A review of class I antiarrhythmics – the sodium channel blockers (e.g. quinidine, procainamide, lidocaine, mexiletine, flecainide, and propafenone). Mechanisms, indications, and side effects are all discussed.
This is video 2 of 7 in this series on antiarrhythmics and a specific topic is class 1 anti rhythmics the sodium channel blockers as mentioned in the introduction sodium channel blockers are subdivided into three subclasses 1a 1b and 1c based upon the strength of their sodium channel blockade however these classes have other distinguishing characteristics which
I’ll go through one at a time first starting with the 1a drugs they have moderate effect at blocking sodium channels but they also have moderate effect at blocking potassium channels as well because of the potassium channel blocking properties 1a drugs increase the action potential duration and thus increase the effective refractory period the effective refractory
Period is the period of time after an action potential has been triggered when the cell has not yet recovered enough for another incoming action potential to depolarize it again if we look at the shape of the fast response action potential in the presence of a 1 a drug we see that the sodium mediated upstroke is less steep and the repolarization is modestly
Delayed examples of 1a drugs include quinidine and procainamide quinidine was one of the first anti rhythmic drugs used specifically for its anti rhythmic properties interestingly quinidine is a stereo isomer of quinine both of which are derived from the bark of the cinchona tree of south america and are used for malaria quinine was first prescribed for atrial
Fibrillation by carl fredrik weinke bach of type 1 second degree av block fame and it was a different cardiologist who later observed that quinidine seemed to be more effective for arrhythmias however quinidine is still not particularly effective and it’s poorly tolerated and thus rarely used today moving on to the 1b drugs these have relatively weak effect at
Blocking sodium but their effect is most prominent in already depolarized tissues that can be seen in a scheme eeeh so these drugs can be helpful at treating arrhythmias in the setting of angina or acute mis when it comes to the action potential duration and a factory refractory period 1b drugs decrease them slightly so the action potential looks like this with
A very slightly decreased slope to the upstroke and mild shortening of the action potential there is not a conclusive explanation for the observed decrease in the refractory period and the shortened action potential while there are a few informal web resources that make claims about a potential mechanism these claims are not backed up by the primary literature as
Far as i can tell regardless this shortening of the action potential does not appear to be a clinically significant effect of 1b drugs examples of 1b drugs include lidocaine and maxilla teen of course you may already be familiar with lidocaine as a topical or subcutaneous anesthetic its use as an anti-arrhythmic however requires iv administration the anti-seizure
Medication phenytoin marketed in the us as dilantin shares its general pharmacological mechanism with class 1 the antiarrhythmics and it is occasionally listed among them however i’ve never encountered a patient on phenytoin for this purpose and last are the 1c drugs as you might guess these are relatively strong blockers of sodium channels one of the two major 1c
Drugs propafenone also has some beta blocking effect as well neither of the one sees affect the action potential duration or refractory period so the action potential of a patient on a 1c drug looks like this a markedly slowed up stroke but otherwise normal the two major 1c drugs are flaking id and the afer mentioned propafenone now let’s talk about indications
Starting back with the 1a s as i’ve mentioned before quinidine is very rarely used these days at least in the us when it is used it is at a last-ditch effort for suppression of vt voc an amide isn’t particularly common either but it is used for both bate and rhythm control and something called pre excited a-fib this is when a patient with wolff-parkinson-white
Syndrome develops atrial fibrillation while most patients with afib are rea controlled with beta blockers or calcium channel blockers those drugs only have a significant impact on the slow response action potential of the sinus and av nodes thus if a patient has an accessory conducting pathway that bypasses the av node which is the defining feature both parkinson
White those av nodal blocking drugs don’t help and there is believed that they are actually very dangerous in pre excited afib procainamide on the other hand slows down conduction in the accessory pathway other indications for procainamide include the pharmacologic cardioversion of afib in patients without wpw cardioversion from particular tachycardia and the
Termination of a rhythm called av rt which stands for av reentrant tachycardia which is a reentrant supraventricular tachycardia seen specifically in patients with wpw the one be agents of lidocaine and maxilla teen are used solely as second or even third line agents for the prevention of vt and flaking id and propafenone are used for outpatient cardioversion of
Afib some patients with symptomatic paroxysmal afib will be prescribed these drugs to use on an as-needed basis they carry the drug around in their pocket and if they feel themselves jump from sinus into afib they pop one of these on their own which can lead to the conversion back to sinus within an hour or two this treatment strategy has been nicknamed the pill
In the pocket approach conventionally patients who use the pill and a pocket approach also take an av nodal blocking drug such as patoka law or diltiazem in the event the afib they feel is actually atrial flutter the d treating atrial flutter with class-one antiarrhythmics alone will be covered in the common pitfall section of the last video in the series other
Indications for 1c drugs include chronic maintenance of sinus rhythm in patients with paroxysmal afib or a flutter in this case the patients would take the drug every day and not on a just as needed basis and last one c’s can help prevent avrt i’ve covered classification and pharmacological effects as well as indications and now i’ll close with side-effects and
Toxicity class one antiarrhythmics have class specific side-effects and drug specific side-effects starting with quinidine and procainamide on account of their potassium channel blocking activity they both prolong the qt interval and they are both negative inotropic means they make the heart contract less vigorously i mentioned that quinidine is rarely used anymore
Partially because it is poorly tolerated it has many side effects the most notable of which are diarrhea which is really common quinidine as well as quinine also causes an interesting syndrome called synchronism which consists of blurred vision tinnitus hearing loss diaphoresis confusion and psychotic symptoms not a good thing to get procainamide is associated
With the development of a lupus like syndrome moving to the one b’s there’s no notable class specific side effects but lidocaine has much cns toxicity pretty much anything bad that can happen to your nervous system can be seen with lidocaine mixable teens most common side effects are related to gi distress such as nausea and vomiting while neither flecainide nor
Propafenone have drug specific side effects they both have a very notable problem with increasing the risk of sudden cardiac death when used in patients with coronary artery disease thus the presence of cid is considered to be an absolute contraindication they are also avoided in patients with heart failure another way to put that one see drugs are only used in
Patients with structurally normal hearts that is hearts that look normal from a macroscopic perspective one see drugs also call its conduction block in the sinus and av nodes leading to sinus bradycardia and av block and like the one a’s one c’s are also negative inotropes that does it for these sodium channel blockers the next video will cover the class two drugs beta blockers
Transcribed from video
Antiarrhythmics (Lesson 2 – Sodium Channel Blockers) By Strong Medicine
