.

Question 17

Explain what is happening in terms of depolarization during the QRS complex.

Answer 17

The action potential spreads through the free wall of the two ventricles produces the R-wave. There are then 3 areas of the ventricles that activate late in the cycle and produce the S-wave. Overall, the activation of the QRS complex is due to the activation of the ventricles.

Question 18

Explain what is happening in terms of depolarization during the T-wave.

Answer 18

Repolarization wave as muscles in the ventricles repolarize.

Question 19

What is the setup of bipolar limb leads? Draw and/or describe the setup.

Answer 19

1: left arm (+) - right arm (-)
2: Left leg (+) - right arm (-)
3: Leg leg (+) - left arm (-)

Question 20

Name the unipolar ECG leads and what part of the body they are connected to.

Answer 20

Unipolar limb leads:
aVR: connected to right arm
aVL: connected to left arm
aVF: connected to left leg

Unipolar chest leads:
V1, V2, V3, V4, V5, V6

Question 21

Describe the setup of negative lead of the voltmeter.

Answer 21

The sum of the voltage of the left arm, the right arm, and the left leg is always equal to zero at any point in time. These three voltages are added together and connected to the minus terminal of the voltmeter, meaning that the voltage of the - terminal is zero.

Question 22

Describe the ventricular action potential in terms of:
a) Shape and parts
b) Duration
c) Resting potential

Answer 22

a) Resting potential -> upstroke -> plateau -> repolarization. The upstroke is very steep.
b) 300 milliseconds, which is much longer than the 10 millicseconds of a neural or skeletal action potential.
c) -90 mV. This is more hyperpolarized than a neural or skeletal action potential.

Question 23

What are the major types of ion channels that exist in ventricular cells?

Answer 23

Na+, Ca2+, K+

Question 24

At rest, what is the dominant ion in the ventricular cell? How does it affect the membrane potential?

Answer 24

At rest, K+ is the dominant ion, which is why the resting potential is around -90 mV

Question 25

Describe the ionic flow(s) responsible for the upstroke stage of the ventricular action potential.

Answer 25

• Initiated by local circuit currents
• Sodium channels will start to open and create an influx of Na+, which will continue depolarization. This causes a quick increase of voltage across the membrane.

Question 26

Describe the ionic flow(s) responsible for the plateau phase of the ventricular action potential.

Answer 26

• The fast inward Na+ ion channels have close, but the Ca2+ channels are open, so Ca2+ is flowing into the cell.
• The K+ channels are also closing.
• These all contribute to a relatively steady but slowly dropping ventricular membrane potential.

Question 27

Describe the ion flow(s) responsible for the repolarization phase of the ventricular action potential.

Answer 27

• At the end, K+ channels will reopen, allowing K+ to flow back into the cell and repolarize it.
• The Ca2+ channels have also closed, cutting off the inflow of Ca2+ to the cell.
  The voltage will therefore fall back down to resting potential.

Question 28

Describe the SA node action potential in terms of:
a) Shape and parts
b) Duration
c) Resting potential

Answer 28

a) Much gentler depolarization slope
b) 0.3 seconds
c) No resting membrane potential

Question 29

What types of ion channels does the sinus node have?

Answer 29

It has no sodium channels, but it does have calcium channels.

Question 30

Describe the ion flow(s) responsible for the upstroke of the sinus node action potential.

Answer 30

The upstroke is generated solely by the inflex of calcium ions through calcium channels.
NO sodium involved.