CARDIAC CYCLE Haripriya Jayakumar
CARDIAC CYCLE The cardiac events that occur from the beginning of one heart beat to the beginning of the next.
History Sir Thomas Lewis, in 1920, fully assembled the cardiac cycle Carl Wiggers, in 1915, first conceived the mechanical events
Systole and diastole- physiologic vs cardiologic PHYSIOLOGIC SYSTOLE CARDIOLOGIC SYSTOLE Isovolumic contraction M1 – A2 maximal ejection PHYSIOLOGIC DIASTOLE CARDIOLOGIC DIASTOLE Reduced ejection phase A 2 – M1 Isovolumic relaxation Filling phase
ELECTRICAL EVENTS
tissue Conduction rate (m/s) SA node 0.05 Atrial pathway 1 AV node Bundle of His Purkinje system 4 Ventricular muscle
Mechanical events of cardiac cycle
The Cardiac Cycle LV Contraction Isovolumic contraction Maximal Ejection LV Relaxation Reduced ejection phase Isovolumic relaxation LV Filling Rapid filling phase Diastasis Atrial systole or kick
Mechanical events Electrical activity of the heart – electrocardiogram (ECG) Pressure and volume changes in both the atria and the ventricles Pressure changes in the arteries – arterial pulse Heart sounds or phonocardiogram
ATRIAL CONTRACTION First phase of the cardiac cycle
ECG Electrical depolarisation of the atrial musculature from the SA node to AV node Represented by the P wave in the electrocardiogram
Atrial Contraction With atrial systole, atrial pressure exceeds the ventricular pressure
Atrial Contraction 20% of the ventricular filling Atrial contribution to ventricular filling varies Inversely with ventricular diastole – up to 40% directly with atrial contractility
Atrial systole produces increase in venous pressure noted as ‘a’ wave in the LA pressure tracing. However blood does not flow back into the vena cava or pulmonary veins: Inertial effect of the venous return ‘’ milking effect’’
With the completion of atrial contraction, Fall in atrial pressure Pressure gradient reversal across the AV valve- float upwards to the preposition Maximum volume of the LV - LVEDV – 120ml – ventricular preload LV EDP – 8-12mm RV EDP – 3-5 mm
Fourth heart sound Heard sometimes with atrial contraction Atrial heart sound Vibration of the ventricular wall with atrial contraction “stiff” ventricle
VENTRICULAR CONTRACTION ISOVOLUMIC CONTRACTION VENTRICULAR CONTRACTION
ECG Depolarisation wave spreads from AV node to septum and walls of the ventricle through Bundle of His and Purkinje fibres
QRS complex in ECG Atrial repolarisation occurs during the same time with Ta being buried inside the QRS complex
The PRESSURE changes in the chambers: VENTRICLES With the depolarization, Ca2+ arrives at the contractile protein – triggers actin myosin interaction : EXCITATION CONTRACTION coupling Myocyte contraction Rapid increase in the ventricular pressure Maximal dP/dT Ventricular pressure increases to LV pressure – 10 mm Hg to 80 mm Hg RV pressure – 4 mm Hg to 15 mm Hg
As the ventricular pressure exceeds the atrial pressure, the AV valves close After the closure of the AV valve and before the opening of the aortic ad pulmonary valves, the ventricles are closed chamber with fixed volume ISOVOLUMIC contraction
The blood volume in this phase = EDV Individual myocyte contraction is not uniformly isometric Some fibres – isometric isotonic eccentric contraction
Ventricle volume is not altered but the chamber geometry changes with heart becoming more spheroid shape - increase in the circumference - decrease in base to apex length
ATRIA: AV valve bulge backward to produce the ‘c’ wave However, since the ventricular contraction also triggers the papillary muscle contraction – prevents valves from becoming incompetent
ARTERIES: The pressure in the systemic and pulmonary arteries constantly decreases during this phase
HEART SOUNDS: First heart sound Vibration of the AV valve, the adjacent myocardium and blood due to the closure of the AV valves When the LV pressure exceeds the LA pressure ( 10-15 mm Hg) the mitral valve closes with a minor inertial delay of ≤ 20 millisec producing M1 Soon to be followed, with a delay of less than 0.04 sec, by T1 produced by the tricuspid valve closure, delayed because of electrical conduction
VENTRICULAR CONTRACTION RAPID / MAXIMAL EJECTION PHASE VENTRICULAR CONTRACTION
Ejection phase of the ventricle : rapid ejection – 70% of the blood volume first 1/3rd of the ejection phase slow ejection – 30 % of the blood volume rest 2/3rd of the ejection phase
ECG: The ventricles are completely depolarised at the beginning of the ejection phase – ST segment
VENTRICLE: The pressure in the left and right ventricle exceeds that of the corresponding great vessel- by a few mm Hg Opening of the aortic and pulmonary valve- clinically silent event
The maximal pressure in the ventricles are attained at the height of ejection – 120 mm Hg in the LV and 25 mm Hg in the right ventricle The pressure gradient across the valve is low because of the relatively large valve opening (ie low resistance) Energy gradient between the blood in the ventricle and great vessels which propel the blood across the aortic and pulmonary valves
ATRIA: The LA pressure initially decreases as the atrial base is pulled down expanding the atrial chamber when the ventricle contract ‘x’ descent Blood continues to flow into atrium from the venous tract and the atrial pressure rises
ARTERIES: The pressure in the great vessels rise due to the rapid ejection of blood from the ventricles, reaching a maximum of 120 mm Hg in the aorta and 25 mm Hg in the pulmonary artery; ie the systolic pressure
VENTRICULAR RELAXATION REDUCED EJECTION PHASE VENTRICULAR RELAXATION
200 ms after the QRS and beginning of ventricular depolarisation, the ventricular repolarisation starts (T wave) SR uptake of Ca2+, with Ca2+ dissociating from Troponin C, preventing the further cross-bridge formation Decline in the ventricular active tension and pressure generation Decrease in the rate of ejection
VENTRICLES: As the blood volume in the ventricle decrease the pressure in the ventricle decrease ATRIA: The pressure in the atria continue to rise gradually due to continued venous filling
ARTERIES - during this phase the blood flow from the left ventricle into the aorta rapidly diminishes, but is maintained by the aortic recoil – the Windkessel effect
Volume change in ejection phase Under resting condition about 70 ml of blood is ejected into the great vessels – stroke volume 60 ml remains in the ventricle – ESV Ratio of stroke volume to the end diastolic volume = ejection fraction 60-65% normal ejection fraction
VENTRICULAR RELAXATION ISOVOLUMIC RELAXATION VENTRICULAR RELAXATION
At the end of systole, the ventricle starts to relax and the ventricular pressure falls When the pressure in the aorta and the pulmonary artery significantly exceeds that of the corresponding ventricles, the aortic and the pulmonary valve closes. Thus, in this phase of ventricular relaxation- the AV valves are closed and so are the aortic and pulmonary valves – ISOVOLUMIC RELAXATION
ECG: The ventricular repolarization is being completed and the end of the T wave appears in the ECG
VENTRICLES: The ventricular relaxation leads to decline in the ventricular pressure to LV pressure : 20 mm Hg RV pressure : near 0 mmm Hg Rate of pressure drop is determined by the rate of relaxation of the myocyte – lusitropy – regulated by the sarcoplasmic reticulum- responsible for sequestering the calcium. The blood volume in each ventricle = end systolic volume of 70 ml
ATRIA: The atrial pressure continues to rise because of the venous return with the formation of v wave. ARTERIES Valve closure is associated with a small blackflow of blood into the ventricle – dicrotic notch or incisura After valve closure, the aortic and pulmonary pressure rises slightly followed by a slow decline in pressure – dicrotic wave
HEART SOUND When the pressure in the aorta and pulmonary artery exceeds the falling ventricular pressures, the aortic and the pulmonary valve closes producing the A2 and P2
VENTRICULAR RELAXATION RAPID FILLING VENTRICULAR RELAXATION
As the ventricles continues to relax, the ventricular pressure falls below that of the atria Opening of the AV valve Rapid filling of the ventricle begins Active diastolic relaxation may also contrbute to the early filling – ‘ ventricular suction during diastole’
VENTRICLE Despite the inflow of blood from the atria, the ventricular pressure initially fall, because the ventricles are still relaxing. Once the relaxation is complete, the ventricular pressure slowly rise as they fill with blood from the atria
This phase accounts for most of the ventricular filing (70%) Although the ventricular volume increases the ventricular pressure does not increase significantly due to chamber relaxation
ATRIA Opening of the AV valve results in rapid drop in pressure – y descent ARTERIES The pressure in the arteries fall after closure of semilunar valves, but never fall to zero due to the elastic property of the vessels
HEART SOUND Normally silent event May give rise to third heart sound May represent tensing of the chordae tendinae and AV valve ring during relaxation and filling
VENTRICULAR RELAXATION SLOW FILLLING PHASE / DIASTASIS VENTRICULAR RELAXATION
As the ventricle is filled with blood they become less compliant – the ventricular pressure starts to increase – reduction in the pressure gradient across the AV valve – reduction in the ventricular filling in the late diastole.
Around 80-90% of ventricular filling occurs passively The arterial pressure continues to fall By the end of this phase, SA node begins to produce the next impulse for the atrial depolarisation- atrial contraction/ kick and the cycle continues
THE LEWIS OR WIGGERS CYCLE
LV CONTRACTION LV RELAXATION Isovolumic contraction (b) Start of relaxation and reduced ejection (d) Maximal ejection (c) Isovolumic relaxation (e) LV filling rapid phase(f) Slow LV filling (diastasis) (g) Atrial systole or kick (a)
RV vs LV PRESSURE WAVE 1/5th dP/dT is less Isovolumic contraction and relaxation phases are shorter
Timings of the events Total ventricular systole – 0.3 s Isovolumic contraction – 0.05s Maximal ejection – 0.1 s Reduced ejection – 0.15 s Total ventricular diastole – 0.5 s Isovolumic relaxation – 0.1 s Rapid filling phase – 0.1s Slow filling phase/ diastasis -0.2s Atrial systole – 0.1 s