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1. Ventilation‐perfusion in health &
Ventilation‐perfusion in health &
disease
Arjun Srinivasan

2. Where was the beginning ?

3. Distribution of ventilation
Distribution of ventilation
• Spatial & anatomical variation
• Rate of alveolar filling
Rate of alveolar filling
• Rate of alveolar emptying

4. Ventilation distribution‐ RC t
Ventilation distribution RC t
f
a
s
t
s
l
o
w
R=1
C=.5
t = .5
R=2
C=1
C 1
t =2
R=1
C=1
C 1
t = 1
volume (% of final)
%
)
100
75
50
25
0
0
1
2
sekundy
seconds
3

5. Clinical relevance
Clinical relevance
• Perfusion is poor & pulsatile at apex
• Pa & Pv proportionately increases from top to
& P proportionately increases from top to
bottom
• PA changes minimally with gravity
h
i i ll i h
i
• Pressures are max at bottom
– Pulm edema starts at bottom
– Redistribution of blood flow to apex antler’s
Redistribution of blood flow to apex – antler’s
horn

6. Its not just gravity!
Its not just gravity!
Lung perfusion in zero gravity situations is more uniform but by no means
equally distributed
Nunn’s Applied respiratory physiology, 6 th edition

7. Understanding V/Q relationships
Understanding V/Q relationships
• C id l
Consider lung as single unit
i l
it
– Relationships between PA O2, PA CO2, alveolar
l
h
b
l l
ventilation & pulmonary blood flow
– Alveolar gas equation
• Consider lung as multiple units of varying V/Q
– Clinical consequences in health & disease

8. Alveolar PO and PCO
Alveolar PO2 and PCO2
• D
Determined by the ratio between ventilation and
i db h
i b
il i
d
blood flow: V/Q
• PO2 and PCO2 are inversely related through alveolar
ventilation
il i
• Increasing V/Q produces higher PAO2 and lower PACO2
• Decreasing V/Q produces lower PAO2 and higher
PACO2

9. Gas Composition in the Alveolar Space
Gas Composition in the Alveolar Space
PiO2 = (barometric pressure‐H2O vapor pressure) xFiO2
= (760 – 47) x 0.21 =150 mmHg
(
)

10. Alveolar Gas Equation
Alveolar Gas Equation
PAO2 = (PiO2) – (PaCO2/R)
PaCO2 approximates PACO2 due to the rapid diffusion of
CO2
R = Respiratory Quotient (VCO2/V02) = 0.8
In a normal individual breathing room air:
PAO2 = 150 – 40/0.8 = 100 mmHg
/
g

11. Ventilation/perfusion ratio (V/Q)
il i / f i
i ( / )
V
Q

12. V/Q distribution in health
West JB, Respiratory Physiology,
The Essentials, 6th ed. 2000, Lippincott, p. 54.

13. V/Q ratio in disease
V/Q ratio in disease
obstruction
alveolus
vessel
50
40
“ideal”
V/Q
↓ V/Q
↑ V/Q
30
PCO2
(mmHg) 20
(
)
dead
space
shunt/
venous
admixture
10
0
0
embolus
40
80
120
PO2 (mmHg)
160

14. Low V/Q Effect on Oxygenation
Low V/Q Effect on Oxygenation
One lung unit has
One lung unit has
normal ventilation and
perfusion, while the
has inadequate ventilation
has inadequate ventilation
Low
L
V/Q
Normal
↑ PCO2
↓ PO2
PO2 50
PO2 100
PO2 ?
PO2 ?

15. CO2=(1.3 x HGB x Sat) + (.003 x PO2)
(
) (
)
%
% Hemog
globin Saturatio
on
100
20
80
16
60
12
40
8
20
4
0
0
0
20
40
60
80
PO mmHg
2
100
Oxy
ygen Content (ml/10 ml)
C
t
00
Oxyhemoglobin Dissociation Curve
Dissociation Curve

16. Low V/Q Effect on Oxygenation
Low V/Q Effect on Oxygenation
Low
L
V/Q
Normal
PO2 50 mmHg
Sat 80%
O2 content 16ml/dl
PO2 50
PO2 100 mmHg
g
Sat 100%
O2 content 20ml/dl
PO2 100
PO2 = 60
Sat 90%
O2 content 18ml/dl

17. HIGH V/Q Effect on Oxygenation
HIGH V/Q Effect on Oxygenation
HIGH
V/Q
Normal
PO2 130mmHg
Sat 100%
O2 content 20 ml/dl
PO2 130
PO2 100 mmHg
g
Sat 100%
O2 content 20ml/dl
PO2 100
PO2 = 100
Sat 100%
O2 content 20 ml/dl

18. High + low V/Q = ?
High + low V/Q = ?
HIGH V/Q
Low
L
V/Q
PO2 50 mmHg
Sat 80%
O2 content 16ml/dl
PO2 50
PO2 130 mmHg
g
Sat 100%
O2 content 20ml/dl
PO2 130
PO2 = 60
Sat 90%
O2 content 18ml/dl

19. Local ↑ V/Q above ~1 has minimal
effect on [O2]
160
23
PaO2
(mmHg)
O2
content
21 ( /
(ml/100 ml)
)
22
120
20
80
19
18
40
17
16
0
0.001
0.01
0.1
1
10
V/Q
Thus ↑ V/Q in one part of lung can’t compensate for ↓ elsewhere
15
100

20. PCO2 in V/Q Mismatch
in V/Q Mismatch
– Shape of CO2 curve
Shape of CO2 curve
• Total ventilation (VE)
( )
must increase for this
compensation
80
CO2 CONTENT (ml/100 ml)
T
m
• Increased ventilation
can compensate for
low V/Q units
low V/Q units
60
40
20
0
20
40
60
P
CO (mmHg)
2
80

21. Infinity
fi i
↑V/Q
Dead space

22. Zero
↓V/Q
Shunt
Venous admixture

23. Ventilation to Perfusion Mismatch
Ventilation to Perfusion Mismatch
Pure
Shunt
Perfusion with
No Ventilation
Pure
Dead Space
Shunt Like
Units
shunt
Dead Space
Like Units
Like Units
Ventilation with
No Perfusion
Dead space

24. 3‐ compartment model
3 compartment model
1
2
3

25. Dead space
Dead space
• Defined as wasted ventilation
• Classification
– Anatomical ( 1 ml / pound of ideal body wt)
– Alveolar
– Physiological
• Anatomical + alveolar
Anatomical + alveolar

26. Physiological Dead Space by Bohr’s
Method (for CO2)
VT X FE = VA X FA
X F V X F
VT = VA + VD
VA = VT – VD
VT X FE = (VT – VD) X FA
VD = FA – FE
VT
FA
T
VD = PACO2 – PECO2 (Bohr Equation), so…
VT
PACO2
VD = PaCO2 – PECO2
PaCO
CO
VT
PaCO2

27. Physiologic Dead Space &
V/Q Mismatch
Hyperinflation
• Airway Obstruction
• Dynamic Hyperinflation
• Tidal Volume
• PEEP
Low Perfusion
Low Perfusion
• Low Cardiac Output
• Pulmonary Vascular
Injury
• Extravascular
Compression
No Perfusion
No Perfusion
• Pulmonary Embolus
• Vascular Obliteration
• Emphysema

28. Shunt equation
Shunt equation
Qt x CaO2 = [(Qt – Qs) x Cc’O2] + [Qs x CvO2]
Qs Cc ' O2 − CaO 2
=
Qt Cc ' O2 − CvO 2

29. Causes of Shunt
Causes of Shunt
• Physiologic shunts
– Bronchial veins, pleural veins
,p
• P th l i h t
Pathologic shunts
– Intracardiac
– Intrapulmonary
• Vascular malformations
• Unventilated or collapsed alveoli

30. Normal shunt fraction
Normal shunt fraction
• ~ 5 %
• Due to
– Physiological shunt
– Gravity related V/Q mismatch
y
• Contributes to A a D O2
Contributes to A‐a D O
– 10 – 15 mm hg

31. A a
A a DO2
• PAO2 ( l l
PAO2 (alveolar gas equation)‐ arterial PaO2
ti ) t i l P O2
• I t t ll ffi i t l
In a totally efficient lung unit with matched V/Q,
it ith t h d V/Q
alveolar and capillary PO2 would be equal
• Admixture of venous blood or V/Q scatter from low
V/Q lung units will increase A a
V/Q lung units will increase A a DO2
• Increases ~ 3 mm hg every decade after 30 yrs of age
Increases 3 mm hg every decade after 30 yrs of age

32. Shunt VS. V/Q scatter
Shunt VS V/Q scatter
• C l l d h
Calculated shunt fraction or A a DO2
f
i
O
– Does not differentiate
• Response of pa O2 to increasing Fi O2
• V/Q scatter
– Pa O2 increase with small increase in FiO2(.21‐.35)
• Pure shunt
– Not much response
p

33. MIGET
•
•
•
50 parallel gas exchange units
characterized by VA‐Q ratio.
Fit predicted PE & Pa to
experimental data.
Output: Ventilation and
perfusion distributions.

34. Compensation of V/Q inequality ?
Compensation of V/Q inequality ?
• ↑V/Q → local hypocapnia → ↑pH → local
bronchoconstriction (↓V/Q )
• ↓V/Q → ↑CO2 → ↑ ventilation
↓V/Q →
– improves CO2 > O2 (dissociation curves)
• ↓V/Q → hypoxic pulmonary vasoconstriction

35. Points to remember
Points to remember
• Ventilation and Perfusion must be matched at the alveolar capillary
Ventilation and Perfusion must be matched at the alveolar capillary
level
• Most important cause of hypoxemia in most respiratory diseases
Most important cause of hypoxemia in most respiratory diseases
• V/Q ratios close to 1.0 result in alveolar PO2 close to 100 mmHg
and PCO2 close to 40 mmHg
and PCO2 close to 40 mmHg
• V/Q greater than 1.0 increase PO2 and Decrease PCO2. V/Q less
than 1.0 decrease PO2 and Increase PCO2
than 1 0 decrease PO2 and Increase PCO2
• Shunt and Dead Space are Extremes of V/Q mismatching.
• A‐a Gradient of 10‐15 Results from gravitational effects on V/Q and
Physiologic Shunt