ARTERIAL BLOOD GASARTERIAL BLOOD GAS

Section of Pediatric PulmonologySection of Pediatric PulmonologyUPCM-Philippine General HospitalUPCM-Philippine General Hospital

Clinical Application of Blood Gases5th Edition

Shapiro, et. Al.

ABG measures respiratoryABG measures respiratory

functionfunction

1. Oxygenation status

2. Acid-base balance

Points to RememberPoints 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 ValuesNormal 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 StatusOxygenation Status

At room air, sea level: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:On oxygen support:PaO2 80-100 corrected hypoxemia

PaO2 > N overcorrected hypoxemia

PaO2 < N uncorrected hypoxemia

Nomenclature for pH & PaCO2 Outside ofnormal range

pH > 7.45 AlkalemiapH < 7.35 AcidemiaPaCO2 > 45 mmHg Respiratory Acidosis

(hypercapnia)PaCO2 < 35 mmHg Respiratory Alkalosis

(hypocapnia)

FiO2 vs PaO2FiO2 vs PaO2

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

FiOFiO22 vs PaO vs PaO22

PAO2 = [ FiO2 (PB – PH20) ] – PaCO2

0.8

PaO2 = FiO2 x 5

What is the expected PAOWhat is the expected PAO22

if you are breathing normally if you are breathing normally at room air?at room air?

= [ .21 (760 – 47) ] – 40 / 0.8

= 150 – 50 = 100

PaO2 = FiO2 x 5

= .21 x 5 = 105

PAO2 = [ FiO2 (PB – PH20) ] – PaCO2

0.8

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

PAO2 vs PaO2PAO2 vs PaO2

FiOFiO22 vs PaO vs PaO22

P(A-a)O2 = PAO2 - PaO2

Normal: < 30 mm Hg at room air

< 50 mm Hg at FiO2 1.0

> 450 mmHg is indicative of severe respiratory failure

PaCO2 – pH RELATIONSHIP

1. For every 10 mm Hg increase in the PaCO2, thepH 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 (pHRespiratory pH (pHRR))

PaCOPaCO2 2 pHpHRR

40 7.440 7.460 7.360 7.380 7.280 7.2

30 7.530 7.520 7.620 7.6

Respiratory pH (pHRespiratory pH (pHRR))

PaCOPaCO2 2 pHpHRR

40 7.440 7.460 7.360 7.380 7.280 7.2

For every in pCO2 of 10 ,

pH by 0.05

Computing for pHComputing for pHRR

•Normal pCO2 = 40 and normal pH = 7.4

pHR = (40 – Actual pCO2) x 0.05 + 7.4 10

If actual pCO2 > 40:

Respiratory pH (pHRespiratory pH (pHRR))

PaCOPaCO2 2 pHpHRR

40 7.440 7.430 7.530 7.520 7.620 7.6

For every in pCO2 of 10 ,

pH by 0.1

Computing for pHComputing for pHRR

•Normal pCO2 = 40 and normal pH = 7.4

pHR = (40 – Actual pCO2) x 0.05 + 7.4 10

If actual pCO2 > 40:

If actual pCO2 < 40:

pHR = (40 – Actual pCO2) x 0.1 + 7.4 10

What is the expected pH What is the expected pH when the pCOwhen the pCO22 is 45? is 45?

If actual pCO2 > 40:

pHR = (40 – Actual pCO2) x 0.05 + 7.4 10

pHR = (40 – 45) x 0.05 + 7.4 10

= 7.375

What is the expected pH What is the expected pH when the pCOwhen the pCO22 is 35? is 35?

If actual pCO2 < 40:

pHR = (40 – Actual pCO2) x 0.1 + 7.4 10

pHR = (40 – 35) x 0.1 + 7.4 10

= 7.45

pHpHRR vs pH vs pH

If pHR compared to actual pH is:

< 0.03 purely respiratory

> 0.03 compensated

pHpHRR vs pH vs pH

If actual pH > pHR partially compensated

If actual pH < pHR mixed

If actual pH = pHR purely respiratory

PaCO2 – Plasma BICARBONATE PaCO2 – Plasma BICARBONATE RELATIONSHIPRELATIONSHIP

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 respiratoryplasma bicarbonate value and the actual plasma bicarbonate value provides a rapid and easyassessment of the metabolic component.

Approximate PaCO2-pH Relationship

PaCO2 pH HCO3(mmHg) (mmol/L)

30 7.50 2220 7.60 20

80 7.20 2860 7.30 26

40 7.40 24

Minute Ventilation vs PaCOMinute Ventilation vs PaCO22

MVMV PaCO PaCO2 2 RangeRange

N 40 35-45 2N 30 25-35

4N 20 15-25The 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.

FiOFiO22-PaO-PaO22 RELATIONSHIP RELATIONSHIP

Inspired Oxygen to PaO2 Relationship in Normal lungs.FiO2 Inspired O2(%) PaO20.30 30 >1500.40 40 >2000.50 50 >2500.80 80 >4001.00 100 >500

DETERMINING BASE DETERMINING BASE EXCESS/DEFICITEXCESS/DEFICIT

Under 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.

BE or BD = (actual pH – pHR) x 2

3

x 100

Base excess or deficitBase excess or deficit

0.15 pH change 10 mEq/L buffer change

BE: actual pH > pHR

BD: actual pH < pHR

BE or BD = (actual pH – pHR) x 200

3

Normal BE or BD: 2

INTERPRETIVE APPROACHINTERPRETIVE 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 determineclassification.

Step 2. Assessment of Arterial Oxygenationa. PaO2b. SaO2

pCO2 < 35

pCO2 35-45

pCO2 > 45

pH < 7.35 pH 7.35 – 7.4 pH 7.4 – 7.45 pH > 7.45

normal normalacidosis

acidosis

acidosis acidosis

acidosis

alkalosis

alkalosis alkalosis

alkalosisalkalosis

pCO2 < 35

pCO2 35-45

pCO2 > 45

pH < 7.35 pH 7.35 – 7.4 pH 7.4 – 7.45 pH > 7.45

normal normalacidosis

acidosis

acidosis acidosis

acidosis

alkalosis

alkalosis alkalosis

alkalosisalkalosis

metabolic metabolic

metabolic metabolic respiratory respiratory

metabolic metabolicrespiratoryrespiratory

comp comp

comp comp

part comp

part comp

Physiologic Mechanisms Physiologic Mechanisms of Hypoxemiaof Hypoxemia

1. Alveolar hypoventilation

2. Ventilation-perfusion mismatch

3. Right-to-left shunt

4. Diffusion limitation

5. Decreased ambient oxygen tension

Physiologic Mechanisms of HypoxemiaPhysiologic Mechanisms of Hypoxemia

Alveolar hypo-

ventilation

V/Q mismatch

R to L shunt

Diffusion limitation

Response

to O2

good good goodpoor

CO2 variable N- CVS N

Clinical

response

A- neuro- muscular

A- CVSP P

CXR N- central

W- alveolar

W- lung

N- CVS minimalW

W –white N - normal P - poor A - absent

Metabolic acidosisMetabolic acidosis

1. Renal failure (RTA)

2. Ketoacidosis (DKA, starvation)

3. Lactic acidosis

Anion GapAnion Gap

= Na – (Cl + HCO3)

Normal: < 15 mEq/L

Anion GapAnion Gap

Increased Normal

Organic acid accumulation

Acute renal failure

Inborn error of metabolism

Lactic acidosis

Late metabolic acidosis

Toxins

Loss of buffer

Renal HCO3 loss

Renal tubular acidosis

Acetazolamide

Renal dysplasia

GI HCO3 loss

Diarrhea

Cholestyramine

Small bowel drainage

Dilutional acidosis

Hyperalimentation acidosis

Metabolic alkalosisMetabolic alkalosis

1. Hypokalemia

2. Hypochloremia

3. Vomiting

4. Massive steroid administration

5. NaHCO3 administration

Respiratory acidosisRespiratory 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 acidosisRespiratory acidosis2. 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 alkalosisRespiratory alkalosis1. 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 ExpectedDisorders Expected compensation compensation

Metabolic 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 compensationDisorder 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

ExercisesExercises

Compute for the pHR and

interpret the ABG values

1. pH 1. pH 7.45 pO 7.45 pO22 65 65 pCO pCO22 32 FiO 32 FiO22 .21 .21

pHpHRR = (40 - 32) x 0.1 + 7.4 = (40 - 32) x 0.1 + 7.4 = 7.48 = 7.48 1010

respiratoryrespiratorycompensatedcompensatedwith mild hypoxemiawith mild hypoxemia

BE/BD = (7.45 – 7.48) x 200 = -2BE/BD = (7.45 – 7.48) x 200 = -233

alkalosisalkalosis

HCO3

2. pH 2. pH 7.3 7.3 pO pO22 120 120 pCO pCO22 30 FiO 30 FiO22 .30 .30

pHpHRR = (40 - 30) x 0.1 + 7.4 = (40 - 30) x 0.1 + 7.4 = 7.5 = 7.5 1010

metabolicmetabolicpartially compensatedpartially compensated

with overcorrected hypoxemiawith overcorrected hypoxemia

BE/BD = (7.3 – 7.5) x 200 = -13BE/BD = (7.3 – 7.5) x 200 = -1333

acidosisacidosis

HCO3

3. pH 3. pH 7.25 7.25 pO pO22 90 90 pCO pCO22 55 FiO 55 FiO22 .40 .40

pHpHRR = (40 - 55) x 0.05 + 7.4 = (40 - 55) x 0.05 + 7.4 = 7.325 = 7.325 1010

uncompensateduncompensated

with corrected hypoxemiawith corrected hypoxemia

BE/BD = (7.25 – 7.325) x 200 = - 5BE/BD = (7.25 – 7.325) x 200 = - 533

acidosisacidosisrespiratoryrespiratory

HCO3 N

4. pH 4. pH 7.42 7.42 pO pO22 200 200 pCO pCO22 35 35 FiOFiO22 .35 .35

pHpHRR = (40 - 35) x 0.1 + 7.4 = (40 - 35) x 0.1 + 7.4 = 7.45 = 7.45 1010

with overcorrected hypoxemiawith overcorrected hypoxemia

BE/BD = (7.42 – 7.45) x 200 = -2 BE/BD = (7.42 – 7.45) x 200 = -2 33

normal acid-base balancenormal acid-base balance

NNNN

HCO3 N

Calculating FiO2 requirement using the Calculating FiO2 requirement using the ABGABG

FiO2 = (desired PaO2) + PaCO2PaO2 RQPAO2

Pb – PH20

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