Session 9 Flashcards
Explain how arrhythmias can arise
Disturbance of impulse generation and/or impulse conduction.
Ectopic pacemaker activity
After depolarisation
Re-entry loop (may lead to atrial fibrillation)
Describe the types of drugs used to treat patients with common cardiovascular disorders and their mechanism of action
Antiarrhythmic drugs:
Class I - local anaesthetics
Class II - B-adrenoceptor antagonists
Class III - K+ channel blockers
Class VI - Ca2+ channel blockers
Adenosine, cardiac glycoside (enhance vagul input to heart) Amiodarone
Inotrophic drugs:
Cardiac glycoside (digoxin) - inhibit Na+ pump, increase in intracellular Na+, NCX doesn’t pump out Ca2+, rise in intracellular Ca2+
Adrenaline, dobutamine - activate B1 adrenoceptors
B-blockers - prevent action of noradrenaline
ACE inhibitors (chronic heart failure):
Prevent formation of vasoconstrictor angiotensin II, promotes vasodilation, decrease blood volume
Organic nitrates (angina):
Release NO –> dilate veins, –> reduce central venous pressure, dilation of collateral coronary arteries
Anti thrombotic drugs:
Heparin (IV), warfarin (orally), aspirin
Describe the classes of anti-arrhythmic drugs and the principles of their therapeutic use
Class I - block v-gated Na+ channels in an open or inactive state e.g. Lidocaine. Prevents firing of APs too close to one another.
Class II - B-blockers e.g. Propanolol. Block sympathetic activity, decrease slope of pacemaker potential in SAN, inhibits adenyl cyclase
Class III - block K+ channels. Prolong action potentials, absolute refractory period is lengthened. Can be pro-arrhythmic.
Class VI - block Ca2+ channels e.g. Verapamil. Decreases slope of pacemaker potential in SAN, decreases AVN conduction, coronary and peripheral vasodilation.
Adenosine - produced endogenously, inhibits adenyl cyclase at AVN, enhances K+ conductance.
Describe the therapeutic uses of B-adrenoceptor antagonists
Reduce force of contraction, reduce workload of heart following MI, reduce heart rate, prevent arrhythmias following MI, treatment of angina and clinically stable heart failure.
Define the term ‘inotrophic drug’ and the circumstances under which these drugs can be used
Inotrophic drug - drugs that affect the force of contraction of the heart.
Negative inotrophic drugs can be used following an MI to reduce the oxygen requirement of the heart and limit further damage e.g. B-blockers.
Positive inotrophic drugs are used in cardiogenic shock or acute reversible heart failure (following cardiac surgery) e.g. Dobutamine.
Describe how drugs can be used in the treatment of heart failure
ACE inhibitors prevent the formation of the vasoconstrictor angiotensin II, promoting vasodilation of arterioles and venous dilation. They decrease both pre-load and after-load to the heart.
ACE inhibits also have a diuretic action as normally angiotensin II promotes aldosterone release from the adrenal cortex (zona glomerulosa). Aldosterone causes Na+ and water retention, increasing blood volume. ACE inhibitors prevent angiotensin II formation so blood volume decreases and pre-load of the heart also decreases.
Explain the mechanism by which organic nitrates alleviate angina
Organic nitrates react with thiols (-SH groups) in vascular smooth muscle which causes NO2- to be released. No2- is reduced to NO, a powerful vasodilator.
NO activates guanylate cyclase, increasing cGMP and lowering intracellular Ca2+ –> relaxation of vascular smooth muscle.
NO acts on the venous system as a venodilator, lowering central venous pressure and pre-load of the heart –> reduced forces of contraction.
NO acts on coronary arteries, improving oxygen delivery to the myocardium.
Name some cardiovascular conditions which have an increased risk of thrombus formation and describe the pharmacological agents used to minimise the risk
Atrial fibrillation, valve disease.
Antithrombotic drugs e.g. Warfarin.
Anti platelet drugs e.g. Aspirin (following MI or in CHD)
Describe how drugs can be used in huge treatment of hypertension
Drugs act to reduce cardiac output and/or peripheral resistance e.g. ACE inhibitors, diuretics, adrenoceptor blockers, calcium channel blockers.
