From PV loop to Starling curve - Critical Care Canada...From PV loop to Starling curve S Magder...
Transcript of From PV loop to Starling curve - Critical Care Canada...From PV loop to Starling curve S Magder...
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
Skeletal Muscle Cardiac Muscle
Passive
Stretch
Total
Tension
Active
Tension
There is no “downward” portion in Cardiac Muscle
(It cannot be stretched beyond L0)
Length Length
L0
L0
Preolad: the tension that sets the initial muscle 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
Volume
Passive
Filling
Curve
End-systolic
Pressure-
Volume
Curve
The Ventricular Pressure-Volume Diagram
Holt 1964
Ca2+
240 msec
180 msec
Duration of Ca2+ entry and contraction is fixed by HR
(≈ 80 b/min)
Volume
Passive
Filling
Curve
Time Varying Elastance -Suga and Sagawa
Slope is
elastance
180 msec
140 msec
100 msec
60 msec
= MAX for the cycle
180 msec
Volume
Time Varying Elastance – ejecting heart
Slope is
elastance
180 msec
140 msec
100 msec
40 msec
40msec
80 msec
140 msec
Mitral
ClosureMitral
Opening
Aortic
Opening Aortic
Closure
P
V
Pressure-Time Pressure-Volume
EDP
&
EDV
“Afterload”ESV
&
ESP
Problem: PV loop not easy to obtain
12
34
5
1 2 3 4 5
1
2
3
4
5
1
2
34
5
From Patterson, Piper, Starling: J. Physiol. 1914
Patterson, Piper, Starling: J. Physiol. 1914
Output in 10 min
Outp
ut in
10 m
in
0 50 100 150 200 250 300 350
CVP (mmH2O)
CVPNote
“downward” part
to the curve
mmH2O
350
300
250
200
150
100
50
0
P
V
1 2
3
4
Filling Pressure
(Pra or Pla)
Q
1
2
3
4
Pressure-VolumeCardiac Function Curve
“Starling Curve”
Change in Preload
P
V
1 2
3
4
End-diastolic volume
Q
1
2
3
4
Pressure-Volume Cardiac Output vs
EDV
Change in Preload(What ever goes in goes out)
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
P
V
Increase in Afterload
New Aortic
Opening P
Decrease
SV
Increased
ESV
Filling Pressure
Q
Cardiac Function Curve
“Starling Curve”
P
V
Decrease in Contractility
Decreased
SV
ESV
Filling Pressure
Q Cardiac Function Curve
“Starling Curve”
Looks like
in Afterload
P
VFilling Pressure
Q Cardiac Function Curve
“Starling Curve”
Looks like:
in Contracitility
in Afterload
Decrease in Heart Rate
P
V Filling Pressure
SVSV vs Filling P
No Change
Decrease in Heart Rate(No effect on SV-filling pressure relationship)
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
“Afterload”
P
V
The Pressure Volume Loop
“Contractility
SV
SV
“Return”
In the steady
state -
SV =
SV “Return”
P
V
Increase in Afterload
New Aortic
Opening P
Filling Pressure
Q
Cardiac Function Curve
“Starling Curve”
SV
“Return”
SV
ESV
EDV
SV return is maintained
No change in Q
Filling Pressure
Q
Cardiac Function Curve with Increase in
Afterload or Decrease Contractility
Predicted no Δ Q
But in reality
Q is decreasedWhy?
EDP
&
EDV
“Afterload”
ESV
&
ESPP
V
The Pressure Volume Loop
“Contractility