Pharmacology of rhythm control Flashcards
Where does electrical conduction of the heart begin?
At the sinoatrial node (SAN) which is near the SVC junction
Describe the journey of electrical impulses
SAN –> AVN –> Bundle of His –> LBB –> RBB –> Purkinje network
Explain the different phases of action potentials in cardiomyocytes (PHASES 0-4)
• Resting TMP in cardiomyoctes (ventricular AP) is -90mV
PHASE 0: is rapid Na+ influx through fast Na+ channels (+55mV) this is depolarisation
PHASE 1: is when you have the transient K+ channel opening, hyperpolarising the cell to 0mV
PHASE 2: is the plateau phase, balancing of Ca2+ influx through LTCC and K+ efflux through delayed rectifier K+ channels
PHASE 3: is when the Ca2+ channels close and the delayed rectifier K+ channels remain open
PHASE 4: (resting phase) is the Na+ and Ca2+ channels closed and recover from inactivation, whilst the open K+ delayed rectifier channels keep the TMP at -90mV
Which cardiac cell types display pacemaker behaviour?
SA node
AV node
Purkinje fibers
Ventricular cardiomyocytes
What is the absolute refractory period?
When the cell is absolutely unexcitable to a new stimulus
What is the effective refractory period?
The ARP + short segment of phase 3 during which a stimulus may cause the cell to depolarise minimally but will not result in a propagated action potential
What is the relative refractory period?
A greater than normal stimulus will depolarise the cell and cause an AP
During which period are cells particularly susceptible to arrhythmias?
During the supra-normal period, with an inappropriately timed stimulus
- This is why one must synchronise the electrical stimulus during cardioversion to prevent inducing ventricular fibrillation
What is the supra-normal period?
A hyperexcitable period during which a weaker than normal stimulus will depolarise the cells and cause an AP
What is bradycardia? (in bpm)
<60bpm
What is tachycardia? (in bpm)
> 100bpm
Different causes of bradycardia?
- Sinus bradycardia
- Sick sinus syndrome
- AV block (1st degree, 2nd degree and CHB)
Different types of tachycardia?
- Supraventricular
- Sinus tachycardia
- AVRT/AVNRT/Atrial tachycardia (Atrioventricular reentrant tachycardia and Atrioventricular nodal reentry tachycardia)
- Atrial flutter/AF - Ventricular
- VT
- VF
- Torsades de pointes
What are bradyarrhythmias?
- Failure of pacemaker cells to generate appropriate electrical impulses AKA disorders of automaticity
- Sinus bradycardia
- Sick sinus syndrome - Failure to propagate electrical impulses appropriately AKA heart block
- AV block (1st/2nd/3rd degree)
What is automaticity?
- The heart rate gradually increases and gradually decreases
- Automaticity is when you get abnormal acceleration of phase 4 activity which occurs in some locations within the heart
What are examples of automaticity?
Sinus tachycardia
Multifocal atrial tachycardia
What are some causes of automaticity?
Metabolic causes Ischaemia Hypoxaemia Acid-base disorders High sympathetic tone Use of sympathomimetic agents
What is the most common mechanism for tachyarrhythmias?
Re-entry
- This is the mechanism for AVRT, AVNRT and VT
What is the most common mechanism for tachyarrhythmias?
Re-entry
- This is the mechanism for AVRT, AVNRT and VT
What is proarrhythmia?
The provocation of a new arrhythmia or the aggravation of a pre-existing one during therapy with a drug at doses considered toxic
What is digoxin used to treat?
Atrial fibrillation
Atrial flutter
Congestive cardiac failure
Mechanism of digoxin?
Mechanism 1: AV Node Inhibition: Digoxin has vagomimetic/parasympathomimetic effects on the AV node. By stimulating the parasympathetic nervous system, it slows electrical conduction in the atrioventricular node, therefore, decreases the heart rate = negative chronotropy (<hr>contractility (inotropy) of the heart, which causes smooth muscle contraction and vasoconstriction
Side effects of digoxin
- Abnormal cardiac rhythms
- GI effects: anorexia, nausea, vomiting, diarrhoea
- Visual effects: xanthopsia, blurring
- Gynecomastia
- CNS effects: confusion, agitation, nightmares and psychoses
What is amiodarone used for?
SVT and VT
What is the most frequently used anti-arrhythmic drug used in hospital settings?
Amiodarone
How is amiodarone administered and why?
Through a central line as it can cause skin necrosis
Side effects of amiodarone
- Abnormal cardiac rhythm (bradycardia, heart block, ventricular arrhythmia)
- Corneal microdeposits
- Hypo and hyperthyroidism
- Photosensitivity reactions
- Blue-grey skin discolouration
- Abnormal LFTs
- Lung fibrosis
Side effects of beta-blockers: BALD FISH
B) Bronchoconstriction and bradycardia
A) Arrhythmias
L) Lethargy
D) Disturbance in glucose metabolism
F) Fatigue
I) Insomnia
S) Sexual dysfunction
H) Hypotension
Indications for lidocaine?
VT
- Used when amiodarone is contraindicated or ineffective
