ARTERIAL BLOOD GAS ANALYSIS Arnel Gerald Q. Jiao, MD, FPPS, FPAPP Pediatric Pulmonologist Philippine Children’s Medical Center

Value ABG measurements can document, specify and quantify cardiopulmonary malfunction and acid- base abnormalities.

Limitation Absolute values should be interpreted in conjunction with the clinical situation.

Guidelines for Interpreting ABG’S The body always tries to maintain a normal ph The lungs compensate rapidly; the kidneys compensate slowly There is no overcompensation. Consider the underlying disease Maintain an adequate level of hemoglobin

Bicarbonate-Carbonic Acid Buffer System: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 -

Advantages of Arterial Over Venous Blood Represents a mixture of blood that has come from various parts of the body Gives information of how well lungs are oxygenating the blood

Normal Arterial Blood Gas Values pH: 7.35 – 7.45 paCO 2 : 35 – 45 mm Hg paO 2 : 80 – 100 mm Hg HCO 3 : 22 – 26 mEq/L BE/BD: - 2 to + 2 O 2 Sat: > 95 %

Clinically Acceptable Levels pH: 7.30 – 7.50 paCO2: 30 – 50 mm Hg paO2 Neonates: 60 – 80 mm Hg 2 months above: 80 – 100 mmHg Decreases with age: Subtract 1 mm Hg from 80 mm Hg for every year past the age of 60

Nomenclature for Clinical interpretation Acidosis: patho- physiologic state where a significant base deficit is present (HCO 3 < 22mEq/L) Alkalosis: patho- physiologic state where a significant base excess is present (HCO 3 > 26mEq/L)

Mathematical interrelationship among pH, pCO 2 and HCO 3 Basis for all Acid-Base interpretation: pH= HCO 3 /pCO 2

Clinical Approach to Interpretation: Steps 1.Assessment of the pCO 2 and pH: ventilatory status and acid-base balance 2. Assessment of Arterial Oxygenation

Step 1 Classify carbon dioxide tension Consider pH and determine classification Consider BE/BD or HCO 3 levels and determine classification

Step 1 Classification of PaCO 2 < 35 mmHg: alveolar hyperventilation (respiratory alkalosis) 35 – 45 mmHg: Normal alveolar ventilation > 45 mmHg: ventilatory failure (respiratory acidosis)

Step 1 Three questions to ask: Is the PaCO 2 abnormal? Is the pH explained by the level of PaCO 2 ? Yes: respiratory No: metabolic Is the pH: Abnormal: acute/uncompensated Normal: chronic/ compensated

Step 1 PaCO 2 < 35 mmHg pH < – pH > 7.45 Partially Compensated Metabolic Acidosis Compensated Metabolic Acidosis Chronic Respiratory Alkalosis Acute Respiratory Alkalosis

 PCO 2 < 35 mm Hg  pH < 7.35  HCO 3 decreased  partly compensated metabolic acidosis

 PCO 2 < 35 mm Hg  pH 7.35 – 7.45  HCO 3 decreased  chronic respiratory alkalosis

 PCO 2 < 35 mm Hg  pH > 7.45  HCO 3 normal  acute respiratory alkalosis

 PCO 2 < 35 mm Hg  pH > 7.45  HCO 3 decreased  partly compensated respiratory alkalosis

 PCO 2 < 35 mm Hg  pH > 7.45  HCO 3 increased  combined respiratory and metabolic alkalosis

Step 1 PaCO 2 35 – 45 mmHg pH < – 7.45pH > 7.45 Acute Metabolic Acidosis Normal Acid- Base Balance Acute Metabolic Alkalosis

 PCO 2 35 – 45 mm Hg  pH < 7.35  HCO 3 decreased  acute metabolic acidosis

 PCO 2 35 – 45 mm Hg  pH 7.35 – 7.45  HCO 3 normal  normal acid-base balance

 PCO 2 35 – 45 mm Hg  pH > 7.45  HCO 3 increased  acute metabolic alkalosis

Step 1 PaCO2 > 45 mmHg pH < pH > 7.45 Acute Resp. Acidosis Chronic Resp. Acidosis Compens Metabolic Alkalosis Partially Compens Metabolic Alkalosis

 PCO 2 > 45 mm Hg  pH < 7.35  HCO 3 normal  acute respiratory acidosis

 PCO 2 > 45 mm Hg  pH < 7.35  HCO 3 decreased  combined respiratory and metabolic acidosis

 PCO 2 > 45 mm Hg  pH < 7.35  HCO 3 increased  partly compensated respiratory acidosis

 PCO 2 > 45 mm Hg  pH 7.35 – 7.45  HCO 3 increased  chronic respiratory acidosis

 PCO 2 > 45 mm Hg  pH > 7.45  HCO 3 increased  partly compensated metabolic alkalosis

Approximate PaCO 2 -pH Relationship PaCO 2 ( mm Hg)pH

Determining the Predicted Respiratory pH Determine the difference between the measured PaCO 2 and 40 mm Hg; then move the decimal point two places to the left If the PaCO 2 > 40, subtract half the difference from 7.40 If the PaCO 2 < 40, add the difference to 7.40

Respiratory pH: pH 7.04 PaCO – 40 = x 0.5 = – 0.18 = 7.22 pH 7.47 PaCO – 18 = = 7.62

Determining Base Excess/ Deficit 1. Determine pCO 2 variance: difference between measured pCO 2 & 40, move decimal point two places to the left 2. Determine the predicted pH: pCO 2 > 40, subtract half pCO 2 variance from 7.40 pCO 2 < 40, add pCO 2 variance to Estimate BE/BD: Difference between measured and predicted pH Move decimal point two places to right. Multiply by 2/3

Base Excess: measured pH > predicted pH Base Deficit: measured pH < predicted pH pH 7.04 pCO2 76 predicted pH – 7.04 = x 2/3 = 12 mEq/L (BD) pH 7.21 pCO2 90 predicted pH – 7.15 = x 2/3 = 4 mEq/L (BE)

Causes of Acidosis Metabolic Diabetes (ketoacidosis) Renal failure (impaired H+ secretion) Diarrhea (loss of base) Tissue hypoxia (lactic acidosis) Respiratory Respiratory insufficiency

Causes of Alkalosis Metabolic Excessive loss of HCl (e.g. pyloric stenosis) Excessive citrate/bicarbonate load Respiratory Hyperventilation (fever, psychogenic)

Treatment Metabolic Acidosis HCO3 administration Empiric: 1-2 meq/kg Calculated: (Desired – actual) x k x KBW = meqs required k = (represents fraction of body wt. where material is apparently distributed)

Treatment Metabolic Alkalosis Volume expansion; Cl and K replacement Respiratory Acidosis Inc. RR, PIP, or both Respiratory Alkalosis Dec. RR

Step 2: Assessment of Arterial Oxygenation Evaluation of Hypoxemia Room Air (Patient < 60 y/o): Mild: PaO2 < 80 mmHg Moderate: PaO2 < 60 mmHg Severe: PaO2 < 40 mm Hg

Step 2 On Oxygen Therapy: Uncorrected hypoxemia: PaO 2 < 80 mm Hg Corrected hypoxemia: PaO 2 = 80 – 100 mm Hg Overcorrected hypoxemia: PaO 2 > 100 mm Hg

FiO 2 (Fractional Inspired Oxygen Concentration)  the measurable amount of oxygen received by the patient  21% - room air  > 21% - supplemental oxygen

Inspired Oxygen to PaO 2 Relationship FiO 2 Predicted Minimal PaO 2 30 % % % %400 If PaO 2 < minimal predicted (FiO 2 x 5), the patient can be assumed to be hypoxemic at room air.

Treatment of Hypoxemia For ventilated patients Increase: FiO2 RR PIP PEEP Inspiratory time Flow rate

Clinically Assess: Cardiac status Peripheral perfusion Blood oxygen transport mechanism Assess 1 and 2 by the vital signs and PE. If 1 and 2 are adequate, then only 3 can be interfering with proper tissue oxygenation.

Criteria for choosing site and Technique for obtaining ABG samples must be based on: Safety Accessibility Patient Comfort

Significant Problems Bleeding Vessel Obstruction = collateral blood flow important Infection

Preparing the sample Syringes Anticoagulants Anaerobic conditions Delay in analysis

Exercises pH7.44 PCO 2 40 PO 2 99 HCO 3 22 BE+2 SaO 2 95 FiO 2 21%

normal acid-base balance with adequate oxygenation Exercises pH7.44 PCO 2 40 PO 2 99 HCO 3 22 BE+2 SaO 2 95 FiO 2 21%

pH7.36 PCO 2 25 PO 2 78 HCO 3 15 BE-10 SaO 2 95 FiO 2 35%

pH7.36 PCO 2 25 PO 2 78 HCO 3 15 BE-10 SaO 2 95 FiO 2 35% chronic metabolic acidosis with uncorrected hypoxemia

pH7.24 PCO 2 60 PO 2 80 HCO 3 26 BE-2 SaO 2 95 FiO 2 60%

pH7.24 PCO 2 60 PO 2 80 HCO 3 26 BE-2 SaO 2 95 FiO 2 60% acute respiratory acidosis with corrected hypoxemia

pH7.55 PCO 2 52 PO 2 70 HCO 3 44 BE+17 SaO 2 97 FiO 2 90%

pH7.55 PCO 2 52 PO 2 70 HCO 3 44 BE+17 SaO 2 97 FiO 2 90% partly compensated metabolic alkalosis with uncorrected hypoxemia

pH7.19 PCO 2 56 PO HCO 3 17 BE-30 SaO 2 94 FiO 2 45%

pH7.19 PCO 2 56 PO HCO 3 17 BE-30 SaO 2 94 FiO 2 45% combined metabolic and respiratory acidosis with overcorrected hypoxemia