EXAM #1: REVIEW OF ECG Flashcards
Where is the Cl- concentration high?
Extracellular
Is the cardiac cell negatively charged or positively charged intracellularly?
Negative
What are the five phases of the cardiac action potential?
0= Rapid upstroke/ depolarization 1= Early repolarization 2= Plateau 3= Rapid repolarization 4= Resting membrane potential
Describe the ion flux in phase 0 of the cardiac action potential.
- Rapid influx of Na+
- FAST Na+ channels
“Rapid depolarization”
Describe the ion flux in phase 1 of the cardiac action potential.
- Na+ channels close
- Voltage-gated K+ channels OPEN and K+ moves OUT
“Early repoarlization”
Describe the ion flux in phase 2 of the cardiac action potential.
- Slow Ca++ channels open
- Opposing movement of Ca++ IN and K+ out= PLATEAU
Why is the plateau phase of the action potential important?
This phase corresponds to systole; the pause allows contraction to occur
Describe the ion flux in phase 3 of the cardiac action potential.
- Ca++ channels close
- K+ channels are still OPEN
- K+ OUT
“Rapid repolarization”
Describe the ion flux in phase 4 of the cardiac action potential.
- Na+/K+ ATPase
- Only K+ channels are open and a SMALL amount of K+ fluxes
“Resting membrane potential”
How many phases are there in the nodal action potential? What are they?
Three phases: 0, 3, and 4
What phases are missing from the nodal action potential?
1 and 2
How does the resting membrane of the nodal action potential differ from the cardiac myocyte action potential?
Cardiac myocyte= -90mV
Nodal= -60 mV
Describe the ion flux in phase 0 of the nodal action potential.
- Upon reaching threshold, Slow Ca++ channels open
- Ca++ INFLUX
Describe the ion flux in phase 3 of the nodal action potential.
- After depolarization, K+ channels open
- K+ EFFLUX
Describe the ion flux in phase 4 of the nodal action potential.
- Funny channels open (HCN)
- K+ efflux progressively declines
- Ca++ influx occurs, leading to slow depolarization
Outline the normal sequence of depolarization in the heart.
1) SA node
2) Atrial syncitium/ internodal tracts
3) AV node
4) Bundle of His
5) Purkinjie fibers
6) Ventricles
What sequence of depolarization occurs in the ventricles?
1) Interventricular septum
2) Apex of the heart
3) Ventricular free walls
What are the two branches of the left bundle branch?
Left aterosuperior fasicle
Left posteroinferior fasicle
What is the conduction velocity of the cardiac depolarization in the SA node?
Less than 0.01 m/sec
What is the conduction velocity of the cardiac depolarization in the atrial myocardium?
1.0- 1.2 m/sec
What is the conduction velocity of the cardiac depolarization in the AV node?
0.02-0.05 m/sec
What is the conduction velocity of the cardiac depolarization in the Bundle of His?
1.2- 2.0 m/sec
What is the conduction velocity of the cardiac depolarization in the bundle branches?
2.0- 4.0 m/sec
What is the conduction velocity of the cardiac depolarization in the Purkinje fibers?
2.0-4.0 m/sec
What is the conduction velocity of the cardiac depolarization in the ventricular myocardium?
0.3 -1.0 m/sec
Where is the conduction velocity the fastest in the heart?
Prukinje fibers
Where is the conduction velocity the slowest in the heart?
AV node
What does the p-wave of the ECG represent?
Atrial myocyte depolarization
What does the PR Interval represent?
Time it takes for the stimulus to spread through the atria and pass through the AV junction
What is the normal PR interval?
0.12- 0.2 seconds i.e. 3-5 small boxes
Note that 1mm = 0.04 sec
What does the QRS complex correspond to?
Ventricular myocyte depolarization
What does the T-wave represent?
Ventricular repolarization
What does the ST segment represent?
Ventricular myocyte plateau
What does 10mm or 2x large boxes represent vertically on ECG paper?
1 mV
What does 7 large boxes between R-waves represent in terms of rate?
43 bpm
Draw the hexaxial diagram.
p. 36, 5-2
What does the QT interval correspond to?
Entire ventricular action potential
How can you specifically determine the mean QRS axis?
1) Take biphasic lead
2) 90 degrees from that
OR
1) Average of 2x tallest QRS complexes
What is the quick way to determine the mean QRS axis?
Looks at leads I, II, and III
What does an upward I, II, and III indicate?
Normal axis (between -30 and 100)
What does an upward I, mid II, and downward III indicate?
“Physiologic left-axis deviation”
What does upward I and downward II and III indicate?
LAD
What does down I and upward III indicate?
RAD (100+)
What does downward I and upward II and III indicate?
Extreme axis deviation
What are the general steps to interpreting and ECG?
1) Rate
2) Rhythm
3) Axis
4) Conduction
5) Infarction
