Mechanisms of cardiac arrhythmias | Fundamentals

The arrhythmias are an abnormality in rate and rhythm of heart.

They may be either bradyaarhythmias or tachyarrhythmias.

There are 3 basic mechanisms by which tachyarrhythmias may occur 
1. Enhanced automaticity 
2. Triggered automaticity 
3. Reentry 

 Enhanced automaticity 

This type of arrhythmias may poccur due to abnormality in the pacemaker cells. Enhanced automaticity may occurif there is increase in slope of the pacemaker potential or the Resting membrane potential becomes less negative. This may happen in case of excessive sympathetic stimulation and mild kyperkalemia

Triggered automaticity 

This occurs in cells which are not having automaticity but automaticy is being triggered by someting. It involves due to generation of after-depolarization
After depolarizations are of two types:

Early after depolarization which occurs during the repolarization phase of cardiac action potential. It occurs whenever action potential gets prolonged. QT interbal prolonation is a sign of prolonationof action potential duration and thus it may lead to early after depolarization and hence predisposes a person to arrhythmias.

Delayed after depolarization occur after the repolarization ends i.e in phase 4 of cardiac action potentia. These after depolarization can lead to generation of impulse before the normal impulse reaches the contractile cells.

Reentry circuits 

Third mechanism for arrhythmias is reentry or movement of an impulse in a circus movement. If an impulse has 2 paths through which it can pass and these conducting paths differe electrically such that one is fast conducting path and th eother is slow conducting path, in that case of renetry of impulse may occur. Reeentry may occur due to an anatomical defect (anatomical reentry) or may be due to functional difference in the electrical propery of the cells (functional reentry).

Wolff-Parkinson syndrome is a type where arrhthmias occur due to anatomical reentry.

Absolute and Relative Refractory period in cardiac muscle cells | States...

The absolute and relative refractory period

Refractory means the cells are unresponsive to stimulus. It may be either absolute or relative.

In the absolute refractory period, the cells are unresponsive to any stimulus no matter how strong it is.

In the relative refractory period, the cells are unresponsive to threshold stimulus but respond to a suprathreshold stimulus that is a higher strength of the stimulus is required than usual to stimulate the cell.

Refractory period occurs due to different states of sodium channels. 

States of sodium channels 

Sodium channels can be in 3 states:
1. Closed (at RMP)
2. Open (as depolarization occurs, lot o channels open at threshold)
3. Inactivated: From the open state, channels become inactivated.

Channels go back to the closed state from the inactivated state as repolarization occurs. At RMP, all become closed. These channels can only open from the closed state and not from the inactivated state. So for the next action potential to occur, channels should go back to the closed state. So till the time, channels are in an inactivated state, the next action potential cannot occur.

Cause of Absolute/Effective refractory period 

From phase 0 to half of phase 3 of the cardiac action potential is the absolute refractory period. It occurs because sodium channels are in the inactivated state during this time.

Cause of relative refractory period 

After half of phase 3 to RMP is the relative refractory period. It occurs because some channels have changed their state to closed state so few channels are available which can open.


 

Ventricular action potential | Ionic basis | Cardiovascular physiology

 Phases of ventricular action potential and its ionic basis

Stable resting membrane potential (phase 4)

Phase of rapid depolarisation (phase 0): Due to opening of voltage gated sodium channels and entry of sodium inside the cells

 Phase of early rapid repolarisation (phase 1): Due to opening of voltage gated potassium channels and exit of potassium outside the cells

 Plateau phase (phase 2) where there is hardly any change in potential: Both voltage gated potassium channels and voltage gated calcium channels are open. Potassium moves out of the cells while calcium moves into the cells. This balances the potenetial and the potential is recorded as a plateau.

Phase of delayed rapid repolarization (phase 3): Calcium channels close while potassium channels remain open. Potassium continues to moves out of the cells causing repolarization.

Total duration of ventricular action potential is approximately 250 ms.