Following is a detailed overview of cardiac cycle phases and chronological divisions.
PROTODIASTOLE
Protodiastole is the first stage of ventricular diastole, hence the name protodiastole. Duration of this period is 0.04 second. Due to the ejection of blood, the pressure in aorta and pulmonary artery increases and pressure in ventricles drops. When intra-ventricular pressure becomes less than the pressure in aorta and pulmonary
artery, the semilunar valves close. Arterio-ventricular valves are already closed (see above). No other change occurs in the heart during this period. Thus, proto-diastole indicates only the end of
artery, the semilunar valves close. Arterio-ventricular valves are already closed (see above). No other change occurs in the heart during this period. Thus, proto-diastole indicates only the end of
systole and beginning of diastole.
Second Heart Sound
Closure of semilunar valves during this phase produces second heart sound.
ISO-VOLUMETRIC RELAXATION PERIOD
Iso-volumetric relaxation is the type of muscular relaxation, characterized by decrease in tension without any
change in the length of muscle fibers. Iso-volumetric relaxation of ventricular muscle is also called isovolumetricrelaxation.
During iso-volumetric relaxation period, once again all the valves of the heart are closed (Now, both the
ventricles relax as closed cavities without any change in volume or length of the muscle fiber. Intra-ventricular pressure decreases during this period. Duration of
iso-volumetric relaxation period is 0.08 second.
Significance of Iso-volumetric Relaxation
During iso-volumetric relaxation period, the ventricular pressure decreases greatly. When the ventricular pressure becomes less than the pressure in the atria, the atrio-ventricular valves open. Thus, the fall in pressure in the ventricles, caused by iso-volumetric relaxation is responsible for the opening of atrio-ventricular valves, resulting in filling of ventricles.
RAPID FILLING PHASE
When atrionventricular valves are opened, there is a sudden rush of blood (which is accumulated in atria
during atrial diastole) from atria into ventricles. So, this period is called the first rapid filling period. Ventricles also relax isotonically. About 70% of filling takes place
during this phase, which lasts for 0.11 second.
Third Heart Sound
Rushing of blood into ventricles during this phase causes production of third heart sound.
SLOW FILLING PHASE
After the sudden rush of blood, the ventricular filling becomes slow. Now, it is called the slow filling. It is
also called diastasis. About 20% of filling occurs in this phase. Duration of slow filling phase is 0.19 second.
LAST RAPID FILLING PHASE
Last rapid filling phase occurs because of atrial systole. After slow filling period, the atria contract and push a small amount of blood into ventricles. About 10% of ventricular filling takes place during this period. Flow of additional amount of blood into ventricle due to atrial systole is called atrial kick.
End-diastolic Volume
Enddiastolic volume is the amount of blood remaining in each ventricle at the end of diastole. It is about 130 to 150 mL per ventricle. Measurement of end-diastolic volume End-diastolic
volume is measured by the same methods, which are used to measure end-systolic
Volume (see Above).
INTRA-ATRIAL PRESSURE CHANGES DURING CARDIAC CYCLE
SIGNIFICANCE
Pressure in the atria is called the intra-arterial Pressure. Intra-arterial pressure is responsible for opening of the Atrio-ventricular valves and ventricular filling. It is also the main factor for the development of venous pulse.
METHODS OF STUDY
Right atrial pressure is recorded directly by cardiac catheterization (Chapter 98). Left atrial pressure is determined indirectly by measuring pulmonary capillary wedge pressure, which reflects the left atrial pressure accurately.
Pulmonary Capillary Wedge Pressure
Pulmonary capillary wedge pressure is the pressure exerted in the pulmonary capillary bed after obstructing the proximal part of pulmonary artery. Pulmonary capillary wedge pressure is measured by using a balloon tippedmulti-lumen cardiac catheter (Swan-Ganz
Catheter) . Tip of the catheter is not open but a pressure transducer is attached to it.
By means of venous puncture, the catheter is guided through right atrium into right ventricle. From the right ventricle, it is advanced towards the proximal portion of pulmonary artery and the balloon is inflated with air by using a syringe. This occludes the pulmonary artery. Then, the catheter alone is advanced further into distal portion of pulmonary artery, leaving the inflated
balloon at the proximal portion. It allows the catheter to float in a wedge position. Now the pressure existing in the pulmonary capillary bed ahead of catheter is called pulmonary capillary wedge pressure (the word wedge refers to being obstructed). When the proximal part of pulmonary artery is obstructed, pressure in the distal part falls rapidly and after about 10 seconds, it becomes equal to left atrial pressure. It is because of the absence of any valve
between pulmonary capillary bed and left atrium. So, the left atrial pressure can be determined by measuring pulmonary capillary wedge pressure..
INTRA-ATRIAL PRESSURE CURVE
Intra-arterial pressure curve is similar to the tracing of Jugular venous pulse, which is known as phlebogram. It Left atrium 7 to 8 mm Hg 0 to 2 mm Hg Right atrium 5 to 6 mm Hg 0 to 2 mm Hg
Left ventricle 120 mm Hg 5 mm Hg Right ventricle 25 mm Hg 2 to 3 mm Hg Systemic aorta 120 mm Hg 80 mm Hg Pulmonary artery 25 mm Hg 7 to 8 mm Hg has three positive waves, a, c and v and three negative waves, x, x1 and y (Fig. 91.3).
‘a’ Wave
‘a’ wave is the first positive wave and occurs during atrial systole. The pressure rises sharply up to 5 mm Hg in right atrium and 7 mm Hg in left atrium. After reaching the peak, the pressure starts decreasing.
‘x’ Wave
‘x’ wave is the first negative wave and appears during the onset of atrial diastole. Because of relaxation of atria, the pressure falls. Atrio-ventricular valves close at the end of this wave.
‘c’ Wave
‘c’ wave is the second positive wave and this appears during iso-volumetric contraction. Rise in pressure is due to the closure of atrio-ventricular valves and the increased intraventricular pressure. When atrio-ventricular valves close, there is a little back flow of blood towards atria. When the intraventricular pressure increases, there is bulging of AV valves into the atria. Because of these two factors, the atrial pressure rises.
‘x1’ Wave
‘x1’ wave is the second negative wave and appears during ejection period. During ejection period, the contraction of ventricular musculature pulls the
atrio-ventricular ring towards the ventricles. This causes fall in atrial pressure.
‘v’ Wave
‘v’ wave is the third positive wave, which is obtained during atrial diastole. It shows a gradual increase in atrial pressure due to filling of blood in atria (venous return).
‘y’ Wave
‘y’ wave is the third negative wave and appears after the opening of AV valves when the blood rushes from atria into ventricles. So, the pressure in the atria falls.
INTRAVENTRICULAR PRESSURE CHANGES DURING CARDIAC CYCLE
SIGNIFICANCE
Intraventricular pressure is the pressure developed inside the ventricles of the heart. It is essential for the circulation of blood, because the flow of blood through systemic and pulmonary circulation depends upon the pressure at which the blood is pumped out of ventricles.
Thus, intraventricular pressure is essential for the circulation of blood.
METHODS OF STUDY
Intraventricular pressure is measured by cardiac catheterization.
MAXIMUM AND MINIMUM PRESSUREIN VENTRICLES
There is some difference in the pressure in right ventricle and left ventricle. The pressure is always more in left ventricle than in the right ventricle.
INTRAVENTRICULAR PRESSURE CURVE
Intraventricular pressure curve has seven segments
(Fig. 91.4).
‘A-B’ Segment
‘AB’ segment is a positive wave and appears during atrial systole. Rise in pressure during this period is due to the entry of a small amount of blood into the ventricles because of atrial systole. The pressure rises to about 6 to 7 mm Hg in the right ventricle and to about 7 to 8 mm
Hg in the left ventricle. ‘B’ indicates the closure of atrio-ventricular valves.
‘B-C’ Segment
‘BC’ segment appears during iso-volumetric contraction.During iso-volumetric contraction period, there is a sharp rise in the intraventricular pressure. ‘C’ denotes the opening of semilunar valves.
‘C-D’ Segment
‘CD’
segment appears during ejection period. During ejection period, the pressure in the ventricles rises to the peak and then falls down. First part of the curve indicates the maximum ejection and the pressure increases to the maximum. Second part of the curve represents the slow
ejection phase when the pressure decreases.Maximum pressure rise in right ventricle is about 25 mm Hg and the maximum pressure rise in left ventricle is about 120 mm Hg, during the peak of this wave.Maximum pressure in the left ventricle is 4 to 5 times more than that in the right ventricle, because of the thick wall of the left ventricle.
‘D-E’ Segment
‘DE’ segment appears during protodiastole. Pressure decreases slightly due to the starting of ventricular relaxation.‘E’ indicates the closure of semilunar valves.
‘E-F’ Segment
‘EF’ segment is obtained during iso-volumetric relaxation.There is a sharp fall in the intraventricular pressure during this phase. Pressure in the ventricle falls below
the pressure in the atria and this causes the opening of atrio-ventricular valves.‘F’ represents the opening of atrio-ventricularvalves.
‘F-G’ Segment
‘FG’ segment appears during rapid filling phase. In spite of filling of blood, pressure decreases in the ventricles. It is because of the relaxation of the ventricles.
‘G-A’ Segment
‘GA’ segment is the last part of intraventricular pressure curve. It is obtained during slow filling phase. Because of continuous relaxation of ventricles during slow filling period, the ventricular pressure decreases further.
AORTIC PRESSURE CHANGES DURING CARDIAC CYCLE
SIGNIFICANCE
Aortic pressure is the pressure developed in the aorta.It is necessary to maintain the blood flow through the
circulatory system.
METHOD OF STUDY
Changes in aortic pressure during the cardiac cycle are recorded by using catheter.
MAXIMUM AND MINIMUM PRESSURE IN AORTA
Pressure in systemic aorta is always higher than that of pulmonary artery. It is because of the higher
pressure in left ventricle than in the right ventricle. Minimum pressure in systemic aorta is
much greater than the minimum pressure in the left ventricle. It is due to the presence of elastic tissues in
the aorta, which enable the aorta to recoil and maintain the minimum pressure at a higher level.
AORTIC PRESSURE CURVE
During the ejection period of the cardiac cycle, the pressure in the aorta increases and reaches the peak.
During diastole, it reduces gradually and reaches the minimum level. At the time of closure of semilunar
valves, an incisura occurs due to back flow of some blood towards the ventricles
VENTRICULAR VOLUME CHANGES DURING CARDIAC CYCLE
SIGNIFICANCE
Volume of blood in the ventricles is an important factor to maintain cardiac output and blood circulation.
METHODS OF STUDY
1. By using Henderson Cardiometer
This study is done only in animals. Cardio-meter is a cup-shaped device with an outlet. At the top, it is closed by means of a rubber diaphragm. A small hole is made in the diaphragm, through which the ventricles of the animal are pushed. Cardiometer is connected to a recording device like Marey tambour (a small stainless steel
capsule covered by rubber membrane) or polygraph, to record the volume changes
2. By Angiography
Angiography is the radiographic study of heart and blood vessels using a radiopaque contrast medium.
During angiography, it is possible to measure the ventricular dimensional area and thickness of ventricular
wall. From the values obtained, the ventricular volume is calculated.
VOLUME OF BLOOD IN RIGHT AND LEFT VENTRICLES
End-diastolic Volume and End-systolic Volume
Amount of blood is the same in both right and left ventricles. Maximum volume of blood in each ventricle
after filling (end-diastolic volume) is 130 to 150 ml Minimum volume of blood left in the ventricles at the
end of ejection period (end of systolic volume) is 60 to 80 ml
Ejection Fraction
Ejection fraction (Ef) is the stroke volume divided by end-diastolic volume, expressed in percentage. See
above for determination and significance of determining ejection period.
VENTRICULAR VOLUME CURVE
Ventricular volume curve recorded by using Henderson cardiometer has seven segments (Fig. 91.6).
‘A-B’ Segment
‘AB’ segment wave is because of atrial systole or last filling phase of ventricles, during which a small amount
of blood enters the ventricles from the atria. It increases the ventricular volume slightly.
‘B’ indicates the closure of atrio-ventricular valves.
‘B-C’ Segment
‘BC’ segment is a positive wave, which is obtained during iso-volumetric contraction. Actually, the ventricular volume is not altered during iso-volumetric contraction However, the slight upward deflection of this wave is
an artifact. It is because the heart thrusts itself into the cardiometer during iso-volumetric contraction.‘C’ represents the opening of semilunar valves.
‘C-D’ Segment
‘CD’ segment occurs during ejection period. Initially, there is a sharp fall in the ventricular volume. This
occurs during rapid ejection. Later, during slow ejection period, the blood leaves the ventricles slowly. So the
ventricular volume decreases slowly.
‘D-E’ Segment
‘DE’ segment part of the ventricular volume curve is recorded during protodiastole. There is no change in
the ventricular volume during protodiastole. ‘E’ denotes the closure of semilunar valves.
‘E-F’ Segment
‘EF’ segment appears during iso-volumetric relaxationperiod of the cardiac cycle. Actually, the ventricular
volume is not altered during iso-volumetric relaxation. However, there is a slight upward deflection in the curve due to artifact. It is because of the entrance of blood into coronary artery from aorta during this period. It
increases the pressure within the cardiometer. ‘F’ indicates the opening of atrio-ventricular valves.
‘F-G’ Segment
‘FG’ segment appears during rapid filling phase. Rapid rise in ventricular volume is due to sudden rush of blood after the opening of atrio-ventricular valves.
‘G-A’ Segment
‘GA’ segment is recorded during slow filling phase. Ventri-cular volume increases slowly because of slow filling.
