Arterial Blood Gas Analysis

Question 1

Clinical Indications for ABGA (10)

Answer 1

Sudden unexplained dyspnea
Cyanosis
Abnormal breath sounds
Severe, unexplained tachypnea
Heavy use of accessory muscles
Changes in ventilator settings
Cardiopulmonary resuscitation
New or progression of arrythmias
Acute hypertension
Acute deterioration in neurological function

Question 2

Bicarbonate/Carbon Dioxide Buffer System

Answer 2

CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+
CO2: more acidic, respiration (expiration), quick method. Hypoventilation → ↑CO2 → ↓pH.
Hyperventilation → ↓CO2 → ↑pH.
HCO3-: more alkalotic, kidney (+liver, excretion), slower method.
Excretion → ↓HCO3- → ↓pH.
Reabsorption → ↑HCO3- → ↑pH.

Question 3

Normal Values pH, PaO2, PaCo2, HCO3-, SpO2

Answer 3

pH: 7.35 - 7.45, ↓ acidosis, ↑ alkalosis
PaO2: 80-100 mmHg
PaCO2: 35-45 mmHg. ↑ acidosis, ↓ alkalosis.
HCO3-: 22-26 mmol/L ↓ acidosis, ↑ alkalosis.
SpO2: 95-100%

Question 4

pH Disturbance - >7.45

Answer 4

↓ CO2 → Hyperventilation → Hypocapnea: Respiratory alkalosis
↑ HCO3- cause: hypovolemia - vomiting - diuretic use. Metabolic alkalosis.

Question 5

pH Disturbance - <7.35

Answer 5

↑ CO2 → Hypoventilation → Hypercapnia. Respiratory acidosis.
↓ HCO3- cause: diarrhea, toxic ingestion, kidney/liver damage/failure. Metabolic acidosis.

Question 6

Interpretation - 3 Steps

Answer 6

1. Alkalosis or acidosis
2. Respiratory or metabolic
3. Compensated, partially compensated or uncompensated.

Question 7

Uncompensated

Answer 7

pH abnormal and either CO2 or HCO3- abnormal.

Question 8

Partially Compensated

Answer 8

pH abnormal and both CO2 and HCO3- are abnormal.

Question 9

Fully Compensated (Diagnosis or no Diagnosis

Answer 9

pH normal and both CO2 and HCO3- are abnormal.
Diagnosis:
- Excessive vomiting: ↓acid → ↑HCO3- → ↑CO2. Fully comp. Metabolic alkalosis.
- Resp. Failure/severe asthma: ↑CO2 → ↑HCO3-. Fully comp. Resp. acidosis.
No Diagnosis: More acidic (7.35-7.39) or alkalotic (7.41-7.45)? If pH:7.4 and PaO2 and SpO2 are low: Respiratory.

Question 10

Level of Oxygenation

Answer 10

PaO2/FiO2: arterial oxygen pressure/inspired oxygen.
Normal values: 80-100mmHg/21% (room air).
Acute respiratory distress syndorme:
Mild: 201-300 (PaO2/FiO2)
Moderate: 101-200
Severe: ≤100