6. Arrhythmias & Acute Affects Flashcards
What are the mechanisms of the interval-duration relationship?
- Increased Ca2+
- Increased Na+
- Sympathetic stimulation
What leads to increased cytosolic Ca2+ and Na+?
More frequent APs
What is an absolute (“effective”) refractory period?
Period of time after an AP is fired during which another AP cannot be fired
What dictates the duration of a refractory period?
AP duration
What is a relative refractory period?
Period of time after the absolute refractory period during which another AP can fire, but the resulting membrane potential will be smaller than normal
What is a supranormal period?
Period of time while membrane is still trying to stabilize during which an AP can trigger again very promptly w/ very little stimulation
What is heterogeneity in regards to AP characteristics? Significance?
- Individual cells are in different excitatory states
- Any spatial or temporal heterogeneity in AP characteristics may be arrhythmogenic
Define tachyarrhythmia.
- Increased firing rate
- HR > 100 bpm
Define bradyarrhythmia.
- Decreased firing rate
- HR < 60 bpm
What can cause tachyarrhytmias?
- Increased automaticy of SA node
- Increased automaticity of latent pacemakers
- Abnormal automaticity
- Unidirectional block and re-entry (“circus rhythms”)
- Triggerd activity: “after-depolarizations”
What can cause bradyarrhythmias?
- Enhanced vagal tone (athletic heart)
- Decreased automaticity of SA node
- SA node dysfunction –> latent pacemakers take over
- AV conduction block
What are after-depolarizations? When do they occur? Consequence?
- Oscillations of the membrane voltage triggered by a preceding AP
- Can be self-renewing and lead to a series of abnormal APs
- Blood is not pumped during this contraction
What is an early afterdepolarization? When do they occur?
Oscillating pattern of membrane potential that occurs during repolarization phase (refractory period)
What is a delayed afterdepolarization? When do they occur?
Prematurely triggered contraction that occurs after repolarization phase is complete
What is a reentrant rhythm?
An electrical impulse that circulates repeatedly, continually depolarizing a circuit of myocardium
What is the critical condition for reentry? In what ways can this condition be met?
The propagating impulse must continue to encounter excitable myocardium
- Electrically heterogenous myocardium (following ischemia)
- Anatomically heterogenous myocardium (scar)
What is the Wolff-Parkinson-White Syndrome?
- Anatomically-fixed reentry condition
- Conduction is faster in the bypass tract than AV node
- Conduction moves slower in ventricles b/c electrical signals travel through ventricular muscle (syncytial) rather than through the rapid, purkinje conduction system
- Sets up a two-circuit route w/ diff refractory periods
- Reentrant supraventricular tachycardia –> HR stays high after exercise has stopped
How can we estimate max HR?
220 - age in years
How does the heart respond to the onset of exercise?
Tachycardia: initial HR increase due to withdrawal of vagal tone and increased SNS tone
What are the expected EKG changes during acute exercise?
- Sinus tachycardia
- Hemodynamic changes: shortened diastole –> less coronary blood flow to heart
What is the only arrhythmia that exercise should reliably provoke?
Sinus tachycardia
What causes many “trivial” arrhythmias? (PACs, PVCs)
- Shortened diastolic interval
- Higher sympathetic stimulation
- Increased myocardial O2 demand
What is a consequence of ventricular tachycardia?
Not enough time for diastolic filling
