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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.