Cardiac Cycle. All events associated with the flow of blood through the heart during one complete heart beat. Approx 0.83sec if heart rate is 72 bpm

Transcription

1 Cardiac Cycle All events associated with the flow of blood through the heart during one complete heart beat Approx 0.83sec if heart rate is 72 bpm Cardiac Cycle 2 Periods Diastole/Systole 4 Phases Ventricular filling Isovolumetric contraction Ventricular ejection Isovolumetric relaxation Wiggers Diagram Pressure/volume relationships 1

2 2 Main Periods of the Cardiac Cycle Systole Ventricles contract AV valves close SL valves open Blood flow into arteries Diastole Ventricles relax SL valves shut AV valves open Blood flows into ventricles Phases of the Cardiac Cycle Ventricular filling Pressure atria > Pressure ventricles AV valves open, SL valves still closed Passive Phase-Blood flows from atria to ventricles Active phase- Atria contract Isovolumetric ventricular contraction Ventricle contracting Ventricle pressure> atrial pressure AV snap shut, SL valves already closed 2

3 Phases of the Cardiac Cycle Ventricular ejection Pressure ventricles > pressure arteries Semilunar valves open, AV valves closed Blood flows into arteries Isovolumetric ventricular relaxation Ventricle relaxes Aortic pressure > ventricular pressure AV and semilunar valves closed Cardiac Cycle Wiggers diagram ECG Aortic pressure Left ventricular pressure Left atrial pressure Left ventricular volume Heart sounds Sherwood, Figure 9-21, pg

4 Electrical events P-wave precedes atrial contraction QRS complex precedes ventricular contraction T-wave precedes ventricular relaxation Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings Ventricular pressure during cardiac cycle Large variation in ventricular pressure to allow for: opening of valves pressure in atria and arteries Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings 4

5 Atrial Pressure Changes Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings Aortic pressure during cardiac cycle Normal value for 80mmHg < aortic BP > 120mmHg Pulse pressure = systolic - diastolic = approx 40mmHg Mean Arterial pressure = 1/3 systolic + 2/3 diastolic = approx 93mmHg Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings 5

6 Ventricular Volume & Stroke Volume EDV = end diastolic volume ESV = end systolic volume SV = volume of blood ejected by the ventricle each beat 5 Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings SV = EDV - ESV Heart Sounds Sounds occur during turbulent blood flow when valves close Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings First sound = soft lubb AV valves close 2nd sound = louder dubb Semilunar valves close 6

7 Cardiac Output Volume of blood pumped out by each ventricle in one minute CO= HR X SV Average CO = 5 liters/min at rest Extrinsic & Intrinsic regulation of CO Extrinsic- neural (ANS) and hormonal (epinephrine) Intrinsic - local Control of Heart Rate Parasympathetic Decreases heart rate Sympathetic Increases heart rate Increases force of contraction Endocrine Catecholamines: Epinephrine and Norepinphrine 7

8 Control of Heart Rate SA node Intrinsic firing rate = 100 impulses/min At rest ( vagal activity inhibits SA node) Average HR = 70 beats/min Achieved via :Parasympathetic (vagus) cholinergic input, K + permeability hyperpolarisation and slower drift to threshold = Inherent SA node pacemaker activity = Parasympathetic stimulation of SA node Control of Heart Rate cont. Initial increases in HR to beats/min Achieved via inhibition of parasympathetic tone (vagal withdrawal) HR > 110 beats/min Achieved via sympathetic stimulation of: SA node: K + permeability: depolarizing effect & faster drift to threshold = SA node pacemaker activity = Sympathetic stimulation of SA node 8

9 Limitations to CO imposed by HR: AV node conduction Decremental conduction Fewer gap junctions Ventricular refractory period Safety mechanism 0.25 to 0.3s Control of Stroke Volume Intrinsic Control End-diastolic volume (Frank Starling law) Extrinsic Control Sympathetic stimulation Ventricular contractility Afterload Sherwood, Fig. 9-20, p

10 Preload: Frank Starling law of the heart Sherwood, 5th edition, fig 9.22, pg. 323 Venous return Blood returning to the right atrium via the great veins Factors that affect venous return include: Posture Skeletal muscle pump Respiratory/abdominal pump Venous tone Atrial pressure gradient 10

11 Contractility Sympathetic stimulation and epinephrine- input to ventricular muscle: Act via β 1 adrenoceptors Increases Ca 2+ entry into cells enhances excitation-contraction coupling greater contractile force of ventricular myocytes Increased SV Limitations to Preload: Ventricular Filling Time Sherwood 4 th edition, fig 9.22, pg

12 Limitations to Preload: Compliance Rigidity Limitations to SV imposed by: Ventricular afterload Increased afterload: Chronic high blood pressure or stenotic valve Exercise -transient Compensation: Hypertrophy May lead to pathological changes in heart structure 12

13 Murmurs:? increased afterload Abnormal sound heard before, between, or after the lubb-dubb of the first and second heart sounds Caused by turbulent blood flow around the valves Sherwood Fig. 9-5, p. 304 Murmurs: Stenotic Valve Mitral stenosis Left atrium needs to generate more pressure Aortic stenosis Left ventricle needs to generate more pressure 13

14 Murmurs: Insufficient valve Mitral regurgitation- left ventricular work Aortic regurgitation- left ventricular work Limitations to Stroke Volume: Ventricular Contraction Duration If contraction too short SV may be decreased Dependent on venous return Copyright 2005 Pearson Education, Inc., Publishing as Benjamin Cummings 14

15 Summary: Control of Cardiac Output Summary Blood flow is controlled by pressure changes Wiggers diagram Cardiac output = Heart rate x Stroke Volume Heart rate- increased by sympathetic activity and epinephrine. Decreased by parasympathetic activity. Stroke volume- increased by increased venous return and increased sympathetic nerve activity Limitations of Cardiac Output 15