Control of heartbeat

               Heart beat

§ A heartbeat is as a result of the contraction and relaxation of the heart (cardiac) muscle.

§ The contraction and relaxation occurs naturally and is not stimulated by nerve impulses and as such cardiac muscles are referred as to myogenic.

§ Coordination

§ To function as an effective pump the different parts of the heart must contract and relax in sequence.

§ This is made possible by an inbuilt control and coordinate system which ensures that  the different cardiac muscles contract and relax together and are in rhythm.

§ The SAN,AVN and purkyne tissue are responsible for this coordination

§ Sinoatrial node(SAN)

§ The SAN or pacemaker is a specialized patch of muscle that starts the cardiac cycle. 

§ It is located in the walls of the right atrium. 

§ Whenever the pacemaker muscles contract. It generates electrical excitation waves that causes the other cardiac muscles to contract together giving them rhythm.

§ Atrioventricular node(AVN)

§ This a patch of conducting fibres that serve as a pathway for the electrical excitation waves through the band of non-conducting fibres that are between the atria and the ventricles. 

§ It is located in the septum.

§ It delays the transmission of the electrical excitation waves to the ventricles to ensure that the atria and ventricles do not contract at the same time.

§ Purkyne tissue

§ This is a bunch of conducting fibres attached to the AVN that receives the electrical excitation waves from the AVN and transmit the wave to the bottom of the septum and upwards through the ventricle muscles so that the ventricle contracts from the bottom up squeezing the blood upwards.

§ How a Heartbeat occurs

Process;

§ Stage 1;

The cardiac muscle cells of the SAN contract together. This generates some electrical excitation waves which spread through the atrial wall and causes the cardiac muscle there to contract together hence both atria contract simultaneously.(Atrial systole)

§ Stage 2;

The electrical excitation wave moves through the AVN as it spreads across the atrial walls which delays transmission for about 0.1 seconds.

§ Stage 3;

The AVN transmits the electrical excitation waves to the purkyne tissue which moves the wave down to the base of the ventricle and sweeps upwards causing the ventricle walls to contract.(Ventricular systole)

§ Stage 4;

Both the ventricles and the atria relax(Ventricular diastole). The muscle in the SAN contracts again stat the whole sequence again.

Fibrillation

§ This occurs when contraction is not coordinated properly and the excitation waves are chaotic.

§ As a result muscle cells in certain parts contract while others relax. This would mean lack of a common rhythm and as such the heart does not relax and contract as a whole.

§ This affects the pumping of blood through the body making fibrillation almost always fatal. 

§ Causes of Fibrillation

§ Fibrillation can be caused by an electric shock.

§ Fibrillation can be caused by damage to large areas of the heart muscle wall.   

§ Types of Fibrillation

§ Atrial fibrillation; is an irregular and uncoordinated contraction of the cardiac muscle of atria.

§ Ventricular fibrillation;  is an irregular and uncoordinated contraction of the cardiac muscle of ventricles

§ Treatments

§ The major treatment is to try and reset the heart’s rhythm.

§ Ideally, to treat  fibrillation, the heart rate and rhythm are reset to normal. To correct your condition, doctors may be able to reset your heart to its regular rhythm (sinus rhythm) using a procedure called cardioversion.

§ Cardioversion

§ Electrical cardioversion. In this brief procedure, an electrical shock is delivered to your heart through paddles or patches placed on your chest. The shock stops yourchest. The shock stops your heart's electrical activity momentarily. When your heart begins again, the hope is that it resumes its normal rhythm.

Cardioversion with drugs. This form of cardioversion uses medications called anti-arrhythmics to help restore normal sinus rhythm                          

§ Electrocardiograms(ECGs

§ The flow of electrical excitation waves can be detected with the use of  electrodes.

§ The electrical potential generated is recorded with time and a graph of voltage against time can be made which is referred to as an Electrocardiogram.

Electrocardiograph( Fibrillation patient)