Bradyarrythmias

Question 1

Why is the sinoatrial node the natural pacemaker of the heart?

Answer 1

Sino-atrial node has most # slow Na+ channels, fastest spontaneous rate = natural pacemaker

Question 2

What antagonizes phase 4 current?

Answer 2

Na+/K+ ATPase pump always tries to keep cell voltage –ve = antagonizes Phase 4 depolarization from slow Na+ current

Question 3

Where does the overdrive suppression occur and why?

Answer 3

a higher pacemaker forces a latent pacemaker to fire faster than its native rate

- ->  excess Na+ enters cell 
- -> ‘revs up’ Na+/K+ ATPase 
- -> slows Phase 4 even more

= OVERDRIVE suppression of latent pacemakers

Question 4

What increases firing rate of SA node?

Answer 4

sympathetic nervous input (‘fight or flight’)
Catecholamines in blood (adrenaline)
–> opens more Phase 4 channels, ↑ slope.
–> Also the threshold is lowered

Question 5

What decreases the SA node rate?

Answer 5

para-sympathetic nervous input (‘relax & rest’ system)
e.g. vagal maneuvers
closes more Phase 4 channels, decreased slope
–> increased threshold voltage
More negative at beginning of diastole

Question 6

What are drugs that decrease SA node rate?

Answer 6

Drugs that decrease Sino-Atrial = decrease Heart Rate
beta-blockers
Cholinergics
some Ca++ channel blockers

Question 7

What are drugs that increase SA node rate?

Answer 7

Drugs that increase Sino-Atrial = increase Heart Rate
Catecholamines (especially -agonists)
Anti-cholinergics (e.g. Atropine)
?theobromides (e.g. caffeine excess)

Question 8

What is sinus brachycardia?

Answer 8

by definition, Sinus rhythm < 60 beats */ min

Clinically, don’t worry about bradycardia unless <50 for most patients

Question 9

What causes bradycardia?

Answer 9

high para-sympathetic / vagal tone:

• physiological (e.g. fit athlete  slow resting HR)
• central nervous system autonomic imbalance
• nausea or other reflexes (e.g. carotid body massage)
• drugs (see previous list)
• age-related degeneration of SA node = Sick Sinus Syndrome
• damage to SA node (e.g. at time of surgery, or infarct)

Question 10

What is sick sinus syndrome?

Answer 10

common age-related degeneration of SA node, usually related to fibrosis

inappropriate sinus bradycardia, and / or inability to appropriately raise HR (e.g. exercise)

often associated with sudden tachycardias arising from chaotic atrial rhythms  ‘tachy – brady syndrome’

can be episodic, unpredictably cause too slow HR  patient (nearly) faints (often post-tachy)

Question 11

Why might fast non-sinus atrial rhythms occur more in sick sinus patients (tachy-brady syndrome)?

Answer 11

If sinus node fails –> other latent atrial pacemakers activated, and if several activated at nearly same time –> chaotic atrial rhythms –> tachycardia

Question 12

Why might the sinus take several seconds to start firing after such a tachycardia  patient faints?

Answer 12

Sinus node already ‘sluggish’ and is overdrive suppressed by preceding tachycardia

Question 13

What are escape rhythms?

Answer 13

if sinus node stops firing / fires too slowly, other latent pacemakers can take over = escape rhythms

Question 14

Which types of AV node blocks actually cause bradycardia?

Answer 14

only 2nd and 3rd cause actual bradycardia

Question 15

What is a first degree AV node block?

Answer 15

1st degree = 1° = every P wave conducts, but >200ms delay between start P & QRS

–> >200ms, >5mm delay from start PQRS

Question 16

What is a secondary degree AV node block?

Answer 16

2nd degree = 2° = only a proportion of P waves conducted by AV node (2 types)

Question 17

What is a third degree AV node block?

Answer 17

3rd degree = 3 ° = NO P waves conducted –> need escape rhythm @ or below AVN

Question 18

What makes up the PR internal?

Answer 18

Time from atrial activation to ventricular activation. 0.120 sec (3 small boxes) to 0.200 sec (5 small boxes):

• Conduction time from SA node to AV node
• AV node conduction time
• Conduction time from AV node to first ventricular activation.

Question 19

What causes a prolonged PR interval?

Answer 19

Increased vagal tone
AV node ischemia
B-blockers and some calcium channel blockers
--> Depress conduction through AV node
Chronic degenerative changes (fibrosis)
Infarct

Question 20

What are the two types of secondary AV node blocks?

Answer 20

Mobitz I = ‘Wenckebach

Mobitz II:

Question 21

What is Mobitz II?

Answer 21

no preceding PR prolongation, sudden blocked P (ie. the P doesn’t lead to QRS complex)
often, fixed ratio of P’s conduct: 2:1, 3:1, 4:1 etc.
–> 2:1 means 2 p-waves for every 1 QRS

Question 22

What is Mobitz I = ‘Wenckebach?

Answer 22

P-waves conduct with progressive increase in PR

• -> finally, AV blocks = non-conducted P, no QRS
• -> After the non conducted P wave the next P-wave conducts with most normal PR
• -> cycle repeats (eg. 2:1, 3:2, 4:3, 5:4 … P: QRS)

Question 23

What is the hallmark of Wenckenbach?

Answer 23

Grouped beating

Question 24

T or F: Wenckenbach is unhealthy

Answer 24

F:
Benign and seen in:
Trained athletes
Children
Sleeping
Vagal tone
Temporarily seen in ischemia to AV node

Question 25

What causes Wenckebach?

Answer 25

can be sign of high vagal /parasympathetic tone (e.g. athletes)
can be consequence of myocardial infarct

Question 26

When is Wenckebach concerning?

Answer 26

generally doesn’t cause major problems, unless already slow P-wave rate, or if 2:1 conduction

Question 27

What is treatment for Wenckebach?

Answer 27

generally responds to anti-cholinergic drug

Question 28

T or F: Mobitz II is more than dangerous than I

Answer 28

T:

more dangerous than Mobitz I, especially if high P:QRS ratios, easily –> complete 3° AV block

Question 29

What usually causes Mobitz II?

Answer 29

Usually due to conduction disturbance below the AV node

often seen with infarcts or age-related degeneration of the His-Purkinje system.

Question 30

What is treatment of Mobitz II?

Answer 30

Treatment is usually required with a pacemaker

Question 31

What is a tertiary AV node block?

Answer 31

• totally dissociated P-waves / QRS’s, therefore no relationship between atrium and ventricle
• regular P-P and R-R, but no relation
• need escape rhythm beneath AV node
• can cause fainting or even death
• age-related degeneration, or infarcts

Question 32

How is a pacemaker wired?

Answer 32

wire threaded into vein  R.atrium  R.Ventricle

Question 33

What is indicated for a pacemaker?

Answer 33

if major bradycardia, no easily reversible cause, need electronic pacemaker