Physiological Shunt measurement Flashcards
What are the 4 indices used for clinical assessment for physiological shunt?
- classical shunt equation
- estimated shunt equation
- alveolar - arterial O2 difference
- oxygenation ratio
Why do we quantify shunt?
- because the are different categories of shunting
e. g. relative & absolute - quantifying shunt gives a numerical value to shunt
definition of classical shunt equation?and equation?
the ratio of shunted cardiac output to total cardiac output
Qsp/Qt(total CO) x 100%
QSp = CcO2 - CaO2 Qt = CcO2 - CvO2
What is Qsp?
- represents physiological shunt
- the different between Capillary O2 and arterial O2 content
- difference shows the amt of o2 that is SHUNTED
characteristic of cco2 ?
pulmonary Capillary O2 content
1.Capillary blood is the most oxygenated because the O2 diffuses from alveolar hasn’t been diluted
- Its o2 content is > CaO2 & CvO2
How to calculate Cco2
ONLY IF Fio2 = 21%, then PaO2 equilibrates with the capillary blood
- ->ScO2 will be 100% saturated (as a .100 mmhg PAO2 correlates with closes to 100% saturation
- ->CcO2 = (1.34 x hb x 1.00) + (0.003 x PAO2)
What is the equation for PAO2?
(PB-47) x Fio2 - PACO2/0.8
Def of Cao2
arterial O2 content from arterial blood sample
Def of Cvo2
mixed venous o2 content from pulmonary artery catheter
Assessment of classic shunt (SHAPIRO & BEACHEY)
equal to or <10 % normal
10 - 19% mild shunting
20 - 29% significant disease requiring PEEP/CPAP
> or equal to 30 % SEVERE disease, needs mechanical ventilation & LIFE THREATENING
What is normal shunt? what about the clinical accepted value?
- 2 - 5 %
2. 10%
What is the caused of mild shunting?treatment?
- general hypoventilation
2. by giving O2 therapy
When is estimated shunt equation used?
IN the ABSENCE of a PA catheter –> cannot obtain CvO2
What assumption do we make for estimated shunt equation?
assumption 1: C(a-v)O2 = 3 .5 - 5 vol%
assumption 2: critical care patient have increase in cardiac output which decreases O2 extraction ??
What is the caveat of the modifed shunt equation
only uses when patient have good cardiovascular response and status
What is the equation of modifted shunt equation?
Qst/Qt
Qst= CcO2-CaO2 Qt = (CcO2-CaO2) + 3.5
definition of the alveolar-arterial o2 tension gradient
Quantifies the efficiency of O2 loading or transfer between alveolar and the capillary
EQuation of alveolar-arterial o2 tension gradient
P(A-a)o2 or A-aDO2
What is the normal value for A-aDO2? how about clincal accept value?
- Assuming patient on 21% Fio2 and STABLE
1. 10 mmhg
2. < 20 mmhg (for ADULTS LESS THAN 60 yrs )
3. < 30 mmhg (for ADULTS OVER 60 yrs)
IF PATIENT is on 100% FIO2
Clinically accept < 50 - 60 mmhg
What is the disadvantage or contraindication of the A-aDO2?
- patient O2 therapy with variable FIO2
WHY? because FIo2 affects PAO2 - A-aDO2 increases with age
- Changes with body position
WHY? because area of ventilation changes in different body position - needs stable C-V condition
the 2 PAO2 equation based on different FIO2
FIO2< 60%
PAO2 = (PB - 47) x FIO2 - (PACO2/0.8)
FIO2 > 60%
PAO2 = (PB - 47) x FIO2 - (PACO2)
Equation of oxygenation ratios?
- PaO2/PAO2 OR
2. PaO2/FIO2
what is the normal numerical value for oxygenation ratio?
equal to or > 75%
What is the percentage of Oxygenation ratio that indicate severe impariment
between 30 - 55%
What is the sigificance of PaO2/Fio2?
looking at the partial pressure of O2 exerts on alveolar PER concentration of O2 in the air mixture
what is the normal PaO2/Fio2 value?
4 - 5 normal
2 - 3.9 moderate
<2 severe
Disadvantage of Oxygenation ratio?
doesnt account for 1) PACO2 and 2)CO changes
*both can changes Pao2
Which shunts equation are the best?
- classical (BUT MORE INVASIVE)
- modified (NON-INVASIVE)
WHY? takes CO and mixed venous values into effect
What are the advantages of the rest of the indices?
P(A-a)O2, PaO2/PAO2 , Pao2/Fio2
- easy to use
- doesn’t require PA catheter
- GUIDE TO O2 therapy
