Cardiac Cycle PDF - 2024-25

Physiology School of Medicine 2024-25 Cardiac Cycle SI208: Cardiovascular Lecturer: Dr K.McCullagh Physiology ‘Wiggers Diagram’ : Pressure, volume, electrical and sound changes related to events in the cardiac cycle. Heart Sound...

Physiology School of Medicine 2024-25 Cardiac Cycle SI208: Cardiovascular Lecturer: Dr K.McCullagh Physiology ‘Wiggers Diagram’ : Pressure, volume, electrical and sound changes related to events in the cardiac cycle. Heart Sounds and what they indicate in terms of blood flow and heart function  The Cardiac Cycle AV Valves Left atrium Semilunar Valves Right atrium Left ventricle Right ventricle Cardiac Cycle ‘Wiggers’ diagram: summary of cardiac cycle events in left atrium, left ventricle & aorta during cardiac cycle Pressure Volume Electrical Sound changes  Pressure changes during cardiac cycle Mid- to late diastole Left atrium and ventricle relaxed Atrial pressure higher than ventricular pressure because atrium is filled with blood entering from the veins The aortic pressure is decreasing Because blood is moving out of the arteries through the vascular system  Mid- to late diastole Contraction of the atrium causes an increase in atrial pressure The “atrial kick” forces the final small amount of blood into the ventricle  Systole After the “atrial kick”, the ventricles begin to contract As ventricles contract, ventricular pressure increases rapidly. Change in pressure forces the AV valve to close - preventing back flow of blood into atrium All valves closed for brief time -Isovolumetric ventricular contraction (Isometric contraction- toothpaste analogy) Isovolumetric Ventricular Contraction Isovolumetric contraction- is like squeezing a toothpaste tube with the cap closed. The volume inside does not change and pressure rises. Note: The myocardial cells in the ventricular wall are contracting isometrically (remember SI207 isometric muscle contractions), during IVC. At some point, the pressure is great enough to force open the cap (Aortic valve) and toothpaste flows out (blood flows into the aorta)  Systole Following the Isovolumetric phase ventricular pressure exceeds aortic pressure This pressure difference forces the aortic valve open and ventricular ejection begins. Blood flows into the aorta, and aortic pressure also increases along with ventricular pressure. These pressures begin to decrease during the latter phase of systole  Early Diastole Ventricles relax, ventricular pressure decreases below aortic pressure, which remains elevated due to the blood that just entered it. Dicrotic notch: Aortic valve closes and some blood recoils back on the valve causing an inflection in aortic pressure All valves closed - isovolumetric ventricular relaxation phase  Diastole When ventricular pressure is below atrial pressure The AV valve opens to allow refilling of the ventricle  Ventricular Volume changes during cardiac cycle EDV SV ESV Stroke volume: = End-diastolic volume - End-systolic volume SV EDV ESV Left ventricular and aortic pressures (at rest) Diastole (DP) Systole (SP) Left ventricular pressure (mmHg) 0 mmHg 120 mmHg Aortic pressure (mmHg) 80 mmHg 120 mmHg Arterial pulse pressure (SP – DP) 40 mmHg Left ventricular volumes (at rest) Maximum volume : end-diastolic volume (EDV) 135 ml Minimum volume : end-systolic volume (ESV) 65 ml Stroke volume (SV) : EDV-ESV 70 ml Cardiac cycle in the right heart: The events on the right side of the heart are essentially the same as on the left - right ventricular volumes = left ventricular volumes Differences - Pulmonary resistance is lower than total peripheral resistance (systemic circulation) - Right ventricular and pulmonary artery pressures are lower than left ventricular pressure (LVP) and aortic pressure (AP) Right ventricular and pulmonary artery pressure (at rest) Diastole Systole Right ventricular pressure (mmHg) 0 mmHg 25 mmHg Pulmonary artery pressure (mmHg) 8 mmHg 25 mmHg Excitation of the Heart  Electrical Activity of the Heart Pacemaker cells in the sinoatrial (SA) node depolarize spontaneously, but the rate at which they do so can be modulated: – Sympathetic neurons secrete norepinephrine or epinephrine from adrenal medulla. Speeds heart rate. – Parasympathetic neurons secrete acetylcholine. Slows heart rate.  Electrical conductance in the heart The signal starts in the SA node (100 signals per minute, Depolarization dampened down by parasympathetic activity to ~75 signals per minute).  Electrical conductance in the heart The wave of depolarization travels through the internodal Depolarization pathway (via gap junctions) to the AV node.  Electrical conductance in the heart The signal then has a 0.1 second delay at the AV node Depolarization to allow the atria to contract and totally fill the ventricles before the ventricles contract.  Electrical conductance in the heart Then the wave of depolarization Depolarization travels through the AV bundle (bundle of His) down towards the Purkinje fibers which go to the apex of the ventricular septum and then turn upwards.  Electrical conductance in the heart Then the wave of depolarization travels through the AV bundle (bundle of His) down towards the Purkinje fibers which go to the Depolarization apex of the ventricular septum and then turn upwards. The Purkinje fibers also supply the papillary muscles which tell them to contract, to help prevent backflow through the valves Papillary (eversion). muscles Depolarization and contractility begin slightly earlier in the apex of the heart and then spread upward. The result is an efficient contraction that moves blood toward the exit valves– like squeezing a tube of toothpaste from the bottom up Repolarization  R P T Q S  Heart Sounds Heart sounds heard through a stethoscope on the chest. 1st _ soft low pitch “lub” AV valve closure. Onset of Systole. 2nd _ Louder “dup” Pulmonary & aortic valves closure. Heart Sounds Beginning of Diastole. Sounds due to vibrations caused by the closing valves https://library.med.utah.edu/kw/pharm/hyperheart/  Heart Sounds - Clinical Issues Abnormal heart sounds are called heart murmurs. Blood flow should be silent as long as it is smoothly flowing. If it hits anything that obstructs it, it will become turbulent and generate sound that can be heard with a stethoscope. Most causes of heart murmurs in adults are valve problems. If the valve is incompetent (doesn’t close correctly) then a swishing sound is heard. If the valve is stenotic (narrowed), a high pitched sound or a click can be heard.  Heart Sounds Vmax End view Note: Laminar flow describes the normal flow through a container as concentric layers with the middle layer moving the fastest.