Why are all the other pacemakers in the normal heart suppressed by the SA node?
It depolarizes the quickest and has the highest rate of automaticity.
What wave of depolarization causes the p-wave on the surface EKG?
SA node-induced depolarization of the right and left ventricles.
What causes the PR interval in the surface EKG?
AV node delay that allows time for the ventricles to fill before systole.
What makes the narrow QRS complex in the surface EKG?
AV node signal transduction down His-purkinje fibers and into left and right branches that causes near simultaneous ventricular contraction.
What makes the T wave in the surface EKG?
Repolarization of the ventricles
What is the definition of normal sinus rhythm?
Rhythm that originates in the SA node and produces a HR of 60-100 bpm.
What is an arrhythmia?
Any cardiac rhythm that is not normal sinus rhythm (to include tachycardia and bradycardia)
Why are arrhythmias bad?
Significantly decreased cardiac output during bradycardia and tachycardia.
What arrhythmia is responsible for the highest number of arrhythmia-related cardiac deaths?
Why does your heart have a difficult time beating when your nodal tissue fails?
Nodal tissue is the only tissue that will spontaneously depolarize. If it fails, cardiac muscle will sit at baseline indefinitely.
What makes the phase 0 slope less in nodal tissue than in muscle?
Muscle is enriched deeply with fast Na+ channels that cause rapid depolarization where nodal tissue is depolarized by L-type Ca++ channels that is slower.
Why is the AV node more difficult to stimulate action potentials as it is stimulated more quickly? How is AV node conduction speed increased during exercise?
Restimulation of the nodal cell results in smaller action potentials due to reduced Ca availability. This is a concept called decremental conduction. The sympathetic nervous system secretes catecholamines that increase Ca channel availability during exercise to increase AV nodal stimulation.
What is responsible for the slurred delta wave that precedes the QRS complex in patients with WPW syndrome?
Bypass of the AV-His pathway results in early cell-to-cell contraction of ventricles.
Why is a-fib so lethal in a patient with WPW syndrome?
Normal decremental conduction in the AV node does not allow conduction at the rate the atria want to contract. In WPW, signal conduction bypasses the AV node and ventricular rate can skyrocket to 200 bpm.
What are the two types of tachycardia?
Supraventricular (atrium/AV node). Ventricular (often lethal).
What are the mechanisms of tachycardia?
Reentry (most common, self-perpetuating depolarization). Triggered (premature beats caused by previous depolarization). Enhanced automaticity (automaticity developed beyond the SA node)
What are the three conditions required for tachycardia caused by reentry?
Adjacent cardiac tissue with different refractory periods, unidirectional conduction block and an area of slow conduction so the wave of depolarization excites previously depolarized tissue.
A 45 year old male comes to see you with tachycardia. He has a history of a single heart attack. You do an EKG and get the results below. What is causing this patient's tachycardia?
Note the wide, uniform QRS waves, indicating non-nodal stimulation of ventricular contraction. This patient has ventricular tachycardia, likely as a result of stimulation circulating around a single point of necrotic tissue from his previous heart attack that is stimulating the ventricles.
A 65 year old male comes to see you with tachycardia. He has a history of a multiple heart attacks. You do an EKG and get the results below. What is causing this patient's tachycardia?
This EKG is typical of ventricular fibrillation. This is a result of stimulation circulating around multiple necrotic areas from his multiple heart attacks.
What do you know if a patient with an arrhythmia has a narrow QRS? A wide QRS?
Narrow = supra ventricular tachycardia (circuit is using AV-His pathway. Wide = WPW, ventricular tachycardia or a supra ventricular tachycardia with a bundle branch block.
A 60 year old male with a history of a heart attack comes in to see you because his heart rate is elevated. You get the EKG below. What is your diagnosis?
Ventricular tachycardia. You know this patient has a scar in the ventricle that is acting as a reentry.
What about an EKG will tip you off to atrial fibrillation in a patient with WPW?
How do you cure this arrhythmia?
Notice the atrial rate is very high and the ventricular rate is slow but irregular. This is characteristic of atrial flutter where an anatomic reentry circuit is causing reentry in the right atrium. You can go in and burn part of the circuit to terminate the arrhythmia.
Why is shocking the heart effective in creating arrhythmias?
It depolarizes the entire heart, like a reset button and stimulation should restart at the SA node.
What are the two types of triggered arrhythmias? What causes them?
Early (low [K], drugs that prolong QT interval, congenital repolarizing mutations) and delayed (digitoxin toxicity, catecholamines and Ca2+) afterdepolarizations. They act as an early stimulus to trigger reentry.
What are the two general causes of bradycardia?
Failure of impulse formation and failure of impulse conduction.
What is your diagnosis in this patient? How do you make the condition go away?
Notice the long PR interval (normally should be no longer than one large block). This is typical of 1st degree AV block that slows conductance from the SA node. This is often from vagal tone and can be eliminated by putting them on a treadmill and making them exercise. If the symptoms don't go away, it is not a benign condition.
What is your diagnosis with this patient? What will usually cause this condition to disappear?
Second degree AV block (Mobitz 1 or Wenkebach). Notice that there are more p waves that QRS complexes due to prolonged PR intervals. This condition will usually disappear with exercise because it is a result of vagal tone.
What is your diagnosis in this patient? What will happen if you put this patient on a treadmill?
Second degree heart block (Mobitz 2). Notice wide QRS interval and no prolonging of PR interval. This tells you there is disease in the His-Purkinje system. If you put this patient on a treadmill, their condition will get worse.
What is your diagnosis? How do you make this go away?
Third degree (complete) heart block. Note that the p-wave and QRS complexes are marching to their own tune. This is fixed with a pacemaker.