KEY POINTSKEY POINTSALVEOLAR VENTILATION–(V

A)ALVEOLAR PERFUSION-

PULMONARY 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 Due to gravitational influence the lower – dependent areas receive lower – dependent areas receive more blood more blood

Upper zone – nondependent areas Upper zone – nondependent areas are less per fusedare less per fused

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

Pulmonary circulation – Alveolar Perfusion Pulmonary circulation – Alveolar Perfusion QQ

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

VentilationVentilation

Due to gravitational influence on intra plural pr [decreased 1cm/H2O per 3cm decrease in lung height] lower zones better ventilated

-6

-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

•Upper zone:less pleural pressure, distended more & hence less compliant

•Lower zone:more pleural pressure,less distended,& hence more compliant

Ventilation pattern - VA

Minute Ventilation V = RR x VV = RR x VTT

Volume of the inspired gas participating in alveolar gas exchange /minute is called ALVEOLAR VENTILATION-VALVEOLAR VENTILATION-VAA

VVA A = RR x V= RR x VTT-V-VDD

Not all inspired gas participating in alveolar gas exchange DEAD SPACE – VDEAD SPACE – VDD

Some gas remains in the non respiratory airways ANATOMIC DEAD SPACEANATOMIC DEAD SPACE

Some gas in the non per fused /low per fused alveoli PHYSIOLOGIC DEAD SPACEPHYSIOLOGIC 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 = PACO2 – PECO2 VT 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 towardsLower zone than Ventilation

•Ventilation Perfusion ratio VA/Q:Less towards lower zone

VA/Q

VA

Q

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 towardsLower zone than Ventilation

•Ventilation Perfusion ratio VA/Q:Less towards lower zone

VA/Q

VA

Q

VENTILATION PERFUSION RATIOVENTILATION PERFUSION RATIO

Wasted ventilationV=normalQ=0V/Q=∞V/Q=∞DEAD DEAD SPACESPACE

Wasted PerfusionV=oQ= normalV/Q=0V/Q=0SHUNTSHUNT

NormalV&QV/Q=1V/Q=1IDEAL IDEAL ALVEOLIALVEOLI

V VV

Q Q Q

The overall V/Q = 0.8 V/Q = 0.8 [ ven=4lpm, per=5lpm]Ranges between 0.3 and 3.0Upper zone –nondependent area has higher ≥ 1Lowe zone – dependent area has lower ≤ 1VP ratio indicates overall respiratory functional

status of lung

V/Q = 0 V/Q = 0 means ,no ventilation-called SHUNTSHUNT

V/Q = ∞ V/Q = ∞ means ,no perfusion – called DEAD DEAD SPACESPACE

Ventilation Perfusion ratio Ventilation Perfusion ratio VVAA/Q/Q

Means – Wasted perfusionShunt – 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 Qs = = CcO2-CaO2 CcO2-CaO2

Qt CcO2-CvO2Qt CcO2-CvO2Normal shunt- Physiologic shunt < 5%

Q

V

V/Q<1

•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 VKP 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

VIRTUAL SHUNT CURVESVIRTUAL SHUNT CURVES

FiOFiO22

PaO

PaO

22

DEAD SPACE DEAD SPACE Not all inspired gas

participating in alveolar gas exchange DEAD SPACE – VDEAD SPACE – VDD

Some gas remains in the non respiratory airways ANATOMIC DEAD ANATOMIC DEAD SPACESPACE

Some gas in the non per fused /low per fused alveoli PHYSIOLOGIC DEAD PHYSIOLOGIC DEAD SPACESPACE

Means – Wasted Ventilation

Dead Space estimated as ratio Vd/Vt Vd/Vt

Dead space expressed as a fraction of total tidal volume Vd/Vt

Vd Vd = = PACO2-PECO2 PACO2-PECO2

Vt PACO2Vt PACO2

Normal dead space ratio < 33%

Q

V

V/Q= ∞

1. SHUNT RATIO Qs = CcO2-CaO2 Qt CcO2-CvO22. MODIFIED = CcO2-CaO2 [CcO2-CaO2]+4

• 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

•CcO2-Pulmonary end capillary O2 content•CaO2-Arterial O2 content•CvO2-Mixed venous O2 content

QUANTIFICATION - SHUNTQUANTIFICATION - 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 - SHUNTQUANTIFICATION - SHUNT5. ARTERIAL O2 INSPIRED O2 RATIO : PaO2/FiO2

Normally >500mmHg Acceptable 250-500 P00r 100-250 Terminal <100

LI Score: <300ALI, <200ARDS SAPS 2

QUANTIFICATION - SHUNTQUANTIFICATION - SHUNT

6. ISO SHUNT TABLE 7. VIRTUAL SHUNT DIAGRAGME

FiO2FiO2

PaO

2P

aO

2

QUANTIFICATION – DEAD SPACEQUANTIFICATION – DEAD SPACE

1. Vd Vd = = PACO2-PECO2 PACO2-PECO2

Vt PACO2Vt PACO2

2. MV x PaCO2 Body Wt

<5 -normal >8 increased dead space

3. PaCo2- EtCO2 GRADIENT 2-5 mmHg

DEAD SPACE

•VKP 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 VKP relationships can eliminate CO2 from the over ventilated alveoli to compensate for the under ventilated alveoli.• Thus, with uneven VKP relationships, PACO2 -to-PaCO2 gradients are small, and PAO2 -to-PaO2 gradients are usually large.