Bradyarrythmias Flashcards
Why is the sinoatrial node the natural pacemaker of the heart?
Sino-atrial node has most # slow Na+ channels, fastest spontaneous rate = natural pacemaker
What antagonizes phase 4 current?
Na+/K+ ATPase pump always tries to keep cell voltage –ve = antagonizes Phase 4 depolarization from slow Na+ current
Where does the overdrive suppression occur and why?
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
What increases firing rate of SA node?
sympathetic nervous input (‘fight or flight’)
Catecholamines in blood (adrenaline)
–> opens more Phase 4 channels, ↑ slope.
–> Also the threshold is lowered
What decreases the SA node rate?
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
What are drugs that decrease SA node rate?
Drugs that decrease Sino-Atrial = decrease Heart Rate
beta-blockers
Cholinergics
some Ca++ channel blockers
What are drugs that increase SA node rate?
Drugs that increase Sino-Atrial = increase Heart Rate
Catecholamines (especially -agonists)
Anti-cholinergics (e.g. Atropine)
?theobromides (e.g. caffeine excess)
What is sinus brachycardia?
by definition, Sinus rhythm < 60 beats */ min
Clinically, don’t worry about bradycardia unless <50 for most patients
What causes bradycardia?
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)
What is sick sinus syndrome?
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)
Why might fast non-sinus atrial rhythms occur more in sick sinus patients (tachy-brady syndrome)?
If sinus node fails –> other latent atrial pacemakers activated, and if several activated at nearly same time –> chaotic atrial rhythms –> tachycardia
Why might the sinus take several seconds to start firing after such a tachycardia patient faints?
Sinus node already ‘sluggish’ and is overdrive suppressed by preceding tachycardia
What are escape rhythms?
if sinus node stops firing / fires too slowly, other latent pacemakers can take over = escape rhythms
Which types of AV node blocks actually cause bradycardia?
only 2nd and 3rd cause actual bradycardia
What is a first degree AV node block?
1st degree = 1° = every P wave conducts, but >200ms delay between start P & QRS
–> >200ms, >5mm delay from start PQRS
What is a secondary degree AV node block?
2nd degree = 2° = only a proportion of P waves conducted by AV node (2 types)
What is a third degree AV node block?
3rd degree = 3 ° = NO P waves conducted –> need escape rhythm @ or below AVN
What makes up the PR internal?
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.
What causes a prolonged PR interval?
Increased vagal tone AV node ischemia B-blockers and some calcium channel blockers --> Depress conduction through AV node Chronic degenerative changes (fibrosis) Infarct
What are the two types of secondary AV node blocks?
Mobitz I = ‘Wenckebach
Mobitz II:
What is Mobitz II?
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
What is Mobitz I = ‘Wenckebach?
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)
What is the hallmark of Wenckenbach?
Grouped beating
T or F: Wenckenbach is unhealthy
F: Benign and seen in: Trained athletes Children Sleeping Vagal tone Temporarily seen in ischemia to AV node
What causes Wenckebach?
can be sign of high vagal /parasympathetic tone (e.g. athletes)
can be consequence of myocardial infarct
When is Wenckebach concerning?
generally doesn’t cause major problems, unless already slow P-wave rate, or if 2:1 conduction
What is treatment for Wenckebach?
generally responds to anti-cholinergic drug
T or F: Mobitz II is more than dangerous than I
T:
more dangerous than Mobitz I, especially if high P:QRS ratios, easily –> complete 3° AV block
What usually causes Mobitz II?
Usually due to conduction disturbance below the AV node
often seen with infarcts or age-related degeneration of the His-Purkinje system.
What is treatment of Mobitz II?
Treatment is usually required with a pacemaker
What is a tertiary AV node block?
- 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
How is a pacemaker wired?
wire threaded into vein R.atrium R.Ventricle
What is indicated for a pacemaker?
if major bradycardia, no easily reversible cause, need electronic pacemaker
