From PV loop to Starling curve. S Magder Division of Critical Care, McGill University Health Centre

Transcription

1 From PV loop to Starling curve S Magder Division of Critical Care, McGill University Health Centre

2 Otto Frank 1890 s

3 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

4 The law of the heart Patterson SW, Piper H and Starling EH J Physiol 48: , 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

5 Determinants of Cardiac Function Heart Rate Stroke Volume Preload Afterload Contractility

6 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

7 Otto Frank 1890 s

8 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

9 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

10 The Ventricular Pressure-Volume Diagram End-systolic Pressure- Volume Curve Passive Filling Curve Volume

11 Holt 1964

12 Duration of Ca 2+ entry and contraction is fixed by HR Ca msec 240 msec ( 80 b/min)

13 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

14 Time Varying Elastance ejecting heart 180 msec Slope is elastance 40msec 140 msec 80 msec 100 msec 140 msec 40 msec Volume

15 Pressure-Time Aortic Opening Aortic Closure P ESV & ESP Pressure-Volume Afterload Mitral Closure Mitral Opening EDP & EDV V

16 Problem: PV loop not easy to obtain

17 From Patterson, Piper, Starling: J. Physiol. 1914

18 Output in 10 min Patterson, Piper, Starling: J. Physiol CVP mmh2o CVP (mmh 2 O) Note downward part to the curve Output in 10 min

19 Change in Preload P Pressure-Volume Q Cardiac Function Curve Starling Curve V Filling Pressure (Pra or Pla)

20 Change in Preload (What ever goes in goes out) P Pressure-Volume Q Cardiac Output vs EDV V End-diastolic volume

21 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

22 Increase in Afterload P Decrease SV New Aortic Opening P Q Cardiac Function Curve Starling Curve Increased ESV V Filling Pressure

23 Decrease in Contractility P Decreased SV Q Cardiac Function Curve Starling Curve ESV Looks like in Afterload V Filling Pressure

24 Decrease in Heart Rate P Q Cardiac Function Curve Starling Curve Looks like: in Contracitility in Afterload V Filling Pressure

25 Decrease in Heart Rate (No effect on SV-filling pressure relationship) P SV SV vs Filling P No Change V Filling Pressure

26 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

27 P The Pressure Volume Loop Contractility Afterload SV SV Return In the steady state - SV = SV Return V

28 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

29 Cardiac Function Curve with Increase in Afterload or Decrease Contractility Q But in reality Q is decreased Filling Pressure Predicted no Δ Q Why?

30 The Pressure Volume Loop P ESV & ESP Contractility Afterload EDP & EDV V