Arterial Blood Gas for HO

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Arterial Blood Gasfor House Officers In Anaesthesia and Intensive Care

Department of Anaesthesiology and Intensive Care

Hospital


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Content

What is ABG?

Normal values for ABGDefinitions

Acid-base balanceInterpretation of ABG

Practical considerations


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What is ABG?


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An ABG is a blood test that

measures the concentration ofoxygen, carbon dioxide andbicarbonate in the blood


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ABG readings

1. pH

2. Respiratory function O2, CO2, SaO2

3. Metabolic measures HCO3, base excess

4. Electrolytes and metabolytes


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Normal Values for ABG


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Normal values

pH

PaO2

PaCO2

HCO3

Base Excess

7.35 - 7.45

>75 mmHg

35 45 mmHg

22 26 mmol/L

-2 - +2

To convert kPato mmHgmultiply by 7.5

Many modern gas machines also measure

K+

,Na+

,Cl-

,SaO2 ,Hb ,COHb ,MetHb ,Lactate


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Definitions


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Definitions

Hypoxaemia - a PaO2 of less than than 60mmHg

Acidaemia - a pH of less than 7.35

Alkalaemia - a pH of greater than 7.45 Acidosis

Respiratory : PaCO2 of greater than 50 mmHg

Metabolic : HCO3 of less than 22 mmol/L

AlkalosisRespiratory : PaCO2 of less than 35 mmHgMetabolic : HCO3 of greater than 26 mmol/L


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Base excess

Observed Buffer Base minus Normal Buffer Base.

It represents the amount of acid or alkali that mustbe added, in vitro, to a liter of fully oxygenated

blood at PCO2 of 40 mmHg and 37oC, to achieve a

normal pH (7.40)Std Bicarbonate and Base Excess are only truly

valid within the closed in-vitro system


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Acid Base Balance


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Acid Base Balance

CO2 HCO3

pH


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Acid Base Balance

CO2 + H2O H+ + HCO3-

Excreted by Lungs Excreted by Kidneys


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pH

Less than 6.8 More than 7.8


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Lungs

Cellular metabolism produces CO2

Concentration of carbonic acid alter bloodpH

pH changes results in lungs altering rateand depth of ventilation


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Kidneys

Maintain blood pH by altering excretion of

HCO3When pH kidneys retain HCO3

When pH kidneys excrete HCO3


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Relationship between PaCO2, HCO3, pH

Henderson-Hasselbach equationpH = pKa + log [HCO3

-]

[H2CO3]

pH = 6.1 + log [HCO3-]

0.03 X PCO2

10 mm Hg PaCO2 0.08 unit pH

10 mmol [HCO3-] 0.15 unit pH


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Interpretation of ABG


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Step 1

Is there hypoxaemia?

Is there significant lung injury?

A a GradientThe gradient between alveolar PAO2 and arterial PaO2 in aperson with healthy lungs is ~ 75 mmHg

i.e. we would expect a person on an FiO2 of 60% tohave a PaO2 ~ 375 mmHg (450-375=75)

The higher the gradient, the worse the lung injury


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Step 2

Assess the pH for acidemia or alkalemia


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Step 3

Is it a respiratory problem?

Alkalosis Acidosis


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Step 4

Is it a metabolic problem?

Acidosis Alkalosis


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Step 5 Compensation

Respiratory compensation is quick

Metabolic compensation is slow

Compensation is not usually complete Patients never over compensate


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Simple Acid-Base Disturbances

Acid-BaseDisturbance

PrimaryAbnormality

SecondaryResponse

Expected Degree of

Compensatory Response

RespiratoryAcidosis

PaCO2 [HCO3-] [HCO3

-] = 0.35 X PaCO2

RespiratoryAlkalosis

PaCO2 [HCO3-] [HCO3

-] = 0.50 X PaCO2

MetabolicAcidosis

[HCO3-

] PaCO2 PaCO2 = (1.5 X [HCO3-

]) + 8

MetabolicAlkalosis

[HCO3-] PaCO2 PaCO2 = (0.9 X [HCO3

-]) + 9


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Causes of Respiratory Acidosis

Respiratory depression Chemical depression

Physical depression

Inadequate chest expansion Skeletal problems

Respiratory muscle weakness External conditions

Airway obstruction

Reduced alveolar-capillary diffusion


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Causes of Respiratory Alkalosis

Anxiety, fear, or improper settings onmechanical ventilators

Direct stimulation of central respiratory centerfrom fever, metabolic acidosis, central nervoussystem lesions, and drugs


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Scenario 1

Arterial blood gas analysis reveals:

FiO2 0.4 (40%)PaO2 52 mmHgpH 7.25PaCO2 67 mmHgHCO3 35 mmol/L

65 year old male with known COPD presents in A&E complaining ofincreased breathlessness. The paramedics have put him on a venturimask to give an FI02 of 40% due to his breathlessness and initial lowsaturations.

Significant findings on your examination is a drowsy patient with a resp

rate of 8, SpO2 of 85% and wide-spread coarse crackles

HypoxiaRespiratory acidosis

with chronic renalcompensationInfective exacerbationof COPD

?Hypoxic drive ?tired


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Scenario 2


FiO2 0.3 (30%)PaO2 160 mmHgpH 7.15

PaCO2 19 mmHgHCO3 10Na 135K 5.4Cl 106

Anion Gap = ?

18 year old male with diabetes has been suffering from diarrhoea andvomiting for 48 hours and because he has been unable to eat he hasnot taken his insulin.

Significant findings on your examination are a resp rate of 40, heartrate of120, BP 95/50, Blood glucose 30mmol/l

Metabolic acidosis withrespiratory compensationDKA

24


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Anion Gap (use if metabolic acidosis)

anion gap DKA

Renal failure

Lactic acidosis

poisoning eg.salicylate, ethyleneglycol

Normal anion gap Chronic diarrhoea,

ileostomy

Addisons disease

Pancreatic fistula Renal tubular acidosis

(Na+ + K+) (HCO3- + Cl-)Normal range 10 18mmol/l

These lists are not exhaustive


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Scenario 3


FiO2 0.21 (21%)PaO2 110 mmHgpH 7.53

PaCO2 23 mmHgHCO3 25.0

17 year old male has taken his fathers BMW (without asking) toimpress his girlfriend and had an accident with a bus where the BMWcame off much the worse.

There is little abnormal to find on examination apart from bruising, aresp rate of 24, a pulse of 110 and a BP of 120/85

Respiratory alkalosisAnxiety


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Scenario 4


FiO2 0.4 (40%)PaO2 61 mmHgpH 7.17

PaCO2 28 mmHgHCO3- 12 mmol l-1

A 75 year old female is on the surgical ward 2 days after a laparotomy for aperforated sigmoid colon secondary to diverticular disease. She has becomehypotensive over the last 6 hours. A nurse has started 40% O2On examination vital signs are: RR 35 min-1, SpO2 92%, HR 120 min-1,warm peripheries, BP 70/40 mmHg, Urine output 50 ml in the last 6 hours

HypoxiaMetabolic acidosis withrespiratory compensationShock secondary to Sepsis


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Scenario 5

A 75 year old man presents to the emergency department after a

witnessed out-of-hospital VF cardiac arrest.The paramedics arrived after 5 minutes, during which CPR had notbeen attempted. The paramedics had successfully restoredspontaneous circulation after 3 shocks but have been unable tointubate him. He is breathing spontaneously with a re breathing mask.

On arrival: comatose (GCS 3)Resp rate 8HR 120 min-1BP 150/95 mmHg.


FiO2 0.85 (85%)PaO2 75 mmHgpH 7.10PaCO2 52 mmHgHCO3 14 mmol/LBE - 10

Mixed respiratory andmetabolic acidosisHypoperfusion and

respiratory failure


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Practical Considerations


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Practical points

1. Delayed analysis Continued O2 use and CO2 production in syringe Invalid after 15 min unless iced

Iced sample can keep 1-2h

May result in sedimentation of rbc roll syringe inhand before test

2. Heparin

Necessary to prevent clotting Dilute blood unless > 50% of syringe volume filled

with blood

Heparin acidic


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3. Air bubbles

PaO2 20kPa, PaCO2 0 in air

Expel air and cap syringe immediately

4. WBC count

O2 consumed by white cells and platelets

5. Pain on sampling

Hyperventilation and breath holding due to pain

of arterial puncture can affect results

6. If the ABG does not make sense, check patients

clinically.


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Any Questions?


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Summary

Identify the hypoxic patient Identify an acidosis or alkalosis

Recognise when compensation is taking place Formulate an initial treatment plan for some

common scenarios Understand the role Arterial Blood Gases play

in patient management

You should now be able to:


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Thank you for your kind

attention

Arterial Blood Gas for HO

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