Post on 05-Apr-2018
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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
Arterial blood gas analysis reveals:
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
Arterial blood gas analysis reveals:
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
Arterial blood gas analysis reveals:
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.
Arterial blood gas analysis reveals:
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