ARTERIAL BLOOD GAS ANALYSIS IN RESPIRATORY DISORDERS

Dr.R.SelvakumarProfessor of AnaesthesiologyK.A.P.Viswanatham Govt Medical College,Trichy-Tamilnadu-India

A knowledge about basics and patient’s clinical historyis important in reading ABG.. just like ECG….

WHAT DO YOU MEAN BY PARTIAL PRESSURE?

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IN A MIXTURE OF GASES,THE PRESSURE EXERTED BY EACH GASIS THE SAME AS THAT WHICH IT WOULD EXERT IF IT ALONEOCCUPIED THE CONTAINER

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5 PSI 3 PSI 8 PSI

DALTON’S LAW OF PARTIAL PRESSURE

THE PROPORTION OF THE PRESSURE EXERTED BY A GASIN THE TOTAL PRESSURE IS EQUATED WITH THE VOLUMEIT OCCUPIES

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PRESSURE OF @ GAS = 5 PSIPRESSURE OF $ GAS = 3 PSITOTAL PRESSURE = 8 PSI

What is the % contribution of pressure of @ gasin the total pressure?5/8 x 100 = 62.5%

So, gas @ will occupy62.5% of the total volume

Total atmospheric pressure -760 mm of Hg

Out of this 760, 78% is contributed byNitrogen and 21 % is contributedby Oxygen

21/100 x 760 = 160

Partial pressure of Oxygen in the atmosphere

What do you mean by tension of a gas?

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The gas which gets dissolved in the water also exerts a pressure.That, we call it as “TENSION”

SAMPLE PaO2 in mm of Hg

PaCO2 in mm of Hg

ARTERIAL 80 -100 35-45VENOUS 37-42 42-50

What do you mean by FIO2?

• The % of O2 in the inspired gas mixture

• In room air , it is 21% or 0.21

How diffusion occurs in alveoli?

Gases crossing theAlveolar-capillaryBasement membranedepending onPartial pressure gradient

Do all alveoli participate in gaseous exchange?

What is the normal O2 content of the blood?

(Hb in gm% X 1.34 X SaO2) + ( 0.003 X paO2) = CaO2 = 20.1 ml

Oxygen in the blood combined with Hb + Oxygen dissolved in plasma as physical solution

How to increase the oxygen content of the blood?

(Hb in gm% X 1.34 X SaO2) + ( 0.003 X paO2) = CaO2 = 20.1ml

(15 X 1.34 X 99) + ( 0.003 X 100 ) = 20.1 ml%

(4 X 1.34 X 99) + (0.003 x 100 ) = 5.6 ml%

(15 X 1.34 x 99) + (0.003 x 600 ) = 21.6 ml%

Which is better?To give 100% O2 or increase the available Hb..?

What is the limitation of pulseoximetry?

With that background knowledge…

Why to measure the ABG..?

What can you infer from it..?

ABG reflects

ALVEOLAR VENTILATION

OXYGENATION

ACID-BASE DISORDERS

ELECTROLYTE STATUS

ALVEOLAR VENTILATION

• CO2 level in the blood depends upon the production and excretion.• Excretion depends upon alveolar ventilation

• Alveolar ventilation depends upon many factorslike central respiratory drive , neuronal pathways,Anterior horn cells, respiratory muscle condition,Respiratory mechanics, intactness of airway and integrity of the pulmonary gas exchange system

If ventilation decreases, then CO2 level increases…..

OXYGENATION

ABG tells you about the status of oxygenation in the form of

paO2, SaO2, Oxygen Content, a/A ratio, (A-a)DO2

We can know the joint status of cardio-pulmonary system

ACID-BASE DISORDER

We can measure the pH, HCO3 level and Base excess/deficit

We can also know about the adequacy of compensatory mechanisms operating in the body.

COMPENSATORY MECHANISMS IN ACID-BASE DISORDERDUE TO RESPIRATORY PROBLEMS

If CO2 accumulates, due to Hypoventilation

CO2 + H2O = H2CO3 = H+ + HCO3

1 molecule of H+ and 1 molecule of HCO3is added to the circulation

But acidosis results….. How?

The impact of H+ increase is more, since the level of H+ in body is in nanomoles and HCO3 is in mmoles

How the body compensates for the addition of excess H+?

It retains HCO3 through kidney.

We call it as metabolic compensation

COMPENSATION RULE:

HCO3 increases by 1 meq/L for every 10 mm rise of CO2 in acute situations.

It increases by 3.5 meq/L for every 10 mm rise in chronic situations.

Memorize plz

COMPENSATION RULE:

If CO2 is washed out due to hyperventilation,HCO3 falls by 2 meq/L for every 10 mm fall in paCO2 in acute situations.

In chronic states, HCO3 falls by 5 meq/L for every10 mm fall of paCO2.

Memorize plz

How to read the ABG..?

PARAMETER NORMAL RANGEpaO2 80 -100 mm of HgpaCO2 35 -45 mm of HGHCO3 22 – 26 meq/LStd HCO3 21 – 27 meq/LBase Excess -2 to + 2 mmol/LCaO2 16 – 20 ml%a/A ratio 0.8(A-a) DO2 10 -20 mm of HgFIO2 0.21Hb 15g%Temp 37.2 C

Examples….

A person choked with a coin in his throat. He is in acute distress.. His ABG reads…

pH - 7.24

paO2 - 42

paCO2 - 72

HCO3 - 27

acidosis

Hypoxemia

FIO2 - 0.21

Acute retention

Slight rise

RESPIRATORY ACIDOSIS - ACUTE

Example -2A COPD patient coming for hernia repair

pH - 7.34paO2 - 68

paCO2- 72

HCO3 - 34

FIO2 - 0.21

BE - 12

Normal / acidosisMild hypoxemia

SaO2 - 95 %

Very high

High level.

High level

SaO2 is high for a low paO2

CHRONIC RESPIRATORY ACIDOSIS WITH METABOLIC ALKALOSIS

pH 7.34paO2 68paCO2 72HCO3 34BE 12SaO2 95FIO2 0.21

• Why it is chronic?

• paO2 is 68, but SaO2 is 95%. How?

• Can it be chronic metabolic alkalosis with compensatory respiratory acidosis?

RULES YOU MUST NEVER FORGET….

pH drifts towards primary acid-base disorder

Compensation never overshoots….

pH 7.34paO2 68paCO2 72HCO3 34BE 12SaO2 95FIO2 0.21

Comment about oxygenation in the first ABG…

pH 7.24paO2 42paCO2 72HCO3 27SaO2 55%FIO2 0.21(A-a)DO2 75a/A ratio 0.38

Severe Hypoxia

Saturation very low

Normal -110 -90 = 20 So ,it is highNormal 90/110 = 0.8Very low.

A patient with head injury with neurogenic hyperventilation

pH 7.51paO2 80paCO2 24HCO3 20SaO2 94FIO2 0.21BE -4

Alkalosis

Decreased

Decreased. ↑ Kidney excretion

Acute Respiratory alkalosis withCompensatory metabolic acidosis

Remember… Acidosis results either due to theIncrease of H+ ions or depletion of HCO3 ions

A patient living in high altitude region

pH 7.48

paO2 72

paCO2 24

HCO3 16

SaO2 96

FIO2 0.21

Mild alkalosis

Mild hypoxemia

Decreased level

Very low level. Kidney excreted much HCO3

For a paO2 of 72, saturation is high

Chronic respiratory alkalosiswith compensatory metabolic acidosis

Some more complicated ABGs…

pH 7.32

paO2 68

paCO2 69

HCO3 34

FIO2 0.21

pH 7.26

paO2 42

paCO2 78

HCO3 26

FIO2 0.40

COPD DEVELOPING LOBAR PNEUMONIA

More acidotic

Further lowered

Looks like normal

Chronic resp.acidosiswith compensatory met.alkalosis

Acute on Chronic resp.acidosiswith compensatory met.alkalosis

Increases further

Home work…. Just try it out

pH 7.4

paO2 98

paCO2 37

HCO3 24

FIO2 1

BE 0.5

SaO2 100

(A-a)DO2 467

Patient on ventilator

Can we wean him?

pH 7.41

paO2 147

paCO2 36

HCO3 23

BE -1

FIO2 0.21

SaO2 99%

Comment on this…

A patient with duodenal ulcer perforation given I.VMorphine for pain relief….

pH 7.14

paO2 80

paCO2 50

HCO3 20

BE -5.2

FIO2 0.5

(A-a)DO2 198

SaO2 98%

Severe acidosis

Moderate hypoxia

Slightly elevated

Slightly lowered

Acute combined metabolicand respiratory acidosis

ARDS patient onventilator

pH 7.37

paO2 105

paCO2 34

HCO3 22

BE -1.3

FIO2 1

(A-a)DO2 438

2 Hours after

the introduction of PEEP 10 cm of H2O

pH 7.37

paO2 94

paCO2 32

HCO3 23.5

BE -0.3

FIO2 0.6

(A-a)DO2 239

Summary

Always remember the Basic physiology

Go through the clinical history and then try to read

pH sways to the primary acid-base disorder side

Compensatory phenomenon never overshoots

If you understand and remember the basic…

If you are methodical….

Reading ABG is a child’s play

Concluding……

dr.r.selvakumarprofessor of anaesthesiologyk.a.p.viswanatham govt medical collegetrichy

thank you