Arterial Blood Gas Arterial Blood Gas AnalysisAnalysis

Dr RDr Réémy Tokomy TokoHull Royal InfirmaryHull Royal Infirmary

OutlineOutline

Brief HistoryBrief HistoryWhy do a BG? Why do a BG? Performing a blood gas Performing a blood gas (phases)(phases)Some BG Parameters Some BG Parameters (measured/calculated)(measured/calculated)Blood gas interpretationBlood gas interpretationScenariosScenariosSummary & ConclusionSummary & Conclusion

1. Brief History of BG Analysis1. Brief History of BG Analysis

1952: Leland Clark invented 1952: Leland Clark invented a method of a method of defoamingdefoaming in a in a bubble bubble oxygenatoroxygenator1953: Dr John Severinghaus1953: Dr John Severinghaus•• Developed analysis of Developed analysis of

PCOPCO22 & pH& pH1954: CO1954: CO22 electrode electrode 1956: PO1956: PO22 measurementsmeasurements1959: 11959: 1stst Blood gas analyser Blood gas analyser BE: Astrup & Andersen BE: Astrup & Andersen (Severinghaus)(Severinghaus)Oxygen Dissociation Curve Oxygen Dissociation Curve (Roughton, Bradley & (Roughton, Bradley & Severinghaus)Severinghaus)

L. ClarkL. Clark

J. SeveringhausJ. Severinghaus

First threeFirst three--function blood gas function blood gas analyseranalyser (1959)(1959)Built by Dr Severinghaus and BradleyBuilt by Dr Severinghaus and Bradley

pH electrode

PCO2 electrode

PO2 electrode

Modern BG Modern BG analysersanalysers

From 1960 Blood Gas Analyser became almost From 1960 Blood Gas Analyser became almost universal universal (Western countries)(Western countries)

BG Analyser have since revolutionised both BG Analyser have since revolutionised both clinical medicine & cardiorespiratory and clinical medicine & cardiorespiratory and metabolic physiologymetabolic physiology

BG BG AnalysersAnalysers are now computerised and are now computerised and multifunctionmultifunction

ABG Analysis is an important laboratory test for ABG Analysis is an important laboratory test for critically ill patients critically ill patients (A&E, Resus room, Theater, HDU, (A&E, Resus room, Theater, HDU, ICU)ICU)

Modern BG AnalysersModern BG Analysers

2. Why do a BG?2. Why do a BG?

To CheckTo Check……OxygenationOxygenation•• PaOPaO22

•• Diffusion capacity of the lungs (ADiffusion capacity of the lungs (A--a a gradient)gradient)

COCO22 Removal (Ventilation)Removal (Ventilation)AcidAcid--base status (pH, HCObase status (pH, HCO33

--, CO, CO22, BE, , BE, lactate)lactate)Electrolytes & metabolites (NaElectrolytes & metabolites (Na++, K, K++, , glucose, etcglucose, etc……))HaemoglobinsHaemoglobins (Hb, MetHb, COHb, FHb)(Hb, MetHb, COHb, FHb)

Aid to management of A-B-C-Defg!

3. Performing BG analysis3. Performing BG analysis

3 Important Phases:3 Important Phases:PrePre--analytical phaseanalytical phase•• largest contributor of bias to BG largest contributor of bias to BG

measurements; measurements; therefore = therefore = weakest linkweakest link

Analytical phaseAnalytical phasePost Analytical phasePost Analytical phase

4. BG Parameters4. BG ParametersMeasured parameters: Measured parameters: •• pH, PaOpH, PaO22, PaCO, PaCO22

•• Oxymetry: Oxymetry: SaOSaO22, tHb,O, tHb,O22Hb, Hb, HbFHbF, MetHb, , MetHb, COHbCOHb

•• Metabolites: Metabolites: LactateLactate, Glucose, , Glucose, tBilirubintBilirubin•• Electrolytes (NaElectrolytes (Na++, K, K++, Ca, Ca2+2+, Cl, Cl--))

Calculated parameters:Calculated parameters:•• Temp corrected values pH, PCOTemp corrected values pH, PCO22, PO, PO22

•• (a)BC, SBC,(a)BC, SBC, SBE, (a)BE, tOSBE, (a)BE, tO22, HHB, HHB•• Anion GapAnion Gap•• PP50, 50, FFShunt, (AShunt, (A--a)DOa)DO22

pHpHMeasured by the Measured by the analyseranalyser

Normal values: 7.35Normal values: 7.35--7.457.45•• Slightly low (acidotic) at birthSlightly low (acidotic) at birth•• May be Neutral (normal), acidotic or May be Neutral (normal), acidotic or alkaloticalkalotic

pHi (6.8 pHi (6.8 -- 7.1 mmHg), not measurable, therefore use 7.1 mmHg), not measurable, therefore use of pH as surrogateof pH as surrogate

Must be interpreted together with other parametersMust be interpreted together with other parameters

pH (ContpH (Cont’’d)d)

Respiratory AcidosisRespiratory Acidosis•• Due mainly to alveolar hypoventilationDue mainly to alveolar hypoventilation

E.g. CNS, lungs, E.g. CNS, lungs, inatrogenicinatrogenic•• Features: Features: ↓↓ pH, pH, ↑↑ PCOPCO22, normal BE, , normal BE, HCOHCO--

33

•• If persistent, may be partly or totally If persistent, may be partly or totally compensated (by compensated (by ↓↓ renal excretion of renal excretion of HCOHCO--

33))•• Compensated respiratory acidosis:Compensated respiratory acidosis:

Slightly Slightly ↓↓ pH, pH, ↑↑ PCOPCO2 2 and and high high bicarbonatebicarbonate

pH (ContpH (Cont’’d)d)Metabolic acidosisMetabolic acidosis•• Caused by: circulatory impairment, Caused by: circulatory impairment,

Renal failure, DKA, DRenal failure, DKA, Diarrhoeaiarrhoea (loss of (loss of HCOHCO--

33))•• Characterised by: Characterised by: ↓↓ pH, pH, ↓↓ HCOHCO--

33, , Normal or low PCONormal or low PCO22

•• If patient breathing spontaneously, If patient breathing spontaneously, partial compensation will occur partial compensation will occur →→hyperventilationhyperventilation

Respiratory AlkalosisRespiratory Alkalosis•• Due to alveolar hyperventilation Due to alveolar hyperventilation •• Features: Features: ↑↑ pH, low PCOpH, low PCO22

pH (ContpH (Cont’’d)d)

Metabolic alkalosisMetabolic alkalosis•• Caused by: Diuretics, GIT loss of Caused by: Diuretics, GIT loss of

acid (vomiting acid (vomiting --HSPHSP), Hypokalemia), Hypokalemia

•• Characterised by: Characterised by: ↑↑ pH, pH, ↑↑ HCOHCO33

•• If patient breathing spontaneously, If patient breathing spontaneously, partial compensation will occur partial compensation will occur →→hypoventilationhypoventilation

Oxygen statusOxygen status

Simplistic way: PaOSimplistic way: PaO22, SaO, SaO22

OO22 status can be assessed by status can be assessed by considering the following considering the following (comprehensive assessment):(comprehensive assessment):

•• OO22 uptakeuptake•• OO22 transporttransport•• OO22 delivery (extraction & delivery (extraction &

utilisation)utilisation)•• Ratios: OI, PaORatios: OI, PaO22/F/FiiOO22

OO22 Status: Status: 1. Uptake1. Uptake

Depends on:Depends on:

•• Alveolar OAlveolar O2 2 tension(FiOtension(FiO22, ambient , ambient pressure)pressure)

•• Degree of shunting (extra & Degree of shunting (extra & intrapulmonary) = intrapulmonary) = FFShuntShunt (4(4--10%)10%)

•• Diffusion capacityDiffusion capacity

Key parameter= PaOKey parameter= PaO22

Normal= 10.6 Normal= 10.6 --14 14 kPakPa in airin air

OO22 Status: Status: 1. Uptake 1. Uptake (Cont(Cont’’d)d)

High PaOHigh PaO22::•• Toxicity (ROP in premature++, lung Toxicity (ROP in premature++, lung

injury)injury)•• Not helpful, no significant gain in SaONot helpful, no significant gain in SaO22

(see ODC)(see ODC)

Low PaOLow PaO22: inadequate uptake: inadequate uptake•• Shunting, V/Q mismatchShunting, V/Q mismatch•• Low FLow FiiOO22

•• Circulatory collapseCirculatory collapse

OO22 Status: Status: 2. O2. O22 transporttransport

Amount of OAmount of O2 2 being transported per litre of blood. being transported per litre of blood. Depends on:Depends on:•• Hb concentrationHb concentration•• Dyshaemoglobins (MetHb, COHb, etcDyshaemoglobins (MetHb, COHb, etc……))•• PaOPaO22, SaO, SaO22

Key parameter= tOKey parameter= tO22

M= 8.4 M= 8.4 --9.9 mmol/L (18.8 9.9 mmol/L (18.8 --22.3mL/dL)22.3mL/dL)F = 7.1 F = 7.1 –– 8.9 mmol/L (15.8 8.9 mmol/L (15.8 –– 19.9 19.9 mL/dLmL/dL))

Low tOLow tO22

Low Hb, low Low Hb, low PaOPaO22, SaO, SaO2 2 (ODC)(ODC)

DyshaemoglobinsDyshaemoglobins

OO22 Status: Status: 3. O3. O22 ReleaseRelease

Delivery/extraction at tissue levelDelivery/extraction at tissue level•• SaOSaO22 –– SvOSvO22 ≥≥ 25% (with SvO25% (with SvO22≥≥70%)70%)

DODO22= CO X CaO2 X 10= CO X CaO2 X 10

CaOCaO22 = SaO= SaO22 X Hb X 1.37 + 0.003 X PaOX Hb X 1.37 + 0.003 X PaO22

•• Perfusion (lactate)Perfusion (lactate)

SvOSvO22 = Mixed venous O= Mixed venous O22saturationsaturation

Saturation (SaOSaturation (SaO22): 95): 95--99%99%

Percentage of oxygenated Hb in relation Percentage of oxygenated Hb in relation to the amount of Hb capable of carrying to the amount of Hb capable of carrying OO22

Common causes of low SaOCommon causes of low SaO2 2 (<90%)(<90%)::•• Impaired OImpaired O22 uptake uptake (FiO(FiO22, Shunts, Diffusion), Shunts, Diffusion)

•• Right shift of ODC Right shift of ODC (may be beneficial in sepsis)(may be beneficial in sepsis)

NB:NB: SaOSaO22 on ABG vs. SOon ABG vs. SO22 on pulse oxymeteron pulse oxymeter

The position of the ODC

Depends primarilyon the pH

OO22 Status: Status: 3. O3. O22 ReleaseRelease

Depends on:Depends on:•• PaOPaO22,, capillary POcapillary PO2, 2, ctOctO22

•• OO2 2 affinity to Hb (affinity to Hb (pp50 on ODC,50 on ODC, 2424--

28mmHg28mmHg))•• Perfusion (lactate)Perfusion (lactate)

pp50 =pO50 =pO22 at half saturation at half saturation and reflects the affinity of Hb and reflects the affinity of Hb for Ofor O22

Carbon dioxide (PCOCarbon dioxide (PCO22))

Depends largely on alveolar Depends largely on alveolar ventilationventilation

Respiratory rateRespiratory rateTidal volumeTidal volumeVentilationVentilation--perfusion matchingperfusion matching

)V-(VxRR nventilatioAlveolar DT=

PaCOPaCO2 2 (Normal: 4.5 (Normal: 4.5 --6 6 kPakPa))

Direct reflection of the adequacy of Direct reflection of the adequacy of alveolar ventilation in relation to the alveolar ventilation in relation to the metabolic ratemetabolic rate

Low PaCOLow PaCO2 2 (<4kPa):(<4kPa):hypocapniahypocapnia•• PrimaryPrimary

Iatrogenic (aggressive ventilation)Iatrogenic (aggressive ventilation)Psychogenic hyperventilationPsychogenic hyperventilation

•• SecondarySecondaryCompensation of Metabolic Compensation of Metabolic AcidosisAcidosis

PaCOPaCO2 2 (Cont(Cont’’d)d)

High PaCOHigh PaCO2 2 (>6.7kPa): (>6.7kPa): hypercapniahypercapnia•• Alveolar hypoventilationAlveolar hypoventilation

Lung diseasesLung diseasesCNS depressionCNS depression

•• Primary: disease relatedPrimary: disease related•• Secondary: sedation, drug OD Secondary: sedation, drug OD

(narcotics e.g. heroin)(narcotics e.g. heroin)

Ventilator treatmentVentilator treatment•• Low Tidal Volume ventilationLow Tidal Volume ventilation•• Permissive hypercapniaPermissive hypercapnia

O2CO2

Basic respiratory Basic respiratory physiologyphysiology

External Respiration⇨ O2 uptake⇨ CO2 removal

Internal Respiration⇨ O2 utilisation⇨ CO2 production

Courtesy of

Dr. C. Gomersal

Brainstem

Spinal cordNerve rootAirway

Nerve

Neuromuscular junction

Respiratory muscle

Lung

Pleura

Chest wall

Sites at which diseases may cause ventilatory disturbances

Actual Bicarbonate Actual Bicarbonate (22 (22 -- 26mmol/L)26mmol/L)

Concentration of HCOConcentration of HCO--33 in the in the

plasma of the sampleplasma of the sample

It is calculated from pH & It is calculated from pH & PCOPCO22 (Henderson(Henderson--Hasselbach Hasselbach equation)equation)

Or mEq/LOr mEq/L

Standard BicarbonateStandard Bicarbonate(22(22-- 26mmol/L)26mmol/L)

Concentration of HCOConcentration of HCO--33 in plasma, in plasma,

equilibrated forequilibrated for•• TToo of 37of 37o o CC•• PaOPaO2 2 ≥≥ 100mmHg100mmHg•• PaCOPaCO22= 40mmHg= 40mmHg

Aim:Aim: to eliminate the respiratory to eliminate the respiratory component of the acidcomponent of the acid--base statusbase status

•• Therefore: Therefore: low SBC low SBC →→ True Met. acidosisTrue Met. acidosishigh SBC high SBC →→ True Met. alkalosisTrue Met. alkalosis

Charge balance in plasma

Na+

140

Cations

Cl-100

HCO3-Lactate-A-

UA-

Anions

H+

Charge balance in plasma

K+, Ca ++ , Mg ++

Actual Base ExcessActual Base Excess--2 to +2mmol/L2 to +2mmol/L

Base deficit (BD) vs. Base excess (BE)Base deficit (BD) vs. Base excess (BE)

BE =BE = deviation in mmol/L of the buffer deviation in mmol/L of the buffer base amount from normal level in bloodbase amount from normal level in blood

Buffer bases = HCOBuffer bases = HCO33--, Hb, Proteins, PO, Hb, Proteins, PO44

--

; N=48 ; N=48 ±±22 mmol/L)mmol/L)

Standard Base ExcessStandard Base Excess--3 to + 3mmol/L3 to + 3mmol/L

SBE =SBE = in vitro expression of BEin vitro expression of BE

It is the BE in total extracellular fluids, It is the BE in total extracellular fluids, of which blood = 1/3)of which blood = 1/3)

Buffering capacities differ in EC Buffering capacities differ in EC compartmentcompartment

SBE = more representative of the in SBE = more representative of the in vivo BE compared to actual BEvivo BE compared to actual BE

LactateLactate0.50.5-- 2mmol/L2mmol/L

Inadequate OInadequate O22 supply supply →→ production of production of lactate by cellslactate by cells

Marker of critical imbalance between Marker of critical imbalance between tissue Otissue O22 demand and Odemand and O2 2 supplysupply

High or High or increasing lactateincreasing lactate::•• HypoperfusionHypoperfusion•• Impaired arterial OImpaired arterial O22 supplysupply

Monitor trend rather than single Monitor trend rather than single valuesvalues

Lactate and mortalityLactate and mortality

Applicable to critically ill patients.

E.g. septic shock in ICU

5. BG Interpretation5. BG Interpretation

What is the pH? What is the pH? Will Will indicate patientindicate patient’’s statuss status•• <7.35 = acidosis<7.35 = acidosis•• >7.45 = alkalosis>7.45 = alkalosis

What is the pCOWhat is the pCO22??What is HCOWhat is HCO33

-- and/or and/or BE/BD? +BE/BD? +?AG or ?AG or AGcAGc

What is the POWhat is the PO22?? (check (check FiOFiO22) ) –– P/F P/F ratio,OIratio,OIElectrolytes, Metabolites, Electrolytes, Metabolites, HbHb’’ss, and others, and others

AcidosisAcidosispH< 7.35pH< 7.35

AlkalosisAlkalosispH>7.45pH>7.45

Respiratory CO2↑(>6kPa)

BD↓ or

↓Bicarb

CO2↓(<4.5)

Metabolic BE↑ or

↑ Bicarb

Check the patient!

Acidosis with high PCOAcidosis with high PCO22= = Respiratory acidosisRespiratory acidosis

Acidosis with Base deficit (Acidosis with Base deficit (--veve base base excess) = excess) =

Metabolic acidosisMetabolic acidosis

Alkalosis with low PCO2 =Alkalosis with low PCO2 =Respiratory alkalosisRespiratory alkalosis

Alkalosis with base excess (+Alkalosis with base excess (+veve base base excess) = excess) =

Metabolic alkalosisMetabolic alkalosis

BG Interpretation(2)BG Interpretation(2)

Mixed pictures are possible: Mixed pictures are possible:

•• Mixed respiratory and Mixed respiratory and metabolic acidosis.metabolic acidosis.

e.g.e.g. severe sepsissevere sepsis

•• Compensated statusCompensated status

BG Interpretation(3)BG Interpretation(3)

BG Interpretation(4)BG Interpretation(4)

2years old admitted 2years old admitted with severe sepsiswith severe sepsis

ABG:ABG:

pH 7.20,pH 7.20,PaCOPaCO22 3.13.1PaOPaO22 9.39.3HCOHCO33-- 1414BD BD --1010

pH: acidosispH: acidosisPCOPCO22: low : low ––doesndoesn’’t explain t explain acidosis acidosis (compensating)(compensating)Low bicarbonate & Low bicarbonate & BD explain the BD explain the acidosisacidosis•• Therefore = Therefore =

Metabolic acidosisMetabolic acidosis

6. Scenarios6. Scenarios

pHpH 7.35 7.35 --7.457.45

PaOPaO22 10.610.6--14 14 kPakPain airin air

PaCoPaCo22 4.5 4.5 –– 6 6 kPakPa

BD or BEBD or BE --2 to +22 to +2

HCOHCO33-- 2222--26mmol/L26mmol/L

Normal values

scenario 1scenario 111 months old baby brought in by mother. 11 months old baby brought in by mother. C/o fever, cough and shortness of breath X C/o fever, cough and shortness of breath X 2 days2 days

On Examination: lethargic, shallow On Examination: lethargic, shallow breathing, RR 68/minute, use of breathing, RR 68/minute, use of accessory muscles, grunting, Temp. 38accessory muscles, grunting, Temp. 38°°C, C, HR 140/min, crackles on the left base.HR 140/min, crackles on the left base.

A.B.G. ???A.B.G. ???

Scenario 2Scenario 210 months old baby, ex 10 months old baby, ex premprem 29 weeks, on 29 weeks, on home Ohome O22. Brought in by mum Re: increased O. Brought in by mum Re: increased O22requirement, difficulty to breathe. She is on requirement, difficulty to breathe. She is on some chronic medications. some chronic medications.

O/E: Audible wheezing, prolonged expiration O/E: Audible wheezing, prolonged expiration phase, saturation 90% in air. RR 60/minute. phase, saturation 90% in air. RR 60/minute. Mildly Mildly hyperinflatedhyperinflated chest.chest.

A.B.G. ???A.B.G. ???

Scenario 2 ContScenario 2 Cont’’ddFlow drive CPAP started. Tolerated well.Flow drive CPAP started. Tolerated well.

O/E: more settled. RR40/min, O/E: more settled. RR40/min, SatsSats 97% on 40% 97% on 40% OO22..

A.B.G. 30min laterA.B.G. 30min later……??????

Scenario 3Scenario 37 year old girl. RTA, passenger in a car involved 7 year old girl. RTA, passenger in a car involved in a head on collision at 35 mph.in a head on collision at 35 mph.Brought in by ambulance. C/O chest pain on the Brought in by ambulance. C/O chest pain on the right side and breathlessness. She is very right side and breathlessness. She is very irritable.irritable.O/E: Confused patient, RR 40/minute, shallow O/E: Confused patient, RR 40/minute, shallow respiratory efforts, bruised right chest.respiratory efforts, bruised right chest.

A.B.G. ???A.B.G. ???

Scenario 4Scenario 412 year old female. Brought to ED by parents. 12 year old female. Brought to ED by parents. She was upset with her mother and ingested an She was upset with her mother and ingested an insecticide few hours ago. Her father works on a insecticide few hours ago. Her father works on a farm.farm.Unconscious patient, drooling++ and Unconscious patient, drooling++ and diaphoretic, has wetted herself twice, constricted diaphoretic, has wetted herself twice, constricted pupils, HR 120/minute, RR10/minute.pupils, HR 120/minute, RR10/minute.

A.B.G. ???A.B.G. ???

Scenario 5Scenario 510 year old boy rescued from a burning house by 10 year old boy rescued from a burning house by firefighters and brought in A&E by ambulance. firefighters and brought in A&E by ambulance. He sustained 28% burns involving his face.He sustained 28% burns involving his face.O/E: He is in pain. His sputum is carbonaceous, O/E: He is in pain. His sputum is carbonaceous, RR 30/minute, HR 100/minute. He is confused RR 30/minute, HR 100/minute. He is confused but saturation reading on the pulse but saturation reading on the pulse oxymeteroxymeter is is 98%.98%.

A.B.G. ???A.B.G. ???

Scenario 6Scenario 66 months old baby boy, known congenital 6 months old baby boy, known congenital cyanotic heart defect (TOF). Brought to you by cyanotic heart defect (TOF). Brought to you by his mother. C/O 1 day history of diarrhoea, poor his mother. C/O 1 day history of diarrhoea, poor feeding and fever.feeding and fever.

O/E: Cyanosed child and irritable. RR 66/minute, O/E: Cyanosed child and irritable. RR 66/minute, HR 145/minute.HR 145/minute. 7.5% dehydrated.7.5% dehydrated.

A.B.G. ???A.B.G. ???

Scenario 7Scenario 7A 7 year old child has been poorly for one A 7 year old child has been poorly for one week with a flu like illness, dry cough, week with a flu like illness, dry cough, vomiting, poor appetite, moderate to vomiting, poor appetite, moderate to severe abdominal pain.severe abdominal pain.

A.B.G. ???A.B.G. ???

Scenario 8Scenario 82yr old Ventilated in PICU. IP 22cmH2yr old Ventilated in PICU. IP 22cmH22O,O,TV 8ml/Kg, RR24, PEEP6, FiO2 35%.TV 8ml/Kg, RR24, PEEP6, FiO2 35%.Sudden deterioration: OSudden deterioration: O22 Sat 85%Sat 85%What will you do?What will you do?

A.B.G. ???A.B.G. ???

Scenario 10Scenario 10A 9 month old baby is brought to A&E by A 9 month old baby is brought to A&E by ambulance. She has been unwell since waking ambulance. She has been unwell since waking up today. Poor feeding and not playing as usual.up today. Poor feeding and not playing as usual.She became lethargic in early afternoon. Mum She became lethargic in early afternoon. Mum called 999.called 999.O/E: Awake, Temp 38.6C, RR 40/minute, O/E: Awake, Temp 38.6C, RR 40/minute, mottled skin, few petechial rash on both thighs.mottled skin, few petechial rash on both thighs.

A.B.G. ???A.B.G. ???

ConclusionConclusion

7. Summary & Conclusion7. Summary & Conclusion

BG is an important test in the BG is an important test in the assessment of respiratory and assessment of respiratory and metabolic adequacymetabolic adequacy•• Neutral pH, Acidosis or AlkalosisNeutral pH, Acidosis or Alkalosis•• OxygenationOxygenation•• COCO22 removalremoval•• Metabolic status, electrolytesMetabolic status, electrolytes

Very useful test in A&E, HDU, ICUVery useful test in A&E, HDU, ICU……Careful interpretation results in Careful interpretation results in better better patient management.patient management.