Basics in Arterial Blood

Gas Interpretation

Crisbert I. Cualteros, M.D.

Obtaining Blood Gas Obtaining Blood Gas SamplesSamples

Radial artery- best siteRadial artery- best site located superficially, easy to palpate located superficially, easy to palpate

& stabilize& stabilize excellent collateral circulation via excellent collateral circulation via

ulnar arteryulnar artery not adjacent to large veinsnot adjacent to large veins probing needle relatively pain-free if probing needle relatively pain-free if

periosteum is avoidedperiosteum is avoided

TechniqueTechnique for Radial Artery for Radial Artery PuncturePuncture

Explain process to patient. Examine skin, Explain process to patient. Examine skin, palpate radial & ulnar arteries. Perform palpate radial & ulnar arteries. Perform modified Allen Test.modified Allen Test.

The Allen TestThe Allen Test

have the patient have the patient clench his/her fistclench his/her fist

press on both press on both radial and ulnar radial and ulnar arteriesarteries

have the patient have the patient unclench fistunclench fist

test for good test for good collateral flow.collateral flow.

Technique for Radial Artery Technique for Radial Artery PuncturePuncture

Position patient- hyperextend wrist. Clean Position patient- hyperextend wrist. Clean site with 70% isopropyl alcohol.site with 70% isopropyl alcohol.

Use latex gloves while doing procedure.Use latex gloves while doing procedure. Local anesthesia may be used.Local anesthesia may be used. Use G20 or G21 needle. Flush syringe with Use G20 or G21 needle. Flush syringe with

sodium heparin (10 mg/ml or 1,000 sodium heparin (10 mg/ml or 1,000 units/ml) & empty. units/ml) & empty. 0.15-0.25 ml of heparin will 0.15-0.25 ml of heparin will anticoagulate 2-4 ml of blood.anticoagulate 2-4 ml of blood.

Technique for Radial Artery Technique for Radial Artery PuncturePuncture

Palpate artery with one hand while holding Palpate artery with one hand while holding properly prepared syringe & needle with otherproperly prepared syringe & needle with otherhand. Hold syringe like a pencil & enter skin at hand. Hold syringe like a pencil & enter skin at 4545o. Advance needle slowly.

Never redirect needle without first withdrawing to Never redirect needle without first withdrawing to subcutaneous tissue.subcutaneous tissue.

Obtain 2-4 ml blood. If possible don’t aspirate.Obtain 2-4 ml blood. If possible don’t aspirate. Remove air bubbles from syringe. Immediately Remove air bubbles from syringe. Immediately

seal syringe with cap.seal syringe with cap. Place sample in ice slush. Analyze blood sample Place sample in ice slush. Analyze blood sample

within 10 minutes.within 10 minutes. Apply pressure to site until bleeding has stopped.Apply pressure to site until bleeding has stopped.

Potential ComplicationsPotential Complications

PainPain

Hematoma, hemorrhageHematoma, hemorrhage

Trauma to vesselTrauma to vessel

ArteriospasmArteriospasm

Air or clotted-blood Air or clotted-blood

emboliemboli

Vasovagal responseVasovagal response

Arterial occlusionArterial occlusion

InfectionInfection

Indications for ABGIndications for ABG

Assess ventilation & acid-base Assess ventilation & acid-base balancebalance

Assess oxygenation statusAssess oxygenation status

Ventilatory/ Ventilatory/ Acid-Base StatusAcid-Base Status

Henderson-Hasselbach Parameters & their Henderson-Hasselbach Parameters & their normal laboratory rangesnormal laboratory ranges

pH= pH= [HCO[HCO33]p]p

PPC02C02

pHpH PCO2PCO2

(mmHg)(mmHg)[HCO3]p[HCO3]p

(mmol/L)(mmol/L)

NormalNormal 7.35-7.457.35-7.45 35-4535-45 22-2622-26

AcidoticAcidotic < 7.35< 7.35 > 45> 45 < 22< 22

AlkaloticAlkalotic > 7.45> 7.45 < 35< 35 > 26> 26

Traditional Metabolic Acid-Base NomenclatureTraditional Metabolic Acid-Base Nomenclature

NomenclatureNomenclature pHpH PCOPCO22[HCO3][HCO3]pp

BEBE

Metabolic acidosisMetabolic acidosisUncompensated (acute)Uncompensated (acute) NN (-)(-)Partly compensated Partly compensated (subacute)(subacute)

(-)(-)

Compensated (chronic)Compensated (chronic) NN (-)(-)

Metabolic alkalosisMetabolic alkalosisUncompensated (acute)Uncompensated (acute) NN (+)(+)

Partly compensated Partly compensated (subacute)(subacute)

(+)(+)

Compensated (chronic)Compensated (chronic) NN (+)(+)

Traditional Respiratory Acid-Base NomenclatureTraditional Respiratory Acid-Base NomenclatureNomenclatureNomenclature pHpH PCOPCO

22[HCO3][HCO3]pp

BEBE

Respiratory Respiratory acidosisacidosisUncompensated (acute)Uncompensated (acute) NN NNPartly compensated Partly compensated (subacute)(subacute)

Compensated (chronic)Compensated (chronic) NN Respiratory Respiratory alkalosisalkalosisUncompensated (acute)Uncompensated (acute) NN NNPartly compensated Partly compensated (subacute)(subacute)

Compensated (chronic)Compensated (chronic) NN

Base Excess/ DeficitBase Excess/ Deficit Blood with large buffering capacity:Blood with large buffering capacity:

significant changes in acid content with little change in free Hsignificant changes in acid content with little change in free H++ concentrations (pH)concentrations (pH)

Acidemia or alkalemia: Acidemia or alkalemia: buffering capacity, > potential for pH buffering capacity, > potential for pH change from any given change in Hchange from any given change in H++ content content

Buffering capacity depends on:Buffering capacity depends on:[HCO[HCO33

--]]RBC massRBC massother factors other factors

Base excess/deficit= (measured pH – predicted pH) x 100 x 2/3Base excess/deficit= (measured pH – predicted pH) x 100 x 2/3Normal metabolic acid-base status:Normal metabolic acid-base status: ++ 3 mmol/L 3 mmol/LRelatively balanced metabolic acid-base status:Relatively balanced metabolic acid-base status: ++ 5 5

mmol/Lmmol/LClinically significant imbalance:Clinically significant imbalance: ++ 10 mmol/L 10 mmol/L

Nomenclature & Criteria for Clinical InterpretationNomenclature & Criteria for Clinical Interpretation

Clinical TerminologyClinical Terminology CriteriaCriteria

Ventilatory failureVentilatory failure (respiratory acidosis) (respiratory acidosis) PPaaCOCO22 > 45 mm > 45 mm HgHg

Acute ventilatory failureAcute ventilatory failure (respiratory acidosis) (respiratory acidosis) PPaaCOCO22 > 45 > 45 mmHgmmHg pH < 7.35pH < 7.35

Chronic ventilatory failureChronic ventilatory failure (respiratory acidosis) (respiratory acidosis) PPaaCOCO2 2 > 45 > 45 mmHgmmHgpH 7.36- 7.44pH 7.36- 7.44

Alveolar hyperventilationAlveolar hyperventilation (respiratory alkalosis) (respiratory alkalosis) PPaaCOCO22 < 35 < 35 mmHgmmHg

Acute alveolar hyperventilationAcute alveolar hyperventilation (respiratory (respiratory PPaaCOCO22 < 35 mmHg < 35 mmHgalkalosis)alkalosis) pH > 7.45pH > 7.45

Chronic alveolar hyperventilationChronic alveolar hyperventilation (respiratory (respiratory PPaaCOCO2 2 < 35 < 35 mmHgmmHg alkalosis)alkalosis) pH 7.36-7.44pH 7.36-7.44

Nomenclature & Criteria for Clinical InterpretationNomenclature & Criteria for Clinical Interpretation

Clinical TerminologyClinical Terminology CriteriaCriteria

AcidemiaAcidemia pH < 7.35pH < 7.35AlkalemiaAlkalemia pH > 7.45pH > 7.45AcidosisAcidosis HCOHCO33

- - < 22 mmol/L< 22 mmol/LBD > 5 mmol/LBD > 5 mmol/L

AlkalosisAlkalosis HCOHCO33-- > 26 mmol/L > 26 mmol/L

BE > 5 mmol/LBE > 5 mmol/LCombinedCombined Respiratory Acidosis & Metabolic AcidosisRespiratory Acidosis & Metabolic Acidosis

Respiratory Alkalosis & Metabolic AlkalosisRespiratory Alkalosis & Metabolic AlkalosisMixedMixed Respiratory Acidosis & Metabolic AlkalosisRespiratory Acidosis & Metabolic Alkalosis

Respiratory Alkalosis & Metabolic AcidosisRespiratory Alkalosis & Metabolic Acidosis

Respiratory Respiratory AcidosisAcidosis

AcuteAcute

pH = pH = 0.08 x (PCO0.08 x (PCO22 – 40) – 40)

1010

ex. PCOex. PCO2 2 = 60= 60

pH = pH = 0.08 x (60 - 40)0.08 x (60 - 40) = 0.16 = 0.16

1010

expected pH = 7.40 – 0.16 = 7.24expected pH = 7.40 – 0.16 = 7.24

HCOHCO33-- increases 0.1 – 1 meq/L per 10 mmHg PCO increases 0.1 – 1 meq/L per 10 mmHg PCO2 2 increaseincrease

Compensation: cellular buffering: HCO3

renal adaptation: H+ secretion, Cl-

reabsorption, net acid excretion

Respiratory acidosisRespiratory acidosis

ChronicChronic

pH = pH = 0.03 x (PCO0.03 x (PCO22 – 40) – 40)

1010

ex. PCOex. PCO22 = 60 = 60

pH = pH = 0.03 x (60 – 40) 0.03 x (60 – 40) = 0.06= 0.06

1010

expected pH = 7.40 – 0.06 = 7.34expected pH = 7.40 – 0.06 = 7.34

HCOHCO33-- increases 1-3.5 meq/L per 10 mmHg PCO increases 1-3.5 meq/L per 10 mmHg PCO22

increaseincrease

Respiratory Respiratory AcidosisAcidosis

COPDCOPD OO22 excess in COPD excess in COPD DrugsDrugs

• BarbituratesBarbiturates• AnestheticsAnesthetics• NarcoticsNarcotics• SedativesSedatives

Extreme ventilation-Extreme ventilation-perfusion mismatchperfusion mismatch

Exhaustion Exhaustion Inadequate MVInadequate MV Neurologic disordersNeurologic disorders

Neuromuscular diseaseNeuromuscular disease• PoliomyelitisPoliomyelitis• ALLALL• G-B syndromeG-B syndrome• Electrolyte deficiencies Electrolyte deficiencies

(K(K++, PO, PO44--))

• Myasthenia gravisMyasthenia gravis Excessive COExcessive CO22

productionproduction• TPNTPN• SepsisSepsis• Severe burnsSevere burns• NaHCONaHCO33 administration administration

Respiratory Respiratory AlkalosisAlkalosis

AcuteAcute

pH = pH = 0.08 x (40 – PCO0.08 x (40 – PCO22))

1010

ex. PCOex. PCO22 = 20 = 20

pH = pH = 0.08 x (40 – 20)0.08 x (40 – 20) = 0.16 = 0.16

1010

expected pH = 7.40 + 0.16 = 7.56expected pH = 7.40 + 0.16 = 7.56

HCOHCO33-- decreases 0-2 meq/L per 10 mmHg PCO decreases 0-2 meq/L per 10 mmHg PCO22

decreasedecrease

Compensation: cellular buffering

renal response: retention of endogenous acids, excretion of HCO3

Respiratory AlkalosisRespiratory Alkalosis

ChronicChronic

pH = pH = 0.03 x (40 – PCO0.03 x (40 – PCO22))

1010

ex. PCOex. PCO22 = 20 = 20

pH = pH = 0.03 x (40 – 20)0.03 x (40 – 20) = 0.06 = 0.06

1010

expected pH = 7.40 + 0.06 = 7.46expected pH = 7.40 + 0.06 = 7.46

HCOHCO33-- decreases 2-5 meq/L per 10 mmHg PCO decreases 2-5 meq/L per 10 mmHg PCO22

decreasedecrease

Respiratory AlkalosisRespiratory Alkalosis

Primary central Primary central disordersdisorders

Hyperventilation Hyperventilation syndrome, anxietysyndrome, anxiety

Cerebrovascular diseaseCerebrovascular disease Meningitis, encephalitisMeningitis, encephalitisPulmonary diseasePulmonary disease Interstitial fibrosisInterstitial fibrosis PneumoniaPneumonia Pulmonary embolismPulmonary embolism Pulmonary edema Pulmonary edema

(some patients)(some patients)

HypoxiaHypoxiaSepticemia, Septicemia,

hypotensionhypotensionHepatic failureHepatic failureDrugsDrugs SalicylatesSalicylates NicotineNicotine XanthinesXanthines Progestational Progestational

hormoneshormonesHigh altitudeHigh altitudeMechanical ventilatorsMechanical ventilators

Metabolic AcidosisMetabolic Acidosis

Anion GapAnion Gap artificial disparity between major plasma cations artificial disparity between major plasma cations

& anions that are routinely measured& anions that are routinely measured major plasma cationsmajor plasma cations – – major plasma anionsmajor plasma anions [Na[Na++]] – – ([Cl([Cl--] + [HCO3] + [HCO3--])]) 12 12 ++ 2 (normal) 2 (normal) Minor cations: KMinor cations: K++, Ca, Ca++++

Minor anions: phosphates, sulfates, organic Minor anions: phosphates, sulfates, organic anionsanions

Metabolic Metabolic AcidosisAcidosis

Anion gap acidosisAnion gap acidosis

~ process increases “minor anions”~ process increases “minor anions”

~ ex. lactatemia, ketonemia, renal failure, excessive ~ ex. lactatemia, ketonemia, renal failure, excessive

organic salt treatment, dehydration, ingestion organic salt treatment, dehydration, ingestion

(salicylates, methanol, ethylene glycol, (salicylates, methanol, ethylene glycol, paraldehyde)paraldehyde)

~ process which decreases “minor cations” rare!~ process which decreases “minor cations” rare! Non-anion gap acidosisNon-anion gap acidosis

~ associated with increased plasma Cl~ associated with increased plasma Cl-- that has replaced that has replaced

HCOHCO33--

~ ex. GI loss of HCO~ ex. GI loss of HCO33-- (diarrhea), renal wasting of HCO (diarrhea), renal wasting of HCO33

--

(RTA), ingestion of acids, parenteral (RTA), ingestion of acids, parenteral hyperalimentation, carbonic anhydrase inhibitors hyperalimentation, carbonic anhydrase inhibitors

Metabolic Metabolic AcidosisAcidosis

Abnormalities:Abnormalities: Overproduction of acidsOverproduction of acids Loss of buffer storesLoss of buffer stores Underexcretion of acidsUnderexcretion of acids

Metabolic AcidosisMetabolic Acidosis

Expected PCOExpected PCO22 = ( [HCO = ( [HCO33--] x 1.5) + 8 ] x 1.5) + 8 ++ 2 2

ex. [HCOex. [HCO33--] = 11] = 11

expected PCOexpected PCO22 = (11 x 1.5) + 8 = (11 x 1.5) + 8 ++ 2 = 22.5- 26.5 2 = 22.5- 26.5

PCOPCO22 decreases 1- 1.5 mmHg per 1 meq/L HCO decreases 1- 1.5 mmHg per 1 meq/L HCO33- -

decreasedecrease

Metabolic Metabolic AcidosisAcidosis

CompensationCompensation pCOpCO22 (hyperventilation) (hyperventilation) Pathway:Pathway:

pCO2

HCO3

ratio H+ conc

Acidification of ECF ECF pH

Stimulation of brainstem RR pCO2

Normalization of pH

HCO3

Metabolic Metabolic AcidosisAcidosis

CompensationCompensation Ionic shiftIonic shift

• KK++ moves extracellularly for H moves extracellularly for H++

• HCOHCO33-- generation, H generation, H++ excretion excretion

Corrected [HCOCorrected [HCO33--] for Anion Gap ] for Anion Gap

Metabolic AcidosisMetabolic Acidosis

Measured serum [HCOMeasured serum [HCO33--] + (anion gap – ] + (anion gap –

12)12)

Metabolic AlkalosisMetabolic Alkalosis

Expected PCOExpected PCO22 = ( [HCO = ( [HCO33--] x 0.75 ) + 20 ] x 0.75 ) + 20 ++ 5 5

ex. [HCOex. [HCO33--] = 34] = 34

expected PCOexpected PCO2 2 = (34 x 0.75) + 20 = (34 x 0.75) + 20 ++ 5 = 40.5- 5 = 40.5- 50.550.5

PCOPCO22 increases 0.5- 1 mmHg per 1 meq/L HCO increases 0.5- 1 mmHg per 1 meq/L HCO33--

increaseincrease

Metabolic Metabolic AlkalosisAlkalosis

Pathway Pathway

HCO3PaCO2

HCO3

ratio H+ conc

Alkalinization of ECF PaCO2 with mild hypoxemia

Normalization of pH

Causes of Metabolic Alkalosis

Hypokalemia*Ingestion of large amounts of alkali or licoriceGastric fluid loss: Vomiting, NG suctioning*Hyperaldosteronism 20 to nonadrenal factors Bartter’s syndrome Inadequate renal perfusion diuretics (inhibiting NaCl reabsorption)*Bicarbonate administration Sodium bicarbonate overcorrection Blood transfusionAdrenocortical hypersecretion (e.g tumor)Steroids*Eucapnic ventilation posthypercapnia

* Common in the ICU

Limits of CompensationLimits of CompensationImbalance [HCO3

-] meq/L PCO2 mmHg

Respiratory AcidosisRespiratory Acidosis

Acute Acute 0.1- 1/ 10 mmHg PCO0.1- 1/ 10 mmHg PCO22

ChronicChronic 1- 3.5/ 10 mmHg1- 3.5/ 10 mmHg

PCOPCO22Respiratory AlkalosisRespiratory Alkalosis

AcuteAcute 0- 2/ 10 mmHg PCO0- 2/ 10 mmHg PCO22ChronicChronic 2- 5/ 10 mmHg PCO2- 5/ 10 mmHg PCO22

Metabolic AcidosisMetabolic Acidosis 1- 1.5/ 1 meq/L 1- 1.5/ 1 meq/L

[HCO[HCO33--] ]

Metabolic AlkalosisMetabolic Alkalosis 0.5- 1/ 1 meq/L 0.5- 1/ 1 meq/L

[HCO[HCO33--] ]

Steps for Analyzing Acid- Base Steps for Analyzing Acid- Base DisturbancesDisturbances

Is patient acidemic or alkalotic? Is patient acidemic or alkalotic? pHpH Is disturbance primarily respiratory or metabolic? Is disturbance primarily respiratory or metabolic?

PCOPCO22, [HCO, [HCO33--]]

If disturbance respiratory, is it acute or chronic?If disturbance respiratory, is it acute or chronic? If disturbance metabolic, is anion gap normal or If disturbance metabolic, is anion gap normal or

abnormal? abnormal? If disturbance metabolic, is the respiratory If disturbance metabolic, is the respiratory

system compensating adequately?system compensating adequately? If disturbance is anion gap metabolic acidosis, are If disturbance is anion gap metabolic acidosis, are

there any other metabolic disturbances present?there any other metabolic disturbances present?

Oxygenation StatusOxygenation Status

Normal ValuesNormal Values

Seated PO2 = 104.2 – 0.27 (age in years)Seated PO2 = 104.2 – 0.27 (age in years)

Supine PO2 = 103.5 – 0.42 (age in years)Supine PO2 = 103.5 – 0.42 (age in years)

Patients Patients << 60 y. o. 60 y. o.

PO2 = 100 PO2 = 100 ++ 20 20

Patients > 60 y. o.Patients > 60 y. o.

PO2 = 80 – (# years > 60)PO2 = 80 – (# years > 60)

Steps for Analyzing Steps for Analyzing Oxygenation StatusOxygenation Status

1. Is the patient hypoxemic or normoxemic?1. Is the patient hypoxemic or normoxemic?

Indices of Oxygenation:Indices of Oxygenation:a. AaDOa. AaDO22 = PAO = PAO22 – PaO – PaO22

PAOPAO22 = FiO = FiO22 (713) – (713) – PaCOPaCO22

0.80.8PaOPaO22 = obtained from blood gas determination = obtained from blood gas determination

b. aAOb. aAO22 = = PaOPaO22

PAOPAO22

c. P/F ratio = c. P/F ratio = POPO22

FiOFiO22

Normal Value:Normal Value: patients patients << 60 y. o. > 400 60 y. o. > 400patients > 60 y. o. expected P/F = 400 patients > 60 y. o. expected P/F = 400 – – [(age in years – 60) x 5][(age in years – 60) x 5]

Actual P/F Ratio < expected =Actual P/F Ratio < expected = hypoxemic hypoxemicActual P/F Ratio Actual P/F Ratio >> expected = expected = normoxemicnormoxemic

2. If hypoxemic, is it uncorrected, 2. If hypoxemic, is it uncorrected, corrected, or overcorrected? corrected, or overcorrected?

With OWith O2 2 supplementationsupplementationPaOPaO2 2 (mmHg)(mmHg)

Uncorrected hypoxemiaUncorrected hypoxemia < 80< 80Corrected hypoxemiaCorrected hypoxemia 80 – 12080 – 120OvercorrectedOvercorrected > 120> 120

FiOFiO22 to PaO to PaO22 Relationship in Normal Lungs Relationship in Normal LungsFiOFiO22 PaOPaO22 (mmHg) (mmHg)0.300.30 > 150> 1500.400.40 > 200> 2000.500.50 > 250> 2500.800.80 > 400> 4001.001.00 > 500> 500

Room Air (patient Room Air (patient << 60 y. o.) 60 y. o.)

PaOPaO22 (mmHg) (mmHg)

Mild hypoxemiaMild hypoxemia 60 to < 8060 to < 80

Moderate hypoxemiaModerate hypoxemia 40 to < 6040 to < 60

Severe hypoxemiaSevere hypoxemia < 40< 40

For each year > 60 subtract 1 mmHg for limits of For each year > 60 subtract 1 mmHg for limits of mild & mild &

moderate hypoxemia.moderate hypoxemia.

At any age, PaOAt any age, PaO2 2 < 40 mmHg indicates severe < 40 mmHg indicates severe hypoxemia.hypoxemia.

3. If normoxemic, is oxygenation3. If normoxemic, is oxygenation adequate or more than adequate or more than

adequate? adequate?

Thank you !Thank you !