2- EKG and Excitation Flashcards
What is the resting membrane potential of SA node cells?
Why?
-55
cell membranes of the sinus fibers are naturally leaky to sodium and calcium
What causes self-excitation in SA node cells?
Leakiness to sodium and calcium lowers resting and threshold potentials
If SA cells leak Na and Ca, why are they not constantly depolarized?
- Ca channel inactivation
- increased number of potassium channels means a large number of K ions leave the membrane and it becomes hyperpolarized for a period of time, protecting it from repolarizing
What is the threshold potential of an SA cell?
-40
How does an impulse from the SA node travel to the AV node?
Three atrial fibers:
Anterior (bachman’s - L atria)
Lateral
Posterior
Join together at the AV node
Where is the AV node located?
Posterior RA wall
Behind the tricuspid
Stokes-Adams Syndrome
When the AV node becomes suddenly blocked, it can take 5-20 seconds for the purkinje fibers to initiate their own intrinsic rate, because at baseline they are oversaturated. This is why people generally faint.
Vagal stimulation in the heart results in release of what neurotransmiter?
Acetylcholine
What effect does acetylcholine have on cardiac muscle cell membrane?
Opens potassium channels, creating hyperpolarization and decreasing excitability
Sympathetic stimulation in the heart results in release of what neurotransmitter?
Norepinephrine
Does sympathetic stimulation alter the conduction of the SA node or the AV node?
both
What are adrenergic receptors?
nerve receptors that are stimulated by either norepinephrine or acetylcholine
What is an adrenergic agonist?
A drug that stimulates a response from an adrenergic receptor
When norepinephrine stimulates Beta 1 receptors, what happens?
Why?
Sympathetic activation
Increased heart rate, conduction, contractility
increases permeability to sodium and potassium channels (hypopolarizes)
On an ECG print out, the darker squares represent ____ seconds.
The smaller subsquares represent _____ seconds
- 2
- 04
What is the instantaneous mean vector?
Which direction does it point?
The summated vector of all the potentials in the heart from moment to moment
Considerably more current flows downward from base to apex, so the arrow points down along the ventricular septum
What vector represents zero, and what degree vector is the standard mean QRS vector?
Exactly horizontal and directed toward the left side
+59 degrees (down and to the left)
What is the axis of a lead?
The direction from negative electrode to positive electrode
What repolarizes first: the inner or outer muscle layers of the ventricle?
The outer. Because during contraction coronary blood flow to the ventricle is substantially decreased, so it takes longer to relax contraction
What is the mean electrical axis of the ventricle?
The instantaneous mean vector of the ventricle
+5
When and why does the mean electrical axis of the heart shift to the left in a healthy heart?
- At the end of deep expiration
- When a person lies down
- In obese people
Caused if the heart is angulated to the left
When and why does the mean electrical axis of the heart shift to the right?
- At the end of deep inspiration
- When a person stands up
- In tall lanky people whose hearts hang downward
If the left ventricle is hypertrophied, which way will the mean electrical axis shift?
To the Left
The normal ventricle is depolarized much faster than the hypertrophied one
What are some causes of depressed QRS voltage?
Old myocardial infarctions (signals take longer to get through the tissue)
Pericardial effusion (fluid short circuits signals)
Pulmonary Emphysema (increased air in the lungs decreases voltage picked up by electrodes)
What is the electrical cause of ST elevation?
The injured muscle remains depolarized when the rest of the cell is completely polarized, so it always causes an altered baseline (J point). The ventricular baseline (J point) becomes different from the atrial baseline (zero).
An injured area of the myocardium is permanently depolarized or polarized?
Depolarized
When observing the injury vector during the J point, how is the location of the injury determined?
when you lay the vecor analysis over a picture of the ventricles, the negative end of the vector will point to the site of injury
What does it mean if the PR interval is less than 0.12 seconds?
The AV node is not conducting that signal, so a secondary pathological pathway must be present
