Lecture 7: Heart Rhythmicity & ECG Flashcards
What are characteristics of AV node?
Receives signal from SA node
Slow rate of depolarization/amplitude action potential b/c of small size
Goes through penetrating bundles
What are the characteristics of the SA node?
Composed of special cardiac muscle fibers
Fibers connect directly to atrial fibers
What are penetrating bundles?
Fibers going through small canal in connective tissue between atrium and ventricles
What is the time frame for signal to pass from SA node to AV node to ventricles?
AV node receives signal from SA node 0.03 seconds after origin
Signal delayed 0.09 seconds in AV node then passed to penetrating fibers
Signal delayed 0.04 seconds in penetrating fibers
Total=0.16 seconds
Why does the conductance slow through the process?
Due to a diminished number of gap junctions along pathway resulting in increased conductive resistance
What is the flow of action potential through the heart?
Base—>Apex (via midline)—>Base (via sides of ventricles)
What is the resting membrane potential of SA node?
-55mV to -60mV
Explain the process of SA nodal depolarization
Slow Na+/Ca++ channels open allowing leakage into cell At -40 mV
Na+/Ca++ channels are activated for 100-150 msec
When Na+/Ca++ channels close, K+ channels open
K+ remains open for a few tenths of a second
What is the ventricular fiber action potential?
-85 mV to -90 mV
What is the term for an action potential originating from the SA node?
Sinus rhythm
What is the name for an action potential originating anywhere else?
Ectopic
What is the parasympathetic modulation of heart rate/action potential?
Vagus nerve uses Ach to slow SA rhythm and decrease AV excitability
Increases permeability to K+ ions
What is the sympathetic modulation of heart rate/action potential?
Norepinephrine stimulates beta-1 and increases depolarization rate by increasing permeability to Na+/Ca++
What does the P wave represent?
Atrial depolarization
What does the QRS waver represent?
Ventricular depolarization
What does the T wave represent?
Ventricular repolarization
What is an ECG representing?
Extracellular potential
T/F: No potential is recorded when the ventricle is completely polarized or depolarized
True
How much time lapses from P wave–>QRS wave and Q wave–>T wave?
0.35 seconds for both
What does limb lead I attach to and which way does it read?
Both arms
Reads left to right
What does limb lead II attache to and which way does it read?
Negative terminal on right arm, positive terminal on left leg
Reads upper right to lower left
What does limb lead III attach to and which way is it read?
Negative on left arm, positive on left leg
Reads upper left to lower left
What is Einthoven’s triangle?
The triangle formed by the 3 leads
What is Einthoven’s law?
If any two electrical potentials of the three leads are known, the third can be determined by summing the first two
Which way does current flow in Einthoven’s triangle?
From negative to positive (typically base to apex)
What is the vector directional angle of Leads I, II, & III?
Lead I- 0˚
Lead II- 60˚
Lead III- 120˚
Which portion becomes repolarized first in the atria?
SA node
What is the electrical axis?
The direction of electrical potential (- to +) from the base of the ventricles to the apex
What conditions cause abnormal ventricular axis deviations?
Change in position of heart in chest
Hypertrophy of one ventricle
Bundle branch block
Fluid in pericardium
Pulmonary emphysema
How long is a normal QRS wave?
0.06-0.08 seconds
What is the current of injury?
The heart remaining partially or totally depolarized all the time
What is the current of injury caused by?
Mechanical trauma Infectious processes Ischemia (most common)
What is the “J” point?
The point where all parts of ventricles, including injured areas, are depolarized
Reference point for analyzing current of injury
At very end of QRS wave
