ABG basics

ABG Basics

1. DRTR SHRESTHA, KMCTH

2. Introduction
▪Testofbloodfromanarterythatmeasures
theamountofcertaingasesdissolvedinit
▪Involvespuncturinganarterywith
aneedleanddrawingasmallvolumeof
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, PaO
2
▪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
▪Ptwithpoordistalperfusion→nostrongarterialpulsations
▪Venouspuncture:lackofpulsatileflowordarkcoloredblood
▪Mixtureofvenousandarterialblood
▪Excessiveskin,abundantsofttissuemayobstructpuncturesite
▪Airbubbleinsample→false↑PaO
2value
▪Insufficientheparin
▪Incompletedismissalofheparinsolution→false↓PaCO
2value
▪Incorrectstorage

9. Normal values
pH 7.35 -7.45
PaCO
2 35 -45 mm Hg
PaO
2 70 -100 mm Hg
HCO
3
¯ 22 -26 mEq/L
SaO
2 93 –98%
%MetHb < 2.0%
%COHb < 3.0%
Base excess -2.0 to 2.0 mEq/L

10. ABG Vs VBG Analysis
Values Arterialblood Venousblood
pH 7.40(7.35-7.45) 7.36 (7.31-7.41)
PaO
2 80-100mmHg 36-40 mmHg
SaO
2 95% 70-75%
PaCO
2 35-45 mmHg 41-51 mmHg
HCO3
-
22-26 mEq/L 22-26 mEq/L
BE -2 to +2 mEq/L -2 to +2 mEq/L

11. BASIC CONCEPTS

12. pH
▪NegativelogofH
+
▪pH7.4,[H
+
]40nEq/L
▪Undernormalconditions,
▪HendersonHasselbalchequation

13. Classification of acid base disorder
▪If PCO
2is responsible for change in [H
+
]
▪RESPIRATORY ACID BASE DISORDER
▪If HCO
3is responsible for change in [H
+
]
▪METABOLIC ACID BASE DISORDER

14. Primary acid base disorder and secondary response

15. Secondary Responses
▪Limitschangein[H
+
]producedbyprimary
acidbasedisorder
▪Accomplishedbychangingothercomponent
ofPaCO
2/HCO
3ratiointhesamedirection.
▪Nocompletecorrectionofthechangein[H
+
]
producedbytheprimarydisorder

16. Metabolic Acidosis
▪Secondaryresponse→increaseinminute
ventilation(V
T,RR)anddecreaseinPaCO
2
▪Appearsin30–120minutes,cantake12-
24hourstocomplete
▪Magnitudeofresponsedefinedbyequation
▪∆PaCO
2=1.2×∆HCO
3

17. ▪UsingnormalPaCO
2of40mmHgandnormalHCO
3of24mEq/L
▪ExpectedPaCO
2
=40–[1.2×(24–currentHCO
3
)]
▪E.g.
▪MetabolicacidosiswithplasmaHCO
3
of14mEq/L,
▪∆HCO
3=24–14=10mEq/L
▪∆PaCO
2=1.2×10=12mmHg,
▪ExpectedPaCO
2=40–12=28mmHg
▪IfPaCO
2>28mmHg→secondaryrespiratoryacidosis
▪IfPaCO
2<28mmHg→secondaryrespiratoryalkalosis

18. Metabolic Alkalosis
▪Secondaryresponse→decreaseinminute
ventilationandincreaseinPaCO
2
▪∆PaCO
2=0.7×∆HCO3
▪ExpectedPaCO
2=
▪40+[0.7×(currentHCO
3
–24)]

19. Respiratory Disorders
Acute Respiratory Acidosis∆HCO
3= 0.1 ×∆PaCO
2
Chronic Respiratory Acidosis∆HCO
3= 0.4 ×∆PaCO
2
Acute Respiratory Alkalosis∆HCO
3= 0.2 ×∆PaCO
2
Chronic Respiratory Alkalosis∆HCO
3= 0.4 ×∆PaCO
2

20. Causes
Respiratory acidosis Respiratory alkalosis
Drugsinduced ventilatory
depression
Mechanicalhyperventilation
Upper airway obstructionCNS Injury
Permissive hypercapnia Hypoxemia,sepsis
Asthma, COPD Anxiety,panic attack
Largecalorie load Salicylateoverdose
Malignant hyperthermia High altitude

21. Metabolicacidosis Metabolicalkalosis
Lactic acidosis Hypovolemia
DKA Vomiting
Renal failure,hepatic failureNGsuction
Alcoholintoxication Diuretics
CO Poisoning Hyperaldosteronism

22. STEPWISE APPROACH TO
ABG ANALYSIS

23. Stage I: Identify Primary Acid-Base Disorder
▪Rule1:IfPaCO
2and/orpHoutsidenormalrange→acid-basedisorder
▪Rule2:IfPaCO
2andpHbothabnormal,comparedirectionalchange
▪2A:IfPaCO
2andpHchangeinsamedirection
→primarymetabolicacid-basedisorder
▪2B:IfPaCO
2andpHchangeinoppositedirections
→primaryrespiratoryacid-basedisorder

24. E.g.
pH = 7.23
PaCO
2= 23 mm Hg

25. ▪Rule3:IfonlypHorPaCO
2isabnormal
→mixedmetabolic&respiratorydisorder(i.e.,equal&opp.disorders)
▪3A:IfPaCO
2isabnormal,directionalchangeinPaCO
2identifiesthe
typeofrespiratorydisorder(e.g.,highPaCO
2indicatesarespiratory
acidosis),andopposingmetabolicdisorder
▪3B:IfpHisabnormal,directionalchangeinpHidentifiestypeof
metabolicdisorder(e.g.,lowpHindicatesametabolicacidosis)and
opposingrespiratorydisorder

26. ▪pH=7.38
▪PaCO
2=55mmHg
▪Mixedrespiratoryacidosisandmetabolic
alkalosis

27. Stage II: Evaluate Secondary Responses
▪Rule4:Foraprimarymetabolicdisorder,
ifthemeasuredPaCO
2ishigherthanexpected→
secondaryrespiratoryacidosis
ifthemeasuredPaCO
2islessthanexpected→
secondaryrespiratoryalkalosis

28. ▪pH=7.32,PaCO
2=23mmHg,HCO
3=16mEq/L
▪pHandPaCO
2changeinsamedirection
→primarymetabolicacidosis
▪∆PaCO
2=1.2×(24–16)=10mmHg
▪expectedPaCO
2=40–10=30mmHg
▪measuredPaCO
2(23mmHg)<expectedPaCO
2
→additionalrespiratoryalkalosis
primarymetabolicacidosiswithsecondaryrespiratory
alkalosis

29. ▪Rule5:Foraprimaryrespiratorydisorder,anormalornear-normalHCO3→acute
▪Rule6:ForaprimaryrespiratorydisorderwheretheHCO3isabnormal,determine
theexpectedHCO3forachronicrespiratorydisorder
▪6A:Forachronicrespiratoryacidosis,iftheHCO3<expected,thereisan
incompleterenalresponse,andiftheHCO3>expected,thereisasecondary
metabolicalkalosis.
▪6B:Forachronicrespiratoryalkalosis,iftheHCO3>expected,thereisan
incompleterenalresponse,andiftheHCO3<expected,thereisasecondary
metabolicacidosis.

30. Stage III: “Gaps” to Evaluate Metabolic Acidosis
▪Aroughestimateoftherelativeabundanceofunmeasuredanions
determinesifametabolicacidosisisduetoanaccumulationofnon-
volatileacids(lacticacid)oraprimarylossofbicarbonate(diarrhea)
▪NormalAG:3-11mEq/L
▪AG=Na
+
–(Cl
-
+HCO3
-
)
▪Forevery1g/dldecreaseinplasmaalbumin,AGdecreasesby
2.5meq/L
▪AGc=AG+2.5(4.5-Albumin)

31. Metabolic acidosis
High AG Normal AG
▪Renal failure
▪Lacticacidosis
▪Diabetic Ketoacidosis
▪Methanol ingestion
▪Salicylatepoisoning
▪Ethylene glycol ingestion
▪Methanol ingestion
▪Diarrhoea
▪Isotonic saline infusion
▪Early renal insufficiency
▪Renal tubular acidosis
▪Diuretics
▪Acetazolamide
▪Spironolactone

32. Strong Ion Difference
▪SID= (Na
+
+K
+
+Ca
2+
+Mg
2+
)-(Cl
-
+Lactate)

33. ▪↑SID→increasepH
▪Dehydration (↑Na+)
▪Chloride loss (NG suction)
▪↓SID→decreasepH
▪Free water excess (dilutionalacidosis)
▪Excessive NS
▪Severe diarrhoea (Na, K loss)
▪↑ lactate

34. Base Excess
▪Amountofstrongbasewhichwouldneedtobe
addedorsubtractedfromasubstanceinorderto
returntonormalpH
▪Usedforassessmentofmetabolicacid-base
disorders
▪Normalvalue:-2to+2meq/L

35. Davenport diagram

36. Measure of gas exchange
A-a O
2gradient
▪Difference in PO
2in alveoli and artery
▪PAO
2-PaO
2
▪5-10 mmHg

37. P/F Ratio
Measure of oxygenation
▪In room air: PaO
2… 100mmHg, FiO
2….0.21
▪PaO
2/FiO
2= 500
▪Provides clues whether patient is heading to ARDS
▪Mild: 200-300mmHg
▪Moderate: 100-200mmHg
▪Severe: <100mmHg

38. Examples

39. pH: 7.49, pCO
2: 29, HCO
3: 22
Primary respiratory alkalosis
Expected HCO
3
=24 –[0.2x (40-29)]
=21.8

40. pH: 7.32, pCO
2: 48, HCO
3:25
Primary respiratory acidosis
Expected HCO3
=24 + [0.1x (48-40)]
=24.8

41. •pH = 7.3
•PCO2 = 29 mm Hg
•HCO3–= 14 mEq/L
•PO2 = 95 mm Hg
Metabolicacidosis
Expected PCO2: 40 –[1.2x(24-14)] = 28
Primary metabolic acidosis with respiratory compensation

42. •pH = 7.6
•PCO2 = 52 mm Hg
•HCO3= 44 mEq/L
•PO2 = 70 mm Hg
Metabolicalkalosis
Expected PCO2: 40 + [0.7x( 44-24)] = 54
Primary metabolic alkalosis with respiratory compensation

43. pH: 7.38
pCO
2: 52
HCO
3: 31
Mixed respiratory acidosis and metabolic
alkalosis