ARTERIAL BLOOD GAS Section of Pediatric Pulmonology UPCM-Philippine General Hospital

Clinical Application of Blood Gases 5 th Edition Shapiro, et. Al.

ABG measures respiratory function 1. Oxygenation status 2. Acid-base balance

Points to Remember Body always tries to maintain normal pH CO 2 – respiratory HCO 3 – metabolic Lungs compensate rapidly Kidneys compensate slowly There is no overcompensation except in chronic ventilatory failure Consider the underlying disease

Normal ABG Values pH clinically acceptable PO for newborn, preterm age 60 PCO clinically acceptable

Oxygenation Status At room air, sea level: At room air, sea level: PaO normal or acceptable PaO 2 < 80 mild hypoxemia PaO 2 < 60 moderate hypoxemia PaO 2 < 40 severe hypoxemia On oxygen support: On oxygen support: PaO corrected hypoxemia PaO 2 > N overcorrected hypoxemia PaO 2 < N uncorrected hypoxemia

Nomenclature for pH & PaCO2 Outside of normal range pH > 7.45Alkalemia pH < 7.35Acidemia PaCO2 > 45 mmHgRespiratory Acidosis (hypercapnia) PaCO2 < 35 mmHgRespiratory Alkalosis (hypocapnia)

FiO2 vs PaO2 The minimally acceptable PaO2 increases by approximately 50 mmHg for every 10% increment of inspired oxygen concentration.

FiO 2 vs PaO 2 PAO 2 = [ FiO 2 (P B – P H 2 0 ) ] – PaCO PaO 2 = FiO 2 x 5

What is the expected PAO 2 if you are breathing normally at room air? = [.21 (760 – 47) ] – 40 / 0.8 = 150 – 50 = 100 PaO 2 = FiO 2 x 5 =.21 x 5 = 105 PAO 2 = [ FiO 2 (P B – P H 2 0 ) ] – PaCO 2 0.8

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

FiO 2 vs PaO 2 P(A-a)O 2 = PAO 2 - PaO 2 Normal: < 30 mm Hg at room air < 50 mm Hg at FiO > 450 mmHg is indicative of severe respiratory failure

PaCO2 – pH RELATIONSHIP 1.For every 10 mm Hg increase in the PaCO2, the pH will decrease by 0.05 unit. 2.For every 10 mm Hg decrease in the PaCO2, the pH will increase by 0.10 unit.

Respiratory pH (pH R ) PaCO 2 pH R

Respiratory pH (pH R ) PaCO 2 pH R For every  in pCO 2 of 10, pH  by 0.05

Computing for pH R Normal pCO 2 = 40 and normal pH = 7.4 pH R = (40 – Actual pCO 2 ) x If actual pCO 2 > 40:

Respiratory pH (pH R ) PaCO 2 pH R For every  in pCO 2 of 10,pH  by 0.1

Computing for pH R Normal pCO 2 = 40 and normal pH = 7.4 pH R = (40 – Actual pCO 2 ) x If actual pCO 2 > 40: If actual pCO 2 < 40: pH R = (40 – Actual pCO 2 ) x

What is the expected pH when the pCO 2 is 45? If actual pCO 2 > 40: pH R = (40 – Actual pCO 2 ) x pH R = (40 – 45) x = 7.375

What is the expected pH when the pCO 2 is 35? If actual pCO 2 < 40: pH R = (40 – Actual pCO 2 ) x pH R = (40 – 35) x = 7.45

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

pH R vs pH If actual pH > pH R partially compensated If actual pH < pH R mixed If actual pH = pH R purely respiratory

PaCO2 – Plasma BICARBONATE RELATIONSHIP 1.An acute PaCO2 increase of 10 mmHg will increase the plasma bicarbonate by 1 mmol/L 2.An acute PaCO2 decrease of 10 mmHg will decrease the plasma bicarbonate by 2 mmol/L The difference between the calculated respiratory plasma bicarbonate value and the actual plasma bicarbonate value provides a rapid and easy assessment of the metabolic component.

Approximate PaCO2-pH Relationship PaCO2pHHCO3 (mmHg)(mmol/L)

Minute Ventilation vs PaCO 2 MV PaCO 2 Range N N N 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.

FiO 2 -PaO 2 RELATIONSHIP Inspired Oxygen to PaO2 Relationship in Normal lungs. FiO2 Inspired O2(%)PaO > > > > >500

DETERMINING BASE EXCESS/DEFICIT Under normal circumstances, a 10 mmol/L variance from the normal buffer base represents a 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, which can be expressed as a 2/3 realtionship.

The difference between the measured pH & the predicted respiratory pH is the metabolic pH change. BE or BD = (actual pH – pH R ) x 2 3 x 100

Base excess or deficit 0.15 pH change  10 mEq/L buffer change BE: actual pH > pH R BD: actual pH < pH R BE or BD = (actual pH – pH R ) x Normal BE or BD:  2

INTERPRETIVE APPROACH Step 1. Assessment of the PCO2 and pH. a. Classify the CO2 tension. b. Consider the pH and determine classification. c. Consider the base excess/deficit or bicarbonate levels and determine classification. Step 2. Assessment of Arterial Oxygenation a. PaO2 b. SaO2

pCO 2 < 35 pCO pCO 2 > 45 pH < 7.35 pH 7.35 – 7.4pH 7.4 – 7.45pH > 7.45 normal acidosis alkalosis

pCO 2 < 35 pCO pCO 2 > 45 pH < 7.35 pH 7.35 – 7.4pH 7.4 – 7.45pH > 7.45 normal acidosis alkalosis metabolic respiratory metabolic respiratory comp part comp

Physiologic Mechanisms of Hypoxemia 1.Alveolar hypoventilation 2.Ventilation-perfusion mismatch 3.Right-to-left shunt 4.Diffusion limitation 5.Decreased ambient oxygen tension

Physiologic Mechanisms of Hypoxemia Alveolar hypo- ventilation V/Q mismatch R to L shunt Diffusion limitation Response to O 2 good poor CO 2  variableN- CVSN Clinical response A- neuro- muscular A- CVSPP CXRN- central W- alveolar W- lung N- CVS minimal W W –white N - normal P - poor A - absent

Metabolic acidosis 1. Renal failure (RTA) 2. Ketoacidosis (DKA, starvation) 3. Lactic acidosis

Anion Gap = Na – (Cl + HCO 3 ) Normal: < 15 mEq/L

Anion Gap IncreasedNormal Organic acid accumulation Acute renal failure Inborn error of metabolism Lactic acidosis Late metabolic acidosis Toxins Loss of buffer Renal HCO 3 loss Renal tubular acidosis Acetazolamide Renal dysplasia GI HCO 3 loss Diarrhea Cholestyramine Small bowel drainage Dilutional acidosis Hyperalimentation acidosis

Metabolic alkalosis 1. Hypokalemia 2. Hypochloremia 3. Vomiting 4. Massive steroid administration 5. NaHCO 3 administration

Respiratory acidosis 1. Hypoventilation a.Inadequate respiratory effort CNS problems Neuromuscular disease Mechanical ventilator settings b.Upper airway not patent c.Decreased lung tissue d.Decreased lung compliance

Respiratory acidosis 2. Abnormal ventilation-perfusion ratio a. Obstruction of small airways b. Atelectasis c. Pneumonia d. Pulmonary edema 3. Increased extrapulmonary shunt a.Pulmonary vasoconstriction RDS, severe infection b.Pulmonary hypoplasia c.Cyanotic heart disease

Respiratory alkalosis 1.With hypoxemia a. Acute pulmonary disease pneumonia and atelectasis, RDS, acute asthma b.Acute myocardial disease MI, pulmonary edema, heart failure, CP bypass 2. Without hypoxemia a.Anxiety, neurosis, psychosis b.Pain c.CNS disease d.Anemia e.Carbon monoxide poisoning

Disorders Expected compensation Metabolic pCO2 = 1.5 x HCO /- 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

Exercises Compute for the pH R and interpret the ABG values

1. pH 7.45 pO 2 65 pCO 2 32 FiO 2.21 pH R = ( ) x = respiratory compensated with mild hypoxemia BE/BD = (7.45 – 7.48) x 200 = -2 3 alkalosis HCO3

2. pH 7.3 pO pCO 2 30 FiO 2.30 pH R = ( ) x = metabolic partially compensated with overcorrected hypoxemia BE/BD = (7.3 – 7.5) x 200 = acidosis HCO3

3. pH 7.25 pO 2 90 pCO 2 55 FiO 2.40 pH R = ( ) x = uncompensated with corrected hypoxemia BE/BD = (7.25 – 7.325) x 200 = acidosis respiratory HCO3 N

4. pH 7.42 pO pCO 2 35 FiO 2.35 pH R = ( ) x = with overcorrected hypoxemia BE/BD = (7.42 – 7.45) x 200 = -2 3 normal acid-base balance NN HCO3 N

Calculating FiO2 requirement using the ABG FiO2 = (desired PaO2) + PaCO2 PaO2RQ PAO2 Pb – PH20

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