Download AANA Journal Course, AANA Journal, October 1991

I
TES
Tedit. T
edits.
445
Definiton
of terms;aeyeaut
Bioimpedance --The resistance to an altrntig,
high frequen~cy current passed through the
ietod
theepbeen ofe
boy. 4 eanh
Cardia outpt-The amount of blood "pumnped" by
heart rate and stroke volumne. Normal cardiac
output is 5.0 liters per minute.
Cardiac index -The cardiac output expressed in proportion to body surface areas as liters " minute-' " meter-'; normal 2.8-4.2.
Diastoli blood pressure-The lowest pressure within
the arterial system during each cardiac cycle;
normal 50-90Omillmeters of Mercury (mmHg).
End diastolic index-The ventricular end diastolic
volume expressed in proportion to the body
surface area as milliliters/meter'.
Ejectionfratin-The difference between veiitricular end diastoli volume and ventricular end
systolic volume as a percentage ofthe~vnrclar end diastolic volume, normal greater than
Hleart nae- The number of tines the heart contracts
each minute; normal 60-100 beats per minute.
to the passage of an elecImpc-earisace
trical current through a conducto.
Stroke volume-The amount of blood ejected from
the left ventricle during each contraction; nor-
n
adoaclrsau
rdtoalivaietcnqe
lw
riva~tivso h iuinlpicpesc
indicator diuto
sn
lo
e
steFc
main
uptdtriain
stay o ada
fiberpiswti umnr reycteeshv
enabled the prattinr to deemn cniuu
hs O
mixed venous oxge saturato
n
However risk mngmn ncsotimn
anlto
concens in~herent tocnrlvnu
ahtrpaeethvn
atr
couraed4 the searc forlssivasie ardicutpt
Additional reinmets inDppe
moir.
andpumnary
echocardiogah ehooyhv rdcdtas
cutaineous and trneohgaDplrmtod
for deriningC.To oivsvemtos
imeac ltymgapyadtasuaeu
Doppler are urnl omrilyaalbe
TrneohgaDplrCOdtmitosrespaga Dopplrprbe
lcmnfa
qur
ungcricott
a
Cadicout
'
';~~
emaurdb aiu
Mtinvasive
inkvasive and noninvaivemehos
Omtosuiiz
cile. Three suc
rn
tedlto
aitoso
methodsar th Fik, indicator
mal 70 milliliters (inL).diuinanthradito.Twnnnvse
Stroke index -The stroke volume expressedas a~.proportion to the body surface area as mnilliliters/
mnethods, trnctaneousDplr(C)adto
ra celctrical biipeac (E)
Systemic vascular resistance-The resistance within
ciples,
ultasoundand impeanc plthsogapyprn
h dilto
o
inta~
repevely,
the arterial system which opposes the outflow<
princile
Fic
The diluto princilth asfrte
h ocnrto C faslt
ehd ttsta
.ovetcnb
egt()o
fte
eerie
i
ovn r
the solute n h o.m V fte
knw.Tewih ivddbrhoue yild
of blood from the~hert;normal 900-,500
dynes-seconds per centhnreter-5
va-
sytei
Systemic vascular reitac index-The..........~
cular resistance expresse. a a proporion to
body surface area in dyesscodspr eti-
....
mete-5
pr meter-2; normal 170260
th areilsse
uigech
cada
ycle;
sstli vlue-Th
Vetrcuaren
Swithin
rsiua blood
.the hart ater cotraction; normal 30-50
.... mL.
Ventricular end diastolic volum.-Thevlueo
blood within the heartatltheend of filling
prior to contraction; niorm~al 100-120 mL.
Eauton and maintenance of hemnodynamic staofthe
fo aprpitaaeent
biity iscrca
critically ill patie. i.the perio~perative period.
Thesepatients require cardiac.outpu CO)~ montoningthemt widel recogized method to ini446
ueDppler
meter'; normal 30-65.
h
,0
CWV.Teeoe
ocnrto
wiP yil a ocnrtono
L ill
100
/L
ueto be acltdi h thrtocnttet
are known~ (V = W/) Agi sing the above exam
pleasletwihn100 grm w'ith acoen
yilsaoutwith a volumef
trto of 1 g
1,00Qm4
The~ Fic pinciple eseially sasta h
sbtanc ~that esta ora mn
amonuntf{
qa tothora'
eidi
ovragvntm
utilization of th usac o the gi~venime,~u
whth.r it............gen t.&To determine
ssoye4a h usac
obne ihhmg
oridctr().Oye
CO h ikpicil
Jora
o
h
meia
Ascaio
fNus
nsteit
Table I
Miscellaneous formulas
Dilutional
C=WN
Fick (simplified)
V/T=W.T-1 C-1
Fick
Q=V02/(CaO2-CvO2)
Mean arterial pressure
MAP=DBP+1/3 (SBP-DBP)
C
= Concentration (arteriovenous oxygen content
gradient in mL 02/100 mL of blood)*
W
= Weight (oxygen consumption inmL 02/min)*
V
= Volume (L/min)*
Derived cardiovascular formulas
T
= Time interval (minutes)*
Q
= Cardiac output in L/min
Stroke volume
Ejection fraction
End diastolic index
Cardiac output
Cardiac index
Stroke index
Ejection fraction
(alternate)
Systemic vascular
resistance
Systemic vascular
resistance index
Left cardiac work index
V02 = Oxygen consumption in mL 02/min
CaO2 = Arterial oxygen content in mL 02/100 mL
blood
Cv02 = Mixed venous oxygen content in mL 02/100
mL blood
DBP = Diastolic blood pressure
SBP = Systolic blood pressure
* Information inparentheses indicates enclosed parameters to derive cardiac output by the simplified
Fick.
Table II
BSA
CVP
EDV
ESV
HR
MAP
PAOP
SV=EDV-ESV (mL/beat)
EF=SV/EDV (percent)
EDI=EDV/BSA (mL/m 2)
CO=HR*SV (L/min)
CI=CO/BSA (L-min-1*m- 2)
SI=SV/BSA (mL/m 2)
EF=SI/EDI (percent)
SVR= [(MAP-CVP)/CO]*80
(dyne-sec/cm5)
SVRI=[(MAP-CVP)/CI]-80
2
(dyne-seccm-5-m- )
LCWI= (MAP-PAOP)*
CI*0.0144 (kg-m/m2)
Body surface area in m2
Central venous pressure
End diastolic volume in mL
End systolic volume in mL
Heart rate
Mean arterial pressure
Pulmonary artery occlusive pressure in
mmHg
447
Table Ill
Normal values
Stroke volume
Ejection fraction
End diastolic index
Cardiac output
Cardiac index
Stroke index
Systemic vascular resistance
Systemic vascular resistance
index
Left cardiac wrk index
Index of contraction
Acceleration index
Systolic time ratio
Thoracic fluid index
Thoracic fluid conductivity
70-100 mL/beat
60%-75%
45-1 00 mL/m 2
5 L/min
2.8-4.2 L~minm- 2
30-65 mLm2
900-1,500
dyne-sec/cm5
1,700-2,650 dynesec~cm- 5 -m-2
3.7-5.7 kg-n/n 2
0.033-0.065/sec
0.5-1.5/sec 2
5%-30%
20-33 ohm (male)
27-48 ohm (female)
0.030-0.050 (male)
0.021-0.037 (female)
Figure 1
Calculation of cardiac output (CO) by the Fick
principle as a function of oxygen consumption
divided by the arteriovenous oxygen difference.
Lungs
Oxygen used=250 mLmin
02=
CO=5,ooo mL/min
200 mL/L
Left
heart
heart
Direction of blood flow
1 Movement of oxygen into blood
250 mL oxygen/min
CO =
5 mL oxygen/i 00 mL of blood
CO = 5,000 mL /min or 5 1/min
-
2
150 mL/L02
Right
Figure 2
This drawing Indicates the proper placement of
TEB electrodes
5 cm
Current
'--___injecting
Current
injecting
5 cm
Frontal view
Posterior view
Reproduced with permission
from BoMed Medical
Manufacturing Ltd.11
,OU-Kg
100-kg
~body
's, car-face, a
Ieffu-
these
449
Figure 3
Figure 4
Thoracic fluid index (F)
as a function of time.
-
40 .
Time relationship of electrocardiograph (ECG),
pressuretimpedance change (API/.Z), and the rate
of impedance change, dZ/dL
ECG
0.020
Standing
Supine
-0.033
-
0.050
I
*
AZ
TmI
U
I BtXZ/d~max.IC
-. 00
10_
/
0
0
lime
- 4
IReproduced with permission from BoMed Medical
Manufacturing
Ltd.11t
conditions resolve or worsen, TFI woukt
or decrease, respectively.
Three factors influence impedance v
1. Physical motion.
2. Respiration.
3. The blood pumping action.
Variation associated with physical n
fleets artifact, while variation associated
ration reflects thoracic cavity dimensioi
and the modulation of venous return. 'I
tion associated with the pump action, puil
vi
(dz/dt) max
IC
AP
B
x
F
0
sure
PF
-
Maximum rate of Impedance change
Index of contractility
Change inimpedance
Change inpressure
Aortic valve opening
Aortic valve closure
Pulmonic valve closure
Reflected wave
Mitral opening snap
Peak flow
Acceleration index
Pre-ejection period
Ventricular ejection time
-
Heart rate period
-
-
-
PEP
VET
HRP
-
Reproduced with permission from BoMed Medical
Manufacturing Ltd.11
Table IV
Bioimpedance-deiiwd formulas
Stroke volume
SV=VEPT*VET-IC (mLlbeat)
tme rtio sec)face
SR=PENET
Systoic
(c)
ST=PEPNE."TR
fratio
Sysc time
t
monitop.AOne model, NC.k P (1 $ed Modical Manfaturing td., Irvine, California), has an
optal oftware pakg with a computer interfor usewit IBM-comnpatible cotnputen .Using
th asoitdcmue
option facilitates interpre-
Ejecionfracion
EF=O84-.64*TR mL/bat)tatin, presentation, and storage ofidata. A segment
of the software is devoted to recommended theraIne(fCotatohCmd/et)mx
petxtic modlities for treatment based on the inferAceeainindex (ohm/sec/)ma/~
ACI=(dZ/dt 2 maxIT
mation prvided.i
(ohm/ec 2 )TEB
compares favorably with CO determina-
VEPT
=
Algorithm for the mass of the thoracic
tissue participating inthe resistivity
tionls derived. by thermal dilution pulmonary ar-
VET
=
PEP
dZP
=
=
Ventricular ejection time
ICcntrationevaluating
=
ndexof
Prdeejecontpraiod
Changjectiniperidncetigatos
=
=
=
=
Change in time
Thoracic fluid index
Rate of change indZ
Rate of change in dt
"close agreement" with thermal dilution COs when
the accuracy of TEB and Doppler
derived COs in intensive care patients.4 Other inexamined severely ill surgical patients
the intensive care unit and measured their CO
with TEB and Doppler methods. Bioimpedance
CO determinations compared very favorably with
Doppler, correlation coefficient of r=0.83. 5 These
findings are similar to those obtained when TEB
has been compared to radionuclide and MUGA
dt
WI
d2Z
dt2
=Chane
dZ
inimpeancein
asina. Therefore, a decrease in impedance will
irror an increase in velocty
Additionally, TEB provides a measure of the
te of change of blood flow (acceleration) identi-
!d as the 'acceleration index (AUI); ACI _(d2Z/
a)/TFI. the ACI correlates with peak accelera>n, a paramneter often measured by Doppler ultraand and used to describe the inotropic state of the
art. This factor can be used to prescribe treatent modalities in the clinical setting.
Frointhis information, stroke volume and ejecn fraction are derived and CO calculated. Stroke
lume is the product of a complex TEB equation
volvng:
1. Index of contraction (IC).
2. Time duration of mechanical systole (VET).
3. The physical volume of electrically particiting tissue (VEPT), with.consideration given to
e,sex, height, weight, and an algorithm for differg metabolic rates of tissue (See Table IV).
Multiplying the stroke volume times the heart
:e yields the CO. In an effort to provide unifor~ty, these parameters are body surface area, indendent and reported as indexed parameters, such
cardiac index, systemic vascular resistance index
VTRI), index of contraction, acceleration index,
t cardiac work index (LCWI), stroke index, endistolic inidex, and ejection fraction.
Additionally, SVRI and LCWI can be deterned by TEB; however, these parameters require
pressure determination and assume a central
rious pressure of 8 mmflg. Blood pressure deter-
)od
tery cat)heterization.-' Wong, and associates found
(multiple-gated
acquisition) scanning-, Addition-
ally, Davies, (cited in Sramek) found good correlation between the Fick method and TEB.1 However,
one must remember that correlation factors only
express the agreement between two methods and
not the accuracy of either.
Gotshall and colleagues compared thermal dilution and bioimpedance-derived cardiac index.9
Thes authors criticized the use of TEB in normal
subjects as TEB tended to overestimate stroke index and cardiac index; however, their sample was
limited to seven subjects and did not address the
individual variability associated with thermal
dilution-derived output and pulsatile flow. 0 In alitically ill patients, however, these authors found
good correlation with thermal dilution and
bioixnpedance-derived cardiac index.
LUftatons
Thoracic electrical bioimpedance isrecognized
to be less accurate in certain situations. Severe dysrhythmics, tachycardia, and pacemaker spikes interfere with the correct determination of systolic
time intervals and ultimately heart rate determination, thereby yielding inaccurate COs. Valvular insufficiency with regurgitant blood flow impairs TEB
determination of CO. Hyperdynamic sepsis is reportedly associated with overestimated COs. Additionally, signal acquisition is impaired by electrode
adherence problems and electrocautery. And lastly,
SVRI and LCWI, being derived parameters using a
formula which incorporates the mean blood pressure and central venous pressure with an assumed
Test Yourself
1.
Describe Ohm's law and the relationship to
thoracic electrical bioimpedence (TEB).
2.
What are the disadvantages of pulmonary artery catheter insertion.
3.
What are the advantages of TEB.
4.
Describe the limitations of TEB.
5.
Describe the factors influencing impedance
variation.
Test Yourself Answers
1. Ohm's law states that for a given conductor
there is a linear relation between voltage and
current, V = IR. Bioimpedance is the resistance within a biological circuit.
2.
The disadvantages of pulmonary artery catheter insertion are that it is invasive, expensive,
and requires specialized training for insertion
and use.
3.
The advantages of TEB are that it is noninvasive, continuous, low cost, clinically accurate,
easy to use, and has good interoperator
reproducibility.
4.
TEB is recognized to be less accurate in situations which interfere with the measurement of
systolic time intervals such as severe dysrhythmias, pacemaker activity, and tachycardia.
Additionally, valvular insufficiency and hyperdynamic sepsis are reported to impair accuracy. Finally, problems with electrode adherence or electrocautery can impair cardiac
output determinations.
5.
There are three components to impedance
variations. They are physical motion, respiration, and the blood pumping motion, pulsatile component.