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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.