Lecture 7 - Cardiac Action Potentials; Drugs and the CVS Flashcards
how is the cardiac resting membrane potential generated?
cell membrane mostly permeable to K
K move out taking positive charge with them
what is the cardiac resting membrane potential?
around -90mV
how is heart rate increased?
sns on san
noradrenaline (alpha 2 receptor) makes the pacemaker potential steeper
how is heart rate decreased?
pns on san
acetylcholine (m3 receptor) makes the pacemaker potential shallower
what is the role of baroreceptors?
role in controlling heart rate
stretch sensitive
pass information to medulla to cause pns innervation of san
slows heart rate
what are cardiovascular drugs used to treat?
arrhythmias heart failure angina hypertension risk of thrombus formation
what do cardiovascular drugs alter?
rate and rhythm of heart
force of myocardial contraction
peripheral resistance and blood flow
blood volume
what are causes of arrhythmias?
ectopic pacemaker activity
after depolarisation
reentry loop
what is ectopic pacemaker activity?
damaged area of myocardium becomes depolarised and spontaneously active
latent pacemaker activated due to ischaemia
dominates over san
what are after depolarisations?
abnormal depolarisations after the ap
maybe caused by high intracellular Ca
longer ap
longer qt
what is the reentry loop?
conduction delay
normal spread of excitation disrupted due to damaged area
incomplete conduction damage
what are the four classes of anti arrhythmic drugs?
- drugs that block voltage gated Na channels
- antagonists of beta-adrenoceptors
- drugs that block K channels
- drugs that block Ca channels
describe class 1 anti arrhythmic drugs
eg lidocaine blocks open or inactive channels dissociates rapidly in time for next ap prevents firing of ap too close together after depolarisations cant cause another ap
describe class 2 anti arrhythmic drugs
eg propanolol, atenolol
block sympathetic action by acting on beta 1 receptors
decreases slope of pacemaker potential
inhibits adenyl cyclase decreasing inotropy
after an am increases sns activity and reduces O2 demand of mycardium
describe class 3 anti arrhythmic drugs
prolong ap by blocking K channels which repolarise
arp is lengthened
can also be pro arrhythmic
describe class 4 anti arrhythmic drugs
eg verapamil decreases slope of pacemaker potential decreases avn conduction decreases force of contraction coronary and peripheral vasodilation
what is adenosine?
produced endogenously acts on A1 receptors at avn enhances K conductance hyperpolarises cells of conduction tissue anti arrhythmic
what is an inotropic drug?
affect force of contraction of the heart
what does a negatively inotropic drug do?
reduces workload of heart
reduces o2 requirement of heart and limits further damage
eg beta blocker
what does a positively inotropic drug do?
when heart needs to beat more strongly
beta-adrenoceptor agonist
eg dobutamine
how are drugs used to treat heart failure?
ace-inhibitors prevent formation of angiotensin II (vasoconstrictor)
promotes vasodilation of arterioles and venous dilation
decreases afterload and preload to heart
what does angiotensin II do?
promotes aldosterone release from adrenal cortex
aldosterone causes Na and water retention increasing blood volume
what causes angina?
o2 supply to heart doesnt meet need ischaemia of heart causes chest pain
due to narrowing of coronary arteries
how is angina treated?
reduced workload to heart
beta blockers
Ca channel blockers
organic nitrates
what do organic nitrates do?
react with thiols in vascular smooth muscle to release NO2-
NO2- is reduced to NO which is a vasodilator
NO activates guanylate cyclase increasing cGMP and lowering intracellular Ca
causes relaxation of vascular smooth muscle
what is the primary action of angina treatment?
acts on venous system
venodilator
lowers central venous pressure and preload
heart fills less so force of contraction lowers
what is the secondary action of angina treatment?
acts on coronary arteries
improves o2 delivery to ischaemic myocardium
which heart conditions increase risk of thrombus formation?
af
valve disease
how are drugs used to treat hypertension?
reduce co and/or tpr
include ace inhibitors, diuretics, adrenoceptor blockers, Ca channel blockers
