KEY POINTS KEY POINTS ALVEOLAR VENTILATION–(V A ) ALVEOLAR PERFUSION- PULMONARY CIRCULATION (Q) VENTILATION – PERFUSION RATIO (V A /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

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 Q

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 Intra pleural pr

Ventilation pattern - V A 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 - V A

V = RR x V T Minute Ventilation V = RR x V T ALVEOLAR VENTILATION-V A Volume of the inspired gas participating in alveolar gas exchange /minute is called ALVEOLAR VENTILATION-V A V A = RR x V T -V D V A = RR x V T -V D Not all inspired gas participating in alveolar gas exchange DEAD SPACE – V D 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 V D = 2ml/kg ; 1ml /pound Dead space ratio V D / V T = 33% V D = PACO2 – PECO2 V T PACO2

Ventilation Perfusion ratio V A /Q Ventilation & Perfusion both are distributed more towards lower zone. Ventilation[V A ] less increased t0wards l0wer zone than Perfusion[Q] Perfusion more increased towards Lower zone than Ventilation Ventilation Perfusion ratio V A /Q: Less towards lower zone V A /Q VAVA Q

Ventilation Perfusion ratio V A /Q Ventilation & Perfusion both are distributed more towards lower zone. Ventilation[V A ] less increased t0wards l0wer zone than Perfusion[Q] Perfusion more increased towards Lower zone than Ventilation Ventilation Perfusion ratio V A /Q: Less towards lower zone V A /Q VAVA Q

VENTILATION PERFUSION RATIO Wasted ventilation V=normal Q=0V/Q=∞ DEAD SPACE Wasted Perfusion V=o Q= normalV/Q=0SHUNT Normal V&QV/Q=1 IDEAL ALVEOLI VVV Q Q Q

V/Q = 0.8 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 SHUNT V/Q = 0 means,no ventilation-called SHUNT V/Q = ∞ DEAD SPACE V/Q = ∞ means,no perfusion – called DEAD SPACE Ventilation Perfusion ratio VA/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 Qs = CcO2-CaO2 Qt CcO2-CvO2 Qt CcO2-CvO2 Normal shunt- Physiologic shunt < 5% Q V V/Q<1

SHUNTS have different effects on arterial PCO 2 (PaCO 2 ) than on arterial PO 2 (PaO 2 ). Blood passing through under ventilated alveoli tends to retain its CO 2 and does not take up enough O 2. Blood traversing over ventilated alveoli gives off an excessive amount of CO 2, but cannot take up increased amount of O 2 because of the shape of the oxygen-hemoglobin (oxy-Hb) dissociation curve. Hence, a lung with uneven V̇P relationships can eliminate CO 2 from the over ventilated alveoli to compensate for the under ventilated alveoli. Thus, with Shunt, PACO 2 -to-PaCO 2 gradients are small, and PAO 2 -to- PaO 2 gradients are usually large.

PAO 2 is directly related to FIO 2 in normal patients. PAO 2 and FIO 2 also correspond to PaO 2 when there is little to no shunt. With no S/T, a linear increase in FIO 2 results in a linear increase in PaO 2. As the shunt is increased, the S/T lines relating FIO 2 to PaO 2 become progressively flatter. With a shunt of 50% of QT, an increase in FIO 2 results in almost no increase in PaO 2. The solution to the problem of hypoxemia secondary to a large shunt is not increasing the FIO 2, but rather causing a reduction in the shunt (fiberoptic bronchoscopy, PEEP, patient positioning, antibiotics, suctioning, diuretics).

PAO 2 is directly related to FIO 2 in normal patients. PAO 2 and FIO 2 also correspond to PaO 2 when there is little to no shunt. With no S/T, a linear increase in FIO 2 results in a linear increase in PaO 2. As the shunt is increased, the S/T lines relating FIO 2 to PaO 2 become progressively flatter. With a shunt of 50% of QT, an increase in FIO 2 results in almost no increase in PaO 2. The solution to the problem of hypoxemia secondary to a large shunt is not increasing the FIO 2, but rather causing a reduction in the shunt (fiberoptic bronchoscopy, PEEP, patient positioning, antibiotics, suctioning, diuretics).

SHUNT

VIRTUAL SHUNT CURVES FiO2 PaO2

DEAD SPACE Not all inspired gas participating in alveolar gas exchange DEAD SPACE – V D 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

Means – Wasted Ventilation Vd/Vt Dead Space estimated as ratio Vd/Vt Dead space expressed as a fraction of total tidal volume Vd/Vt Vd = PACO2-PECO2 Vd = PACO2-PECO2 Vt PACO2 Vt PACO2 Normal dead space ratio < 33% Q V V/Q= ∞

1. SHUNT RATIO Qs = CcO2-CaO2 Qt CcO2-CvO2 2. 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 SaO x PaO2 CcO2-Pulmonary end capillary O2 content CaO2-Arterial O2 content CvO2-Mixed venous O2 content

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 acceptable – poor < 0.20 –very poor

QUANTIFICATION - SHUNT 5. ARTERIAL O2 INSPIRED O2 RATIO : PaO2/FiO2 Normally >500mmHg Acceptable P00r Terminal <100 LI Score: <300ALI, <200ARDS SAPS 2

QUANTIFICATION - SHUNT 6. ISO SHUNT TABLE 7. VIRTUAL SHUNT DIAGRAGME FiO2 PaO2

QUANTIFICATION – DEAD SPACE Vd = PACO2-PECO2 1. Vd = PACO2-PECO2 Vt PACO2 Vt PACO2 2. MV x PaCO2 Body Wt <5 -normal >8 increased dead space 3. PaCo2- EtCO2 GRADIENT 2-5 mmHg

DEAD SPACE

V̇P inequalities have different effects on arterial PCO 2 (PaCO 2 ) than on arterial PO 2 (PaO 2 ). Blood passing through under ventilated alveoli tends to retain its CO 2 and does not take up enough O 2. Blood traversing over ventilated alveoli gives off an excessive amount of CO 2 but cannot take up a proportionately increased amount of O 2 because of the flatness of the oxygen-hemoglobin (oxy-Hb) dissociation curve in this region. Hence, a lung with uneven V̇P relationships can eliminate CO 2 from the over ventilated alveoli to compensate for the under ventilated alveoli. Thus, with uneven V̇P relationships, PACO 2 -to-PaCO 2 gradients are small, and PAO 2 -to-PaO 2 gradients are usually large.