The heart:
• cells that generate impulses = pacemaker system
• cells that conduct impulses = conduction system
• cells that respond to electrical impulses = working myocardium
rhythmicity: the frequency and regularity of pacemaking activity are intrinsic to the heart
all-or-none response: an impulse arising at any part of the heart→ copmlete contraction of
both ventricle and atria or to none
•
•
•
a pacemaker potential- no constant resting potential = the cell potential in sinoatrial
node→ slow depolarization immediately after each repolarization
MDP = maximum diastolic potential- the most negative diastolic cell potential (-70
mV)
PP = prepotential = the slow diastolic depolarization→ threshold potential (TP)→
triggering of action potential (AP)
AP
→ voltage-gated Ca channels (associated with dihydropiridine receptors) in the
sarcolemma of myocardial cells opened
→ Ca influx from extracellular fluid
→ local increase in cytosolic Ca triggers ligand-gated ryanodine-sensitive Ca channels in
the sarcoplasmic reticulum
→↑ Ca in cytosol
→ electromechanical coupling
→ myocardial contraction
→ active Ca transport back
1. into Ca stores by Ca-ATPase (SERCA),
2. to extracellular fluid by Ca-ATPase and 3Na/Ca exchange carrier driven by
electrochemical Na gradient established by Na-K-ATP-ase
rate of impulse generation in SA node (pacemaker)→ heart rate (sinus rhythm at rest:
60-100 beat per min)
• the intrinsic rhythms of other pacemakers- slower than sinus rhythm (slope of their
PPs- flatter)
Working myocardium: voltage-gated fast Na channels
•
chronotropism- rate of impulse genration by the pacemaker (firing frequency of SA
node)→ heart rate
•
dromotropism (velocity of impulse conduction; AV node!!!)
•
inotropism (heart contractily = force of working muscle contr.)
pathological changes in impulse generation or conduction (visible in ECG)
disturbances in impulse generation (normal- nomotopic impulses generated):
1. sinus tachycardia (> 100/min)
2. sinus bradycardia (< 60/min) (1,2: the rate is regular)
3. sinus arrhythmias (the rate varies)
ectopic pacemakers:
focus in atrium, AV node, ventricles→ abnormal ectopic (heterotopic) impulses generated
(+/- nomotopic –sinus impulses genrated):
1. atrial tachycardia→ ventricular response rate > 200/min (only second/third stimulus
transmitted to the ventricles (Purkinje= frequency fibers)
2. atrial flutter >350/min→ totally irregular ventricular stimulation
3. extrasystoles in ventricles→ ventricular tachycardia→ inadequate ventricular filling → ↓
stroke volume
4. ventricular fibrillation (uncoordinated contr.)
AV conductance disturbed→ AV block:
1. first-degree: prolonged, normal impulse conduction in AV node (PQ interval > 0,2 sec)
2. second-degree: every second/third impulse conducted (2:1, 3:1)
3. third-degree: no impulses conducted→ ventricular atopic pacemakers take over
(ventricular bradycardia with normal atrial excitation rate)→ heart rate 40-55/min (AV node
is pacemaker) or 25-40/min (ventricular pacemaker)→ sudden cardiac arrest (AdamesStock attack; syncope)
Bundle- branch block
arrhythmia→ abnormality in
• impulse generation
• impulse conduction
• both
because of
◊ ischemia of heart muscle (infarction, ↑oxygen demand, anemia)
◊ endocrinological disturbances (hypothyroidism)
◊ electrolyte imbalance
◊ excess of catecholamines/ACh/↑ sensitivity to transmitters
◊ autonomic nervous system influence
◊ heart muscle infarction→ scar formation
◊ drugs (TCA, thyroid hormones prep., antiarrhythmics)
◊ toxins
◊ alcohol
◊ stimulants: caffeine, nicotine
Antidysrhythmic drugs
Class I: drugs that block voltage-sensitive sodium channels (fast current)
They are subdivided: Ia, Ib, Ic
Class II: beta-adrenoceptor antagonists
Class III: drugs that substantially prolong the cardiac action potential (↑repolarization
phase)
Class IV: calcium channel antagonists
Effects of antidysrhythmic drugs on the different
phases of the cardiac action potentials
Pacemarker potential
Phase 4
Class II
Rapid depolarisation
Phase 0
Class I
Plateau
Phase 2
Class IV
Class II
Repolarisation
Phase 3
Class III
(and Ia)
Vaughan Williams’s classification of antidysrhythmic drugs
Ia Dysopyramide: sodium channel block
intermediate dissociation
Ib Lidokaine: sodium channel block
fast dissociation
Ic Flecainide: sodium channel block
slow dissociation
II Propranolol: beta-adrenoceptor
antagonism
III Amiodarone
Sotalol: potassium channel block
IV Verapamil: calcium channel block
Antidysrhythmic drugs unclassified in the Vaughan William’s system
Atropine used in sinus bradycardia
Adrenaline (epinephrine) used in cardiac
arrest
Isoprenaline used in heart block
Digoxin used in rapid atrial fibrillation
Adenosine used in supraventricular tachycardia
Calcium chloride used in ventricular tachycardia
Magnesium chloride used in ventricular fibrillation and in case of digoxin toxicity