Electrocardiography

Question 1

What is an electrocardiogram?

Answer 1

Recording/image of electrical activity of heart

Question 2

What is an electrocardiograph?

Answer 2

Instrument to record electrical activity of the heart. Abbreviation for electrocardiogram and electrocardiograph is the same, ECG, but what it stands for is obvious from the context.

Question 3

What is the set up for the ECG?

Answer 3

-There are five leads connected to the patient (chest, right arm, left arm, right leg, left leg) and the ECG is basically a voltmeter, a very sensitive voltmeter (since amplitude is so small) that picks up a signal when there is a wave of depolarization or repolarization of the heart causing local circuit currents which as seen flow either within the cells or outside the cells. The ECG measures the outside. It takes an extracellular recording on the surface of the body because the interstitial space is all connected and can be measured extracellularly.
- Voltage is the potential difference, + minus the - end, the + end is connected to one of the leads selected and the - end is connected to another selected lead. The 0 in voltage is picked arbitrarily. For an ECG, the voltage of the right leg is picked as the reference lead and must always be connected (RL lead)

Question 4

What is an important characteristic of an electrode?

Answer 4

• It is moist and has like salt water (ions) so it becomes a good conductor and we get good electrical contact with cells and no voltage is lost in the air.

Question 5

What are the typical ECG waves? What is the amplitude?

Answer 5

P
Q neg
R
S neg
T
(this sequence is seen for each heartbeat)
Typically the amplitude is about 1 mV (highest peak R) while an intracellular recording would probably give like 100 mV

Question 6

How can we explain the negative and positive deflections in the ECG?

Answer 6

If a wave of depolarization is headed towards the positive electrode, it means that at the positive electrode, it is more positive and the deflection will be positive. If instead it heads in the direction of the negative electrode, it means that the negative electrode it is more positive and the deflection will be negative. For a wave of repolarization, if it is headed towards the positive electrode, it means it is more negative there and the deflection will be negative. If it is headed in the direction of the negative electrode, it means it is more negative there and the deflection will be positive.

Question 7

Are gap junctional channels the same as other ionic channels?

Answer 7

No, gap-junctional channels are huge and many things can go through it, what’s important is that there is a restoration of current and charges. For ionic channels, they are more specific, only let specific ions flow through them.

Question 8

How does the sequence of activation correlate with the ECG?

Answer 8

In the diastolic period, resting period of the heart, the resting potential is basically 0, therefore there is no recording on the ECG. The little bit of activation that comes from the SA node is barely felt and it is only when the wave of depolarization spreads through the atria do we see the P wave appear. The P wave stops once all of the atrial muscle has been depolarized. At about the peak of the P wave, the action potential also reach the AV node through which they pass quite slowly (physiological delay) and then very quickly pass through the His bundle and purkinje fibers (at the end of the flat segment). Once the action potentials are going through these, you see nothing because again, they are very small structures. Then the first part of the ventricular muscle to get activated is the interventricular septum from left to right and that is the inverted Q wave. The R wave is the simultaneous activation of both the ventricular free walls. Then three areas of the ventricle get activated quite late and they are the cause of the S wave. The T wave is caused by the repolarization wave of the ventricles.

Question 9

What does a lead refer to in electrocardiography?

Answer 9

• Can be the actual wire conductor itself
• Can be a particular combination of wires connected to the input terminals of the ECG

Question 10

What are bipolar limb leads?

Answer 10

• There are three bipolar limb leads
  I: VLA - VRA
  II: VLL - VRA
  III: VLL - VLA
  (1st one to positive and 2nd one to negative)
  They are called bipolar limb leads because it is always between two limbs and only between limbs (no chest). Also, the right leg, reference lead, is always also connected to the ECG to be the arbitrary 0 (in all ECG leads).

Question 11

What are unipolar ECG leads?

Answer 11

• just one point for voltage recording
• There are three unipolar limb leads (aVR, aVL, aVF (left foot)) and 6 unipolar chest leads V1 to V6. V1 is right of sternum then V2 on left of sternum and the 4 other ones are in a particular intercostal space with V6 basically mid armpit.
• It was found that if you add VLA + VRA + VLL = 0 in voltage so that gets plugged to the - end of the voltmeter and the exploring lead gets plugged to the + end of the voltmeter and explore any of the 9 points described before.

Question 12

How are unipolar ECG leads helpful for detecting the spread of waves?

Answer 12

• The unipolar limb leads constitute one plane where the vector is projected
• The unipolar limb leads constitute a second plane where the vector of propagation is projected
• With the projection of the vector on two different planes we can get the full reconstitution of the 3D vector of propagation.

Question 13

What constitutes the clinical ECG?

Answer 13

• 12 leads are recorded, the three bipolar limb leads and the 9 other unipolar leads
• the recording of each of the leads may be different but when put together we get a bit of extra information about the depolarization and repolarization waves at each lead to make sure that normal PQRST waves are recorded at least somewhere across the 12 leads

Question 14

What could make a novice in cardiology freak out when seeing a clinical twelve-lead ECG?

Answer 14

• normal PQRST waves are not present on each lead
• Some leads have weird stuff happening like no P wave, an inverted T wave, only a Q wave, perhaps no Q wave but yes an RS complex, and many more odd things
• remember that it is only if the big picture, when combining the results of the 12 leads, is odd like no P wave anywhere, then freak out lmao

Question 15

What does an action potential do quite concretely?

Answer 15

Well when there is an action potential, this causes local circuit currents which produce the depolarization and repolarization waves that are detected by the ECG for example.

Question 16

What are the components of a ventricular action potential?

Answer 16

• 1 cardiac cycle is about 1 second where heart rate is about 60 bpm and 2/3 of the cycle will be at resting potential.
  Then for about 1/3 will be the action potential which starts obvi at resting potential then there is an upstroke caused by the opening of Na+ channels, then there is a pretty long and present plateau which is only in cardiac muscle, not in smooth nor skeletal muscle and finally the repolarization step

Question 17

How is the action potential in ventricular muscle different from action potentials elsewhere (ex: other muscle type or neuron)?

Answer 17

• An action potential in a cardiac muscle cell lasts much longer than a nerve or skeletal muscle action potential
• Plateau not in other muscle types
• Resting potential is about - 90 mV, so more hyperpolarized than in neurons where it is about - 60 mV or - 70 mV

Question 18

What are the important ionic channels in cardiac cells and why?

Answer 18

• For an action potential, we need ionic channels which are proteins that sit on the lipid bilayer membrane that changes conformation (open/close) to let ions flow in and out of the cell since the diffusion coefficient in the lipid membrane for ions is 0 (ions insoluble in membrane)
• In a ventricular cell, we need particularly an INa, an ICaL (long lasting bcs it stays open for a bit), an ICaT (transient) and many kinds of IK
• Different mixes of channels exist in different cell types depending on the specific cell type needs

Question 19

What is the ionic basis underlying the ventricular AP?

Answer 19

At rest the membrane permeability (through channels, not lipid bilayer obvi) to K is much higher than anything else, so the voltage is EK around the K nernst potential, - 90 mV. The upstroke in both ventricular and atrial AP is caused by the opening of Na+ channels (activation wave). The increase is due to local circuit current which cause Na+ to open and then through a positive feedback loop with 1 ms, more voltage is higher and the more Na+ channels open and the more voltage is go up and the more Na+ channels open, etc. The upstroke is so quick, the activation and inactivation (closing of channels) is very quick (INA, fast inwards Na+ current). When the increase in voltage is sensed, one of the many K+ channel types closes so permeability to K+ decreases. The calcium channels also sense the increase in voltage but will wait a bit before opening then stay open for a little bit and then close. At the end of the upstroke all Na+ channels have closed. During the plateau, some Ca2+ channels open and some K+ channels open and the two flows in and out of the cell are nearly equal but not quite. There is still a bit more K+ leaving than entering so plateau trends down. Then repolarization happens as more and more K+ channels open.

Question 20

How does the sinus node action potential work?

Answer 20

• There is no resting potential, they are pacemaker cells. We instead call it a pacemaker potential which is the potential needed for spontaneous pacemaker cell pacemake (generate spontaneous action potentials, spontaneous diastolic depolarization)
• There is no INA bcs no Na+ channels in SA node, the upstroke is caused by ICaL (influx of calcium ions) and the upstroke of the action potential happens when the threshold voltage is reached

Question 21

What are the differences between FAST and SLOW APs? Which parts of the heart have slow vs fast APs?

Answer 21

• Slow vs fast bcs upstroke rises more slowly (1-10 V/sec) than in fast (100-1000 V/sec)
• Slow vs fast bcs conduction velocity is only 0.01-0.05 m/sec in slow and 0.5-5 m/sec in fast
• Slow AP => SA node and AV node
• Fast AP => ventricular muscle, atrial muscle, bundle of his, bundle branches, purkinje fibers

Question 22

Why is the physiological delay important in the AV node?

Answer 22

So that blood flows well, prevents two guys (ventricle and atrium) from working in opposite directions leading to nothing (so atria contract and a little later ventricles, not at the same time)

Question 23

How does the ECG relate to action potentials?

Answer 23

• Atrial action potentials happen in the P wave. P wave corresponds to the upstrokes from the very 1st atrial cell to the very last one. At some point AV node also depolarizes but basically 0 because of how tiny it is.
• Small plateau where “nothing happens” moving from AV node to His bundle to bundle branches to Purkinje fibers in that tiny time frame
• No repolarization peak for atria because smaller voltage and gets masked by the QRS complex
• The QRS wave corresponds to the upstrokes from the very first ventricular cell to the very last
• The T wave corresponds to the ventricular cells repolarizing so happens at the same time when comparing graphs

Question 24

Why is the action potential duration shorter for atrial cells than for ventricular cells?

Answer 24

Smaller calcium influx in atrial cells, so plateau is shorter.