Cardiovascular Pharmacology Flashcards
What are the P wave, QRS complex and T wave representations on the ECG for?
P wave = Depolarisation of the Atria
QRS complex = Depolarisation of the ventricles
T wave = Repolarisation of the ventricles
In a Ventricular Action Potential there are 5 phases (0-4). What are they? (Contractile Myocytes)
0 = Depolarisation 1 = Rapid Repolarisation 2 = Plateau 3 = Repolarisation 4 = Baseline Potential
Pacemaker cells have a different Action Potential sequence to contractile myocytes. Highlight the action potential they go through.
0 = depolarisation
3 = repolarisation
4 = baseline potential
repeat
Membrane depolarisation and repolarisation is a results of ion flows across the membrane of the cell. What ion channels are responsible for depolarisation and repolarisation?
Depolarisation:
Sodium INFLUX, Calcium INFLUX
Repolarisation:
Potassium EFFLUX, Active Na+-K+-ATPase
What do the following in the ECG represent? PR Segment PR Interval ST Segment QT Interval
PR Segment = AV Delay
PR Interval = Atrial Systole
ST Segment = Ventricular Plateau
QT Interval = Ventricular Depolarisation & Repolarisation
What are the ion mechanisms for a contractile myocyte action potential? Describe with the 5 phases (0-4)
0 = Rapid Na+ influx via Voltage Activated Sodium Channels (iNa) 1 = Transient K+ efflux (iTO1) 2 = Ca2+ influx via L-Type Voltage Activated Calcium Channels (iCa(L)) 3 = K+ efflux via: slow (iKs) & rapid (iKe) delayed rectifier potassium channels, also via iK1 4 = K+ efflux via iK1
What is absolute/effective refractory period?
The refractory period when the generation of the next action potential is not even possible at the strongest stimulus.
What is relative refractory period?
When the generation of the next action potential is possible, but requires a stronger stimulus.
Describe the mechanism of refractoriness
- Depolarisation stimulus opens Voltage Gated Sodium Channels
- They are rapidly INACTIVATED. If depolarisation is maintained (plateau), channels cannot be reopened until…
- Repolarisation below the threshold voltage (-65mV) recovers Na+ channels from inactivation moving them into the closed state.
Why does prolonging refractoriness in the heart a problem?
Causes a long QT interval which can promote cardiac dysrhythmias.
Describe the ion mechanisms that occur in a Pacemaker Potential.
4 = Pacemaker current 4 = Voltage gated T-type Calcium influx, iCa(T) 0 = Voltage Activated L-Type Calcium influx, iCa(L) 3 = Voltage Activated K+ Channels efflux, iK
What 2 ways activate iF (funny current)?
Membrane repolarisation (hyperpolarisation) Directly by cAMP (kinase dependent phosphorylation)
How is the iF current modulated by the Peripheral Nervous System?
Sympathetic: STIMULATION via β1 adrenoceptors; increases current.
Parasympathetic: INHIBITION via Muscarinic M2 receptors; decreases current
What functions does the iF current have?
Cardiac Automaticity
Heart Rate control by the PNS and Adrenaline
What drug will inhibit the iF current?
Ivabradine
Describe the Ion Mechanisms for the modulation of the Pacemaker potential via the Peripheral Nervous System.
Sympathetic: 1) β1 Adrenoceptor is stimulated 2) Increase Adenylyl Cyclase 3) Increase Cellular cAMP 4) Increase iF current 5) Increase the rate of pacemaker depolarisation 6) Increase conduction velocity RESULT: INCREASED HEART RATE
Parasympathetic: 1) M2 receptor is stimulated 2) Decrease Adenylyl Cyclase 3) Decrease Cellular cAMP 4) Decrease iF current 5) Decrease the rate of pacemaker depolarisation 6) Decrease conduction velocity RESULT: DECREASED HEART RATE
What are the 3 major pathways and 1 minor pathway of contraction through calcium entry and release?
MAJOR PATHWAYS:
1) Activation of L-VACC (Voltage Activated Calcium Channels) - responsible for phase 2 plateau - provides the influx of Ca++ ions.
2) Activation of RyRs on the Sarcoplasmic Reticulum (SR)
3) Calcium-Induced Calcium release from the Sarcoplasmic Reticulum
MINOR PATHWAY:
4) Ca++ influx via Ca++/Na+ Exchanger (NCX)
What are the activators and inhibitors of RyR2 (cardiac Ryanodine Receptors)?
Activators:
Ca++ ions
Ryanodine (at nanomolar concs)
Inhibitors:
Ryanodine (At micromolar concs)
What are the sensitisers for RyR2?
Caffeine
Halothane
CATECHOLAMINES
What are the MAJOR and MINOR calcium pathways for cardiac muscle relaxation?
- Ca++ REUPTAKE into the Sarcoplasmic Reticulum via SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase) (70-80%)
- Ca++ extrusion by Na+-Ca++ exchanger (NCX) (20-30%)
- Ca++ extrusion by Ca++-ATPase (Ca++-pump) ~1% Ca++ removal
- Ca++ uptake into mitochondria via mitochondrial Ca++ uniporter (~1% Ca++ removal)
What regulates the Cardiac Isoform of SERCA (SERCA2a)?
Phospholamban (PLB)
How does PLB regulate Ca++ reuptake?
RE-UPTAKE
1) β1 adrenoceptors are stimulated from catecholamines
2) PKA is activated.
3) PKA promotes the phosphorylation of PLB
4) Phosphorylated-PLB promotes faster re-uptake and therefore faster relaxation
The Sodium-Calcium Exchanger (NCX) has two different modes. What are they and how do they work?
1) Forward (Normal) Mode - Responsible for repolarisation in phase 3 of AP.
Activated by a build up of cytosolic Ca++ during the cardiac AP
Extrude 1 Ca++ ion in exchange for 3 Na+ ions.
2) Reverse Mode - Responsible for the depolarisation in phase 0 of AP
Activated by a build up of cytosolic Na+
Exclude 3Na+ ions for 1 Ca++ ion
Increase in cytosolic [Ca++] (MINOR ROLE)
Sympathetic stimulation has the effects of both increasing AND decreasing the concentration of cytosolic Calcium. Why?
Increase in [Ca++]:
1) increase in calcium influx via L-voltage activated calcium channels activation
2) Increase in calcium release via RyR2 activation
3) Increase in calcium sensitisation via Tropinin I phosphorylation
Decrease in [Ca++]:
1) Increase in calcium reuptake via increase in SERCA activity (PLB-mediated)
What drug binds to the Ca++-TNC complex to prolong systole action?
Levosimendan
Describe the ion mechanism in cardiac glycosides (eg. Digoxin)
Digoxin will inhibit the Na+/K+-ATPase by binding to the extracellular K+ binding site of the ATPase.
There will be an indirect activation of the REVERSE MODE of the NCX as a result of the accumulation of the intracellular Na+, thus resulting in an increase in cytosolic Ca++.
What is digoxin used for?
Congestive Heart Failure
Antidysrhythmic: Atrial Fibrillation & Atrial Flutter
In terms of coronary circulation, what are the roles of the left and right coronary artery?
Left Coronary Artery: supplies the left and right sides of th heart (about 85% of CBF)
Right Coronary Artery: Supplies the SA node, AV node and right side of the heart
What cardiac metabolites will cause vasodilation of the coronary arteries?
Adenosine
Potassium
Hypoxia (decreased tissue partial oxygen)
In an emergency with heart block, what drugs are used?
Atropine (IV), or Isoprenaline (IV)
There are several mechanisms behind tachydysrhythmias, what are they?
Automaticity:
Enhanced (Sympathetic overreactivity)
Abnormal (Ectopic pacemaker)
Triggered:
Early after depolarisation (EAD) - TORSADES DE POINTES
Delayed After Depolarisation (DAD)
Reentry
There are three types of Re-entry. What are they?
AV Node and Atria (AVNRT) Purkinje Fibres (VT) Accessory Pathways (AVRT)
Re-entry causes what on an ECG?
An atrial echo
What is the common cause of Atrial Fibrillation?
Random ectopic activity from the pulmonary vein in the left atrium
Classify antidyrhythmic drugs according to the Vuaghan-Williams Classification
Class I: Na+ Channel Blockers
Class II: β-Blockers
Class III: Drugs that prolong the action potential duration
Class IV: Calcium Channel Blockers
What is the main mechanism of Class I antidysrhythmics?
Blockage of FAST voltage activated Na+ Channels
Class I antidysrhythmics are classified based on their dissociation rate and are therefore classified into three sub-types. What are they?
Class Ia: Intermediate Dissociation Rate
Class Ib: Fast Dissociation Rate
Class Ic: Slow Dissociation Rate
What are the drugs in each subtype of Class I Antidysrhythmics?
Ia: Disopyramide, Procainamide, Quinidine
Ib: Lignocaine, Mexilitine
Ic: Flecainide, Propafenone, Moricizine
Class Ia drugs have what effects on the Cardiac Action Potential and what effects on the ECG?
Cardiac Action Potential: decreases Vmax, increases duration
ECG: increases QRS duration, increases QT interval
Class Ib drugs have what effects on the Cardiac Action Potential and what effects on the ECG?
Cardiac Action Potential: No effect on Vmax, Decrease in duration
ECG: QRS duration unchanged, but a modest decrease in the QT interval
NB Blocks extra heart beats
Class Ic drugs have what effects on the Cardiac Action Potential and what effects on the ECG?
Cardiac Action Potential: Decrease in Vmax, Small increase/no effect on Action Potential
ECG: Increase in QRS duration
modest increase in QT interval
From each Class I antidysrhythmic subtypes, list the most relevant drug, and its therapeutic use.
Class Ia: Disopyramide - Atrial Fibrillation - Ventricular Tachycardia Class Ib: Lidocaine -Ventricular Tachycardia & Ventricular Fibrillation Class Ic: Flecainide -Paroxysmal Supraventricular Tachycardia (due to AVNRT) -Atrial Fib. & Flutter -WPW syndrome
What are the clinical uses of Class II Antidysrhythmics (β-Blockers)?
Rate control in Supraventricular Tachycardias (due to increased sympathetic activity)
Rate and rhythm control in Atrial Fibrillation & Flutter
Ventricular Fibrillation
What are the drugs that are classified under Class III antidysrhythmics?
Amiodarone, Sotalol, Bretylium