Antiarrhythmic drugs - drdhriti
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A Power point presentation on "Antiarrhythmic drugs" suitable for UG MBBS level students

A Power point presentation on "Antiarrhythmic drugs" suitable for UG MBBS level students

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  • The gross electrical activities that can be observed in ecg are due to the action potentials generated in different cardiac tissues.
  • The fast sodium channel can be modeled as being controlled by a number of gates . Each gate (or gating variable) can attain a value between 1 (fully open) and 0 (fully closed). The product of all the gates denotes the percentage of channels available to conduct Na + . Following the model of Hodgkin and Huxley , the sodium channel contains three gates: m , h , and j . In the resting state, the m gate is closed (zero) and the h and j gates are open (one). Hence, the product denoting the percentage of conducting channels is also zero. Upon electrical stimulation of the cell, the m gate opens quickly while simultaneously the h and j gates close more slowly. For a brief period of time, all gates are open ( i.e. non-zero) and Na + can enter the cell following its electrochemical gradient . If, as above, the resting membrane potential is too positive, the h or j gates may be considerably less than one, such that the product of m , h and j becomes too small upon depolarization.
  • Electrical activities in different part of the cardiac tissues can be recorded externally over the skin by placing electrodes on both side of the heart – called ECG or EKG
  • Abnormal pacemaker activity can occur if myocardial cells are damaged eg ischaemic heart disease also via catecholamine overactivity
  • Ventricles can be excited with each sinus node depolarization. The will generally travel via AV node and accessory pathway if present. However accessory pathways have fast action tissues and AV node has slow action tissues. Thus in patients with WPW syndrome has premature atrial impulses and cannot excite accessory pathway as not enough threshold can be generated there but can excite AV node slowly and conducts the impulse. The impulse will pass down the ventricular wall and somewhere meets the accessory pathway which is by now may be no longer refractory and the impulse traverse back to the atrium and again reach the AV node and reenter ventricle.
  • Atrial fibrillation - Atria remains dilated and quiver like bag of worms Torsades de pointes – polymorphic ventricular tachycardia
  • Pharmacologically, minimum interval between the two propagating action potentials. It is closely related to AP duration (APD). An AP can be evoked in fast channel fibres before complete repolarization. Because Na+ channel recover in a voltage-dependent manner, above the threshold potential. In contrast, the Ca++ channels recover in time dependent manner.

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