Arterial Blood Gas (ABG)/ Analisis Gas Darah (AGD)

Arterial blood gas (ABG) includes blood pH, PO2 , Pco2, bicarbonate.

Indications
1. Evaluation of respiratory and metabolic acidbase disturbances including poisoning respiratory failure, or severe chronic obstructive pulmonary disease (COPD) exacerbations.
2. Undifferentiated shock.
3. Unexplained coma or confusion/obtundation.

Normal values
- Blood pH 7.38 - 7.42
- PCO2 38 - 42mmHg
- PO2 83 - 108mmHg
- HCO3 22- 26 mmol/L


Abnormalities and causes
1. Elevated pH: Vomiting, volume contraction, hyperaldosteronism, early CHF, drugs causing alkalosis, or anxiety.
2. Decreased pH: Ketoacidosis, lactic acidosis, renal failure, respiratory failure, chronic obstructive lung disease, drugs causing acidosis (isoniazid, iron, salicylates), or ethylene glycol.
3. Increased PCO2 : Respiratory failure, hypoventilation, COPD, CNS depression,
metabolic alkalosis, or drugs depressing respiration (alcohol, barbiturates, opiates,
benzodiazepines).
4. Decreased PCO2 : Hyperventilation, anxiety, interstitial lung disease, cirrhosis, metabolic acidosis, hyperthyroidism, PE, or aspirin.
5. Increased PO: Oxygen therapy, excessive air bubbles in specimen, hyperventilation, or aspirin.
6. Decreased PO2 : COPD, pneumonia, interstitial lung disease, PE, CHF, shock, CNS
depression, right-to-left cardiac shunts, or drugs that depress respiration.
7. Elevated HCO3 : Respiratory acidosis, metabolic alkalosis (vomiting, Cushings
syndrome, volume depletion), diuretics, or glucocorticoids.
8. Decreased HCO3 : Respiratory alkalosis, metabolic acidosis (ketoacidosis, lactic
acidosis, renal failure, diarrhea), carbonic anhydrase inhibitors, ethylene glycol,
methanol, aspirin, or pregnancy.

Pearls and pitfalls
1. Prior to obtaining the sample, a patient must be evaluated for both radial and ulnar arterial blood supply. The Allenís test determines the presence of collateral flow from both the radial and ulnar artery in the hand. If abnormal, radial artery cannulation should be avoided to prevent possible ischemic injuries.
2. ABG specimens must be handled appropriately and run expeditiously. Errors arise from excess air bubbles in the sample, excess heparin in the syringe, and not immediately placing the sample on ice.
3. If a patient is hypoxemic, attempt to determine the etiology. Do not just treat with supplemental oxygen.
4. The ABG measurement in a patient with carbon monoxide poisoning will help by
identifying the presence of a metabolic acidosis with a normal PO2.
5. ABG samples are not necessary if pulse oximetry is sufficient to guide management.

Reference : Mahadevan&Garmel. 2005. An Introduction to Clinical Emergency Medicine. Cambridge University Press.