Arterial Blood Gas Determination and its relevance to the Anaesthetist. - Sample Collection - Consideration/Complication - How to interpret ABG's - Compensation - Anion Gaps/ Base Excess -Clinical Presentation - Anaesthetic consideration

1. Arterial Blood Gas
Dr John Afam - Osemene
Registrar, Department of Anaesthesia
Ahmadu Bello University Teaching Hospital
Zaria

2. Introduction
An arterial blood gas sampling is used to analyze gas exchange and acid base
status
● To document respiratory failure and assess its severity.
● To monitor patients on ventilators and assist in weaning
● To assess acid base imbalance in critical illness
● To assess response to therapeutic interventions and mechanical ventilation
● To assess pre-op patients.

3. Collection Procedure
Radial Artery
● Superficial
● Collateral Circulation
● Easy to Palpate/Locate
● Not close to large veins
Others : Brachial, Femoral, Dorsalis Pedis, Posterior Tibial
Allen’s Test - Done to ensure collateral supply to the hand

4. Collection Procedure
● Prepare Materials (Aseptic)
● Counsel and inform patient on the procedure.
● Allen’s Test
● Proper Positioning ?hyperextend wrist
● Clean Site
● Anaesthetic
● Ensure Heparinized Syringe (0.05mls of Sodium Hep/ml)
● Insert at 45^ (Do not move needle around deep)
● Collect sample, expel air, place in Ice
● Apply pressure to area

6. Points to Note
● Complications
■ Pain
■ Infection
■ Arteriospasm, Occlusion
■ Hemorrhage
■ Hematoma
● Temperature :
● Air Bubbles : Take sample slowly, Draw in glass syringe
● FiO2
● Infection, Negative Allen’s
N.B Steady State

7. Acid/Base balance
Needed to maintain normal pH (Homeostasis)
Extreme pH - Incompatible with life (Denaturation of proteins)
Maintaining the PH
● Bufffer systems
○ Bicarbonate Buffer
■ Bicarbonate is the most important buffer in the extracellular fluid compartment
○ Heamoglobin
○ Phosphates etc
● Respiratory Compensation
○ Hours
● Renal Compensation
○ Days

11. Components
● pH : 7.35-7.45
○ Measure of how acidic or alkaline the Blood is
● PaCO2 : 35 - 45mmhg
○ Partial Pressure of CO2 in the blood.
○ Acidic component
● HCO3 : 22- 26 mmol/l
○ Concentration of Bicarbornate
○ Basic component
● PaO2 : 80 - 100mmhg (In room air)
○ Partial pressure of O2 in arterial blood.
○ Measure of Oxygenation
○ P/F >400
● O2 Saturation

12. Interpreting ABG
Is there alkalemia or acidemia present?
pH < 7.35 acidemia
pH > 7.45 alkalemia
This is usually the primary disorder
Remember: an acidosis or alkalosis may be present even if the pH is in the normal
range (7.35 – 7.45)
You will need to check the PaCO2, HCO3- and anion gap

13. Respiratory or Metabolic
What is the PaCO2?
● PaCO2 : 35 - 45mmhg
○ > 45mmhg - Acidic
○ < 35 mmhg - Alkaline
What is the Bicarbonate?
● HCO3 : 22- 26 mmol/l
○ < 22 mmol/l - Acidic
○ >26 mmol/l - Alkalaline

14. Respiratory or Metabolic
Is the disturbance respiratory or metabolic?
What is the relationship between the direction of change in the pH and the
direction of change in the PaCO2?
In primary respiratory disorders, the pH and PaCO2 change in opposite directions;
in metabolic disorders the pH and PaCO2 change in the same direction.

15. ROME
Simple Mnemonic to identify the primary cause of acid base imbalance
Respiratory Opposite,
● pH PaCO2 = Respiratory Alkalosis
● pH PaCO2 = Respiratory Acidosis
Metabolic Equal
● pH HCO3 = Metabolic Alkalosis
● pH HCO3 = Metabolic Acidosis

16. What is the Primary Disorder?
Ngozi
● PH- 7.52
● pCO2 - 28
● HCO3 - 22
Umar
● pH- 7.23
● pCO2 - 40
● HCO3 - 16
Femi
● PH- 7.34
● pCO2 - 55
● HCO3 - 29
King
● PH- 7.45
● pCO2 - 52
● HCO3 - 35

17. Examples
Ngozi | Respiratory Alkalosis
● PH- 7.52
● pCO2 - 28
● HCO3 - 22
Umar | Metabolic Acidosis
● pH- 7.23
● pCO2 - 40
● HCO3 - 16
Femi | Respiratory Acidosis
● PH- 7.34
● pCO2 - 55
● HCO3 - 29
King | Metabolic Alkalosis (compensated)
● PH- 7.45
● pCO2 - 52
● HCO3 - 35

18. Compensation
The body constantly tries to ensure a normal pH.
Usually, compensation does not return the pH to normal (7.35 – 7.45)
Patients commonly present with a mixed acid base disorder
If the observed compensation is not the expected compensation, it is likely that
more than one acid-base disorder is present.

19. Compensation
● Is there appropriate compensation for the primary disturbance?
Disorder Expected compensation
Metabolic acidosis PaCO2 = (1.5 x [HCO3-]) +8 ± 2
Acute respiratory acidosis Increase in [HCO3-]= ∆ PaCO2/10
Chronic respiratory acidosis (3-5 days) Increase in [HCO3-]= 4(∆ PaCO2/10)
Metabolic alkalosis Increase in PaCO2 = 40 + 0.6(∆HCO3-)
Acute respiratory alkalosis Decrease in [HCO3-]= 2(∆ PaCO2/10)
Chronic respiratory alkalosis Decrease in [HCO3-] = 5(∆ PaCO2/10)

20. Anion Gap
Calculate the anion gap (if a metabolic acidosis exists):
AG= [(Na+ + K+)-( [Cl-] + [HCO3-] )
A normal anion gap is approximately 10-18 mmol/L.
In patients with hypoalbuminemia, the normal anion gap is lower;
The “normal” anion gap in patients with hypoalbuminemia is about 2.5 mmol/L
lower for each 1 gm/dL decrease in the plasma albumin concentration

21. ● If an increased anion gap is present, assess the relationship between the
increase in the anion gap and the decrease in [HCO3-].
● Assess the ratio of the change in the anion gap (∆AG ) to the change in
[HCO3-] (∆[HCO3-]): ∆AG/∆[HCO3-]
○ This ratio should be between 1.0 and 2.0 if an uncomplicated anion gap metabolic acidosis is
present.
● If this ratio falls outside of this range, then another metabolic disorder is
present:
○ If ∆AG/∆[HCO3-] < 1.0, then a concurrent non-anion gap metabolic acidosis is likely to be
present.
○ If ∆AG/∆[HCO3-] > 2.0, then a concurrent metabolic alkalosis is likely to be present.

22. Anion Gap Acidosis
Electroneutrality must be maintained
((Na + KCL + Unmeasurable Cations) - (Cl + Hco3 + Unmeasurable Anions))
Na - (Cl + Hco3) = <12
Increased above this level, indicates an increase in unmeasurable anions
Maintained by albumin

23. Respiratory Acidosis
Characterized by High CO2 and Low PH (Commonly with low PaO2 and SaO2)
Retention of CO2
● Hypoventilation
● Airway Obstruction
● Lung collapse

24. Signs/ Symptoms
Signs of the Cause
Reduced LOC
Anxiety/Confusion
Dyspnea
Vomiting
Muscle weakness
Arrythmias

25. ● Airway obstruction
○ - Upper
○ - Lower
■ COPD
■ asthma
■ other obstructive lung disease
● CNS depression
● Sleep disordered breathing (OSA or OHS)
● Neuromuscular impairment
● Ventilatory restriction
● Increased CO2 production: shivering, rigors, seizures, malignant
hyperthermia, hypermetabolism, increased intake of carbohydrates
● Incorrect mechanical ventilation settings

26. Therapy
Correct cause
Improve ventilation
OD?
● Naloxone
● Flumezanil

27. Metabolic Acidosis
Low pH , Low HCO3
● Increase in metabolic acids (DKA,LA, Poisoning)
● Loss of Bases (Diarrhea, Renal Failure)

28. ● Signs of the cause
● Altered LOC
● Confusion/Drowsiness
● Nausea and Vomiting
● Tachypnea
● Hyperkalemia
○ Muscle weakness
Arrythmias

29. Elevated anion gap:
● Methanol intoxication
● Uremia ; Chronic Renal Failure
● Diabetic ketoacidosisa, alcoholic ketoacidosis, starvation ketoacidosis
● Paraldehyde toxicity
● Isoniazid, INfection
● Lactic acidosis
○ Type A: tissue ischemia
○ Type B: Altered cellular metabolism
● Ethanol or ethylene glycol intoxication
● Salicylate intoxication

30. Normal anion gap: will have increase in [Cl-]
● GI loss of HCO3-
○ Diarrhea, ileostomy, proximal colostomy, ureteral diversion
● Renal loss of HCO3-
○ proximal RTA
○ carbonic anhydrase inhibitor (acetazolamide)
● Renal tubular disease
○ ATN
○ Chronic renal disease
○ Distal RTA
○ Aldosterone inhibitors or absence / Endocrinopathies
○ NaCl infusion, TPN, NH4+ administration

31. Respiratory Alkalosis
Characterized by low CO2 and high PH (Commonly with low PaO2 and SaO2)
Loss of CO2
● Hyperventilation
● Hypoxemia
● Mechanical Ventilation

32. Signs/ Symptoms
Signs of the Cause
Light headed
Numbness/tingling
Dypsnea
Confusion
Lethargy

33. ● CNS stimulation: fever, pain, fear, anxiety, CVA, cerebral edema, brain
trauma, brain tumor, CNS infection
● Hypoxemia or hypoxia: lung disease, profound anemia, low FiO2
● Stimulation of chest receptors: pulmonary edema, pleural effusion,
pneumonia, pneumothorax, pulmonary embolus
● Drugs, hormones: salicylates, catecholamines, medroxyprogesterone,
progestins
● Pregnancy, liver disease, sepsis, hyperthyroidism
● Incorrect mechanical ventilation settings

34. Paper Bag
Ventilator Settings
Meds

35. Metabolic Alkalosis
High PH High HCO3
Loss of Metabolic Acids or increase in Alkaline bases
Increase in alkaline substances
Loss of acids
Hypokalemia

36. Sign/Symptoms
Signs of the cause
Altered LOC
Headache
Numbness/Tinglinf
Bradypnea
Arrythmias
Hypokalemia

37. ● Hypovolemia with Cl- depletion
○ GI loss of H+
■ Vomiting, gastric suction, villous adenoma, diarrhea with chloride-rich fluid
○ Renal loss H+
■ Loop and thiazide diuretics, post-hypercapnia (especially after institution of mechanical
ventilation)
● Hypervolemia, Cl- expansion
○ Renal loss of H+: edematous states (heart failure, cirrhosis, nephrotic syndrome),
hyperaldosteronism, hypercortisolism, excess ACTH, exogenous steroids, hyperreninemia,
severe hypokalemia, renal artery stenosis, bicarbonate administration

38. Kcl
Na Cl
Dialysis

39. Anaesthetic considerations (Acidosis)
Cautious Sedation
Rational Opiod Use
Increased risk of aspiration
Increased risk of hypotension
Increased risk of arrythmias
Hyperkalemia

40. Anaesthetic considerations (Alkalosis)
Reduced Cerebral Blood flow
Respiratory depression
Arrythmias
Increased duration of block

41. Thank you for Listening
Questions?

42. References
● NPMCN / WACS Anaesthesia Update 2020. ABG Material, delivered by Dr.
Chizoba Peters
● Morgan & Mikhail's Clinical Anesthesiology, 5th Edition. DiLorenzo, Amy N.,
MA; Schell,
● Oxford Textbook of Anaesthesia. Edited by Jonathan G Hardman, Philip M
Hopkins, and Michel M.R.F Struys. Abstract.