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Transcript
Cardiac Physiology Pump Function Jim Pierce Bi 145a Lecture 12, 2009-10 Cardiac Pump • The Heart Pumps Blood • • • • … by contraction and relaxation Contraction is called systole Relaxation is called diastole The Cardiac Cycle is the cycle through one systole and one diastole Cardiac Pump • When the heart pumps, it generates: • Pressure Changes • Volume Changes • We talk about both – Blood Pressure, Arterial/Venous Pressure – Cardiac Output, Venous Return Cardiac Pump • We can measure pressures and volumes during the cardiac cycle • These will help us understand the heart Echocardiography Swan-Ganz Catheter Swan-Ganz Catheter Arterial Pressure Swan-Ganz Catheter Cardiopulmonary Function • When we combine cardiac output with oxygen carrying capacity of the blood, we begin to evaluate … • Delivery of Oxygen Swan-Ganz Parameters Volumes • There are a variety of ways to measure vascular volumes. • Volume per Time, or Flux – Thermodilution across compartments – Oxygen Extraction across compartments • Absolute Volume – Echocardiogram (imaging study) – Thermodilution in a compartment – Actual Dilution (distribution across all compartments) Pressure Versus Volume • Pressure and Volume are related • Increasing Pressure will Increase Volume • Decreasing Pressure will Decrease Volume • Increasing Volume will Decrease Pressure • Decreasing Volume will Increase Pressure Compliance • Compliance is the change in pressure caused by a change in absolute volume • Compliance = P / V • Point Compliance = dP / dV Compliance (Computation) Compliance (Real) Contractility • Contractility is the change in Volume per Time caused by a change in Pressure • Contractility = (dV/dT) / dP Contractility Compliance and Contractility Contractility determines EMPTYING Compliance determines FILLING Pressure Volume Loop Contractility Area = Work Compliance Cardiac Cycle • Thus, each part of the cardiac cycle is dominated by a relationship between volume and pressure. Cardiac Cycle • Systole – Muscle is Contracting – A contracting “sphere” generates Pressure – Pressure causes a change in Volume – This is measured by CONTRACTILITY – This is affected by • Function of Muscle • Initial Volume (PRELOAD) • Initial Pressure (AFTERLOAD) Cardiac Cycle • Diastole – Muscle is Relaxing – Veins return blood to the heart – As the heart fills with blood, the absolute volume and pressure change – This relationship is measured by COMPLIANCE – This is affected by • Connective Tissue • Venous Pressure • Venous Resistance Cardiac Cycle • Both systole and diastole can be divided into early and late phase Cardiac Cycle • We begin at the end of diastole… • Here, the ventricles are relaxed and maximally filled with blood, including an extra “fuel injection” fuel injection from the atria Cardiac Cycle • Early Systole – The Pressure in the Ventricle is the same as in the great veins – The Ventricle contracts – The AV valves close – Since the Aortic and Pulmonic valves were already closed, the heart is a closed ball – As the heart contracts, the pressure in the ball rises at a fixed volume. Cardiac Cycle • Early Systole … • Is … • ISOMETRIC CONTRACTION! Pressure Volume Loop Early Systole Cardiac Cycle • Late Systole – The Pressure in the Ventricles is the same as in the great arteries – The A/P valves open – Further contraction of the ventricles causes blood flow at a relatively constant pressure – (this is because the aorta is compliant as well and increase in volume causes only a small increase in pressure) Cardiac Cycle • Late Systole … • Is … • ISOTONIC CONTRACTION! Pressure Volume Loop Late Systole Cardiac Cycle • Early Diastole – The Ventricles begin to relax – As the Ventricular pressure falls below the great artery pressure, the A/P valves close – Since the AV valves were already closed, the heart is a closed ball – As the heart relaxes, the pressure in the ball falls at a fixed volume. – ISOMETRIC RELAXATION Pressure Volume Loop Early Diastole Cardiac Cycle • Late Diastole – When the pressure inside the heart falls below the pressure of the great veins AND the papillary muscles have relaxed, the AV valves open – The blood flows down its pressure gradient and the ventricles fill passively at a fixed pressure (because the ventricle has compliance) – ISTONIC RELAXATION Pressure Volume Loop Late Diastole Cardiac Cycle • End Diastole – Is unique because the atria contract – This leads to an increase in pressure in three places: • The great veins • The atria • The ventricles Pressure Volume Loop End Diastole Cardiac Cycle • End Diastole – Atrial Contraction • Early Systole – Isometric Contraction • Late Systole – Isotonic Contraction • Early Diastole – Isometric Relaxation • Late Diastole – Isotonic Relaxation • End Diastole Cardiac Cycle • Why does this work? • The heart is like a sphere. • The volume of the sphere is a function of the radius. • The surface diameter / area is a function of the radius • Thus the surface area can be expressed as a function of the volume. • Since the muscle fiber length is a function of the surface area … Cardiac Cycle • The muscle fiber length is a function of the Cardiac Volume • Just like with a muscle or with a sphincter, we can draw a “VOLUMEFORCE” graph and a “VOLUMESHORTENING” graph (for isometric and isotonic contraction respectively) Cardiac Cycle • Similarly, PRESSURE and VOLUME are related. • So we can draw a “PRESSUREFORCE” and “PRESSURESHORTENING” graph, as well. Cardiac Cycle • Thus, if we know two things: – Ventricular COMPLIANCE (during diastole) – Ventricular CONTRACTILITY (during systole) • We can use PRESSURE and VOLUME interchangably. (very useful!) Cardiac Cycle • We discover that: – 1) Initial Volume is PRELOAD • Also called END DIASTOLIC VOLUME • Is related to END DIASTOLIC PRESSURE – 2) AFTERLOAD is the outflow pressure • Also called BLOOD PRESSURE • If we know the compliance and resistance (V=IR), then can be related to CARDIAC OUTPUT (Volume per time) Pressure Volume Loop Cardiac Pump • So now we ask: • 1) What determines PRELOAD? • 2) What determines AFTERLOAD? • 3) How does the heart turn PRELOAD into CARDIAC OUTPUT against an AFTERLOAD? Cardiac Output • First: – Systemic venous return must equal right cardiac output – Right cardiac output must equal pulmonary venous return – Pulmonary venous return must equal left cardiac output – Left cardiac output must equal systemic venous return Cardiac Output • Thus COright = COleft • Flux is constant, even though pressure is not. Cardiac Output • Second: – Blood comes in from Venous Return – Despite lots of flow, there is little change in pressure – Thus, the Venous return is from a capacitant system and provides preload to the heart Cardiac Output • Third: – Blood goes into the Arterial Tree – With the same amount of flow, there are much higher pressures – Thus, the Arterial Tree is a resistance system, and that resistance is the afterload on the heart. Cardiac Output • Is any vessel just a capacitor or resistor? • Of course not. Cardiac Output • Capacitant Veins have venous resistance to control flow rates – (just like V=IR, P = JR, so J = P / R) • Resistant Arteries have capacitance – This capacitance allows them to dilate slightly to receive more volume at a given pressure, and is appropriately called compliance. (V / P) Beginning Diastole End Diastole Beginning Systole End Systole Ventricular Pressure Central Venous Pulse Cardiac Output Guyton’s Model Venous Return Venous Return Venous Resistance Frank - Starling Curve Contractility Cardiac Output Blood Flux (CO versus VR) Pressure versus Afterload Velocity versus Afterload Ventricular Pressure Blood Flux (CO versus VR) Cardiac Cycle Economic Effects Questions?