V/Q and Oxygen

Anuja AbayadeeraPart 1B Anaesthsiology

What is?

Hypoxia

Hypoxaemia

Aim oxygen delivery to tissues

Hypoxia

Hypoxaemia

Aim oxygen delivery to tissues

OXYGEN CASCADE

150

95100

PIO2

PAO2 PaO2

10

40

Tissue PO2

mmHg

OXYGEN CASCADE

PIO2 = 0.21 x760 mmHg= 159 mm Hg

PIO2 = 0.21 x(760 47)= 149 mm Hg

PIO2 = 0.21 x760 mmHg= 159 mm Hg

PIO2 = 0.21 x(760 47)= 149 mm Hg

PAO2

Depends on removal of oxygen by pul.capillary blood Replenishment by alveolar ventilation (PACO2)

Removal = oxygen consumption = 250ml/min

Depends on removal of oxygen by pul.capillary blood Replenishment by alveolar ventilation (PACO2)

Removal = oxygen consumption = 250ml/min

PAO2Inspired gas come to alveoli,PCO2 increase from 0 to 40 mmHg.If O2 leaving alveoli = CO2 diffusing into alveoliPAO2 = PIO2- PACO2 ; 149 - 40 = 109 mmHgVCO2/ VO2 = 200/250 = 0.8 = R (respiratory exchange ratio)

Ideal alveolar gas equationPAO2 = PIO2- PACO2/ R or PAO2 = PIO2 PACO2 x1.2PAO2 = 150-50(40/0.8) =100

Inspired gas come to alveoli,PCO2 increase from 0 to 40 mmHg.If O2 leaving alveoli = CO2 diffusing into alveoliPAO2 = PIO2- PACO2 ; 149 - 40 = 109 mmHgVCO2/ VO2 = 200/250 = 0.8 = R (respiratory exchange ratio)

Ideal alveolar gas equationPAO2 = PIO2- PACO2/ R or PAO2 = PIO2 PACO2 x1.2PAO2 = 150-50(40/0.8) =100

OXYGEN CASCADE

150

95100

PIO2

PAO2PaO2

10

40

Tissue PO2

mmHg

Normal Gas ExchangeAffected by

Diffusion Shunt V/Q scatter

PAO2 PaO2

Affected by

Diffusion Shunt V/Q scatter

PAO2 PaO2

Diffusion

Ficks Law0

Vgas = A x D x (P1 P2)T

D = diffusion coefficient = solmw

Ficks Law0

Vgas = A x D x (P1 P2)T

D = diffusion coefficient = solmw

Diffusion

Perfusion limited

Diffusion limited

DiffusionP1 P2 =60

Exercise

P1-P2 =30

High Altitude

Diffusion

At rest blood spends sec in the capillary At rest, PO2 of blood reaches that of alveolar

gas in 1/3 rd of the time in the capillary

On exercise time reduced to sec Diffusion process challenged by

Exercise Alveolar hypoxia Thick blood gas barrier

At rest blood spends sec in the capillary At rest, PO2 of blood reaches that of alveolar

gas in 1/3 rd of the time in the capillary

On exercise time reduced to sec Diffusion process challenged by

Exercise Alveolar hypoxia Thick blood gas barrier

Shunt

Blood that enters the arterial system withoutgoing through ventilated areas of lung

Physiological Bronchial veins to pulmonary veins Thebesian veins to left ventricle

2%-5% of cardiac output

Blood that enters the arterial system withoutgoing through ventilated areas of lung

Physiological Bronchial veins to pulmonary veins Thebesian veins to left ventricle

2%-5% of cardiac output

V/Q Scatter

If pulmonary blood perfuses ventilated lung regionsnormal gas exchange occurs.

Riley Analysis

Absolute shunt Absolute dead space

Relative shunt Relative dead space

Oxygen carbon dioxide diagram

V/Q PAO2 PACO23.3 132 28

1.0 100 40

0.6 89 42

Regional differences in V/Q giveregional differences in PAO2 &PACO2 in alveoli & similarly inend capillary blood.

Depression of arterialPO2 by V/Q inequality.

High V/Q units cannotchange the desaturatingeffect of low V/Q units.

Note that PaO2 is notequal to PO2. (no Hbinvolved)

CaO2 change by V/Q inequality

The reduction in O2 content in arterial blood caused by alveoli with lowV/Q is more than the increase in O2 content caused by alveoli with highV/Q.

Normal A a gradient

5-10 mmHg when breathing room air30-56 mmHg when breathing 100% oxygen

Due to Physiological shunt Normal V/Q scatter Diffusion

5-10 mmHg when breathing room air30-56 mmHg when breathing 100% oxygen

Due to Physiological shunt Normal V/Q scatter Diffusion

Abnormal gas exchange

Effect hypoxaemia low PaO2CO2 elimination may be affected

Causes of hypoxaemiaPaO2

1. Alveolar O2 partial pressureoverall hypoventilation

2. Alveolar to arterial O2 partial pressuregradient

A aabnormal diffusion, pathological shunt,V/Q mismatch (relative shunt; low V/Qratio)

1. Alveolar O2 partial pressureoverall hypoventilation

2. Alveolar to arterial O2 partial pressuregradient

A aabnormal diffusion, pathological shunt,V/Q mismatch (relative shunt; low V/Qratio)

Effect of overall hypoventilationDrug overdose

Muscle paralysisPCO2 = VCO2

VA

Abnormal A -a

Diffusion no effectUnless thick alv.capillary membrane Diffusion no effectUnless thick alv.capillary membrane

Abnormal A -a

Shunt pathological shuntscardiac A V shuntspulmonary absolute shunt (venous admixture)

Causes Pneumonia Pulmonary oedema Alveolar collapse

Shunt pathological shuntscardiac A V shuntspulmonary absolute shunt (venous admixture)

Causes Pneumonia Pulmonary oedema Alveolar collapse

Absolute shunt

Depression of arterial PO2 byshunted blood

V/Q mismatchCommonest cause

Regional hypoventilation

Partial airway obstruction:asthma, COPD, lowcompliance

V/Q mismatch on O2 & CO2 Low V/QTheoretically cause hypoxaemia andhypercapnia.Actually, have normal or low PaCO2Rising PaCO2 stimulates ventilation aboverequirementWasted or dead space ventilation

Low V/QTheoretically cause hypoxaemia andhypercapnia.Actually, have normal or low PaCO2Rising PaCO2 stimulates ventilation aboverequirementWasted or dead space ventilation

A 61 year old man with myasthenia gravis isadmitted to ETU. He c/o progressive weakness andshortness of breath.ABG on air shows: PaO2 = 59mmHg

PaCO2= 63 mmHgpH= 7.22; HCO3- = 25meq/L

Vital capacity and maximum inspiratory force are low. Aprevious ABG on air is found which is

PaO2 =80 mmHg; PaCO2=40 mmHg; pH= 7.39;HCO3- = 24meq/L

Why is he having hypoxaemia? Is it abnormal gasexchange ?

A 61 year old man with myasthenia gravis isadmitted to ETU. He c/o progressive weakness andshortness of breath.ABG on air shows: PaO2 = 59mmHg

PaCO2= 63 mmHgpH= 7.22; HCO3- = 25meq/L

Vital capacity and maximum inspiratory force are low. Aprevious ABG on air is found which is

PaO2 =80 mmHg; PaCO2=40 mmHg; pH= 7.39;HCO3- = 24meq/L

Why is he having hypoxaemia? Is it abnormal gasexchange ?

28yr old man after chest trauma developsARDS and is ventilated in ITU.ABG on 50% oxygen:PaO2 =45mmHg; PaCO2= 38mmHg;pH= 7.41; SaO2= 80%.Ventilator is adjusted. PEEP of 10 cmH2Oadded.ABG on 50% oxygen after 1 hr;PaO2 = 65mmHg; PaCO2= 36mmHg;pH= 7.42; SaO2= 92%.How do you explain this change?

28yr old man after chest trauma developsARDS and is ventilated in ITU.ABG on 50% oxygen:PaO2 =45mmHg; PaCO2= 38mmHg;pH= 7.41; SaO2= 80%.Ventilator is adjusted. PEEP of 10 cmH2Oadded.ABG on 50% oxygen after 1 hr;PaO2 = 65mmHg; PaCO2= 36mmHg;pH= 7.42; SaO2= 92%.How do you explain this change?

55 yr old male is ventilated in the ITU for leftlower lobe pneumonia.ABG done when lying on the left sidePaO2 = 68mmHg; PaCO2 = 40mmHgpH= 7.43; SaO2 = 92%.2hrs later when lying on the right sidePaO2 = 110mmHg; PaCO2= 40mmHgpH= 7.42; SaO2=99%.How did the oxygenation improve?

55 yr old male is ventilated in the ITU for leftlower lobe pneumonia.ABG done when lying on the left sidePaO2 = 68mmHg; PaCO2 = 40mmHgpH= 7.43; SaO2 = 92%.2hrs later when lying on the right sidePaO2 = 110mmHg; PaCO2= 40mmHgpH= 7.42; SaO2=99%.How did the oxygenation improve?

22yr old patient is admitted with bronchialasthma.ABG on air:PaO2= 60mmHg; PaCO2= 35mmHg; pH= 7.35;SaO2= 90%

Treated with bronchodilators and oxygen isincreased to 28%; ABG 1 hr laterPaO2= 90mmHg; PaCO2=38mmHg; pH=7.37;SaO2= 96%.Was the hypoxaemia due to hypoventilation, shuntor V/Q mismatch?

22yr old patient is admitted with bronchialasthma.ABG on air:PaO2= 60mmHg; PaCO2= 35mmHg; pH= 7.35;SaO2= 90%

Treated with bronchodilators and oxygen isincreased to 28%; ABG 1 hr laterPaO2= 90mmHg; PaCO2=38mmHg; pH=7.37;SaO2= 96%.Was the hypoxaemia due to hypoventilation, shuntor V/Q mismatch?

Low PaO2Cause Mechanism P(A-a)O2 Response

to O2Hypoventilation VA PACO2 Normal good

Shunt Venous bloodmixing with arterialblood

(venous admixture)

Increased poorShunt Venous bloodmixing with arterialblood

(venous admixture)

Increased poor

Ventilation-perfusionmismatch

Underoxygenatedblood mixing witharterial blood(venous admixture)

Increased good

Causes of hypoxaemiaPaO2

1. Alveolar O2 partial pressure

2. Alveolar to arterial O2 partial pressuregradient

A - a

1. Alveolar O2 partial pressure

2. Alveolar to arterial O2 partial pressuregradient

A - a

Measurement of shunt

A a gradientNormal 5 10 mmHg breathing 21% O2

30 56 mmHg breathing 100% O2A ideal alveolar gas equationa blood gases Normal value increase with age. Varies with FiO2- limit value

A a gradientNormal 5 10 mmHg breathing 21% O2

30 56 mmHg breathing 100% O2A ideal alveolar gas equationa blood gases Normal value increase with age. Varies with FiO2- limit value

Measurement of shunt

PaO2 / PAO2More stable with FiO2 changesLower normal limit 0.75 Useful to follow patients lung function

when FiO2 is changed Used to predict FiO2 required to achieve a

desired PaO2

PaO2 / PAO2More stable with FiO2 changesLower normal limit 0.75 Useful to follow patients lung function

when FiO2 is changed Used to predict FiO2 required to achieve a

desired PaO2

Measurement of shunt

PaO2 /FiO2Oxygenation ratio (P/F ratio)Affected by PaCO2Least accurate indicator of shunt

None of these consider CvO2(mixed venous oxygen content)Misleading in patients with cardiovascular instability

PaO2 /FiO2Oxygenation ratio (P/F ratio)Affected by PaCO2Least accurate indicator of shunt

None of these consider CvO2(mixed venous oxygen content)Misleading in patients with cardiovascular instability

Shunt equationCalculation (shunt fraction)QT x CaO2 = QS x CVO2 + (QT Qs) x CcO2QS = CcO2 - CaO2= oxygen lost by mixing with QsQT CcO2 - CVO2 = total amount of oxygen uptake

Most reliable method for oxygen transfer efficiency

a - arterialC end capillary = oxygen delivery equation ; ideal alveolar equationV mixed venous

Calculation (shunt fraction)QT x CaO2 = QS x CVO2 + (QT Qs) x CcO2QS = CcO2 - CaO2= oxygen lost by mixing with QsQT CcO2 - CVO2 = total amount of oxygen uptake

Most reliable method for oxygen transfer efficiency

a - arterialC end capillary = oxygen delivery equation ; ideal alveolar equationV mixed venous

Shunt

Clinical significanceShunt fraction percentage Clinical significance

Points to remember

V/Q abnormalities are more likely to createhypoxaemia than hypercapnia

High V/Q regions cannot compensate forhypoxaemic effects of low V/Q regions.

V/Q abnormalities are more likely to createhypoxaemia than hypercapnia

High V/Q regions cannot compensate forhypoxaemic effects of low V/Q regions.