ARTERIAL BLOOD GAS

Section of Pediatric PulmonologyUPCM-Philippine General Hospital

Clinical Application of Blood Gases5th EditionShapiro, et. Al.

ABG measures respiratory

function1. Oxygenation status2. Acid-base balance

Points to Remember Body always tries to maintain normal pH CO2 respiratory HCO3 metabolic Lungs compensate rapidly Kidneys compensate slowly There is no overcompensation except in chronic ventilatory failure Consider the underlying disease

Normal ABG Values pH 7.35-7.45 7.3-7.5 clinically acceptable

PO2 80-100 60-80 for newborn, preterm < 1 mm Hg for every year > age 60 PCO2 35-45 30-50 clinically acceptable

Oxygenation Status At room air, sea level: PaO2 80-100 normal or acceptable PaO2 < 80 mild hypoxemia PaO2 < 60 moderate hypoxemia PaO2 < 40 severe hypoxemia

On oxygen support:PaO2 80-100 corrected hypoxemiaPaO2 > N overcorrected hypoxemia PaO2 < N uncorrected hypoxemia

Nomenclature for pH & PaCO2 Outside ofnormal rangepH > 7.45AlkalemiapH < 7.35AcidemiaPaCO2 > 45 mmHgRespiratory Acidosis(hypercapnia)PaCO2 < 35 mmHgRespiratory Alkalosis(hypocapnia)

FiO2 vs PaO2The minimally acceptable PaO2 increases byapproximately 50 mmHg for every 10% incrementof inspired oxygen concentration.

FiO2 vs PaO2PaO2 = FiO2 x 5

What is the expected PAO2if you are breathing normally at room air? = [ .21 (760 47) ] 40 / 0.8 = 150 50 = 100 PaO2 = FiO2 x 5 = .21 x 5 = 105

PAO2 vs PaO2In normal individuals the PAO2 is more or less equal to the PaO2 with a normal shunt of about 5 %

FiO2 vs PaO2Normal: < 30 mm Hg at room air < 50 mm Hg at FiO2 1.0 > 450 mmHg is indicative of severe respiratory failure

PaCO2 pH RELATIONSHIPFor every 10 mm Hg increase in the PaCO2, thepH will decrease by 0.05 unit.

For every 10 mm Hg decrease in the PaCO2, the pH will increase by 0.10 unit.

Respiratory pH (pHR)PaCO2 pHR40 7.460 7.380 7.230 7.520 7.6

Respiratory pH (pHR)PaCO2 pHR40 7.460 7.380 7.2

Computing for pHRNormal pCO2 = 40 and normal pH = 7.4pHR = (40 Actual pCO2) x 0.05 + 7.4 10If actual pCO2 > 40:

Respiratory pH (pHR)

Computing for pHRNormal pCO2 = 40 and normal pH = 7.4pHR = (40 Actual pCO2) x 0.05 + 7.4 10If actual pCO2 > 40:If actual pCO2 < 40:

What is the expected pH when the pCO2 is 45? = 7.375

What is the expected pH when the pCO2 is 35? = 7.45

pHR vs pH If pHR compared to actual pH is: < 0.03 purely respiratory > 0.03 compensated

pHR vs pHIf actual pH > pHR partially compensated

If actual pH < pHR mixed

If actual pH = pHR purely respiratory

PaCO2 Plasma BICARBONATE RELATIONSHIPAn acute PaCO2 increase of 10 mmHg will increase the plasma bicarbonate by 1 mmol/L

An acute PaCO2 decrease of 10 mmHg will decrease the plasma bicarbonate by 2 mmol/LThe difference between the calculated respiratoryplasma bicarbonate value and the actual plasma bicarbonate value provides a rapid and easyassessment of the metabolic component.

Approximate PaCO2-pH RelationshipPaCO2pHHCO3(mmHg)(mmol/L)307.5022207.6020807.2028607.3026407.4024

Minute Ventilation vs PaCO2 MV PaCO2 Range

N40 35-45 2N30 25-35 4N20 15-25

The existence of a significant minute MV to PaCO2 disparity should alert the clinician to the possibility that a deadspace-producing pathologic condition may be present.

FiO2-PaO2 RELATIONSHIPInspired Oxygen to PaO2 Relationship in Normal lungs.FiO2 Inspired O2(%)PaO20.3030>1500.4040>2000.5050>2500.8080>4001.00 100>500

DETERMINING BASE EXCESS/DEFICITUnder normal circumstances, a 10 mmol/L variance from the normal buffer base representsa pH change of approximately 0.15 unit.

If we move the pH decimal point two places to the right, we have a 10 to 15 relationship, whichcan be expressed as a 2/3 realtionship.

The difference between the measured pH & thepredicted respiratory pH is the metabolic pHchange.x 100

Base excess or deficit 0.15 pH change 10 mEq/L buffer change BE: actual pH > pHRBD: actual pH < pHRNormal BE or BD: 2

INTERPRETIVE APPROACHStep 1. Assessment of the PCO2 and pH.a. Classify the CO2 tension.b. Consider the pH and determineclassification.c. Consider the base excess/deficit or bicarbonate levels and determineclassification.Step 2. Assessment of Arterial Oxygenationa. PaO2b. SaO2

pCO2 < 35pCO2 35-45pCO2 > 45pH < 7.35 pH 7.35 7.4pH 7.4 7.45pH > 7.45normalnormal

pCO2 < 35pCO2 35-45pCO2 > 45pH < 7.35 pH 7.35 7.4pH 7.4 7.45pH > 7.45normalnormal

Physiologic Mechanisms of HypoxemiaAlveolar hypoventilationVentilation-perfusion mismatchRight-to-left shuntDiffusion limitationDecreased ambient oxygen tension

Physiologic Mechanisms of HypoxemiaAlveolar hypo-ventilation V/Q mismatchR to L shuntDiffusion limitationW white N - normal P - poor A - absent

Metabolic acidosis Renal failure (RTA) Ketoacidosis (DKA, starvation) Lactic acidosis

Anion Gap = Na (Cl + HCO3) Normal: < 15 mEq/L

Anion GapIncreased Normal Organic acid accumulationAcute renal failureInborn error of metabolismLactic acidosisLate metabolic acidosisToxinsLoss of buffer Renal HCO3 loss Renal tubular acidosis Acetazolamide Renal dysplasia GI HCO3 loss Diarrhea Cholestyramine Small bowel drainage Dilutional acidosis Hyperalimentation acidosis

Metabolic alkalosis Hypokalemia Hypochloremia Vomiting Massive steroid administration NaHCO3 administration

Respiratory acidosis1. HypoventilationInadequate respiratory effort CNS problemsNeuromuscular diseaseMechanical ventilator settingsUpper airway not patentDecreased lung tissueDecreased lung compliance

Respiratory acidosis2. Abnormal ventilation-perfusion ratioa. Obstruction of small airwaysb. Atelectasisc. Pneumoniad. Pulmonary edema3. Increased extrapulmonary shuntPulmonary vasoconstriction RDS, severe infectionPulmonary hypoplasiaCyanotic heart disease

Respiratory alkalosisWith hypoxemia a. Acute pulmonary diseasepneumonia and atelectasis, RDS, acute asthmaAcute myocardial diseaseMI, pulmonary edema, heart failure, CP bypass2. Without hypoxemiaAnxiety, neurosis, psychosisPainCNS diseaseAnemiaCarbon monoxide poisoning

Disorders Expected compensationMetabolic pCO2 = 1.5 x HCO3 + 8 +/- 2 Acidosis

Metabolic pCO2 increase by 7 mmHg for each Alkalosis 10 mEq/L increase in HCO3

Disorder Expected compensation

Respiratory acidosis Acute HCO3 increase by 1 for each 10mmHg increase in pCO2 Chronic HCO3 increase by 3.5 for each 10mmHg increase in pCO2 Respiratory Alkalosis Acute HCO3 decrease by 2 for each 10mmHg decrease in pCO2 Chronic HCO3 decrease by 4 for each 10mmHg decrease in pCO2

ExercisesCompute for the pHR and interpret the ABG values

1. pH 7.45 pO2 65 pCO2 32 FiO2 .21 compensatedwith mild hypoxemiaHCO3

2. pH 7.3 pO2 120 pCO2 30 FiO2 .30 partially compensatedwith overcorrected hypoxemiaHCO3

3. pH 7.25 pO2 90 pCO2 55 FiO2 .40 uncompensatedwith corrected hypoxemiarespiratoryHCO3N

4. pH 7.42 pO2 200 pCO2 35 FiO2 .35 with overcorrected hypoxemiaHCO3 N

Calculating FiO2 requirement using the ABGFiO2 = (desired PaO2) + PaCO2PaO2RQPAO2 Pb PH20