KEY POINTS
ALVEOLAR VENTILATION–(V A)
ALVEOLAR PERFUSIONPULMONARY CIRCULATION (Q)
VENTILATION – PERFUSION
RATIO (VA/Q)
VENTILATION PERFUSION
MISMATCH
SHUNT
DEAD SPACE
Pulmonary blood flow 5l/min
Total pulmonary blood volume -500ml
to 1000ml
These volume going to be spreaded
all along the alveolar capillary
membrane which has 50 to 100 m²
surface area
Pulmonary blood flow 5l/min
Total pulmonary blood volume -500ml
to 1000ml
These volume going to be spreaded
all along the alveolar capillary
membrane which has 50 to 100 m²
surface area
Due to gravitational influence the lower
– dependent areas receive more blood
Upper zone – nondependent areas are
less per fused
Pulmonary circulation – Alveolar Perfusion Q
ZONE-I: Only exist if Ppa very low in
hypovolemia / PA in PEEP
ZONE-II: Perfusion α Ppa-PA
arterial-alveolar gradient
ZONE-III: Perfusion α Ppa-Ppv
arterial-venous gradient
ZONE-IV: Perfusion α Ppa-Pist
arterial-interstitial gradient
Ventilation is unevenly distributed in the lungs.
Rt lung more ventilated than Lt lung [53% & 47%]
Due to gravitational influence on intra plural pr
[decreased 1cm/H2O per 3cm decrease in lung height]
lower zones better ventilated
Ventilation
-6
Due to gravitational
influence on intra plural pr
[decreased 1cm/H2O per
3cm decrease in lung
height] lower zones better
ventilated
-3
-1
Intra pleural pr
Ventilation pattern - VA
•Pleural pressure [Ppl] increased
towards lower zone
•Constricted alveoli in lower
zones & distended alveoli in upper
zones
•More compliant alveoli towards
lower zone
•Ventilation: distributed more
towards lower zone
Ventilation pattern - VA
•Upper zone:
less pleural pressure,
distended more
& hence less compliant
•Lower zone:
more pleural pressure,
less distended,
& hence more compliant
Minute Ventilation V = RR x VT
Volume of the inspired gas participating in alveolar gas
exchange /minute is called ALVEOLAR VENTILATION-VA
VA = RR x VT-VD
Not all inspired gas participating in alveolar gas exchange
DEAD SPACE – VD
Some gas remains in the non respiratory airways
ANATOMIC DEAD SPACE
Some gas in the non per fused /low per fused alveoli
PHYSIOLOGIC DEAD SPACE
Lower zone i.e. dependent part of
alveoli are better ventilated
than the middle & upper zones i.e.
nondependent
Dead space ventilation - wasted ventilation
ventilation of unperfused alveoli
Dead space VD = 2ml/kg ; 1ml /pound
Dead space ratio VD/ VT = 33%
VD
VT
= PACO2 – PECO2
PACO2
Ventilation Perfusion ratio VA/Q
•Ventilation & Perfusion both are
distributed more towards lower zone.
•Ventilation[VA] less increased
t0wards l0wer zone than Perfusion[Q]
•Perfusion more increased towards
Lower zone than Ventilation
•Ventilation Perfusion ratio VA/Q:
Less towards lower zone
VA/Q
Q
VA
Ventilation Perfusion ratio VA/Q
•Ventilation & Perfusion both are
distributed more towards lower zone.
•Ventilation[VA] less increased
t0wards l0wer zone than Perfusion[Q]
•Perfusion more increased towards
Lower zone than Ventilation
•Ventilation Perfusion ratio VA/Q:
Less towards lower zone
VA/Q
Q
VA
VENTILATION PERFUSION RATIO
V
Q
V
Q
Q
Wasted
ventilation
V=normal
Q=0
V/Q=∞
DEAD SPACE
V
Normal
V&Q
V/Q=1
IDEAL
ALVEOLI
Wasted
Perfusion
V=o
Q= normal
V/Q=0
SHUNT
Ventilation Perfusion ratio VA/Q
The overall V/Q = 0.8 [ ven=4lpm, per=5lpm]
Ranges between 0.3 and 3.0
Upper zone –nondependent area has higher ≥ 1
Lowe zone – dependent area has lower ≤ 1
VP ratio indicates overall respiratory functional status of
lung
V/Q = 0 means ,no ventilation-called SHUNT
V/Q = ∞ means ,no perfusion – called DEAD SPACE
V
V/Q<1
Q
Means – Wasted perfusion
Shunt – 1. Absolute Shunt : Anatomical shunts – V/Q = 0
2. Relative shunt : under ventilated lungs –V/Q ≤ 1
Shunt estimated as Venous Admixture
Venous Admixture expressed as a fraction of total cardiac
output Qs/Qt
Qs = CcO2-CaO2
Qt CcO2-CvO2
Normal shunt- Physiologic shunt < 5%
•SHUNTS have different effects on arterial PCO2 (PaCO2 ) than on arterial
PO2 (PaO2 ).
• Blood passing through under ventilated alveoli tends to retain its CO2
and does not take up enough O2.
•Blood traversing over ventilated alveoli gives off an excessive amount of
CO2, but cannot take up increased amount of O2 because of the shape of
the oxygen-hemoglobin (oxy-Hb) dissociation curve.
• Hence, a lung with uneven V̇P relationships can eliminate CO2 from the
over ventilated alveoli to compensate for the under ventilated alveoli.
• Thus, with Shunt, PACO2 -to-PaCO2 gradients are small, and PAO2 -to-
PaO2 gradients are usually large.
•PAO2 is directly related to FIO2 in normal patients.
•PAO2 and FIO2 also correspond to PaO2 when there is little to
no shunt.
•With no S/T, a linear increase in FIO2 results in a linear
increase in PaO2.
•As the shunt is increased, the S/T lines relating FIO2 to PaO2
become progressively flatter. With a shunt of 50% of QT, an
increase in FIO2 results in almost no increase in PaO2 .
•The solution to the problem of hypoxemia secondary to a large
shunt is not increasing the FIO2 , but rather causing a reduction
in the shunt (fiberoptic bronchoscopy, PEEP, patient
positioning, antibiotics, suctioning, diuretics).
•PAO2 is directly related to FIO2 in normal patients.
•PAO2 and FIO2 also correspond to PaO2 when there is little to
no shunt.
•With no S/T, a linear increase in FIO2 results in a linear
increase in PaO2.
•As the shunt is increased, the S/T lines relating FIO2 to PaO2
become progressively flatter. With a shunt of 50% of QT, an
increase in FIO2 results in almost no increase in PaO2 .
•The solution to the problem of hypoxemia secondary to a large
shunt is not increasing the FIO2 , but rather causing a reduction
in the shunt (fiberoptic bronchoscopy, PEEP, patient
positioning, antibiotics, suctioning, diuretics).
SHUNT
PaO2
VIRTUAL SHUNT CURVES
FiO2
DEAD SPACE
Not all inspired gas
participating in alveolar gas
exchange
DEAD SPACE – VD
Some gas remains in the non
respiratory airways
ANATOMIC DEAD SPACE
Some gas in the non per fused
/low per fused alveoli
PHYSIOLOGIC DEAD SPACE
V
V/Q= ∞
Q
Means – Wasted Ventilation
Dead Space estimated as ratio Vd/Vt
Dead space expressed as a fraction of total tidal volume Vd/Vt
Vd = PACO2-PECO2
Vt
PACO2
Normal dead space ratio < 33%
1. SHUNT RATIO Qs = CcO2-CaO2
Qt CcO2-CvO2
2. MODIFIED
= CcO2-CaO2
[CcO2-CaO2]+4
•
•
•
•
•CcO2-Pulmonary end capillary O2
content
•CaO2-Arterial O2 content
•CvO2-Mixed venous O2 content
PcO2=PAO2
PAO2=PiO2-PaCO2/0.8
=FiO2x6
PiO2 =PB-PH2OxFiO2
CaO2 = O2 carried by Hb + Dissolved O2 in plasma
= 1.34 x Hb% x SaO2 + 0.003 x PaO2
QUANTIFICATION - SHUNT
3. ALVEOLAR – ARTERIAL O2 GRADIENT : PAO2-PaO2


Varies with FiO2 & age
7-14 to 31-56mm Hg
4. ARTERIAL – ALVEOLAR RATIO : PaO2/PAO2


FiO2 independent
>0.75
-normal
0.40-0.75-acceptable
0.20-0.40– poor
< 0.20 –very poor
QUANTIFICATION - SHUNT
5. ARTERIAL O2 INSPIRED O2 RATIO : PaO2/FiO2




Normally >500mmHg
Acceptable 250-500
P00r 100-250
Terminal <100
LI Score: <300ALI, <200ARDS
SAPS 2
QUANTIFICATION - SHUNT
6. ISO SHUNT TABLE
PaO2
7. VIRTUAL SHUNT DIAGRAGME
FiO2
QUANTIFICATION – DEAD SPACE
1. Vd = PACO2-PECO2
Vt
PACO2
2. MV x PaCO2
Body Wt

<5
>8
-normal
increased dead space
3. PaCo2- EtCO2 GRADIENT
2-5 mmHg
DEAD SPACE
•V̇P inequalities have different effects on arterial PCO2 (PaCO2 ) than
on arterial PO2 (PaO2 ).
• Blood passing through under ventilated alveoli tends to retain its
CO2 and does not take up enough O2.
•Blood traversing over ventilated alveoli gives off an excessive amount
of CO2 but cannot take up a proportionately increased amount of O2
because of the flatness of the oxygen-hemoglobin (oxy-Hb)
dissociation curve in this region.
• Hence, a lung with uneven V̇P relationships can eliminate CO2 from
the over ventilated alveoli to compensate for the under ventilated
alveoli.
• Thus, with uneven V̇P relationships, PACO2 -to-PaCO2 gradients are
small, and PAO2 -to-PaO2 gradients are usually large.