Introduction to Cardiac Cycle and Cardiac Output

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Introduction to Cardiac Cycle and Cardiac Output


ObjectivesExplain the ECG waves and correlate them with mechanical events. Describe the phases of cardiac cycle. Identify the origin of heart sounds. Define heart rate, stroke volume, venous return and cardiac output. Explain the Starling’s law of the heart. List the function of autonomic nervous system on the heart.


Electrocardiogram ECG (EKG)


Electrocardiogram ECG (EKG)Surface electrodes record electrical activity deep within body Reflects electrical activity of whole heart not of single cell! EC fluid = “salt solution” (NaCl)  good conductor of electricity to skin surface ECG tracing =  of all electrical potentials generated by all cells of heart at any given moment


Electrocardiogram (ECG, EKG) Electrodes placed on arms and legs (limb leads) + six positions on chest (chest leads). Each lead records different electrical activity (each looks at heart from a different position)


3 waves appear with each heart beat: P wave: represents atrial depolarization QRS complex: represents ventricular depolarization T wave: represents ventricular repolarization


Depolarization = signal for contraction Segments of ECG reflect mechanical heart eventsSince:


Correlation of ECG waves with Atrial and Ventricular Systole


ECG cont.Comparing records with one another & normal records, allows determination : if conductivity pathway is normal if the heart is enlarged if certain regions of heart are damaged ECG time intervals or segments: P-Q interval S-T interval Q-T interval


ECG cont.


Cardiac CycleOne cardiac cycle (CC): all events associated with one heart beat. (Systole and diastole of atria + systole and diastole of ventricles) At HR 75 beats /min, CC lasts 0.8 secs. In each CC, atria & ventricles alternatively contract & relax  Force blood from areas of higher pressure  Areas of lower pressure


Cardiac Cycle cont.Cardiac Cycle comprises the following phases: Atria systole: atria contracting but Ventricles relaxing. Lasts 0.1 sec Ventricular systole: Ventricular Contracting while atria relax (atrial diastole). Lasts 0.3 sec. Relaxation Phase (Complete cardiac diastole ), Both atria and ventricular relax. Lasts about 0.4 sec


Heart Sounds Auscultation: act of listening to sounds within the body, usually done with a stethoscope sounds are caused by vibrations set up within the walls of ventricles and major arteries during valve closure. During each CC4 heart sounds Only first & second (S1 &S2) heard in normal heart


First heard sound (S1) lubb: - louder & longer than second - caused by closure of A-V valves (soon after beginning of ventricular systole) Second heart sound (S2) dupp: - shorter & not as loud as first - caused closure of SL valves (beginning of ventricular diastole) S3 is due to turbulance during ventric filling S4 is due to turbulance during atrial systole


Heart soundsS1 &S2 are best heard at the surface of chest in specific locations


Cardiac output (CO)CO is the volume of blood ejected from the left ventricle (or the right ventricle) into the aorta (or pulmonary trunk) each minute. CO equals the stroke volume (SV) multiplied by heart rate (HR) CO = SV X HR SV volume of blood ejected by the ventricle during each contraction HR number of heart beats per minute. CO = SV X HR = 70 ml/beat X 75 beats/minute = 5.25 L/min


Cardiac Output cont.CO may  to meet demands During exercise CO can increase to 20-25 L/ min Cardiac reserve (CR): The difference between the maximum CO and CO at rest. Average CR: 4 -5 X resting value (20-25 L/m) higher in athletes (35 L/m) little or no reserve in heart disease


Regulation of cardiac outputCO= SV x HR Depends on the regulation of SV and HR


Cardiac Terminology End diastolic volume (EDV): blood in the ventricles at the end of diastole. Ejection fraction: fraction of EDV that is ejected (%), used to measure heart efficiency. End systolic volume (ESV): blood that remains in the blood at the end of systole. Preload: the degree of stretch on the heart before it contracts. Afterload: the pressure that must be exceeded before ejection of blood from the ventricles can occur.


Regulation of stroke volumeTHREE factors regulate SV: Preload (EDV) Contractility Afterload


1. Preload: effect of stretchingFrank-Starling (F-S) law of the heart: Within limits, the more the heart fills during diastole (preload – stretching), the greater the force of contraction during systole. the preload is proportional to EDV, the greater the EDV, the more forcefull the next contraction “The heart will pump what it receives”- Starling’s law of the heart


Preload cont.Two factors determine EDV: Duration of ventricular diastole (HR) Ventricular filling occur during diastole Tachycardia ( HR) diastole duration ventricular filling end diastolic volume CO Venous return (volume of blood flowing back to the heart through systemic veins)


2. Contractility Contractility: strength of contraction at any given preload +ve inotropism:  contractility Include: - sympathetic stimulation - Hormones; adrenaline and noradrenaline -ve inotropism:  contractility May result from: - inhibition of the sympathetic system - anoxia - acidosis


3. Afterloadpressure in pulmonary tract is a bout 20 mm.Hg and in the aorta is about 80 mm Hg. This pressure must be overcome before the semilunar valves open. Depend on: Elasticity of large arteries Peripheral resistance of arterioles An  in afterload   SV  more blood remains in ventricle at end of systole (ESV) Conditions that  afterload include: Hypertension Narrowing of arteries by atherosclerosis


Regulation of heart rateSeveral factors, the most important are 1. Nervous factors 2. Chemical factors 3. Others

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Last Updated: 8th March 2018

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