Blood Gas Interpretation

Ayman I. Abou Mehrem, MDAssistant Consultant

King Abdulaziz Hospital

Blood Gas Interpretation

• Components of blood gas (BG) analysis

• Normal arterial BG (ABG)• Indications for BG analysis• Abnormalities in ABG• Stepwise analysis of ABG• Quiz

Components of BG

• Measured Values:– pH

– PaCO2

– PaO2

• Calculated Values:– HCO3

– O2Sat

– BE

Components of BG

Blood Gas

Oxygenation Ventilation Acid-Base

PaO2

SaO2

PCO2

pHHCO3-BE

Normal ABG

• pH 7.35 - 7.45

• PCO2 35 – 45 mmHg

• PO2 70 - 100 mmHg

• SaO2 ≥ 93 %

• HCO3 22 - 26 mEq/L

• BE -2 - +2 mEq/L

Normal ABG

Normal ABG

• In preterm babies the acid-base balance is a bit different.

• We use usually the term “Acceptable Blood Gas” instead of normal blood gas.

• This is to avoid more aggressive interventions to normalize their blood gas, which may lead to harm.

Target Blood Gas in Neonates*

< 28 weeks’

GA

28-40 weeks’

GA

Term with PPHN

Infant with BPD

pH ≥ 7.25 ≥ 7.257.30-7.50

7.35-7.45

PaCO2

45-55 45-55 30-40 55-65

PaO2 45-65 50-70 80-120 50-80

* Goldsmith and Karotkin, Assisted Ventilation of the Neonate, 4th edition, Saunders

Indications

• Assessment of ventilation and oxygenation status in patients with respiratory disease

• Assessment of acid-base imbalance in sepsis, metabolic, and renal diseases

Hypoxia

RespiratoryAcidosis

RespiratoryAlkalosis

MetabolicAlkalosis

MetabolicAcidosis

• ↓ PaO2

• ↓ O2 Saturation

• Causes:– Respiratory: RDS, Pneumonia– Cardiac: Cyanotic CHD, CHF– Abnormal Hemoglobins

PaCO2

HCO3

pH

• Primary acid-base disorders

• Compensation

• Mixed acid-base disorders

• One of the four acid-base disturbances that is manifested by an initial change in HCO3

- or PaCO2

• Types:

– Respiratory acidosis

– Respiratory alkalosis

– Metabolic acidosis

– Metabolic alkalosis

• A primary disorder where the first change is an elevation of PaCO2, resulting in decreased pH.

• Causes:– Airway: e.g. laryngeal edema,

severe micrognathia– Lungs: e.g. RDS, pneumonia– CNS: respiratory depression due to

medications, CNS infection, hemorrhage, etc.

• A primary disorder where the first change is a lowering of PaCO2, resulting in an elevated pH.

• Rare in neonates• Causes:

– Iatrogenic: for ventilated babies– Hyperventilation: e.g. urea cycle

disorders

• A primary acid-base disorder where the first change is a lowering of HCO3

-, resulting in decreased pH.

• Causes:– Dehydration– Shock– Sepsis– Metabolic disorders

• A primary acid-base disorder where the first change is an elevation of HCO3-, resulting in increased pH.

• Causes:– Iatrogenic: loop diuretics– Rare diseases: cystic fibrosis,

congenital chloride diarrhea

• The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.

• For respiratory disorders (i.e. resp. acidosis or alkalosis) the body develops metabolic compensation through the kidney (i.e. HCO3).

• For metabolic disorders (i.e. metabolic acidosis or alkalosis) the body develops respiratory compensation through the lungs (i.e. CO2).

• Combination of two primary acid-base disorder with different range of compensation.

• Usually happen in patients with chronic diseases or multiple primary pathologies

• Step One:Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. If it is above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic.

• Step Two:If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a respiratory or metabolic problem. To do this, assess the PaCO2 level. Remember that with a respiratory problem, as the pH decreases below 7.35, the PaCO2 should rise. If the pH rises above 7.45, the PaCO2 should fall. Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions, then the problem is primarily respiratory in nature.

• Step ThreeAssess the HCO3 value. Recall that with a metabolic problem, normally as the pH increases, the HCO3 should also increase. Likewise, as the pH decreases, so should the HCO3. Compare the two values. If they are moving in the same direction, then the problem is primarily metabolic in nature.

pH PaCO2 HCO3

Respiratory Acidosis ↓ ↑ normal

Respiratory Alkalosis ↑ ↓ normal

Metabolic Acidosis ↓ normal ↓

Metabolic Alkalosis ↑ normal ↑

Primary Acid-Base Disorders(No compensation)

pH PaCO2 HCO3

Respiratory Acidosis ↓ ↑ ↑

Respiratory Alkalosis ↑ ↓ ↓

Metabolic Acidosis ↓ ↓ ↓

Metabolic Alkalosis ↑ ↑ ↑

Partially Compensated Acid-Base Disorders

pH PaCO2 HCO3

Respiratory Acidosis

normal, but < 7.4

↑ ↑

Respiratory Alkalosis

normal, but > 7.4

↓ ↓

Metabolic Acidosis

normal, but < 7.4

↓ ↓

Metabolic Alkalosis

normal, but > 7.4

↑ ↑

Fully Compensated Acid-Base Disorders

Quiz 1• Baby boy, 28 wks GA, admitted

3 hrs ago, intubated initially, given surfactant, then extubated immediately to nasal CPAP, pressure 5 cm H2O, FiO2 0.5.

• ABG now: pH=7.20, PCO2=68, PO2=40, HCO3=22, SaO2=85%

• Interpret above blood gas

Quiz 2• Baby girl, born at term by

emergency CS, because of cord prolapse and severe fetal distress. She was flat, needed thorough resuscitation (intubation, UVC, 2 doses of epinephrine)

• Now she is 6 hrs old, ventilated, FiO2 0.3, and had focal seizure.

• ABG: pH=7.15, PCO2=30, PO2=60, HCO3=6, SaO2=92%

• Interpret above blood gas

Quiz 3

• Hundred day-old baby girl, was born at 27 wks GA, had stormy course.

• Now she is on NC 1 LPM, FiO2 0.3

• ABG: pH=7.34, PCO2=65, PO2=60, HCO3=33, SaO2=92%

• Interpret above blood gas

Quiz 4• Seven day-old, baby boy, born at

29 wks GA.• He had large PDA, led to

pulmonary hemorrhage, which treated conservatively.

• Indomethacin cannot begiven because of Lt side grade 4 IVH, TFI was restricted to 120 ml/kg/d and furosemide was given 1.2 mg q12 hrs.

• ABG: pH=7.47, PCO2=40, PO2=60, HCO3=30, SaO2=92%