Dr Suryawan bin Tasref Department of Anaesthesiology & Intensive Care HSNZ
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Dr Suryawan bin Tasref
Department of Anaesthesiology
& Intensive Care
HSNZ
VASODILATORS
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CONTENT
Introduction
Classifications
Conclusion
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INTRODUCTIONvasodilator = drugs that relax the smooth muscle in blood vessels, which causes the vessels to dilate.
Dilation of arterial (resistance) vessels leads to a reduction in systemic vascular resistance, which leads to a fall in arterial blood pressure.
Dilation of venous (capacitance ) vessels decreases venous blood pressure
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CLASSIFICATION Based on their site of action (arterial vs venous)
Arterial dilators--primarily dilate resistance vessels (e.g., hydralazine)
Venous dilators--primarily affect venous capacitance vessels ( e.g., nitroglycerine).
Mixed arterial and venous dilator properties ( e.g., alpha-adrenoceptor antagonists, angiotensin converting enzyme inhibitors)
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INTRO-CLINICAL USEVasodilators are used to treat hypertension, heart failure and angina
Arterial dilators that act primarily on resistance vessels are used for hypertension and heart failure, but not for angina because of reflex cardiac stimulation.
Venous dilators are very effective for angina, and sometimes used for heart failure, but are not used as primary therapy for hypertension.
Most vasodilator drugs are mixed (or balanced) vasodilators in that they dilate both arteries and veins;
there are some drugs that are highly selective for arterial or venous vasculature
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CLASSIFICATIONBy mechanism of action
1. Antiadrenergic Agents
2. Vasodilators
3. Renin-Angiotensin System Inhibitors
4. Calcium Channel Blocker
5. Phosphodiesterase inhibitor
6. Beta adrenoceptor agonist
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1. Antiadrenergic Agents
Alpha-1 ReceptorsPrazosin
Phenoxybenzamine
Phentolamine
Beta ReceptorsPropanolol, Metoprolol, Atenolol, Esmolol
Alpha-Beta receptorsLabetolol
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1. Antiadrenergic Agents
Central Nervous SystemClonidine
Methyldopa
Autonomic GangliaTrimetaphan
Nerve endingsReserpine
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2. Vasodilators
Vascular Smooth MuscleHydralazine
Minoxidil
Diazoxide
Nitroprusside
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3. Calcium Channel Blocker
Vascular Smooth MuscleVerapamil
Nifedipine
Diltiazem
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4. Renin-Angiotensin System
Angiotensin Converting Enzyme InhibitorsCaptopril
Enalapril
Lisinopril
Angiotensin II Receptor AntagonistsLosartan
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1. Antiadrenergic Agents
A number of drugs that inhibit the adrenergic nervous system are available, including some that act
centrally on vasomotor center activity
peripherally on neuronal catecholamine discharge
by blocking alpha- and/or beta-adrenergic receptors
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Simplified schematic view of the adrenergic nerve ending showing that norepinephrine (NE) is released from its storage granules when the nerve is stimulated and enters the synaptic cleft to bind to alpha1 and beta receptors on the effector cell (postsynaptic). In
addition, a short feedback loop exists, in which NE binds to alpha2 and beta receptors
on the neuron (presynaptic), to inhibit or to stimulate further release, respectively
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1. Antiadrenergic Agents
Alpha-1 ReceptorsPrazosin
Phenoxybenzamine
Phentolamine
Beta ReceptorsPropanolol, Metoprolol, Atenolol, Esmolol
Alpha-Beta ReceptorsLabetolol
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PRAZOSINquinazoline derivative produces peripheral vasodilation; vascular tone in both resistance(arterioles) and capacitance(veins) vessels is reducedresulting in decreased VR, CO and BPnot assd with reflex tachycardia (presyn alpha-2)selective and competitive postsynaptic alpha-1 receptor blockade (5000 affinity than alpha-2 receptor)
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Pharmacokinetics Prazosin
administered orally (2 to 40mg/day) in divided bd doses
60% bioavailability
protein binding 90%
nearly completely metabolized by the liver
elimination half-time is about 3 hours (prolonged by cardiac failure)
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Side Effects Prazosin
vertigofluid retentionorthostatic hypotension‘first dose phenomenon’ ; dizziness, faintness, syncope soon after the administration of the first dosedryness of the mouth, nasal congestion, nightmares, urinary frequency, lethargy, and sexual dysfunction
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PHENOXYBENZAMINE
non-competitive and irreversible blocker
100x affinity for alpha-1
active metabolites which actually binds to the receptor; slow onset
poorly absorbed; 20-30%, dosage 40-60mg/day
elimination half-life; 24hours
indications; pre-operative preparation or long term Mx in phaechromocytoma
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PHENTOLAMINE
competitive antagonismless selective alpha blocker (3-5x alpha-1)also acts at histamine and Ach receptorsadministered intravenously
as a diagnostic test for phaechromocytoma; risk of CVS collapseto ctrl HPT during surgical removal of phaechromocytoma
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1. Antiadrenergic Agents (cont)
Alpha-1 ReceptorsPrazosinPhenoxybenzaminePhentolamine
Beta ReceptorsPropanolol, Metoprolol, Atenolol
Alpha-Beta ReceptorsLabetolol
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Mechanism of Action
competitive antagonist
binding is reversible
chronic administration is associated with an increase in the number of beta-adrenergic receptors (up-regulation)
produces some degree of membrane stabilization in the heart
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Classification selective and nonselective for beta-1 and beta-2 receptors partial or pure antagonists on the basis of the presence or absence of intrinsic sympathomimetic activity(ISA)antagonists with ISA cause less direct myocardial depression and heart rate slowing (better tolerated in patients with poor LV function)
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CLASSIFICATION
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Clinical Effect
negative inotropic and chronotropic effects
conduction speed (AVN) is slowed
decreased the rate of spontaneous phase 4 depolarization
antidysrhythmic effect
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Side Effects
increased airway resistance
unmask the signs of hypoglycaemia
precipitate cardiac failure
PVD and Raynaud’s phenomenon
hyperkalaemia
memory loss and mental depression
withdrawal hypersensitivity; up-regulation
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PROPANOLOL
non-selective blocker
lacks of ISA; pure antagonist
equal antagonism at beta-1 and beta-2
the first beta-antagonist introduced
the standard drug to which all other beta antagonists are compared
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Pharmacokinetics Propanolol
rapidly and almost completely GIT absorptionextensive hepatic first-pass metabolism (70%); poor bioavailabilityextensively bound to plasma proteins (90% to 95%) clearance is by hepatic metabolism to active metabolite, 4-hydroxypropranolol (equivalent in activity to propanolol)
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Pharmacokinetics Propanolol
elimination half-time is 2 to 3 hours,
elimination reduced when hepatic blood flow decreases
alterations in hepatic enzyme activity may also influence the rate of hepatic metabolism
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ESMOLOL
selective beta-1 blocker; lacks of ISA
rapid-onset and ultra short-acting
preventing or treating haemodynamic instability intraoperatively in response to noxious stimulation, e.g. during intubation
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Pharmacokinetics Esmolol
rapid metabolism in blood by hydrolysis of the methyl ester
inactive acid metabolites
elimination half-time 10 minutes
poor lipid solubility; limits transfer into the CNS or across the placenta
dosage: (peri-operative)= 0.5 to 1mg/kg over 15-30sec
infusion 50-300mcg/kg/min
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1. Antiadrenergic Agents (cont)
Alpha ReceptorsPrazosin
Terazosin
Beta ReceptorsPropanolol, Metoprolol, Atenolol
Alpha-Beta receptorsLabetolol
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LABETOLOL
Both alpha & beta (ratio1:7)
selective alpha-1 antagonist (1/5 to 1/10 as potent as phentolamine); presynaptic alpha-2 receptors are spared
nonselective beta-1 and beta-2 antagonist (1/4 to 1/3 as potent as propranolol)
useful in Mx of PIH
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Pharmacokinetics Labetolol
extensive first pass metabolism30-40% bioavailabilitymetabolism is by conjugation to glucuronic acid< than 5% excreted unchanged in the urineelimination half-time is 5 to 8 hours (prolonged in liver disease and unchanged in renal dysfunction)
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Dosage and Administration Labetolol
oral dose of 100-400mg per day
severe HPTmultiple bolus dose 20-40mg every 10-15 mins (up to 300mg)
continous infusion 2mg/min (up to 150mg)
intraop/postop HPT or induced hypotension during anaesthesia
lower starting multiple bolus dose of 5-10mg
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Side Effects Labetolol
orthostatic hypotension (most common)
beta-antagonists effects (bronchospasm, congestive heart failure, heart block, fatigue, mental depression
fluid retention
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1. Antiadrenergic Agents
Central Nervous SystemClonidine
Methyldopa
Autonomic GangliaTrimetaphan
Nerve endingsReserpine
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CLONIDINE
centrally acting alpha-2 agonist
stimulates alpha-2 inhibitory neurons in the medullary vasomotor center
resulting in reduction of SNS outflow from the CNS to peripheral tissues
manifested as decreases in BP, HR and CO
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Pharmacokinetics Clonidine
well absorbed after oral administration
daily adult dose is 0.2 to 0.3 mg orally (bd)
60% of the drug excreted unchanged in the urine
duration of action; 8 hours
elimination half-time; 8.5 hours
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Side Effects Clonidine
dry mouth
sedation
withdrawal syndrome; hyperadrenergic states resembling phaechromocytoma
retention of Na + and water
skin rashes
constipation
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METHYLDOPA
serves as an alternative substrate to dopa
decarboxylated to methyldopamine and beta-hydroxylated to alpha methylnorepinephrine
inhibits SNS from the vasomotor center to the periphery
resulting in decrease SVR and BP
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Pharmacokinetics Methyldopa
daily adult dose is 250 to 2000mg (bd)
incomplete absorption (25-50%)
low protein binding 15%
maximal effect within 4 to 6 hrs after an oral dose and persists for as long as 24 hrs
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Side Effects Methyldopa
Sedation
Hepatic dysfunction, necrosis; maybe fatal
Positive Coombs' test (10-20%)
Rebound hypertension
Retention of Na + and water
Sexual dysfunction
Bradycardia
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1. Antiadrenergic Agents
Central Nervous SystemClonidineMethyldopa
Autonomic GangliaTrimetaphan
Nerve endingsReserpineGuanethidine
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TRIMETAPHAN
peripheral vasodilator and adrenergic ganglionic blocker
directly relaxes capacitance vessels and blocks autonomic nervous system reflexes
decreases CO and SVR; hence lowering BP
increases in HR; most likely reflect PNS ganglionic blockade
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Side effects Trimetaphan
Mydriasis
Reduced gastrointestinal activity; ileus
Urinary retention
Histamine release
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1. Antiadrenergic Agents
Central Nervous SystemClonidine
Methyldopa
Autonomic GangliaTrimetaphan
Nerve endingsReserpine
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RESERPINE
interferes with the cathecholamines uptake into the storage vesicles
depletes stores of catecholamines
decreased CO and bradycardia leading to hypotension
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Side Effects Reserpine
Orthostatic hypotension (prominent)
Sedation and drowsiness
Mental depression
Signs of PNS predominance include bradycardia, nasal stuffiness, xerostomia, increased gastric H+ secretion, and exaggerated gastrointestinal motility (abdominal cramps and diarrhea)
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2. Vasodilators
Vascular Smooth MuscleHydralazine
Nitroprusside
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HYDRALAZINE
phthalazine derivative decreases BP by a direct relaxant effect on vascular smooth muscle (on arterioles greater than veins)pronounced on the coronary, cerebral, renal, and splanchnic circulationsinterference with Ca2+ transport in vascular smooth muscle
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Clinical Uses Hydralazine
usual adult dose is 20 to 40 mg qid
treatment of a hypertensive crisis; 2.5 to 10 mg IV (effect begins within 15 mins and lasts 3 to 4 hours)
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Pharmacokinetics Hydralazine
extensive hepatic first-pass metabolism
metabolized partially by acetylation; slow and rapid acetylators
elimination half-time; 3 hours
< than 15% of the drug excreted unchanged in the urine
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Side Effects Hydralazine
Na + and water retention vertigo, diaphoresis, nausea, and tachycardiamyocardial stimulation can evoke angina pectorislupus erythematosus-like syndrome (10% to 20%)drug fever, urticaria, polyneuritis, anemia, and pancytopenia, peripheral neuropathies
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NITROPRUSSIDE
causes relaxation of arterial and venous vascular smooth muscle onset is almost immediate, and its duration is transient, continuous intravenous infusion to maintain a therapeutic effectextreme potency; necessitates careful titration of dosage and frequent monitoring of blood pressure
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Mechanism of Action
produce NO, which activates the GC enzymeresults in increased conc of cGMP in smooth muscleleading to vasodilatation in arteries and veinsmay be sec to decreased Ca2+ entry into muscle cells or increased uptake by the SR
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Clinical Uses Nitroprusside
hypertensive emergencies
induced hypotension during surgery
congestive cardiac failure; improve CO, due to decrease in LV impedance
rapid injection, 1 to 2 mg kg -1 IV obtund haemodynamic changes produced during intubation
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Thiocyanate
thiocyanate is cleared slowly by the kidney, with an elimination half-time of 4 to 7 days; (accumulates with prolonged therapy or in renal failure) clinical toxicity is rare (100x less toxic than cyanide)skeletal muscle weakness, nausea, and mental confusion (> 10 mg dl –1)
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Cyanide Toxicity
dose-dependentshould be suspected
in any patient who is resistant to the hypotensive effects of the drug despite adequate infusion rates (up to 8 mg kg -1 min -1 IV)in a previously responsive patient who becomes unresponsive to the BP-lowering effects of the drug despite increasing doses (tachyphylaxis)
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Cyanide Toxicity (cont)
mixed venous PO2 is elevated
development of metabolic acidosis
decreased cerebral oxygen use
CNS dysfunction in awake patients
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Treatment of Cyanide Toxicity
immediate discontinuation
100% oxygen
NaHCO3
sodium thiosulfate, 150 mg kg -1 IV over 15 minutes, is a recommended therapy (acts as a sulfur donor to convert cyanide to thiocyanate)
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Treatment of Cyanide Toxicity
severe case; slow administration of sodium nitrate, 5 mg kg -1 IV (convert Hb to metHb)
metHb acts as an antidote; converting cyanide to cyanmethemoglobin
hydroxycobalamin (vit B12); reacts with cyanide to form cyanocobalamin
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3. Calcium Channel Blocker
Vascular Smooth MuscleVerapamil
Nifedipine
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Pharmacological Effects
decreased myocardial contractility
decreased HR
decreased conduction rate through the AVN
vascular smooth muscle relaxation with assd vasodilation and reductions in SVR and BP; affect primarily arterial rather than venous vascular tone
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Clinical Uses
Essential Hypertension
Supraventricular Tachydysrhythmias
Coronary Artery Vasospasm
Exercise-Induced Angina Pectoris
Cerebral Artery Vasospasm
Cerebral Protection
Myocardial Protection
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NIFEDIPINEdihydropyridine derivative
arterial vasodilator (minimal effect on veins)
peripheral vasodilation and the resulting decrease in BP activate baroreceptors, leading to reflex tachycardia
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Pharmacokinetics Nifedepine
absorption of an oral or SL dose is about 90%
onset within about 20 minutes
protein binding; 90%.
hepatic metabolism is nearly complete
elimination 80% in renal, remaining in bile
elimination half-time is 4 to 6 hours.
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Clinical Use in Hypertension
rapid reduction in BP; orally or SL or by biting the capsule and swallowing its content
10-20mg produce significant hypotension within 15-30 mins
chronic therapyneeds tds dosing
combination with beta blocker to blunt reflex tachycardia
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VERAPAMIL
synthetic derivative of papaverine
supplied as a racemic mixture
direct depressant effects on SAN and delay conduction via AVN (Tx of SVT)
mild vasodilating properties (Tx of angina and essential HPT)
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Clinical Use in Hypertension
reduces elevated PVR in essential HPT
prevent significant increases in HR (effects on SAN)
chronic therapy; mild natriuretic effect, Na+ retention does not occur
dose ranges from 160 to 480 mg daily (bd)
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4. Renin-Angiotensin System
Angiotensin Converting Enzyme InhibitorsCaptopril
Enalapril
Lisinopril
Angiotensin II Receptor AntagonistsLosartan
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ACE Inhibitors
ACE converts inactive AT I to active AT IIAT II then acts to raise BP through;
Its potent vasoconstrictor effectBy stimulating secretion of aldosterone by the adrenal cortex; which acting through kidney, causes Na+ retention and expands IV volume
ACE is also responsible for the metabolism of bradykinin, which is a potent vasodilator
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The four sites of action of inhibitors of the renin-angiotensin system. J-G = Juxtaglomerular apparatus; CE = converting enzyme
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CAPTOPRIL
competitive inhibitor of ACE; therefore prevents the generation of AT II
inhibits AT II mediated vasoconstriction and aldosterone secretion
also inhibits breakdown of bradykinin; further contribute to its hypotensive effect
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Pharmacokinetics Captopril
well absorbed after oral administration (60-70%)
onset; 15 minutes
half-life; 1.7 hrs
plasma protein binding is low (20-30%)
excreted both through metabolism and by urinary excretion of unchanged drug
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Side Effects Captopril
skin rash sometimes acc. by fever and joint discomfort (10%) and pruritis
loss of taste sensation (1% to 2%)
proteinuria and elevations in [creatinine]
neutropenia (0.3%)
abrupt fall in BP after initial dose; esp in pts who are volume depleted
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Side Effects Captopril
angioedema; drug-induced inhibition of the metabolism of bradykinin
cough and exacerbation of dyspnea and wheezing in COAD (kinin effects)
increase serum K + levels, esp in pts with impaired renal function
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4. Renin-Angiotensin System
Angiotensin Converting Enzyme InhibitorsCaptopril
Enalapril
Lisinopril
Angiotensin II Receptor AntagonistsLosartan
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LOSARTAN
AT II receptor blocker (type AT1); hence blocks the vasoconstrictor and aldosterone-secretion effects of AT II
25-50 mg once daily
extensive 1st pass metabolism
active metabolites is 10-40x more potent
can cause fetal morbidity and mortality
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PHOSPHODIESTERASE INHIBITOR mechanism of action
. Cyclic-AMP is broken down by an enzyme called cAMP-dependent phosphodiesterase (PDE).
Inhibition of this enzyme increases intracellular cAMP, which further inhibits myosin light chain kinase thereby producing less contractile force (i.e., promoting relaxation)
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Phosphodiesterase inhibitors
a) bypiridine derivatives
- amrinone , milrinone
b)imidazole - enoximone,pyroximone
c) methylxanthines - theophylline, caffein
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PHOSPHODIESTERASE INHIBITOR
cardiovascular action
Systemic circulation
-Vasodilatation
- Increased organ perfusion
- decreased SVR
- decreased atrial pressure
cardiopulmonary
- increased contractility & HR
-increased SV & ejection fration
- decreased ventricular pre-load
- decreased pul;monary capillary wedge pressure
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β2 adrenoceptor agonistVascular smooth muscle has β2-adrenoceptors that have a high binding affinity for circulating epinephrine and a relatively lower affinity to norepinephrine released by sympathetic adrenergic nerves
These receptors are coupled to a Gs-protein, which stimulates the formation of cAMP.
, in vascular smooth muscle an increase in cAMP leads to smooth muscle relaxation
cAMP inhibits myosin light chain kinase that is responsible for phosphorylating smooth muscle myosin.
increases in intracellular cAMP caused by β2-agonists inhibits myosin light chain kinase thereby producing less contractile force (i.e., promoting relaxation)
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(β-agonists)-mech. of action
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Beta-AgonistsCardiac effects
• Increase contractility(positive inotropy)
• Increase relaxation rate(positive lusitropy)
• Increase heart rate(positive chronotropy)
• Increase conduction velocity(positive dromotropy)
Vascular effects
• Smooth muscle relaxation(vasodilation)
Other actions
• Bronchodilation
Hepatic glycogenolysis
• Pancreatic release of glucagon
• Renin release by kidneys
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QUESTIONS
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Thank You