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