Give the conventional definitions of each:
- Slow heart rate (<60 bpm)
- Fast heart rate (>100 bpm)
What is a slow physiologic rhythm called?
What is a fast physiologic rhythm called?
abnormal bradycardic rhythm (<60bpm)
excludes phsyiologic bradycardia such as sleep or rest
abnormal tachycardic rhythm (>100bpm)
excludes physiologic sinus rhythms, such as exercise or stress
Define supraventricular tachycardia (SVT)
abnormal tachycardia requiring participation of either atrial or AV nodal tissue
Chaotic: atrial fibrillation
Define ventricular tachycardia (VT)
abnormal tachycardia originating in the His bundle or Purkinje system. Does not require involvement of the atrium or AV node.
Chaotic: vetricular fibrillation
All disorders of heart rhythm arise as a consequence of what (2) effects?
- alterations in impulse function
- alterations in impulse conduction
Explain overdrive suppression
The hyperpolarizing current increases when a cell is forced to fire faster than its intrinsic pacemaker rate. The more frequently the cell is depolarized, the greater the quantity of Na+ ions that enter the cell per unit time. As a result of the increased intracellular Na+, the Na+K+-ATPase pump becomes more active, thereby tending to restore the normal transmembrane Na+ gradient. This increased pump activity provides a larger hyperpolarizing current, opposing the depolarizing current If, and further decreases the rate of spontaneous depolarization. Thus, overdrive suppression decreases a cell’s automaticity when that cell is driven to depolarize faster than its intrinsic discharge rate.
Give the normal intrinsic rates of the following:
Where does normal rhythm usually originate?
What is the appriximate normal resting potential of sinud node and AV nodal pacemaker cells?
What about working myocytes?
Normal automaticity is regulated by what?
What factors determine automaticity?
autonomic tone (sympathetic and parasympathetic)
- Rate of diastolic depolarization (mostly If)
- Maximum negative diastolic potential
- Threshold potential
The occasional long pause in the attached image is indicative of what? Is this pathological? What are the symptoms?
Near syncope (increased parasympathetic tone)
Sx: nausea, sweating, 'not feeling well'
What is a ventricular escape?
An escape arising from infranodal tissue
What dominates at rest (sympathetic or parasympathetic tone)?
Describe briefly the sympathetic and parasympathetic effects on automaticity
- Rate augmented (increased) by sympathetic tone
- Beta-andrenergic stimulation increases the 'open' probability of the pacemaker current (increases If), increasing the rate of diastolic depolarization
- Lowers the threshold potential (makes more negative)
- Rate depressed (decreased) by parasympathetic tone
- Decreases the open probability of the pacemaker current (decreases If)
- Increases the threshold potential (makes more positive)
- Increases probability of ACh-sensitive K+ channels (IKACh) being open at rest, leading to a more negative membrane diastolic potential
Why might automaticity be augmented under pathologic fibrosis?
Pacemaker cells are no longer inhibited by neighboring working myocardium.
Normally, intracellular current between more negative working cells and less-negative pacemaker cells actually makes the pacemaker cell more negative, decreasing automaticity.
What is a Junctional escape?
An escape rhythm arising in the AV node.
Narrow QRS without a preceding P wave, typically 50-60bpm
What is abnormal automaticity?
Abnormal impulse or automaticity arising from any tissue not normally capable of pacemaker activity. Often a consequence of cellular injury, coronary ischemia, myocardial disease, etc
What is triggered activity?
Single or repetitive cellular activity following a prior action potential due to oscillations in membrane potential. Can be triggered by tachycardia or bradycardia
Define EAD (early afterdepolarization)
Define DAD (delayed afterdepolarization)
- membrane oscillations which occur within the action potential
- usually occur during phase 2 (Ca current) or phase 3 (reactivation of fast Na channels)
- Promoted by conditions which prolong the action potential (QT prolongation)
- mechanism underlying Tosades de Pointes VT
- Often precipitated by QT-prolonging drugs
- membrane oscillations occurring after completion of full repolarization (phase 4)
- promoted by conditions that lead to higher than normal intracellular Ca
- promoted by catecholamines, inhibited by Ca channel blockers
- mechanism underlying Idiopathic VT and digitalis-induced PVC or VT
Name the three key requirements for reentry:
- Two distinct paths
- slowed conduction
- unidiretional block - tissue capable of conduction in one direction but not the other. Often 'functional' - that is, depending on timing errors elsewhere
What condition often shows delta waves?
Wolf-Parkinson-White Syndrome (WPW)
Functional conduction (slight delays, not complete blocks) obstacles lead to waves and 'whorls' or re-entry.
What does this lead to (what is it called)?
What is an ectopic beat?
Abnormal automaticity leading to beats arising from unusual locations
may contribute to tachycardia