Physiology: Cardiac Cycle PDF

(005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21...

(005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21 OUTLINE  HR 72 beats /min  Duration of cardiac cycle is 1/72 min/ beat I. CARDIAC CYLCE  About 0.0139 minutes per beat, or A. PHASES OF CARDIAC CYCLE 0.833 second per beat II. PRESSURE VOLUME LOOP  When the heart rate increases - duration of cardiac III. RELATIONSHIP OF THE cycle decreases, including contraction and relaxation ELCTROCARDIOGRAM TO THE CARDIAC phases. CYCLE  Duration of action potential and the period of contraction (systole) also decreases, but not A. SCALAR ELECTROCARDIOGRAPHY by as great a percentage as does diastole B. STANDARD LIMB LEADS  Heart beating very fast rate does not remain C. ARRYTHMIAS relaxed long enough to allow complete filling of the cardiac chambers before the next I. CARDIAC CYCLE contraction.  Sequence of mechanical and electrical events that repeats every heart beat  Sequence of cardiac events that occur from the beginning of one heartbeat to the beginning of the next heartbeat  Each cycle is initiated by spontaneous generation of an action potential sinus node (pacemaker of the heart) o Action potential  Superior vena cava both atria A-V bundle  ventricles  A delay of more than 0.1 second during passage of cardiac impulse from atria to ventricles allowing atria to contract ahead of ventricular contraction, thereby pumping blood into the ventricles before the strong ventricular contraction begins.  Atria: Act as primer pumps for the ventricles. Figure 1. Wiggers diagram. Note: Events of the cardiac cycle for left ventricular function, showing changes in left atrial  Ventricles: Provide the pressure, left ventricular pressure, ventricular volume, major source of power for electrocardiogram and the phonocardiogram. moving blood through the body’s vascular system. o Sinus node A. PHASES OF CARDIAC CYCLE - Located in the superior lateral wall of the COVERAGE OF SYTOLE: right atrium near the opening of the superior vena cava 1. Atrial systole  Subdivided into of 2 periods:  FOURTH HEART SOUND is heard in adults 1. Diastole – period of relaxation, the heart with associated disease fills with blood  Onset of atrial systole occurs soon after the 2. Systole – period of contraction beginning of the P wave (ATRIAL a. At a normal heart rate of 72 DEPOLARIZATION) of the ECG beats/min: It comprises about  Triggered activity is always coupled to a 0.4 of the entire cardiac cycle preceding action potential and is caused  Blood flows from higher to lower pressure; contraction by after depolarizations. increases the pressure, relaxation lowers the pressure  Early depolarizations (EADs)-  AV valves open when atrial pressure is higher than the occur when the prevailing heart ventricular pressure; close when the pressure gradient rate is slow is reversed  Delayed depolarization (DADs)-  Semilunar valves open when ventricular pressures are heart rate is high. higher than the aortic pulmonary pressures; close  Begins as atrium contracts → small rise in when reversed atrial (a wave) and ventricular pressure  Duration of cardiac cycle (including the systole  Atrial contraction causes increase pressure in and diastole): reciprocal of heart rate the atrial and ventricular pressures  Mitral valve is open and aortic valve is closed Page 1 of 7 PREPARED BY: CMED 1C  (005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21  Aortic pressure gradually drops as the elastic  Aortic pressure continues to drop and walls of the artery recoil from the previous reaches its minimum pressure at about systolic ballooning 80mmHg  Ventricle at its maximum volume at  Massive contraction of the ventricle distorts approximately 120ml (END DIASTOLIC the shape of the atrium → atrial pressure C VOLUME) or the volume of blood that fills the wave ventricles during diastole  At the end of this phase: ventricular pressure  QRS complex is seen at the end of this phase equals and exceeds aortic pressure. in preparation for ventricular contraction OPENING OF AORTIC VALVE  Atrial systole is responsible for the small (progressively increasing pressure in the left increases in the atrial, ventricular and venous ventricle → exceeds the pressure in the aorta pressure, as well as ventricular volume. → aortic valve opens → ejection or outflow) Contribution of atrial contraction to ventricular filling is governed to a great extent by the  Ejection or outflow – subdivided into two periods: heart rate and position of the AV valves. At  Blood beings to pour out of ventricles slow heart rates, filling practically ceases  Approximately 60% of blood in the ventricle at toward the end of diastasis and atrial the end of diastole is ejected during systole contraction contributes little additional filling. During tachycardia, however, diastasis is 3. Rapid ventricular ejection/ Maximum ejection abbreviated and atrial contribution can  Ventricular pressure continues to rise, become substantial followed by a rapid elevation of aortic  Atria is regarded as the primer of the pressure, ventricular pressure drops ventricles  When the left ventricular pressure increases  At rest 20% volume of blood contribution slightly to more than 80mmHg, the ventricular (atrial contraction usually causes an pressure pushes the aortic valve to open additional 20% filling of the ventricles during  Left ventricle slightly exceeds aortic pressure resting period) and aortic blood flow accelerates  Heavy exercise, rise to as high as 40%  Semilunar valves open allowing ejection of  Atrial contraction causes a slight rise in intra- blood from the left ventricle going to aorta atrial pressure  Is the first third of the ejection period of blood  70% of blood (from 60% as 2. Isovolumic contraction mentioned above) flows out during  Systole commences this period  The phase between the start of ventricular  Aortic cusps open → blood rapidly ejected into systole and opening of the semilunar valves: the aorta → rapid drop in ventricular volume aortic and pulmonic (when ventricular and increase in pressure in the ventricle and pressure rises abruptly) aorta brought by contracting forces  An additional 0.02 to 0.03 second is  Atrial filling via venous pressure against a required for the ventricle to build up closed mitral valve → gradual rise in the sufficient pressure to push atrium pressure semilunar valves (aortic and  Midway in the cycle: muscle stops contracting pulmonary) open against the and starts repolarizing → T wave appears; pressures in the aorta and decrease of aortic and ventricular pressures pulmonary arteries  Although the ventricle is contracting, the 4. Reduced ventricular ejection/ Slow ejection volume does not change; remains at 120ml  Decrease in ventricular volume is less rapid,  Start of QRS complex → ventricular both ventricular and aortic pressure fall off contractions begin → ventricular pressures  Are the last two thirds of the ejection period rapidly rise and exceed the atrial pressure →  Remaining 30% of blood (from semilunar valves (aortic and pulmonic) open the 60%-as mentioned above) and atrioventricular valves (mitral and flows out during this period tricuspid) close  Although the pump has stopped the blood  The closure of mitral and tricuspid valves flows continue at a reduced rate due to inertia coincides with the peak of R wave on ECG,  End of the ejection phase: ventricular initiation of the FIRST HEART SOUND (S1), pressure falls below aortic pressure and earliest rise in ventricular pressure Page 2 of 7 PREPARED BY: CMED 1C  (005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21 COVERAGE OF DIASTOLE:  Slight increase in atrial and ventricular pressure because of the continuous venous 5. Isovolumic relaxation return  At the end of systole, repolarization of the  LA pressure will be slightly higher than LV ventricle is complete. Ventricular relaxation  Aortic pressure continues to fall begins suddenly, allowing both the right and  Phonogram is silent, no recorded heart sound left intraventricular pressures to decrease  Near the end diastasis: P wave appears on rapidly; end of the T wave the ECG as the atrial myocardium prepares  Ventricle at rest, thereby, ventricular pressure for atrial systole decreases rapidly back to their low diastolic levels End-Diastolic volume – the volume during diastole wherein  Closure of the aortic valve, followed by normal filling of the ventricles increases the volume of each closure of pulmonic valve. Closure of the ventricle to about 110 to 120 milliliters semilunar valves corresponds to the  When large amounts of blood flow into SECOND HEART SOUND (S2) ventricle during diastole, volume can become  AV valves remain closed during most of this as great as 150 to 180 milliliters in a healthy phase heart  The ventricular volume is constant (isovolumetric) and at its lowest value End-systolic volume – the remaining volume in each ventricle because the valves are closed (about 40 to 50 milliliters).  Elevated pressures in the distended large  When heart contracts strongly, volume may arteries immediately push blood back toward decrease to as little as 10 to 20 milliliters the ventricles, closing the aortic and pulmonary valves. Stroke volume output – the volume as the ventricle empty  Momentary backflow of blood → aortic valve during systole decreasing about 70 milliliters snapping closed → dicrotic notch or incisura  An increase in the end-diastolic volume and in the aortic pressure curve can be seen decrease in end-systolic volume can cause  Atrial pressure continues to rise as blood an increase to stroke volume output to more dams up against closed mitral valve → atrial than double that which is normal. v wave  When ventricular pressure becomes less than Ejection Fraction – the fraction of the end-diastolic volume that atrial pressure → MITRAL VALVE OPENS is ejected, usually equal to about 0.6/ 60%. 6. Rapid ventricular filling  The left ventricular pressure is slightly lower than the atrial pressure, hence, this reversal of pressure gradient causes the opening of mitral valve  Mitral valve open → the opening facilitates the rapidly flow of blood from the atria into the ventricles → steep slope on the ventricular volume curve (increase ventricular volume)  Gradual drop of aortic pressure because the aortic valve is closed  THIRD HEART SOUND (S3) hear at the end of this phase, possibly caused by turbulent flow into a nearly filled chamber  No activity is seen on the ECG 7. Diastasis or slow ventricular filling (longest phase)  Mitral valve is open but little blood flows from left atrium to the left ventricle; ventricular volume slowly rises then plateau.  The period flows in the middle of the ventricular diastole; only small amount of Figure 2. Wiggers Diagram. Note: A wave, slight rise. QRS starts blood enters the ventricle at the latter part of atrial systole in preparation for the ventricular contraction. Page 3 of 7 PREPARED BY: CMED 1C  (005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21 there is corresponding increase in the diastolic pressure, from 2-3 mmHg to 5-7 mmHg facilitating the closure of mitral valve   Phase II: Period of Isovolumic Contraction  Segment BC  Point B closure of mitral valve  Volume → no changes, because all valves closed  (120 ml throughout)  There is an increase pressure inside the left ventricle to equate the pressure in the aorta at 80mmHg  LV pressure = Aortic pressure  Aortic end diastolic pressure  Phase III: Period of Ejection  1st Segment CD rapid ejection  AV opens (LV pressure > Aortic Pressure); the ventricular pressure increases  Volume decreases from 120-75 ml Figure 3. Wiggers diagram. Note: The top 3 curves show the because the aortic valve has opened pressure changes in the aorta (aortic pressure – upper dotted causing the blood to flow out from lines), left ventricle (ventricular pressure – solid red line) and the the ventricle going to aorta left atrium (atrial pressure – lower dotted line). Each cardiac cycle  High pressure 120mmHg; the is initiated by spontaneous generation of action potential in the systolic pressure rises even higher sinus node and these action potentials spread rapidly to both atria than 80mmHg because there is still then through AV bundle going into the ventricles. Once action potential is generated, it is conducted to both atria which more contraction of ventricle stimulates the atrial systole.  2nd segment CD slow ejection  LV starts to relax  LV pressure 130 mmHg to II. PRESSURE VOLUME LOOP 100mmHg  Phase I: Period of Filling  Volume decreases from 75ml to  Segmented AB (Point A to Point B) 50ml  During point A, mitral valves open  Atrial pressure higher than ventricular pressure  Phase IV: Period of Isovolumic Relaxation  Left Ventricular (LV) volume at minimal  Segmented DA = 50ml = ESV (end-systolic volume)  Aortic valve closes  ESV – the amount/volume of blood  No change in volume (50ml) but that remains in the ventricle after the decrease in the ventricular pressure diastole or ventricular relaxation (LV relaxation)  Diastolic pressure of around 2-3  Ventricle returns to its starting point, mmHg with about 50 ml of blood left in the  At point B, the mitral valve closes ventricle and an atrial pressure of 2  Blood flows from LV to LA; an increase to 3 mmHg. of LV volume at about 120ml = Left  Point A is the end systolic volume Ventricular End-Diastolic Volume and pressure (LVEDV)  As venous blood flows into the ventricle from the left atrium, the ventricular volume normally increases about 120 ml, called end-systolic volume, an increase of 70 ml.  Once the amount of EDV (end- diastolic volume) reached 150ml, Page 4 of 7 PREPARED BY: CMED 1C  (005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21 a. P wave – caused by spread of depolarization through the atria and is followed by atrial contraction, which causes. a slight rise in the atrial pressure curve b. QRS wave – appear about 0.16 seconds after onset of the P wave  Appear as a result of electrical depolarization of the ventricles, which initiates contraction of the ventricles and causes the ventricular pressure to begin rising  This complex begins slightly before the onset of ventricular systole c. T wave – represents the stage of repolarization of the ventricles when the ventricular muscle fibers begin to relax Figure 4. Volume-Pressure Diagram  Occurs slightly before the end of ventricular contraction IV. RELATIONSHIP OF THE ELECTROCRDIOGRAM TO THE CARDIAC CYCLE  Electrocardiogram  Shows the electrical charges generated by the heart and is recorded by the electrocardiograph from the surface of the body  By analyzing the details of these fluctuations in electrical potential, physician gains insight into: a. Anatomical orientation of the heart b. Relative sizes of heart chambers c. Various disturbances in rhythm and conduction d. Extend, location and progress of ischemic damage to myocardium e. Effects of altered electrolyte Figure 4. Electrocardiogram showing P, Q, R, S, T waves concentrations f. Influence of certain drugs A. SCALAR ELECTROCARDIOGRAPHY (notably digitalis, antiarrhythmic agents, and calcium channel  A recording of changes in the difference in potential antagonists) between two points (temporal changes) on the surface of the skin over time and an indifferent electrode or  Lead is the electrical connection from the between pairs of points on skin surface. patient’s skin to the recording device (electrocardiograph) o Precise configuration of the ECG B. STANDARD LIMB LEADS varies from individual to individual,  Willem Einthoven and in any given individual, the - He devised the original ECG lead system at pattern varies with the anatomical the beginning of the 20th century location of the leads - In this system:  Tracing  Resultant cardiac vector  Is the graphic display of the electrical impulse o Is the vector sum of all cardiac recorded in an ECG? electrical activity at any moment  Generally, it consists of: o Directional electrical force lies in the of an equilateral triangle (called Einthoven’s triangle) whose apices Page 5 of 7 PREPARED BY: CMED 1C  (005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21 are located in the left and right shoulders and the pubic region Figure 6. Electrocardiographic tracings of the Heart Rhythms. A) Normal sinus rhythm, B) Sinus Tachycardia, C) Sinus Bradycardia  Bradycardia – results from the decrease of firing rate of the SA node Figure 5. Einthoven’s triangle, illustrating the o Heart rate decreases electrocardiographic connections for standard limb leads I, II,  Tachycardia – results from the increase firing of the and III. SA node  The electrodes are connected to the right and left o Heart rate increases forearms rather that to the corresponding shoulders because the arms represent simple electrical TEST YOUR KNOWLEDGE extensions of leads from the shoulders. 1. During isovolumic relaxation phase of the cardiac  Similarly, the leg represents an extension of the lead cycle: system from the pubis, and thus the third electrode is a. AV valves and semilunar valves are open generally connected to an ankle (usually the left) b. AV valves and semilunar valves are closed o Lead I – records the potential difference c. AV valves are close and semilunar valves are open between the left and right arm. d. AV valves are open and semilunar valves are open  If potential at the left arm (VLA) exceeds the potential at the right arm (VRA), 2. The cardiac events that occur from the beginning of tracing is deflected upward from the one heart beat to the beginning of the next heart beat isoelectric line. are called the cardiac cycle. During the cardiac cycle: o Lead II – records the potential difference a. Contraction of atria before the ventricles between the right arm and the left leg. b. Simultaneous contraction of atria and ventricles  Tracing is deflected upward when the c. Ventricles contraction before the atria potential of the left leg (VLL) exceeds (VRA). 3. In a typical human cardiac cycle, the volume of blood o Lead III – records the potential difference in the left ventricle just before the start of ventricular between the left arm and the left leg. systole is approximately ________. This volume if  Tracing is deflected upward when VLL referred to as the end-diastolic volume. exceeds VLA. a. 15 mL b. 50 mL B. ARRYTHMIAS c. 60 mL d. 120 mL  Are disturbances in either impulse initiation or impulse propagation. 4. Period wherein the ventricular pressure continues to  Disturbances in impulse initiation: Arise from the SA rise, followed by a rapid elevation of aortic pressure node and those that originate from various ectopic foci. and a drop in the ventricular pressure.  Disturbances in impulse propagation: Reentrant a. Isovolumic contraction rhythms and conduction blocks b. Slow ejection Page 6 of 7 PREPARED BY: CMED 1C  (005) CARDIOVASCULAR PHYSIOLOGY: CARDIAC CYCLE DR. MAEFLOR OFILAS | 01/11/21 c. Rapid ejection d. 140 to 160 milliliters d. Isovolumic relaxation 14. True or False: When the heart rate increases, duration 5. During this phase, the mitral valve opens causing a of cardiac cycle also increases. rapid flow of blood into the ventricles causing a steep a. True slope on the ventricular volume curve and a gradual b. False drop of aortic pressure. It is in the end of this phase wherein the third heart sound can be heard. 15. Is a sequence of mechanical and electrical events that a. Diastasis b. Maximum ejection occur from the beginning of one heartbeat to the c. Reduced ejection beginning of the next heartbeat. d. Rapid filling a. Electrocardiogram 6. The second heart sound is caused by: b. pressure volume loop a. Closure of semilunar valves c. Cardiac cycle b. Vibrations of ventricular wall during systole d. Action potential c. Closure of AV valves d. Ventricular filling 16. The volume of blood each ventricle pumps out during a cardiac cycle is about ___________ 7. The dicrotic notch on the aortic pressure is caused by: a. 70 ml a. Closure of mitral valve b. 50 ml b. Closure of aortic valve c. 150 ml c. Closure of tricuspid valve d. Rapid filling of the left ventricle d. 120 ml 8. During which phase of cardiac cycle is aortic pressure 17. The P wave of the ECG represents _________ highest? a. ventricular depolarization a. Reduced ventricular ejection b. atrial depolarization b. Atrial systole c. ventricular repolarization c. Isovolumic contraction d. atrial systole d. Rapid ventricular filling 18. Which of the following is not true for ventricular 9. Atrial systole is preceded by: systole? a. a-wave a. The ventricles relax b. p-wave b. The ventricles contract c. c-wave c. The semilunar valves close d. t-wave d. The atrioventricular valves open 10. If the ejection fraction increases, the _____ decreases a. Cardiac output 19. It represents ventricular contraction and ejection b. Heart rate phases. c. Pulse pressure a. Systole d. End-systolic volume b. Diastole c. Latch state 11. Results from a decrease of firing rate of the SA node d. Plateau phase and an increase in heart rate a. Arrythmias 20. There is a small rise in atrial pressure during phase 4 b. Bradycardia due to the bulging of mitral valve into the atrium when c. Systole the ventricle begins to contract, this is called d. Tachycardia a. c-wave 12. Phase in the pressure volume loop, wherein the aortic b. r-wave valves close and the ventricular pressure decreases. c. a-wave a. Phase I d. p-wave b. Phase II ANSWERS: B, A, D, C, D, A, B, A, B, D, D, D, A, B, C, A, B, A, A, C c. Phase III d. Phase IV REFERENCES 1. Berne, R. M., Koeppen, B. M., & Stanton, B. A. 13. Normal filling of the ventricles during diastole can (2010). Berne & Levy Physiology. Philadelphia, PA: increase the volume of each ventricle to about ______. Mosby/Elsevier. a. 110 to 120 milliliters** 2. Costanzo, L. S. (2014). Physiology (Fifth edition). b. 70 to 90 milliliters Philadelphia, PA: Saunders/Elsevier. c. 150 to 180 milliliters 3. Guyton, A.C., Hall, J. E. Guyton and Hall Textbook of Medical Physiology. 13th ed., W B Saunders, 2015. Page 7 of 7 PREPARED BY: CMED 1C