Drugs & CVS Flashcards
What are the 4 classifications of anti-arrhythmic drugs?
- Class 1 : drugs that block voltage sensitive sodium channels
- Class 11 : antagonists of beta-adrenoreceptors
- Class 111: drugs that block potassium channels
- Class 1V: drugs that block calcium channels
Give an example of a class 1 drug
Lidocaine
Explain how a class 1 drug works eg lidocaine
Loclass 1 drugs blocks voltage gated Na+ channels in open or inactive state.
Damaged myocardium can spontaneously depolarise causing arrhythmia’s eg AF. Lidocaine works in this condition by blocking Na+ channels after an action potential , therefore preventing the initiation of another action potential.
Sometimes used after an MI. INTRAVENOUSLY
Give examples of class 11 drugs
Propranolol , atenolol ( BETA BLOCKERS j
Outline the mechanism of action of class || drugs
- They block the sympathetic action by blocking B1 receptors in the heart to slow the heart rate ( negative chronotropy).
- Blocking the b1- receptors causes a reduced heart rate due to a decreased slope of the pacemaker potential in the SA node as the SA nodes increases the time taken to initiate an action potential
- AND slows conduction in the AV node to increase the time between depolarisation of the atria and the ventricles.
When are beta blockers used ?
- Used following an MI. This is because MI often causes increased sympathetic activity and arrhythmia may be partly due to the increased sympathetic activity. Beta blockers prevent ventricular arrythmias.
- Can prevent supraventricular tachycardia as beta blockers slow conduction in the AV node. It also slows ventricular rate in patients with AF.
- Reduce myocardial ischaemia by reducing oxygen demand.
- In heart failure
Give an example of a class ||| drug
Amiodarone
Outline the mechanism of action of class 111 drugs
They block potassium channels which prolongs the action potential and lengthens the absolute refractory period to slow the heart rate down. This prevents another action potential from forming. However , these drugs can actually be pro-arrhythmic as they promote early after depolarisations.
Why is amiodarone considered a good class 111 drug ?
Because it has other actions in addition to blocking K+ channels for example it is also a beta blocker and calcium channel blocker.
When do we use class 111 drugs ?
- Can be used to treat most arrhythmias , however it is not first line in most cases due to side effects .
- Treat tachycardia associated with Wolff-Parkinson white syndrome.
- Suppress ventricular arrhythmias post MI.
Give examples of class 1V drugs ?
Verapamil ( act on the heart to give negative chronotropic and inotropic effects )
Diltiazem ( act on the heart to give negative inotropic and chronotropic effects )
Dihydropyridine eg Amlodipine which work on smooth muscle vasculature by promoting systemic vasodilation. These are NOT effective in preventing arrhythmias.
What is the mechanism of action of class 1V drugs ?
- Can act on the myocardium to decrease the slope of the action potential at the SA node and decrease AV node conduction by blocking calcium channels
This leads to negative inotropic and negative chronotropic effects. - Class 1V drugs can also block calcium channels on smooth muscle cells to reduce contraction. This leads to the dilation of blood vessels to reduce total peripheral resistance. Which then decreases arterial blood pressure. And reduces the workload of the heart by reducing afterload.
What is Adenosine ? And what is its importance
An endogenous substance that is produced in our body in small amounts at psychological levels.
It can be given IV which acts on a1 receptors to enhance potassium conductance and hyperpolarised cells. This hyperpolarisation inhibits the movement fo calcium and essentially re sets the SA node. It has a very short half life.
The patient will feel like their heart has stopped when adenosine is given.
It is important because it is an anti-arrhymic drug but does not belong to any of the classes. It is useful for terminating re entrant SVT.
What are ACE-inhibitors ( ACEi) ?
Very important in reducing hypertension and heart failure.
They inhibit the action of angiotensin converting enzyme. This , this would prevent the conversion of angiotensin 1 to angiotensin 11.
This is important because angiotensin 11 acts on the kidneys to increase Na+ and water reabsorption. Angiotensin 11 is also a vasoconstrictor. And angiotensin 11 also stimulates release of aldosterone from the adrenal cortex.
What are the common side effects on patients who take Angiotensin converting enzyme inhibitors ?
CNS cause a dry cough due to excess bradykinin building up in the lungs this is because ACE is used to break down bradykinin but now that it is inhibited , bradykinin builds up.
