Cardiac cyclen
- 2. CARDIAC CYCLE The cardiac events that occur from the beginning of one heart beat to the beginning of the next. Events : Electrical Mechanical Electrical and mechanical events occur in a coordinated manner to generate effective contractions Duration – 0.8 s
- 3. ELECTRICAL EVENTS The rhythmical activity of the heart is controlled by electrical impulses A specialised conduction system generates and propagates impulses Cardiac muscle as a functional syncytium enables rapid and uniform passage of the impulses Ensures : all parts of the ventricle contract simultaneously. atrial contraction → 1/6th sec before ventricular contraction
- 5. SINOATRIAL NODE Junction of SVC with right atrium Specialized neurocardiac tissue ; almost no contractile muscle filament Normal pacemaker Connects directly with the surrounding atrial muscle fibres so that action potential generated spreads immediately into the atrium. Properties of Self- excitation and Rythmicity
- 6. What causes the Self Excitation? • Membrane in natural state permeable to Na and Ca ions • RMP in SA nodal fibre is less negative : -60 to -70 mv • This voltage closes fast Na channels but slow Na - Ca channels are open • The ions flow inward → membrane permeability further increased → Threshold reached → Action potential
- 7. Causes of Self Excitation Half way through the action potential, slow Na-Ca channels begin to close. K channels open for a prolonged period Inside of the membrane becomes more negative = HYPERPOLARIZATION This persists for 0.5s after the Action potential ends The K conductance weans off → RMP reached → Na –Ca opening overbalances K closure → threshold for Action potential→ Action potential
- 8. SINO ATRIAL NODE Self–excitation & AP Resting Memb Potential Recovery of AP Hyperpolarisation
- 9. Conduction through the other parts From SAN impulses through the 3 bands (1m/s) and atria (0.3m/s) Delay in the AVN – of 0.09 s. Due to reduced gap junction number. Allows time for the ventricles to fill completely before they contract Rapid transmission through Purkinje fibres (4m/s) due to many gap junctions Transmission in ventricular muscle- 0.3 – 0.5m/s
- 10. AP in a Ventricular Muscle Fibre 1 2 3 0 4 Phase 0 : rapid depolarization – opening of fast Na channels Phase 1 : rapid repolarisation – closure of Na channels Phase 2 : Plateau – slow prolonged opening of Ca channels Phase 3 : final repolarisation – closure of Ca channels Phase 4 : RMP (-85 to -90 mv) – opening of K channels
- 11. REFRACTORY PERIOD • Refractory to re-stimulation during Action potential •Absolute refractory period = 0.25 – 0.3 s. Phase 0 to 2 and half of ph 3 • Relative refractory period = phase 4 • Refractory period of the atria is shorter than that of ventricle
- 12. Variation in length of action potential and associated phenomena with cardiac rate.1 Heart Rate Heart Rate 75/min 200/min Duration, each cardiac 0.80 0.30 cycle Duration of systole 0.27 0.16 Duration of action 0.25 0.15 potential Duration of absolute 0.20 0.13 refractory period Duration of relative 0.05 0.02 refractory period Duration of diastole 0.53 0.14 1 All values are in seconds.
- 13. Effect of autonomic nervous system on conduction
- 14. Effect of parasympathetic system Vagal stimulation causes release of Ach Two effects: ↓ rate of rhythm of SAN – RMP becomes more negative ↓ excitability of A-V junctional fibres Slowing of heart rate, complete block in transmission may occur Strong vagal stimulus → ventricles stop beating for 5-20s → purkinje fibres take over → ventricles contract at 15-40 beats per minute → VENTRICULAR ESCAPE
- 15. Effect of sympathetic system Sympathetic stimulation → Noradrenaline released Three effects : ↑ rate of SAN discharge – RMP more positive ↑ rate of conduction and overall excitability ↑ the force of contraction – increases Ca permeability Heart rate may increase 3 times and strength of contraction may increase 2 times the normal
- 16. EXCITATION-CONTRACTION COUPLING How AP causes myofibrils to contract
- 17. MECHANICAL EVENTS The heart contracts and relaxes alternately during every heart beat and this occurs in concordance with the electrical events. The mechanical events can be studied in various phases,which are explained henceforth.
- 20. ATRIAL SYSTOLE Initiated by p wave of ECG :Atrial Depolarisation Atrial contraction causes increased pressure in atrial chambers,forcing more blood through the open AV valves Ventricular filling : 80% - direct flow from SVC 20% - atrial contraction.
- 21. ATRIAL SYSTOLE At high heart rates,however,atrial contraction may account for 40% of ventricular filling : Atrial kick Atrial contribution to ventricular filling directly proportional to atrial contrctility,and inversely proportional to duration ATRIAL SYSTOLE of ventricular diastole. After atrial contraction is completed,atrial pressure starts falling: Pressure Gradient Reversal across AV Valves EDV,120ml,represents ventricular preload;associated with End Diastolic Pressure 8-12mm Hg in LV & 3-6mm in RV.
- 23. ….. Atrial systole JVP – ‘a’ wave ECG – P wave preceeds the atrial systole. PR segment – depolarization proceeds to the AVN. The brief pause allows complete ventricular filling Heart sounds - S 4 – pathological. Vibration of the ventricular wall during atrial contraction. Heard in ‘stiff’ ventricle like in hypertrophy and in elderly. Also heard in massive pulmonary embolism, cor pulmonale, TR
- 24. ISOVOLUMETRIC CONTRACTION Increase in ventricular pressure > atrial pressure → AV valves close Begins with the appearance of QRS Complex: Ventricular Depolarisation Period between AV valve closure and semilunar valve opening → contraction occurs without emptying Tension develops without change in muscle length
- 25. AV Valves close as IV pressure exceeds atrial pressure Early in this phase,rapid increase in !V pressure: maximal dP/dT Contraction of papillary muscles and chordae tendinae prevent AV valve leaflets from bulging back into atria & becoming incompetent Closure of AV Valves-First Heart Sound (S1);normally split (0.04s)
- 26. … Isovolumetric contraction JVP – ‘c’ wave → due to the bulging of the Tricuspid valve into RA secondary to increased pressure in the ventricle. ‘x’ descent ECG – Interval between QRS complex and T wave (QT interval) Heart Sounds – S1 : closure of the AV valves. Normally split as mitral closure preceeds tricuspid valve closure. valve
- 27. EJECTION • When LV pres > 80 mm Hg RV pres > 8 mm Hg, The semilunar valves open. • Rapid Ejection – 70% emptying in first 1/3 • Slow Ejection – 30% in last 2/3 • The pressure in the ventricle keeps decreasing until it becomes lower than that of the great vessels
- 28. EJECTION Ventricular pressure exceeds outflow tract pressure by a few mm Hg.Pressure gradient across valve usually low,due to relatively large valve orifice ( Low resistance) LA pressure initially decreases as atrial base is pulled downwards Approximately 200 m sec after QRS and the beginning of ventricular contraction, ventricular repolarization occurs as shown by the T-wav of ECG. Repolarization leads to a decline in ventricular tension ,and t rate of ejection falls. Ventricular pressure falls slightly below outflow tra pressure; however, outward flow still occurs due to kinetic (or inertial) energy of the blood.
- 29. …………. ejection JVP – no waves ECG – T wave Heart sounds – none Aortic pressure - Rapid rise in the pressure = 120 mm Hg Even at the end of systole pressure in the aorta is maintained at 90 mm Hg because of the elastic recoil
- 30. ISOVOULUMETRIC RELAXATION • When ventricle pressure < arterial pressure→ backflow of blood → forces semilunar valves to close. • For 0.03-0.06 s, ventricle relaxes despite no change in its volume • Meanwhile, atria fill up and atrial pressure gradually rises • Pressures in ventricle keep falling till it is < atrial pressure
- 32. …… isovolumetric relaxation JVP – ‘v’ wave – due to venous return to the atria from SVC and IVC ECG - no deflections Heart sounds – S2 : closure of the semilunar valves. Normally split because aortic valve closes slightly earlier than the pulmonary valve Aortic pressure curve – INCISURA - when the aortic valve closes. Caused by a short period of backflow before the valve closes followed by sudden cessation of the backflow when the valve closes.
- 33. VENTRICULAR FILLING • Begins with the opening of AV valves • Rapid filling – first 1/3 of diastole • Reduced filling (Diastasis) – middle 1/3 of diastole • Atrial contraction – last 1/3 of diastole • As the atrial pressures fall, the AV valves close and left ventricular volume is now maximum → EDV (120 ml in LV)
- 35. ……… ventricular filling JVP – ‘y’ descent ECG – no deflections Heart sounds - S3 - Pathological in adults. Seen in dilated congestive heart failure, MI, MR, severe hypertension. Normal in children.
- 36. VARIOUS PRESSURE VALUES CHAMBERS Right Atrium Right Ventricle NORMAL RANGE (mm of Hg) 2–6 ( systolic) 15 – 25 (diastolic) 2 -8 Pulmonary Artery (systolic) (diastolic) Left Atrium Left Ventricle 15 – 25 8 - 15 6 - 12 (systolic) 100 – 140 (diastolic) 3 – 12
- 37. SUMMARY The conduction system and the atrial and ventricular muscle in the normal heart work in an extremely well coordinated manner to ensure correct opening and closure of the atrio - ventricular and semilunar valves and movement of the blood through the heart in an appropriate direction with minimal regurgitation.
- 38. REFERENCES 1 . William F. Ganong. The heart as a Pump. Review of Medical Physiology, Lange, 2010 2. Guyton and Hall. Heart Muscle, the heart as a Pump and function of the heart valves. Textbook of Medical Physiology.Philadelphia, Mosby Elsevier, 2006 3.Cardiovascular Physiology & Concepts 2nd Edition:Lippincott Williams & Wilkins 4 Ronald D. Miller. Cardiac Physiology.Miller’s Anaesthesia.Philadelphia, Mosby Elsevier, 2010