Antiarrhythmics of the first group
This information is not intended for purposes of treatment without physician.
Description of the pharmacological group
Antiarrhythmic means of the I class
All antiarrhythmic medicines, which relate to 1 class, possess the ability to one degree or another block rapid transmembrane Na+- channels and to, therefore, slow down the maximum speed of depolarization. In view of the presence in this class of the ability to slow down the rapid phase of the depolarization of cardiomyocytes, in their medical literature they sufficiently frequently call Membrane stabilizing agents. Under the effect of antiarrhythmics of the I class:
- increases the threshold of excitability, i.e., it decreases the excitability of cardiomyocytes;
- it slows down (it is suppressed) the rate of conducting electric pulses on the myocardium;
- spontaneous diastolic depolarization is impeded.
For antiarrhythmics of the I class is characteristic the so-called phenomenon of rate-dependence (syn.: use-dependence - frequency dependence), essence of which consists in the fact that their electrophysiological effects and, therefore, antiarrhythmic activity increase with the growth of the rhythm of the heart contractions (direct frequency dependence).
To the greatest degree this phenomenon is inherent in the antiarrhythmic medicines of IB class; therefore Lidocaine and other preparations of this class are more effective with “the rapid”, than with “slow” tachyarrythmias.
At the same time, despite the fact that all antiarrhythmics of the I class possess the general fundamental mechanism of action - block rapid Na+- transmembrane channels, they differ from each other on the ability to influence the action potential of heart cells and the refractory period: antiarrhythmics of IA class lengthen action potential and refractory period; antiarrhythmics of IB class shorten and/or does not change the form of action potential; antiarrhythmics of 1C of class slow down conducting pulses, sharply decrease the rate of depolarization.
Classification of Membrane stabilizers
The I class includes the series of the preparations, which are distinguished by some special features of action. Conditionally they are subdivided into three subgroups:
subgroup 1A - quinidine, procainamide hydrochloride, ethmosine, disopyramide;
the subgroup of 1B - local anesthetics (Lidocaine, Trimecaine, Bumecaine), and also Mexiletine (sin.: Mextil) and Diphenine;
the subgroup of 1C - Ajmaline, ethacyzin, lappaconitine.
Some antiarrhythmic preparations possess to one degree or another the properties of different classes.
Different groups of antiarrhythmic means and separate preparations are distinguished according to the degree of influence on the transport through the cellular membranes of ions (Na+, K+, Ca2-), and also interconnected with this action on the electrophysiological processes in the myocardium, on the depolarization of the electrical membrane potential of cardiac hystiocytes.
Thus, the preparations of subgroup 1A and 1C in essence suppress the transport of sodium ions through “rapid” sodium channels of cellular membrane. The preparations of the subgroup of 1B increase the permeability of the membranes for potassium ions. quinidine simultaneously with the oppression of the transport of sodium ions decreases the entering into the cardiomyocytes of calcium ions. The same combined action renders the new antiarrhythmic preparation of bonnecor.
The preparations of the I class reduce the maximum speed of depolarization, increase the threshold of excitability, slow down conductivity along the beam of His and the fibers of Purkinje, slow down the restoration of the reactivity of the membranes of cardiomyocytes.