flow-directed approach to shock - critical care · pdf filedivision of critical care, ......
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Hemodynamic monitoring: Guyton at the bedside
S Magder
Division of Critical Care,
McGill University Health Centre
Flow-directed approach to shock
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What are the key points that
Guyton made?• 1. Cardiac output is not just about “heart
rate and stroke volume”
– Key factor is how blood gets back to the heart
– Guyton sited Ernest Starling
• venous return is dependent upon an
upstream pressure which he called
“mean systemic pressure”
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Increase the
initial
volumeGreater
flow
Stressed
volume
Importance
of volume
Stressed Volume
Cv x RvQ =
Determinants of flow
Cv
Rv
Unstressed
Volume
Stressed
Volume
2x
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2. Established a method to measure “mean
circulatory filling pressure” (MCFP)
3. Identified the determinants of venous
return – contsructed venous return
curves (return function)
What are the key points that
Guyton made (continued)?
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The height of the water
determines the outflow
MSFP
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4. Cardiac output is determined by the
interaction of “cardiac function” and
“return function”
What are the key points that
Guyton made (continued)?
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Heart has a
“restorative”
function
Volume stretches the veins and creates the “recoil”
pressure that drives flow back to the heart
Heart has a
“permissive”
function. It lowers
the outflow
pressure and
allows veins to
empty
which refills the veins
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In contrast to your tub, the veins are the
source of fluid in the circulation
MCFP
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Concepts:
Capacitance and Stressed volume
Pressure vs Total Volume
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MCFPMCFP
Unstressed
volume
Concept of Stressed and Unstressed Volume
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Volume (ml)
0 1000 2000 3000 4000 5000 6000
MS
FP
(m
mH
g)
0
2
4
6
8
10
Pressure-Volume Relationship of
the Vasculature
StressedUnstressed
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MSFP
RvHeart
unstressed volume
stressed volume
Circulatory Model
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Pra = MSFP
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Pra < MSFP
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Principle:
The heart controls cardiac
output (venous return) by
regulating Pra and not by
controlling Part
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Pra
Q
-1/Rv
Pra = MSFP
A
Pra < MSFP
B
MSFP
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PRA(CVP) should always be used
to determine the optimal volume
for cardiac output
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Pra
Q
Increase in Cardiac Function Curve
↑ Heart Rate
↑Contractility
↓Afterload
Higher Q for a given Pra
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Return Function
“working CVP”
Cardiac Function
Pra
Q
“Cardiac limited”
“Return limited”
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Change in Cardiac Function
Q
Pra Increase in gradient
for venous returngradientgradient
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Change in Volume
PMCFP ↑
MCFPQ
Pra
Q
V
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Volume (ml)
0 1000 2000 3000 4000 5000 6000
MC
FP
(m
mH
g)
0
2
4
6
8
10
Volume (ml)
0 1000 2000 3000 4000 5000 6000M
CF
P
(mm
Hg
)
0
2
4
6
8
10
Change in Capacitance(can change by 10-15 ml /kg)
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Change in Capacitance
MCFP
Q
Pra
Q
PMCFP
MCFP
↑ MCFP
V
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Pra
Q
Change in venous resistance:
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Change in volume on plateau of
cardiac function curve
MCFP
Q
Pra
No
change
in Q
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Fluid Challenge
1 Assess the value
of Pra ( NOT the
wedge).
2 Give sufficient
fluid to raise Pra
by ~2mmHg and
observe Q.
Type of fluid is not of importance if given fast enough
Pra
Q
+ve
-ve
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Change in CVP of even 1 mmHg should
be sufficient to test the Starling response
Pra (mmHg)
Q (l/min)
0 10
5
Slope = 500 ml/mmHg
plateau
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When Pra < Pleural Pressure, the great
veins collapse when they enter the thorax
Lowering Pra no longer effects
Cardiac output
=“vascular waterfall”
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The amount of water
flowing over a
waterfall is not
affected by the
height of the
waterfall
Concept of
vascular
waterfall
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Q
Pra
Lowering Pra
further will not
increase Q
VR =-Pra
Rv
MCFPmax
Stressed
volume must
increase to
increase Q
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Q
Pra
When there is
“venous collapse” an
increase in HR
decreases SV but Q
does not change
-consider
significance for “SV”
devices
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Patient has low urine output and is
thought to be “pre-renal”
How does volume increase the flow
of urine?
1. Increase renal blood flow
2. By increasing blood pressure
3. By increasing cardiac output
You decide to try giving volume
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For most tissues it is Oxygen Delivery (DO2) that counts
What do we have to play with?
DO2 = Q x Hb x k x(Sata).
Volume(preload)
↑ Contractility↑ HR
↓Afterload
Blood ↑ PO2Usually not much gain
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How do you apply these principles
to manage volume clinically?
• 1st: is there a hemodynamic problem to fix?
• Is inadequate cardiac output the problem?
• Is the inadequate cardiac output due to the heart function or return?
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BP = Cardiac Output x SVR
First Question to ask:
Is the cardiac output decreased
Or
Is the cardiac output normal or increased
Measured variable
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Where does “volume” show up in
the equation?
Part = Q x SVR (- Pv)
Starlings Law
“Preload”Venous Return
“ Elastic recoil”
MSFP
Vs
Cv
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If Q is normal or elevated it is a
SVR problem
Causes of a low SVR:– Sepsis (sirs)
– Drugs
– Arterio-venous shunts
– Spinal/epidural injections
– Spinal injury
– Cirrhosis
– Thyroid disease
– Anaphylaxis
– Corticosteroid deficiency
– Anemia
– Beriberi
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BP = Cardiac Output x SVR
Heart Rate
Stroke Volume
Afterload
Contractility
Preload
Stressed volume
Compliance
Resistance
Pra
Cardiac Function Return Function
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Is it a “circuit” or a “cardiac”
problem?
Pra is the clue:
•Actual level is only of some help–CVP > 10-12 mmHg usually indicates cardiac limit, but limit can be higher or lower
•Examine the response to a fluid challengeor
•Examine the respiratory variation in Pra
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1. Normal cardiac
function and
blood volume
Q
Pra
Pra = 0 mmHg
2. Depressed cardiac
function but low
blood volume
Pra
Pra = 0 mmHg
Q
3. Decrease in
volume with normal
cardiac function
Q
Pra
Three examples of low CVP
Q = 5 L/min
Q = 3 L/min
Give volume
and inotrope
Give volume
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1. Increase volume
with normal cardiac
functionQ
Pra
2. Decrease cardiac
function and normal
blood volume
Q
Pra
3. Decrease RVR without
change in cardiac
function (?sepsis)
Q
Pra
Three examples of high CVP
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BP ↓ = Q ↓ x SVR
CVP fell with the fall in Q
Decrease in
volume with
normal cardiac
function
Q
Pra
Give volume
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BP ↓ = Q ↓ x SVR
CVP rose with the fall in Q
Q
Pra
Decreased
cardiac function
- Give inotrope
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1. Decrease in
cardiac output
Q
Pra
2. Decrease in
cardiac output
Q
Pra
Fall in Q with
fall in CVP
Fall in Q with
rise in CVP
- decrease in return
function (decreased volume)
- decrease in cardiac
function
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0
4
8
12
16
20
No Insp
Fall
+ve Insp
Fall
mmHG
Initial Right Atrial Pressure
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Wedge Pra
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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Inspiratory fall in Pra No Inspiratory fall in Pra
Q
Pra
↓ Ppl Q
Pra
↓ Ppl
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L/m
in (
delta
)
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
+ve Resp -ve Resp
Inspiratory fall No Inspiratory Fall
+ve
-ve
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Part = Q x SVR (+K)
CircuitStressed volume
Compliance
Resistance
Pra
Sepsis
Drugs
SpinalHeartHeart Rate
Afterload
Contractility
Preload
Dobutamine
Milrinone
VolumeNE NE
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Crit Care Med 2010 Vol. 38, No. 11
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Fluid ProtocolCI < 4 and
CVP < 12 ?
Protocol Fluid Bolus
Check CI
and
CVP
CVP incr. > 2and
CI incr. < 0.3
CVP incr. < 2and
CI incr. > 0.3
CI < 2.2
or
MAP < 70
or
SBP < Target
or
CVP <3
or
UO < 20
Total
Protocol
Fluid
>1L/24hr?
Catecholamine
Protocol
Yes
Yes
NoCVP incr. < 2
and
CI incr. < 0.3
CVP incr. > 2and
CI incr. > 0.3
Inadequate
challenge
Cardiac
response ok
Pt not volume responsive
Review fluid
criteria
No
Observe
or wean
Saline
YES
CI < 2.2
or
MAP < Target
or
SBP < Target
or
CVP < 3
or
Urine < 20 cc/hr
triggers
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Primary OutcomeCatecholamines between 8:00 and 9:00 AM
HES
119
Crystalloid118
YES 13 34
10.9% 28.8 %
.38 ( 0.21, 0.68)
p = 0.001
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Clinical Scenario
• Post operative cardiac surgery
• Initial blood pressure is 80 mmHg
What do you want to know
• CI = 3.2 l/min/m2, Pw= 12 mmHg, Pra= 10mmHg,
What is wrong and what do you want to do?
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Clinical Scenario
• Patient comes back from aorto-coronary
bypass surgery. The initial hemodynamics
are:
– Q = 2.2 l/min/m2 , Pra = 12 mmHg,
Pw = 8 mmHg, Part = 110/70 mmHg
• One hour later
– Q = 1.8 l/min/m2, Pra = 6 mmHg,
Pw = 7 mmHg, Part = 90/70 mmHg
What would you do?
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Clinical Scenario
• Patient comes back from aorto-coronary
bypass surgery. The initial hemodynamics
are:
– Q = 2.2 l/min/m2 , Pra = 8 mmHg,
Pw = 6 mmHg, Part = 110/70 mmHg
• One hour later
– Q = 1.8 l/min/m2, Pra = 12 mmHg,
Pw = 8 mmHg, Part = 80/60 mmHg
What would you do?
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1. Normal cardiac
function and
blood volume
Q
Pra
Q = 5 L/min
Pra = 0 mmHg
2. Depressed cardiac
function but low
blood volume
Pra
Q = 3 L/min
Pra = 0 mmHg
Q
3. Decrease in
volume with normal
cardiac function
Q
Pra
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1. Increase volume
with normal cardiac
functionQ
Pra
2. Decrease cardiac
function and normal
blood volume
Q
Pra
3. Decrease RVR without
change in cardiac
function (?sepsis)
Q
Pra
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Part = Q x SVR (+K)
CircuitStressed volume
Compliance
Resistance
Pra
Sepsis
Drugs
SpinalHeartHeart Rate
Afterload
Contractility
Preload
Dobutamine
Milrinone
VolumeNE NE
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Inspiratory fall in Pra No inspiratory fall in Pra
Q
Pra
Q
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Increase the
initial
volumeGreater
flow
Stressed
volume
Importance
of volume
Stressed Volume
Cv x RvQ =
Determinants of flow
Cv
Rv
Unstressed
Volume
Stressed
Volume