What is preload and how do I measure it

1
What is preload and how do I measure it?

• S Magder
• Division of Critical Care,
• McGill University Health Centre
• Montreal, Canada

2
Issues to be disscused

• What is preload?
• Volume vs pressure
• Right heart vs left heart
• Assessment of preload
• optimal preload
• Fluid challenge
• Dynamic test ( measure volume responsiveness)
• Technical issues
• Leveling
• Transmural pressure
• Waveform analysis

3
Fallacious Physiological reasoning

• Ignoring the series effects of the R and L hearts
• Use of Ppao and LVEDV to determine fluid
  responsiveness
• Expecting R heart values to directly predict L
  heart values
• Failure to consider the plateau of the cardiac
  function curve
• Use of a single preload value out of context of
  the overall hemodyamics
• Failure to consider transmural pressure and
  implication during the ventilatory cycle
• Effect of Time preload effect is
  instantaneous and long term measurements include
  physiological adjustments
• Spurious correlations in evaluating responses

4
What is Preload?

• Preload is a term that was first developed in the
  19th century for the analysis of skeletal muscle
  function
• Preload is the force or load which gives the
  final stretch which determines the force-length
  relationship (it is dependent upon the elastance
  of the muscle)

5
Definition of Preload in Physiology Texts

• A preload can be viewed as a weight that
  stretches a muscle before it is stimulated to
  contract. Katz, Physiology of the Heart
• Preload is the load present before contraction
  has started and is provided by the venous return
  that fills the atrium and empties into the left
  ventricle during diastole Opie, The heart
• The muscle is first stretched to a given preload
  (weight).. Best and Taylor Textbook of
  Physiology ed West
• An increase in the initial muscle length,
  induced by a change in passive stretch of the
  muscle, produces a resting load that has been
  termed preload The Heart and Cardiovascular
  system ed Fozzard et al
• It (sic cardiac function)is an expression of
  the well known Frank-Starling relationship and
  reflects the fact that the cardiac output
  depends, in part, on the preload (this is, the
  central venous, or right atrial pressure). Berne
  and Levy, Physiology
• In the intact heart, ventricular end-diastolic
  wall stress or tension is analogous to the
  preload of isolated muscle and ultimately
  determines the resting length of the sarcomeres
  E Braunwald, Heart Disease

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Tension
Time
7
There is no downward portion in Cardiac
Muscle (It cannot be stretched beyond L0)
Skeletal Muscle
Cardiac Muscle
Tension
Length
Length
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Patterson, Piper, Starling J. Physiol. 1914
CVP
Note downward part to the curve
mmH2O 350 300 250 200 150 100
50 0
Output in 10 min
0 50 100 150 200 250 300 350 CVP
(mmH2O)
Output in 10 min
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The law of the heartPatterson SW, Piper H and
Starling EHJ 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
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(No Transcript)
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Change in Preload
Pressure-Volume
Cardiac Function Curve Starling Curve
P
Q

V
Filling Pressure (Pra or Pla)
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Significance of Starlings 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
13
Should preload be determined from the right or
the left heart?

• The right and left ventricles are in series
• Thus, the left heart can only pump what the right
  heart gives it
• The right atrium (CVP) is where the heart
  interacts with the venous return

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What determines the CVP ?

• Pra is the preload of the heart and thus a
  determinant of cardiac function
• Pra is also the outflow pressure for venous
  return and therefore a determinant of the return
  funtion
• The actual value of the Pra/CVP is determined by
  the interaction of Cardiac function and return
  function

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Cardiac output is determined by the intersection
of return function and cardiac function
working Pra
E2
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Plateau of the cardiac function curve

• Concept of wasted preload

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Cardiac Function Curve
Q
wasted preload
Pra
Increasing Pra in the plateau phase does not
increase cardiac output
18
Holt 1964
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Implication of plateau of the cardiac function
curve

• Further volume loading will not help and can only
  due harm
• I call this wasted preload
• When the right heart is functioning on the flat
  part of its function curve left side events
  become irrelevant for the determination of
  cardiac output. (the series effect)
• In this state Ppao is meaningless for the
  determination of cardiac output

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Should we use pressure or volume to monitor
preload?
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Change in Preload (What ever goes in goes out)
Pressure-Volume
Cardiac Output vs EDV
P
Q
5
6
4

7
3
2
1
V
End-diastolic volume
22

• The volume that is important is that of the R
  heart (the series effect) and it is hard to
  assess with great precision
• For example A pt has a cardiac output (Q) 4
  l/min, HR 80 b/min, SV 50 ml and the EDV is
  140 ml (EF 36)
• - A change in EDV of 10 ml (7 change) would
  increase the SV to 60 ml and Q to 4.8 l/min which
  is in the normal range
• However, this is in the range of error of most
  devices (Vincent et al 1986)

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Volume -continued

• The bellows like shape of the RV makes
  echocardiographic measurements difficult
• At higher volumes there is often tricuspid
  regurgitation which limits thermodilution
  methodologies
• Volume measurements cannot identify wasted
  preload, they can only identify a high value

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Wagner LeathermanChest 1998 1131048
change in SV
Baseline Pra (mmHg
25
Wagner LeathermanChest 1998 1131048
change in SV
RVEDVI (ml/m2)
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Wagner LeathermanChest 1998 1131048
Volume (ml/m2)
Pressure (mmHg)
Ppao
RVEDVI
Pra
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What is optimal preload?
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Consider the following

• In the sitting position the CVP of a normal
  person is lt 0 mmHg,
• yet cardiac output (Q) and blood volume are
  normal
• There is no need for fluid infusion

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Continued

• At peak exercise CVP is normally 5-6 mmHg and
  cardiac output is gt20 l/min
• The cardiac output is maximal and likely so is
  volume, yet the CVP is not elevated
• (However, increasing volume may increase the
  cardiac output)

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Pra vs Cardiac Output during incremental
exercise
Notarius et al Am Heart J 1998
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Ways that CVP can decrease (1)
Q
Increase in cardiac function without circuit
adaptation
Pra
32
Ways that CVP can decrease (3)
Q
Decrease Rv without change in cardiac function
Pra
33
Ways that CVP can decrease (2)
Q
Decrease in MCFP without change in cardiac
function
Could be volume loss or decrease in capacitance
Pra
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Concept

• CVP should not be considered alone
• - compare the assessment of the clinical
  significance of an increase in PCO2
• first question to ask is what is the pH?
• Similarly, when given a value of CVP, the next
  question should be
• what is the cardiac output ?
• (or a surrogate)

35
Large overlap between responders and non
responders
49
69
Responders
Non-responders
Bafaqeeh Magder ATS 2004
36
Some Patients at all values of CVP fail to
respond to fluids
25
45
82
100
Number of Pt
mmHg
Bafaqeeh Magder ATS 2004
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Dynamic Testing for volume responsiveness

• Response to a fluid challenge
• Gold Standard
• Arterial Pressure and Aortic SV Variation
• Can only be used when there are no inspiratory or
  expiratory ventilatory efforts and constant
  ventilatory conditions.
• Respiratory variation in CVP/Pra

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Fluid Challenge

• Assess the value of Pra ( NOT the wedge).
• Give sufficient fluid to raise Pra by 2mmHg and
  observe Q.

-ve
ve
Type of fluid is not of importance if given fast
enough
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Principles of Volume challenge

• To test Starlings law the fluid needs to be
  given quickly the faster it is given the less
  that is needed
• It makes no sense to test preload responses
  over long periods of time (eg Kumar et al 2004)
• The type of fluid is not critical if given
  quickly enough
• There needs to be a change in CVP to know that
  Starlings Law has been tested

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Who might respond at higher CVP

• Patients with
• Increased PAP
• Increased pleural pressure
• Increased mediastinal pressure

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Inspiratory fall in Pra
No Inspiratory fall in Pra
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Approach 1. Assess adequacy of inspiratory
effort from wedge 2. Evaluate the change in Pra
Eg of no fall in Pra with inspiratory effort
Magder et al JCCM 1992
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No Inspiratory Fall
Inspiratory fall
ve
-ve
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Technical issues

• Small changes in CVP can have large effects on
  cardiac output
• Therefore technical issues need to be considered
  very carefully
• The 3 primary considerations are
• Leveling
• Transmural pressure
• Where on the wave form

45
Change in CVP of even 1 mmHg should be sufficient
to test the Starling response
plateau
5
Slope 500 ml/mmHg
0
10
46
Measurement of MCFP in Patient
Pacemaker turned off
MCFP
10 mmHg
6 mmHg
The gradient for VR is normally very small ie
in this case it is only 4 mmHg
?small changes in CVP are physiologically very
significant
47
Principle 1 Pressures are relative to a point
that you arbitrarily pick

• Therefore, what you pick as a reference point is
  crucial
• We use a level 5 cm below the sternal angle
• In 20 pt the difference from the mid-axillary
  position was 2.6.7 mmHg
• ie 10 mmHg at the sternal angle 13 mmHg at
  the mid-axillary

48
Leveling

• The critical value for CVP/Pra depends on the
  leveling position
• Note, we use the sternal angle
• In 20 pt the difference from the mid-axillary
  position was 2.6.7 mmHg
• ie 10 mmHg at the sternal angle 13 mmHg at
  the mid-axillary

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Where do you pick the reference point?
3 mmHg diff
50
Advantage of position based on the sternal angle
A
B
51
Change in Transducer Position
What happened here?
175
Part
20
0
PA
0
20
CVP
0
52
Transmural pressure

• The pressure that determines the stretch of an
  elastic structure is the transmural pressure
• The difference between inside and outside the
  chamber (Pin-Pout)
• The pressure outside the heart is pleural
  pressure and not atmospheric pressure

53
End-Expiration
mmHg
24
12
0
Insp
Insp
54
Where do you make the measurement?
End-Expiration
Real value?
You cant really because of the forced expiration
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Where on the waveform do you make the measurement?

• The most common rational for measuring CVP/Pra is
  to assess the preload of the heart

56
Ventricular pressure-time curve
Systole
Diastole
Aorta
Ventricle
Atrium
57
Where do you make the measurement? ve pressure
ventilation (AC)
CVP
End-expiration base of the a wave
58
Timing of waves
Atrial Fibrillation
mmHg
40-
20-
0-
v
c
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Recommendations

• Preload should refer to the pressure in the heart
  at end-diastole and not the volume
• The CVP/Pra gives the preload for the whole heart
  and should always be used instead of Ppao for the
  optimization of cardiac output
• There is a plateau to the cardiac function curve
  and the CVP/Pra in this region should be
  considered as wasted preload
• Optimal preload must be considered in relation
  to the subjects overall hemodynamic status -
  there is no ideal preload
• A CVP of 10-12 mmHg (sternal angle based level)
  is a reasonable empiric value for acute
  resuscitation

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Recommendations - continued

• The hemodynamic response to a change in preload
  is much more important than a single preload
  value preload should never be considered in
  isolation
• Fluid responsiveness means that there is preload
  reserve but does not mean that the subject needs
  fluid
• A fluid challenge and flow response remain the
  gold standard for assessing fluid responsiveness
• The functional range of CVP/Pra is small so that
  technical factors are important
• Importance of leveling
• Transmural pressure
• Position on waveform