From PV loop to Starling curve S Magder Division of Critical Care, McGill University Health Centre
Otto Frank 1890 s
Frank-Starling Relationship ( The Law of the Heart ) The greater the initial stretch of the heart wall the greater the force produced and the greater the output = the Preload - The preload is the force which sets the initial length before the muscle contracts Significance: What goes in will go out
The law of the heart Patterson SW, Piper H and Starling EH J Physiol 48: 465-513, 1914 the mechanical energy set free on passage from the resting to the contracted state depends on the area of chemically active surfaces, i.e. on the length of the muscle fibers, The Linacre Lecture on the Law of the Heart ( 1915) The energy of contraction, however measured, is a function of the length of the muscle fibre
Determinants of Cardiac Function Heart Rate Stroke Volume Preload Afterload Contractility
There is no downward portion in Cardiac Muscle (It cannot be stretched beyond L 0 ) Preolad: the tension that sets the initial muscle length Total Tension Active Tension Skeletal Muscle L 0 Passive Stretch Cardiac Muscle L 0 Length Length
Otto Frank 1890 s
Afterload Load after the onset of shortening has meaning for isotonic contractions (shortening against a constant load Has no meaning for isometric (no shortening) as used for preload studies. Greater afterload --- the less the degree and velocity of muscle shortening Consider lifting 5 Kg above your head versus 40 kg
Contractility Defines the velocity and extent of shortening for a given preload and a given afterload ie your heart can change from the equivalent of Fiat to a Ferrari
The Ventricular Pressure-Volume Diagram End-systolic Pressure- Volume Curve Passive Filling Curve Volume
Holt 1964
Duration of Ca 2+ entry and contraction is fixed by HR Ca 2+ 180 msec 240 msec ( 80 b/min)
Time Varying Elastance -Suga and Sagawa = MAX for the cycle 180 msec Slope is elastance 140 msec 180 msec 100 msec 60 msec Passive Filling Curve Volume
Time Varying Elastance ejecting heart 180 msec Slope is elastance 40msec 140 msec 80 msec 100 msec 140 msec 40 msec Volume
Pressure-Time Aortic Opening Aortic Closure P ESV & ESP Pressure-Volume Afterload Mitral Closure Mitral Opening EDP & EDV V
Problem: PV loop not easy to obtain 1 2 3 4 5 1 2 3 4 5 1 4 3 5 2 5 4 3 1 2
From Patterson, Piper, Starling: J. Physiol. 1914
Output in 10 min Patterson, Piper, Starling: J. Physiol. 1914 CVP mmh2o 350 300 250 200 150 100 50 0 50 100 150 200 250 300 350 0 CVP (mmh 2 O) Note downward part to the curve Output in 10 min
Change in Preload P Pressure-Volume Q Cardiac Function Curve Starling Curve 3 4 4 2 1 2 3 1 V Filling Pressure (Pra or Pla)
Change in Preload (What ever goes in goes out) P Pressure-Volume Q Cardiac Output vs EDV 4 3 2 4 1 1 2 3 V End-diastolic volume
Significance of Starling s Law Consider a situation where the SV of the RV is 101 ml and that of the LV is 100 ml, ie a 1% difference. The heart rate is 70 b/min - In 1.5 hr the total blood volume would be in the lungs What goes in must come out The Starling mechanism provides the fine tuning to match in-flow to out flow
Increase in Afterload P Decrease SV New Aortic Opening P Q Cardiac Function Curve Starling Curve Increased ESV V Filling Pressure
Decrease in Contractility P Decreased SV Q Cardiac Function Curve Starling Curve ESV Looks like in Afterload V Filling Pressure
Decrease in Heart Rate P Q Cardiac Function Curve Starling Curve Looks like: in Contracitility in Afterload V Filling Pressure
Decrease in Heart Rate (No effect on SV-filling pressure relationship) P SV SV vs Filling P No Change V Filling Pressure
Summary P-V curve gives detailed, specific view of cardiac function but readily obtainable: precise volume measurements are hard to obtain Useful for understanding the potential physiological changes Cardiac function curve is easy to obtain but non-specific: Affected by heart rate, afterload, & contractility
P The Pressure Volume Loop Contractility Afterload SV SV Return In the steady state - SV = SV Return V
Increase in Afterload P New Aortic Opening P Q Cardiac Function Curve Starling Curve ESV SV SV Return EDV V SV return is maintained No change in Q Filling Pressure
Cardiac Function Curve with Increase in Afterload or Decrease Contractility Q But in reality Q is decreased Filling Pressure Predicted no Δ Q Why?
The Pressure Volume Loop P ESV & ESP Contractility Afterload EDP & EDV V