Wednesday, February 16, 2011

The Cardiac Cycle

The Cardiac cycle is basically divided into two phases.

  1. Systole
  2. Diastole
The relaxed state of the ventricles is known as Diastole. In this phase the blood passively flows from right and left atia to right and left ventricles respectively. The blood flow in between atria and ventricles is through Mitral and Tricuspid valves. The venous blood reaches the Right atrium through superior venacava and inferior vanacava. On the other hand pulmonary veins deliver oxygenated blood to the left atrium. 

The phase of systole represent the contraction of ventricles. The right and left ventricle contract and the blood is ejected into aorta and pulmonary artery. The atrioventricular valves are closed during the systole so no blood enters into the ventricles. 

The cardiac cycle is divided into 7 phases. 
  1. Atrial Contraction
  2. Isovolumetric Contraction
  3. Rapid Ejection
  4. Reduced Ejection
  5. Isovolumetric Relaxation
  6. Rapid Filling
  7. Reduced Filling
Phase 1 - Atrial Contraction -
The first phase of the cardiac cycle is atrial contraction which is depicted by P wave of Electrocardiogram.
P wave actually signifies the depolarization of atria which causes its contraction. The artia contracts and as a result pressure in the chamber increases and the blood flows through open atrioventricular valves and reaches the ventricles. The blood does not move back to venacava due to inertial effect of venous return but does produce a small rise in the venous pressure which can be seen as a wave of the left atrial pressure. The peak of a wave is always followed by x decent.
The contraction of atria is  responsible for 10 % of the ventricular filling in a resting indivudual. Most of the blood passively flows from left pulmonary veins to right atrium and further into left ventricle through open mitral valve. This passive filling of left ventricle takes place before the contraction of atria. If the heart rate is on the higher side the atria can be responsible for even 40% of ventricular filling. This phenomenon is also known as atrial kick. The atrial contribution to the filling of the ventricle is directly proportional to the contraction of the atria and inversely proportional to the duration of ventricular diastole.
The pressure in the atria falls after the completion atrial contraction. The AV valves float upwards due to reversal of pressure gradient. This is the time when maximum volume is found in the ventricles. This is known as End Diastolic Volume (EDV). The end diastolic volume of left ventricle is about 120 ml which signifies the preload of left ventricle. The ventricular preload is associated with end diastolic pressures of 8-12 mmHg and 3-6 mmHg in left and right ventricles respectively.
In some individuals a heart sound is noted during the phase of atrial contraction. This sound is known as S4. The cause of this sound is vibration in the ventricular walls during atrial contraction. This sound is heard in patients with decreased ventricular compliance (stiff ventricles), ventricular hypertrophy or in elderly population.

Phase 2 - Isovolumetric Contraction -

The QRS complex of the ECG represents this phase of cardiac cycle. The QRS wave of ECG means ventricular depolarization. Ventricular depolarization starts excitation-contraction coupling which as a result trigger myocyte contraction and rapid increase intraventricular pressure. The rate of pressure development is maximal in this phase. This is also known as maximal dP/dt.
As the intraventricular pressure exceeds intraatrial pressure, the AV valves close. Ventricular contraction also triggers the contraction of papillary muscles which are attached to chordae tendineae. This prevents the bulging back of AV valve in the atria. In this manner an AV valve does not becomes leaky or incompetent. The closure of AV valve causes the first heart sound -S1. The first heart sound is normally split (0.04 sec)  because mitral valve closure precedes tricuspid valve.
In the period between closure of the AV valves and opening of the aortic and pulmonary valves, the pressure in the ventricles rises without any change in the volume of the ventricles, which means no ejection takes place. There is no change in the volume because all the valves are close. That is why this phase is known as isovolumetric contraction. But at the myocyte level the contraction is not isometric. Every myocyte goes under a length change. Some myocytes contract isotonically or concentrically which means shortening contraction and others contract isometrically that is no change in length of fibre. There are fibers which contract eccentrically that is lengthening contraction. Therefore the shape of the ventricle changes and it becomes more spheroid in shape. The circumfrence of ventricle increases and the atrial base to apex length decreases. The pressure in the ventricles increases with the rate of contraction of the muscle fibers, which is determined by the excitation contraction coupling. The c wave visible in the Left atrial pressure is due to bulging back mitral valve leaflets in to atria.

Phase 3 - Rapid Ejection -

In this phase the blood ejects from right and left ventricles to Pulmonary artery and aorta respectively. The process of ejection starts when pressure in the ventricles increases and exceeds the pressure in the aorta and pulmonary artery. Higher pressures in ventricles result in the opening of aortic and pulmonary valves. During the phase of rapid ejection the ventricular pressure exceeds the outflow tract pressure by few mmHg.
The opening of the valves is a silent process so no heart sound is heard during this phase. The sounds can be heard in case of valvular diseases and intracardiac shunts. The pressure in the left atria initially decreases  as the atrial base is pulled downward and as a result atria expands. Blood flows into the atria through venous inflow tracts and the pressure in the atria rises until the AV valves open at the end of phase 5.

Phase 4 - Reduced Ejection -

As represented by T wave of Electrocardigram, ventricular repolarization starts after 200 msec of ventricular contraction or QRS complex. Repolarization decreases the tension in the ventricles and as a result rate of ejection also decreases. The pressure in the ventricles falls below the pressure in the outflow tracts but the blood still flows passively into outflow tracts due to inertia. The pressure in the left and right atria slowly rise due to continuous venous flow.

Phase 5 - Isovolumetric relaxation -

The pressures in the ventricles sufficiently fall at the end of phase 4, as the aortic and pulmonary valves abruptly close. This causes the second heart sound (S2). This is the beginning of isovolumetric relaxation. The closure of the valves is associated with small backflow of blood in the ventricles which is represented by dicrotic notch in pulmonary artery and aortic wedge pressures. As the valve closes the pressure in the aortic and pulmonary artery slowly rise represented by dicrotic wave, which is followed by slow decline in pressure.
The rate of fall in pressure within the ventricles is proportional to the rate of relaxation of the muscle fibres, which is known as lusitropsy. The relaxation of the muscle fibers is regulated by the sarcoplasmic reticulum. Sarcoplasmic reticulum helps in relaxation of the muscle fibers by re-sequestering the calcium after the contraction. The pressure within the ventricles fall but the volume remains he same because all the valves are closed. End systolic volume of the left ventricle is about 50 ml. The difference between end diastolic volume and end systolic volume is known as stroke volume which is about 70 ml. The pressure in the left atrium continues to rise due to continuous venous return. The peak LAP at the end of this phase is denoted by v Wave.

Phase 6 - Rapid Filling -

The ventricles continue to relax and intraventricular pressure drops below the intraatrial pressure. At this point of time the AV valves open and the ventricles start filling. Despite continous filling of the ventricles the pressure in the ventricles still fall because muscle fibres of ventricles are still relaxing. After complete relaxation of the ventricles the pressure rises gradually with the filling of blood. The sudden opening of the AV valves causes rapid fall in the atrial pressures. The LAP before the opening of valves is denoted by v wave which is followed by y descent.
Ventricular filling is normally silent. But sometimes a third heart sound is heard which is due to tensing of chordae tendineae and AV ring during ventricular filling and relaxation. This heart sound is normal and children but pathological in adults due to ventricular dilation.

Phase 7 - Reduced Filling -

The pressure in the ventricles continue to rise with filling of blood. As the pressure in the ventricles increases the rate of filling falls. Normally 90 % of ventricle is filled in resting heart. Aortic and pulmonary pressure continue to fall during this period.

Informative Lecture on CARDIAC CYCLE


Cardiac Cycle Quiz Part 1

Cardiac Cycle Quiz Part 2

Cardiac Cycle Quiz


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