Electrical Activity of the Heart Flashcards
IK1 meaning. When do they open?
inward rectifying K + channels that are voltage gated channels that open at rest
How do inward Ca+ channels for phase 4 of nodal cells differ from L type Ca2+ channels?
open at a more negative membrane potential ~ -70 mV
What currents are responsible for phase 4 of nodal cell action potential?
inward Ca2+ current
Inward Na+ current If (funny current)
Outward K+ current
What is the S wave?
1st downward deflection after an R wave
What is PR interval?
start of the P wave to start of QRS complex
What is QT interval? What does it represent?
start of QRS complex to end of the T wave; represents duration of the AP
What is ST segment? What does it correspond to in the action of the cardiac myocytes AP?
End of S wave to beginning of T wave; ventricles are depolarized; corresponds to plateau phase of AP
What is J point?
end of the S wave; represents isoelectric point
What is the trick to being able to tell bpm (beats per minute) based off of boxes on ECG?
Count big boxes from R wave to R wave
1 = 300
2 = 150
3 = 100
4 = 75
5 = 60
6 = 50
What does one big box on the y axis (volts) on the ECG represent? (how many V)
0.5 mV
What does one big box on the x-axis of the ECG represent?
0.2 sec (200 msec)
What are the steps to reading an ECG?
Step 1: determine rate and rhythm
Step 2: Waves Examine the trace for the presence of P, QRS, and T can they be seen and do they look normal?
Step 3: PR interval and determine if it is in normal range about 120 - 200msec (3-5 small boxes). Look to see if PR is consistent
Step 4: Estimate the MEA (mean electrical axis)
What is the MEA?
indicates the net direction (vector) of current flow during ventricular depolarization. Each lead can be represented by an angle.
Be able to explain the quadrant method for determine MEA.
What are the causes of left axis deviation?
- left heart enlargement, either left ventricular hypertrophy or dilation
- conduction defects in the left ventricle, except in the posterior bundle branch
- acute MI on rt. side tends to shift the axis left unless the rt ventricle dilates
What does the image here represent? Explain.
First degree heart block
Long PR interval (> 200 sec; one big box)
Slowed conduction through AV node
rate and rhythm typically normal here
What pathology does this image represent? Explain.
Second degree heart block (Mobitz Type I )
In second degree, every QRS complex is preceded by a P wave, but not every P wave is preceded by QRS complex
Mobitz I (Wenckebach)
when there is progressive prolongation of PR interval until a ventricular beat is missed and then the cycle begins again.
This arrhythmia will have an unsteady rhythm
Which condition is being seen here? Explain.
Second degree heart block Mobitz type II
In second degree, every QRS complex is preceded by a P wave, but not every P wave is preceded by QRS complex
PR interval is consistent (i.e it doesn’t lengthen this separates it from Wenckebach.)
Rhythm can be steady or unsteady
Which conditions is being seen here? Explain.
complete heart block
complete dissociation of P waves and QRS complexes
Steady rhythm (usually)
Verrrrrrrrry slow rate
Which condition is being seen here? Explain.
Atrial flutter
fast atrial rate,
“saw tooth” appearance of waves
no discernable T waves
rhythm typically steady
What condition is being seen here? Explain.
a fib
lack of coordinate conduction results in no atrial contraction
unsteady rhythm (usually) no discernible P waves
What is the condition being seen here? Explain?
Wolff- Parkinson-White Syndrome
short PR interval; steady rhythm and normal rate (usually)
slurred upstroke of R wave and widened QRS complex
