Physiological Shunt measurement

Question 1

What are the 4 indices used for clinical assessment for physiological shunt?

Answer 1

• classical shunt equation
• estimated shunt equation
• alveolar - arterial O2 difference
• oxygenation ratio

Question 2

Why do we quantify shunt?

Answer 2

• because the are different categories of shunting
  e. g. relative & absolute
• quantifying shunt gives a numerical value to shunt

Question 3

definition of classical shunt equation?and equation?

Answer 3

the ratio of shunted cardiac output to total cardiac output

Qsp/Qt(total CO) x 100%

QSp = CcO2 - CaO2
Qt = CcO2 - CvO2

Question 4

What is Qsp?

Answer 4

• represents physiological shunt
• the different between Capillary O2 and arterial O2 content
• difference shows the amt of o2 that is SHUNTED

Question 5

characteristic of cco2 ?

Answer 5

pulmonary Capillary O2 content
1.Capillary blood is the most oxygenated because the O2 diffuses from alveolar hasn’t been diluted

2. Its o2 content is > CaO2 & CvO2

Question 6

How to calculate Cco2

Answer 6

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)

Question 7

What is the equation for PAO2?

Answer 7

(PB-47) x Fio2 - PACO2/0.8

Question 8

Def of Cao2

Answer 8

arterial O2 content from arterial blood sample

Question 9

Def of Cvo2

Answer 9

mixed venous o2 content from pulmonary artery catheter

Question 10

Assessment of classic shunt (SHAPIRO & BEACHEY)

Answer 10

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

Question 11

What is normal shunt? what about the clinical accepted value?

Answer 11

1. 2 - 5 %

2. 10%

Question 12

What is the caused of mild shunting?treatment?

Answer 12

1. general hypoventilation

2. by giving O2 therapy

Question 13

When is estimated shunt equation used?

Answer 13

IN the ABSENCE of a PA catheter –> cannot obtain CvO2

Question 14

What assumption do we make for estimated shunt equation?

Answer 14

assumption 1: C(a-v)O2 = 3 .5 - 5 vol%

assumption 2: critical care patient have increase in cardiac output which decreases O2 extraction ??

Question 15

What is the caveat of the modifed shunt equation

Answer 15

only uses when patient have good cardiovascular response and status

Question 16

What is the equation of modifted shunt equation?

Answer 16

Qst/Qt

Qst= CcO2-CaO2
Qt = (CcO2-CaO2) + 3.5

Question 17

definition of the alveolar-arterial o2 tension gradient

Answer 17

Quantifies the efficiency of O2 loading or transfer between alveolar and the capillary

Question 18

EQuation of alveolar-arterial o2 tension gradient

Answer 18

P(A-a)o2 or A-aDO2

Question 19

What is the normal value for A-aDO2? how about clincal accept value?

Answer 19

• 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

Question 20

What is the disadvantage or contraindication of the A-aDO2?

Answer 20

1. patient O2 therapy with variable FIO2
   WHY? because FIo2 affects PAO2
2. A-aDO2 increases with age
3. Changes with body position
   WHY? because area of ventilation changes in different body position
4. needs stable C-V condition

Question 21

the 2 PAO2 equation based on different FIO2

Answer 21

FIO2< 60%
PAO2 = (PB - 47) x FIO2 - (PACO2/0.8)

FIO2 > 60%
PAO2 = (PB - 47) x FIO2 - (PACO2)

Question 22

Equation of oxygenation ratios?

Answer 22

1. PaO2/PAO2 OR

2. PaO2/FIO2

Question 23

what is the normal numerical value for oxygenation ratio?

Answer 23

equal to or > 75%

Question 24

What is the percentage of Oxygenation ratio that indicate severe impariment

Answer 24

between 30 - 55%

Question 25

What is the sigificance of PaO2/Fio2?

Answer 25

looking at the partial pressure of O2 exerts on alveolar PER concentration of O2 in the air mixture

Question 26

what is the normal PaO2/Fio2 value?

Answer 26

4 - 5 normal
2 - 3.9 moderate
<2 severe

Question 27

Disadvantage of Oxygenation ratio?

Answer 27

doesnt account for 1) PACO2 and 2)CO changes

*both can changes Pao2

Question 28

Which shunts equation are the best?

Answer 28

1. classical (BUT MORE INVASIVE)
2. modified (NON-INVASIVE)

WHY? takes CO and mixed venous values into effect

Question 29

What are the advantages of the rest of the indices?

Answer 29

P(A-a)O2, PaO2/PAO2 , Pao2/Fio2

• easy to use
• doesn’t require PA catheter
• GUIDE TO O2 therapy