dr. Husnil Kadri, M.Kes

Biochemistry Departement Medical Faculty Of Andalas University Padang

Arterial Blood Gases Aids in establishing a diagnosis Helps guide treatment planAids in ventilator managementImprovement in acid/base management allows for optimal function of medicationsAcid/base status may alter electrolyte levels critical to patient status/care

LogisticsWhen to order an arterial line --Need for continuous BP monitoringNeed for multiple ABGsWhere to place (with antikoagulant)A. RadialA. Femoral A. BrachialA. Dorsalis PedisA. Axillary

The ComponentsDesired Ranges:pH ; 7.35 - 7.45PaCO2 ; 35-45 mmHgPaO2 ; 80-100 mmHgHCO3 ; 21-27O2sat ; 95-100%Base Excess ; +/-2 mEq/L

Arterial Blood GasesReflect oxygenation, gas exchange, and acid-base balancePaO2 is the partial pressure of oxygen dissolved in arterial bloodSaO2 is the amount of oxygen bound to hemoglobin

Base Excess Definition: The amount of a strong acid (like HCl) needed to bring blood to 7.40.Assumes 100% oxygenation, 37oC, and pCO2 of 40.

Normal = 0Used to calculate the metabolic component of an acid-base disturbance.

Base Excess calculationsCalculated the same way, in practice, as SID:Buffer Base (SID) = HCO3- + A-HCO3 calculated by pH & pCO2 (blood gas machine)BE = Buffer Base expected buffer base (expected if pH = 7.4 and pCO2 = 40) A- calculated using pH & hemoglobin (whole blood)OR A- calculated using albumin & phos (plasma)

*Indicators of hypoxaemia and hypoxia

Arterial blood gasesLab FindingsPO280-100 mm Hg (normal)60-80 mm Hg (mild hypoxemia)40-60 mm Hg (moderate hypoxemia)

Is it Respiratory or Metabolic?Respiratory Acidosis

Respiratory Alkalosis

Metabolic Acidosis

Metabolic AlkalosisIncreased pCO2 >50

Decreased pCO2

Compensated or Uncompensatedwhat does this mean?Evaluate pHis it normal? Yes

Next evaluate pCO2 & HCO3

pH normal + increased pCO2 + increased HCO3 = compensated respiratory acidosis

pH normal + decreased HCO3 + decreased pCO2 = compensated metabolic acidosis

Compensated vs. UncompensatedIs pH normal? NoAcidotic vs. AlkaloticRespiratory vs. MetabolicpH50 + normal HCO3 = uncompensated respiratory acidosispH30 + normal pCO2 = uncompensated metabolic alkalosis

Causes of AcidosisRespiratoryHypoventilationImpaired gas exchangeMetabolicKetoacidosisDiabetesRenal Tubular AcidosisRenal FailureLactic AcidosisDecreased perfusionSevere hypoxemia

Causes of AlkalosisRespiratoryHyperventilation due to:HypoxemiaMetabolic acidosisNeurologicLesionsTraumaInfection

MetabolicHypokalemia

Gastric suction or vomiting

Hypochloremia

*Mixed Metabolic Acidosis and Chronic Respiratory AlkalosisExamples:SepsisAddition of respiratory alkalosis to metabolic acidosis further decreases HCO3- but pH may remain normalLactic acidosis plus respiratory alkalosis due to severe liver disease, pulmonary emboli, or sepsis

*Mixed Metabolic Alkalosis and Chronic Respiratory AcidosisExamples:Patient with COPD receiving glucocorticoids or diureticspCO2 and HCO3- are increased by both conditions, but pH is neutralized

*Mixed Alkalosis, SevereExample:Postoperative patient with severe hemorrhage stimulating hyperventilation [respiratory alkalosis] plus massive transfusion and nasogastric drainage [metabolic alkalosis]

*Mixed Chronic Respiratory Acidosis and Acute Metabolic AcidosisExamples:COPD [chronic respiratory acidosis] with severe diarrhoea [metabolic acidosis]. pH is too low for pCO2 of 55 mmHg in chronic respiratory acidosis, indicating low pH due to mixed acidosis, but HCO3- effect is offset

*Mixed Metabolic Acidosis and Metabolic AlkalosisExamples:Gastroenteritis with vomiting [metabolic alkalosis] and diarrhoea [metabolic acidosis due to loss of HCO3-]; surprisingly normal findings with marked volume depletion

*Serum Values in Acid-Base Disturbances

ConditionNa+mmol/LCl-mmol/LHCO3-mmol/LpCO2 mmHgpHNormal14010525407.40Metabolic acidosis14011515317.30Chronic respiratory alkalosis13610225407.44Mixed metabolic acidosis and chronic respiratory alkalosis13610814247.39Metabolic alkalosis1409236487.49Chronic respiratory acidosis140100-10228507.37Mixed metabolic alkalosis and chronic respiratory acidosis1409040677.40

Serum Values in Acid-Base Disturbances*

ConditionNa+mmol/LCl-mmol/LHCO3-mmol/LpCO2 mmHgpHNormal136-145100-10624-2635-457.35-7.45Metabolic alkalosis1398935477.49Respiratory alkalosis13610220307.44Mixed alkalosis, mild1399232397.53Mixed alkalosis, severe1399232307.63Mixed chronic respiratory acidosis and acute metabolic acidosis13610222557.22Mixed metabolic acidosis and metabolic alkalosis14010325407.40

*Summary of Pure and Mixed Acid-Base DisordersSource: Adapted from Friedman HH. Problem-oriented medical diagnosis, 3rd ed. Boston: Little, Brown. 1983

Decreased pHNormal pHIncreased pH pCO2Respiratory acidosis with or without incompletely compensated metabolic alkalosis or coexisting metabolic acidosisRespiratory acidosis and compensated metabolic alkalosisMetabolic alkalosis with incompletely compensated respiratory acidosis or coexisting respiratory acidosisNormal pCO2Metabolic acidosisNormalMetabolic alkalosis

ReferencesAnisman, S. Base Excess & Strong Ion Theories. ppt. 2003.Klee, V. Arterial Blood Gas Analysis.ppt. 2012.Perkins, J. ABG Interpretation. ppt. 2012.Rashid, FA. Respiratory Mechanisms in Acid-Base Homeostasis.ppt. 2005.

*********HYPOKALEMIAkidneys hold on to K+ and excrete H+ causing increase blood baseGASTRIC SX/VOMIT--loss of HCLHYPOCHLOREMIACL- depleted so HCO3- increased to maintain electrical balance*