2. Introduction
▪ Test of blood from an artery that measures
the amount of certain gases dissolved in it
▪ Involves puncturing an artery with
a needle and drawing a small volume of
blood
3. ▪ Identification of acid-base disorders
▪ Measurement of partial pressure of respiratory gases involved in
oxygenations and ventilation
▪ Monitoring acid-base status
▪ Assessment of response to therapeutic intervention as mechanical
ventilation in respiratory failure
▪ Quantification of oxyhemoglobin, PaO2
▪ Quantification of levels of carboxyHb, MethHb
▪ Quantification of lactate level
▪ Procurement of blood sample in an acute emergency
Indications
4. Contraindications
Absolute
▪ Abnormal modified Allen’s
test
▪ Local infection
▪ AV fistula/ vascular grafts
▪ Peripheral vascular disease
of the limb
Relative
▪ Coagulopathy
▪ Anticoagulants therapy
5. ▪ Radial artery
▪ Brachial artery
▪ Femoral artery
▪ Radial artery
▪ Easy to access
▪ Not deep artery→ facilitates palpation, stabilization and
puncturing
▪ Collateral blood circulation
5
Choice of artery
6. Allen’s Test
▪ Presence of collateral circulation from ulnar
artery in case occlusion in radial artery
6
7. Procedure
▪ Insert needle at 45°
▪ Withdraw needle and apply
digital pressure
▪ Check bubbles in syringe
▪ Place the capped syringe in
container of ice immediately
▪ Maintain firm pressure on
the puncture site for 5
minutes
8. Potential Errors
▪ Pt with poor distal perfusion→no strong arterial pulsations
▪ Venous puncture: lack of pulsatile flow or dark colored blood
▪ Mixture of venous and arterial blood
▪ Excessive skin, abundant soft tissue may obstruct puncture site
▪ Air bubble in sample →false ↑PaO2 value
▪ Insufficient heparin
▪ Incomplete dismissal of heparin solution→false ↓PaCO2 value
▪ Incorrect storage
9. Normal values
pH 7.35 - 7.45
PaCO2 35 - 45 mm Hg
PaO2 70 - 100 mm Hg
HCO3
¯ 22 - 26 mEq/L
SaO2 93 – 98%
%MetHb < 2.0%
%COHb < 3.0%
Base excess -2.0 to 2.0 mEq/L
12. pH
▪ Negative log of H+
▪ pH 7.4, [H+] 40nEq/L
▪ Under normal conditions,
▪ Henderson Hasselbalch equation
13. Classification of acid base disorder
▪ If PCO2 is responsible for change in [H+]
▪ RESPIRATORY ACID BASE DISORDER
▪ If HCO3 is responsible for change in [H+]
▪ METABOLIC ACID BASE DISORDER
15. Secondary Responses
▪ Limits change in [H+] produced by primary
acid base disorder
▪ Accomplished by changing other component
of PaCO2/HCO3 ratio in the same direction.
▪ No complete correction of the change in [H+]
produced by the primary disorder
16. Metabolic Acidosis
▪ Secondary response→ increase in minute
ventilation (VT, RR) and decrease in PaCO2
▪ Appears in 30 – 120 minutes, can take 12 -
24 hours to complete
▪ Magnitude of response defined by equation
▪ ∆PaCO2 = 1.2 × ∆HCO3
17. ▪ Using normal PaCO2 of 40 mmHg and normal HCO3 of 24 mEq/L
▪ Expected PaCO2 = 40 – [1.2 × (24 – current HCO3)]
▪ E.g.
▪ Metabolic acidosis with plasma HCO3 of 14 mEq/L,
▪ ∆HCO3= 24–14=10mEq/L
▪ ∆PaCO2= 1.2×10 =12mmHg,
▪ Expected PaCO2= 40–12 =28 mmHg
▪ If PaCO2 >28mmHg→secondary respiratory acidosis
▪ If PaCO2 <28 mm Hg→secondary respiratory alkalosis
23. Stage I: Identify Primary Acid-Base Disorder
▪ Rule 1: If PaCO2 and/or pH outside normal range→acid-base disorder
▪ Rule 2: If PaCO2 and pH both abnormal, compare directional change
▪ 2A: If PaCO2 and pH change in same direction
→ primary metabolic acid-base disorder
▪ 2B: If PaCO2 and pH change in opposite directions
→ primary respiratory acid-base disorder
25. ▪ Rule 3: If only pH or PaCO2 is abnormal
→ mixed metabolic & respiratory disorder (i.e., equal & opp. disorders)
▪ 3A: If PaCO2 is abnormal, directional change in PaCO2 identifies the
type of respiratory disorder (e.g., high PaCO2 indicates a respiratory
acidosis), and opposing metabolic disorder
▪ 3B: If pH is abnormal, directional change in pH identifies type of
metabolic disorder (e.g., low pH indicates a metabolic acidosis) and
opposing respiratory disorder
26. ▪ pH = 7.38
▪ PaCO2 = 55 mm Hg
▪ Mixed respiratory acidosis and metabolic
alkalosis
27. Stage II: Evaluate Secondary Responses
▪ Rule 4: For a primary metabolic disorder,
if the measured PaCO2 is higher than expected→
secondary respiratory acidosis
if the measured PaCO2 is less than expected→
secondary respiratory alkalosis
28. ▪ pH = 7.32, PaCO2 = 23 mm Hg, HCO3 = 16 mEq/L
▪ pH and PaCO2 change in same direction
→primary metabolic acidosis
▪ ∆PaCO2 = 1.2 × (24 – 16) = 10 mm Hg
▪ expected PaCO2 = 40 – 10 = 30 mm Hg
▪ measured PaCO2 (23 mm Hg)< expected PaCO2
→additional respiratory alkalosis
primary metabolic acidosis with secondary respiratory
alkalosis
29. ▪ Rule 5: For a primary respiratory disorder, a normal or near-normal HCO3→ acute
▪ Rule 6: For a primary respiratory disorder where the HCO3 is abnormal, determine
the expected HCO3 for a chronic respiratory disorder
▪ 6A: For a chronic respiratory acidosis, if the HCO3< expected, there is an
incomplete renal response, and if the HCO3> expected, there is a secondary
metabolic alkalosis.
▪ 6B: For a chronic respiratory alkalosis, if the HCO3> expected, there is an
incomplete renal response, and if the HCO3< expected, there is a secondary
metabolic acidosis.
30. Stage III: “Gaps” to Evaluate Metabolic Acidosis
▪ A rough estimate of the relative abundance of unmeasured anions
determines if a metabolic acidosis is due to an accumulation of non-
volatile acids (lactic acid) or a primary loss of bicarbonate (diarrhea)
▪ Normal AG: 3-11mEq/L
▪ AG = Na+ – (Cl- + HCO3-)
▪ For every 1g/dl decrease in plasma albumin, AG decreases by
2.5meq/L
▪ AGc= AG + 2.5 (4.5- Albumin)
31. Metabolic acidosis
High AG Normal AG
▪Renal failure
▪Lactic acidosis
▪Diabetic Ketoacidosis
▪Methanol ingestion
▪Salicylate poisoning
▪Ethylene glycol ingestion
▪Methanol ingestion
▪Diarrhoea
▪Isotonic saline infusion
▪Early renal insufficiency
▪Renal tubular acidosis
▪Diuretics
▪Acetazolamide
▪Spironolactone
33. ▪ ↑SID→increase pH
▪ Dehydration (↑Na+)
▪ Chloride loss (NG suction)
▪ ↓SID→decrease pH
▪ Free water excess (dilutional acidosis)
▪ Excessive NS
▪ Severe diarrhoea (Na, K loss)
▪ ↑ lactate
34. Base Excess
▪ Amount of strong base which would need to be
added or subtracted from a substance in order to
return to normal pH
▪ Used for assessment of metabolic acid-base
disorders
▪ Normal value: -2 to +2 meq/L
