Lecture 14 Cardiac Function and the Cardiac Cycle Study Guide PDF

Length-Tension Relationship Objective 2 • The ability of the ventricle muscle to generate force to pump blood out during systole increases steeply as fiber length increases • Greater degrees of overlap of actin and myosin filaments… greater cross-bridge cycling….greater tension • Curve levels off...

Length-Tension Relationship Objective 2 • The ability of the ventricle muscle to generate force to pump blood out during systole increases steeply as fiber length increases • Greater degrees of overlap of actin and myosin filaments… greater cross-bridge cycling….greater tension • Curve levels off when overlap is maximal • During diastole (relaxation), blood fills the ventricle and exerts pressure on the ventricle muscle, which stretches the muscle fibers and increases their length. • The End Diastolic Volume (EDV) is that volume of blood in the ventricle at the end of the relaxation phase. • Cardiac muscle normally operates ONLY on the ascending limb, for the optimal length-tension relationship. • In a normal heart… increasing preload leads to increasing sarcomere length toward the optimum actin myosin overlap (Lmax =2.2µm) 6 Objective 3 Indices of Systolic Performance - Terms to Know Ventricle function is described by 3 parameters: Stroke volume (mL) = the volume of blood ejected by the ventricle on each beat [Typical stoke volume is ~ 70mL on average RVSV=LVSV]* !"#$%& '$()*& = ,-. ./01"$(/2 '$()*& − ,-. 141"$(/2 '$()*& Ejection fraction = the fraction of the end-diastolic volume ejected in each stroke volume (measurement of ventricular efficiency; normally ~0.55 or 55%) !"#$%& '$()*& ,5&2"/$- 6#02"/$- = ,-. ./01"$(/2 '$()*& Cardiac Output (mL/min) = total volume of blood ejected per unit time [Cardiac Output (CO) ≈ 5000mL/min in a 70-kg man – based on stroke volume of 70mL and heart rate of 72 beats/min] 70#./02 $)"8)" = !"#$%& '$()*& ×:&0#" #0"& *RVSV = Right Ventricular Stroke Volume and LVSV is Left Ventricular Stroke Volume 7 Frank Starling Relationship Objective 4 Starling’s Law of the Heart – Increasing venous return or ventricular preload (x-axis) leads to an increasing stroke volume or cardiac output (y-axis) Frank-Starling Curve ­ Venous return Increased Active Tension Increased Passive tension • Governs normal ventricular function and ensures that the volume of blood the heart ejects in systole equals the volume it receives in venous return. • In a steady state, cardiac output = venous return • Changes in contractility shift the FrankStarling curve upward or downward for any level of EDV • Positive inotropic agents increase contractility so they increase ejection fraction • Negative inotropic agents decrease contractility so they decrease ejection fraction 8 Interpreting Cardiac Function Curves Frank-Starling Curve E C B Objective 4 • Graphical depiction of effects of preload versus contractility • Each curve is generated by keeping contractility constant and following ventricular performance as preload increases. Thus: • All points on a single ventricular function curve have the same contractility • AND: D • If contractility increases, the curve shifts up N A • Green curve = greater contractility F • If contractility decreases, the curve shifts down • Orange curve = less contractility • All curves will have an ascending limb, a peak point, and possible a descending limb 10 Afterload Frank-Starling Curve Stroke volume (ml) 100 Afterload Control 50 0 Afterload Objective 5 Afterload = the force that the ventricular muscle must generate to eject the blood into the aorta Blood already in the aorta pushes back against blood being ejected from the ventricle. • Acceptable indices of afterload: • Mean aortic pressure • Hypertension: increased afterload • Hypotension: decreased afterload • Peak left ventricular pressure • Influence of afterload on ventricular ejection Left Ventricular Enddiastolic Pressure (mmHg) • An acute increase in afterload reduces the volume of blood ejected (cardiac output or stroke volume) • The blood not ejected remains in the left ventricle and increases preload in the next cycle 11 Cardiac Cycle Objective 8 Time (msec) 0 Electrocardiogram (ECG) 100 200 QRS complex 300 P 400 500 600 700 T 800 QRS complex P 120 90 Pressure (mm Hg) 60 30 7 phases Aorta Dicrotic notch Left venticular pressure Left atrial pressure 0 Heart sounds S1 S2 S3 S4 135 Left ventricular volume (mL) 65 Atrial systole Atrial systole The sequence of electrical changes, pressures, and mechanical events within the heart and great vessels leading to and away from the heart during each beat is known as the cardiac cycle Ventricular systole Ventricular diastole Isovolumic Ventricular Isovolumic Ventricular ventricular ejection ventricular filling contraction relaxation Atrial systole 1. Atrial Systole 2. Isovolumetric Ventricular Contraction 3. Rapid Ventricular Ejection 4. Reduced Ventricular Ejection 5. Isovolumetric Ventricular Relaxation 6. Raid Ventricular Filling 7. Reduced Ventricular Filling or Diastasis Ventricular Systole Ventricular Diastole Atrial systole Figure 14-26 from Silverthorn Human Physiology 5th Ed 14 P-V loop = 1 complete cardiac cycle 120 Aortic valve opening Left 90 Ventricular Pressure 60 Mitral (mm Hg) valve closing 30 Left ventricular volume (mL) Mitral valve opening 0 135 EDV Stroke Volume (SV) ESV 65 Aortic valve closing 120 Left Ventricular Pressure (mm Hg) Aortic valve closing 4 3 ESV Aortic valve opening 80 5 SV 40 Mitral valve opening 0 6 65 7 100 2 Mitral valve closing EDV 1 1. Atrial systole 2. Isovolumic ventricular contraction 3. Rapid ventricular ejection 4. Reduced ventricular ejection 5. Isovolumic ventricular relaxation 6. Rapid ventricular filling 7. Reduced ventricular filling 135 Left ventricular volume (mL) 25 Objective 9, 10 Normal Ventricular Pressure-Volume Loop • Isovolumetric Contraction (1 à 2) Vent ri 80 40 • B/c all valves are closed, no blood is ejected (isovolumetric) • Ventricular ejection (2 à 3) 2 Stroke Volume Ventricular Filling 4 0 E je c t io n 65 100 Left ventricular volume (mL) 1 EDV 135 Isovolumetric Contraction 3 ESV Isovolumetric Relaxation Left Ventricular Pressure (mmHg) 120 cular • At point 1 LV is filled with ~ 140mL of blood (EDV) and pressure is low b/c muscle is relaxed • On excitation, the ventricle contracts, pressure goes up and mitral valve closes • Aortic valve opens at point 2 (LVP > aortic pressure) • Blood is ejected into the aorta and volume decreases • Ejected volume = Stroke volume • Volume of blood remaining at point 3 = ESV • Isovolumetric relaxation (3 à 4) • Ventricles relax at point 3 • LVP < aortic pressure… aortic valves close. B/c all valves are closed, ventricular volume is constant (isovolumetric) • Ventricular Filling (4 à 1) • Once LVP < LAP, the mitral valve opens and filling begins • Ventricular volume increases to ~140 ml (EDV) 26