Too much of a good thing
 Intravenous fluid boluses are used liberally in
medicine to achieve a variety of goals.
 In critical care we use them most often to improve
cardiovascular parameters and replace known losses
 Prompt, appropriate resuscitation with fluid can
prevent the need for vasoactive drugs
Too much of a good thing
 Excessive or inappropriate use of fluid can cause
problems both acutely with APO or worsening of ALI
and over the longer term with potentially delayed
extubation, persistent oedema predisposing to
infections or potential surgical complications like
anastomotic breakdowns.
Preload assessment
 Since it is harmful to give too much fluid as well as to
give too little we need to have a good idea when its
indicated.
 When treating a cardiovascular parameter it is
important to treat the patient not the numbers
 E.g. Hypotension with no end organ dysfuncion
 After establishing that the cardiac output/Blood
pressure needs to be improved the next important
question is: “Will this patient benefit from increased
pre-load?”
What is pre-load?
What is pre-load?
 Ventricular preload is the end-diastolic volume
producing the initial passive stretch of the
myocardium prior to active contraction.
 Volume not pressure.
What is pre-load?
 According to Starling ,One of 3 Determinants of
Cardiac stroke volume
 Pre-load
 Sympathetic Tone
 After-load
Why are we interested?
Why are we interested?
 Increasing Pre-load increases Stroke volume.
 Stroke volume x Heart rate = Cardiac Output
 Cardiac output influences
 Systemic Blood pressure
 End organ perfusion



Oxygenation
Nutrients
Acid and waste removal
 The answer to life the universe and everything
Frank and Starling
 Cardiovascular Physiologists
 Developed animal models where RA
was canulated and supplied with
blood from a resovior. Height of
resovior determined pressure
(Preload)
 Aorta was attached to a variable
resistor (Afterload) and fed back to
resovior.
 The Vagus and Sympathetic nerves
could be artificially Stimulated.
Image from P Cam “Physiology for the Anaesthetist”
Frank and Starling
 The Ventricular volumes could
then be measured.
 These graphs show how increased
end diastolic volume increased
the stroke volume to a point then
output was decreased as
overstretch occurred.
 Of note, these are volumes not
pressures. The pressure curve has
more of a plateau
Image from P Cam “Physiology for the Anaesthetist”
Frank and Starling
 This shows the same
part of the curve we are
interested in using, but
substitues pressure as
preload
 Volume is proportional
to pressure
Frank and Starling
 Is it really that simple?
Guyton
 The heart is a demand pump.
 It can’t pump what it doesn’t
receive from venous return.
 In health the maximum Cardiac
output achievable from solely
increasing Preload is 10-15L/min
 Adding sympathetic stimulation
this increases to 20-30L/min
 So there is usually some reserve
Image from Current Opinion in Critical Care 2010,
16:289–296 “Fluid status and Fluid Responsiveness”
Guyton
 Guyton’s venous return and
cardiac output curves can be
plotted simultaneously to show
their point of intersection.
 If you change the pre-load after a
few cardiac cycles a new
intersection point will be reached
with a new cardiac output/venous
return.
 There is a plateau to the CO curve
an increase in pre-load will not
always work.
Preload assessment
 The most important part about pre-load assessment is
not what the pre-load currently is (CVP) but weather
an increase will lead to a clinically significant effect.
 The assessment of Pre-load and prediction of fluid
responsiveness can be divided into Static and Dynamic
measurements.
 Each seeks to find a surrogate for the gold standard
measurement of preload; the Left (or right)
Ventricular End Diastolic volume (or pressure)
Static vs dynamic predictors
 Static predictors seek to measure a single value in order to make
a prediction on fluid responsiveness
 Pressures CVP / Ppao
 Volumes; Global end diastolic volume
 Dynamic predictors apply a controlled and reversible preload
variation and measure the heamodynamic response.
 Variation with respiration:



Stroke volume Variation / Pulse Pressure variation
Plethysmographic variation
IVC variation
 Systolic Pressure Variation
 Passive leg raise test
 Valsalva manoeuvre
Ref: Crit Care Clin 26 (2010) “Preload Assessment”
Static predictors
 Pressures
 CVP
 Ppao
 Volumes
 Ventricular volumes (thermodilution)
 Left Ventricular End Diastolic Area (U/S)
Static predictors
 Central Venous Pressure
 Gives you a surrogate for Left Ventricular end diastolic
pressure.
 Very unreliable




Needs to have a reproducible reference point to use absolute
values (e.g. “CVP of less than 8 needs fluid”)
 Stopcock of transducer needs to be zeroed level with right
atrium.
Varies with Respiratory cycle,
Varies (Inconsistently) given different PEEP values
Totally unreliable if any right sided valvular disease.
Static predictors
 Central Venous Pressure
 Marik and Rodrigo did an update meta-analysis
published in July this year By the Society of Critical Care
Medicine
 This reinforced that there was little to no correlation
between CVP values and fluid responsiveness. Making a
plea stating “this app to fluid resuscitation should be
abandoned.”
Ref: “Does the Central Venous Pressure Predict Fluid Responsiveness? An Updated
Meta-Analysis and a Plea for Some Common Sense*” Crit CareMed 2013; 41:1774–1781
Static predictors
 Pulmonary arterial occlusion pressure
 Swan-Ganz catheter is passed into a branch of the
pulmonary artery, a baloon inflated and the pressure
beyond the balloon measured.
 This pressure is thought to be proportional to Left atrial
pressure and an idea of Left Ventricular preload gleaned
much simmilar to using the static measurement of CVP.
 It is an Invasive and potentially dangerous procedure
and whilst a swan ganz is still the gold standard for
measureing Cardiac output, performing Ppao has gone
out of vouge
Ref: “Static Measures of Preload Assessment” Crit Care Clin 26 (2010) 295–305
Static predictors
 Pulmonary arterial occlusion pressure
 A 2002 review by Michard and Teboul of 12 studies
found that Ppao could not reliably predict fluid
responsiveness in any of the studies.
 In general the best use for CVP or Ppao are in their
negative predictive value.
 A high CVP (e.g. >10 mm Hg in one study) is less likely
to respond to a fluid bolus
 A high Ppao (e.g. >11mm Hg in 2007 Osman Etal) is less
likely to respond to a fluid bolus.
Ref: “Static Measures of Preload Assessment” Crit Care Clin 26 (2010) 295–305
Static predictors
 Pressures
 CVP
 Ppao
 Volumes
 Ventricular volumes (thermodilution)
 Left Ventricular End Diastolic Area (U/S)
Static predictors
 Volumes
 Right Ventricular End diastolic volume(thermodilution)


A Pulmonary artery catheter with a rapidly responsive
thermister in the tip and a means of warming or cooling a
known quantity and temperature of blood in the right atrium
allows measurement of the SV, CO and RVEF. The RVEDV can
then be derived from the SV and the RVEF.
Diebel etal. In 1994 compared RVEDVI in trauma patients to
fluid responsiveness (20% increase in CI) and found that cutoffs of <90, 90-140, and >140 mL/m^2 had predictive values of
64%, 27% and 0% respectively.
Static predictors
 Volumes
 Global End Diastolic Volume (and index)

PiCCO, uses a Central line and a specially designed Femoral
Arterial line to calculate, by thermo-dilution the GEDVI
 Numerous studies exist showing good correlations
between increases in GEDVI and SVI and one even
compared these to CVPs which did not correlate to
changes in SVI
Static predictors
 In summary;
 Pressures (CVP and Ppao) are only really useful to
predict lack of response to fluids in the ICU and even
that is subject to debate.
 Volumes measured by a swan ganz catheter (RVEDV) or
PiCCO (GEDVI) have a better correlation with fluid
responsiveness than static Pressures but, by nature
require specialised invasive monitoring.
Dynamic Predictors
 Dynamic predictors apply a controlled and reversible
preload variation and measure the heamodynamic
response.
 Variation with respiration:




Stroke volume Variation / Pulse Pressure variation
Plethysmographic variation
IVC variation
Systolic Pressure Variation
 Passive leg raise test
 Valsalva manoeuvre
Frank and Starling
 For a constant Afterload
and Sypathetic
stimulation, An
Increase in Pre-load will
produce an increase in
Stroke volume as
described by this curve.
Frank and Starling
 Small variations in pre-
load, like those due to
respiration can cause
changes in the stroke
volume or pulse pressure.
 These small dynamic
changes can be used to
obtain an idea of the
position on their Cardiac
output curve and the
potential for response to a
fluid bolus.
Frank and Starling
 During positive pressure
ventilation inspiration
right ventricular Preload
is decreased due to IVC
and RA compression
leading to decreased
venous return.
 In turn this decreases RV
output, Pulm Blood flow,
LV Filling and LV output.
Frank and Starling
 This also shows how static
measures of pressure will
be missleading in the
setting of a failing
ventricle.
 The upper line represents
a normal ventricle whilst
the lower is failing, the
same Pra representing
dramatically different
curve and potential for
fluid responsiveness.
Dynamic Predictors
 Variation with respiration:


Stroke volume Variation
 Measured with a PiCCO or LiDCO via pulse contour analysis.
 Shows promise as a predictor of fluid responsiveness but
studies have given inconsistent results
Systolic Presure Variation
 In it’s simplest for its a “swing” in the art line trace of a
ventilated patient.
 Preisman S et al showed that a variation of 8mmHg of systolic
pressure correlates to fluid responsiveness with 82% sensitivity
and 86% specificity
Preisman S et al Br J Anaesth 2005;95:746–55
Dynamic Predictors
 Variation with respiration:

Pulse Pressure Variation
 Expressed as a percentage; PPV=100x(Ppmax-Ppmin)/[(Ppmax
+ PPmin/2]
 More sensitive and specific than SPV.
 Difference of 13% PPV would lead to at least 15% increase in
Cardiac output with 500ml colloid (HES) (sens 94%, Spec
96%)
Dynamic Predictors
 Variation with respiration:
 Plethysmography assessed for “swing” similar to art line during
respiration. Numerical value calculated usually giving a
%change
 Currently unreliable according to studies
 Auto-gain in many systems dampens changes in the trace
 Significant intra- and inter-patient variability.
 No accepted standard or signal processing between
manufacturers to facilitate reproduction between different
institutions.
Dynamic Predictors
 Variation with respiration:
 IVC variation
 The IVC pressure as it passes thru the diaphragm has nearly
the same pressure as the Right Atrium. It also receives nearly
all of the added pressure of Positive pressure ventilation.
 During positive pressure inspiration the abdominal IVC
expands as the abdominal pressure (usually) is significantly
lower. This distension (and collapse) of the IVC only occurs in
a fluid responsive patient. If it is full (i.e. Right heart failing)
than ventilation will have little effect on IVC distension.
 Using an Echo and M-mode the distension can be measured
Dynamic Predictors
 Variation with respiration:
 IVC variation
 Barbier et al showed that if you calculate the dispensability as a
percentage (Dmax-Dmin)/Dmin% if it was greater than 18%
than it predicted fluid responsiveness with 90% sens and 90%
spec.
Dynamic Predictors
 Variation with respiration Caveats :
 Must be Positive pressure ventilation, NOT triggering/PSV
modes (I.e. Heavily sedated/narcotised, paralysed or just
hyperventilated beyond pt’s resp drive)
 Sinus Rhyth.
 Limited by same things as Art Line (bubbles, kinks, excessive
length etc)
 Evidence of Fluid responsiveness does not equate to fluid
Deficiency.
 Limited data on extremes of ventilation E.g. Smaller tidal
volumes with higher resp rates
Dynamic Predictors
 Passive Leg Raising
 Auto-transfusion from leg ~150-750 mLs depending on pt and
technique.
 Usable in Spontaneously ventilated patients.
 Usable in Non-Sinus Rhythm.
 Produces a change in CO within 30sec. Which resolves
spontaneously (regardless of leg position due to blood redistribution)
 Most sensitive when pt starts semi-recumbent at 45deg then is
laid flat and legs elevated.
 Requires some means to rapidly assess changes in Cardiac
output (usually TOE or TTE)
 Needs more studies to make more easily clinically applicable.
Dynamic Predictors
 Valsalva meneuver



Assessing primarily blood pressure trace as pt progresses thru
the 4 phases of the Valsalva meneuver.
The presence of the variations in blood pressure, usually
quantified using Pulse pressure Variation methods described
earlier, with PPV of 52% or greater predicted fluid
responsiveness.
Still inadequate studies, i.e. Difficult to justify using a Vigelo
or PiCCO with such a simple bedside test.
Dynamic Predictors
Dynamic Predictors
Dynamic Predictors
 Dynamic predictors apply a controlled and reversible
preload variation and measure the heamodynamic
response.
 Variation with respiration:



Stroke volume Variation / Pulse Pressure variation
Plethysmographic variation
IVC variation
 Systolic Pressure Variation
 Passive leg raise test
 Valsalva manoeuvre
In Summary
 The Frank-Starling principle of ventricular stretch producing




increased shortening/Stroke volume is key to assessing if a
patient will respond to a fluid bolus
Healthy individuals will all respond to a fluid bolus! Just because
a patient may respond doesn’t mean they need one. Use your
clinical judgement on the adequacy of their cardiac output prior
to assessing for responsiveness.
CVP only has some use as a predictor of NON-responsiveness
Dynamic measures ranging from Leg raise and Systolic pressure
variation up to SVV and IVC distension are all potentially helpful
tools in determining preload responsiveness. Each having their
pros and cons.
Some potentially good research project ideas there!
Reference
1.
2.
3.
4.
5.

Power and Kam, Principles of Physiology for the Anaesthetits 2e
2008
“Dynamic Indicies of Preload” T. Miko Enomoto, Louise Harder, Crit
Care Clin 26 (2010) 307–321
“Static Measures of Preload Assessment” Richard A. Nahouraii,
Susan E. Rowell, Crit Care Clin 26 (2010) 295–305
“Optimal volaemic status and predicting fluid responsiveness” Lorna
Eyre, Andrew Breen Continuing Education in Anaesthesia, Critical
Care & Pain | Volume 10 Number 2 2010
“Fluid status and fluid responsiveness” Sheldon Magder Curr Opin
Crit Care 16:289–296
“Does the Central Venous Pressure Predict Fluid Responsiveness? An
Updated Meta-Analysis and a Plea for Some Common Sense” Paul E.
Marik, Rodrigo Cavallazzi, Crit Care Med 2013; 41:1774–1781