Arterial blood gas analysis is used to determine the adequacy of oxygenation and ventilation, assess respiratory function and determine the acid–base balance. These data provide information regarding potential primary and compensatory processes that affect the body’s acid–base buffering system.
Interpret the ABG in a step-wise manner:
- PaO2. Determine the adequacy of oxygenation
- Normal range: 80–100 mmHg (10.6–13.3 kPa)
- Determine pH status
- Normal pH range: 7.35–7.45 (H+ 35–45 nmol/L)
- pH <7.35: Acidosis is an abnormal process that increases the serum hydrogen ion concentration, lowers the pH and results in acidaemia.
- pH >7.45: Alkalosis is an abnormal process that decreases the hydrogen ion concentration and results in alkalaemia.
- PaCO2. Determine the respiratory component
- Normal range: PaCO2 35–45 mmHg (4.7–6.0 kPa)
- PaCO2 <35 mmHg (4.7 kPa):
- Primary respiratory alkalosis (hyperventilation) if pH >7.45 and HCO3– normal.
- Respiratory compensation for metabolic acidosis if pH <7.35 and HCO3– (decreased).
- PaCO2 >45 mmHg (> 6.0 kPa):
- Primary respiratory acidosis (hypoventilation) if pH <7.35 and HCO3– normal.
- Respiratory compensation for metabolic alkalosis if pH >7.45 and HCO3– increased.
- In chronic respiratory acidosis the metabolic compensation may be acute or chronic. In acute compensation for every10 Δ pCO2 = 0.08 Δ pH and in chronic for every10 Δ pCO2 = 0.03 Δ pH
- HCO3– Determine the metabolic component
- Normal HCO3– range 22–26 mmol/L
- HCO3– <22 mmol/L: Primary metabolic acidosis if pH <7.35. Renal compensation for respiratory alkalosis if pH >7.45.
- HCO3– >26 mmol/L: Primary metabolic alkalosis if pH >7.45. Renal compensation for respiratory acidosis if pH <7.35.
Rules and Resources
- Acid Base disorders worksheet – Joshua Steinberg MD
- Acid Base Made Easy – ICU OnePager
- ABG walk though
- Anion Gap – Delta ratio – Strong ion difference
- Base Excess vs Standard Base Excess
- Bedside Rules for Assessment of Compensation – Kerry Brandis
- Use: Screening test for detecting abnormal low MW solutes (e.g. ethanol, methanol & ethylene glycol [Reference])
- An elevated osmolar gap (>10) provides indirect evidence for the presence of an abnormal solute which is present in significant amounts [Reference]
- Osmolar gap = Osmolality – Osmolarity
- Osmolality (measured)
- Units: mOsm/kg
- Measured in laboratory and returned as the plasma osmolality
- Osmolarity (calculated)
- Units: mOsm/l
- Osmolarity = (1.86 x [Na+]) + [glucose] + [urea] + 9 (using values measured in mmol/l)
- Osmolarity = (1.86 x [Na+]) + glucose/18 + BUN/2.8 + 9 (using US units of mg/dl)
- NOTE: even though the units of measured (mOsm/kg) and calculated (mOsm/l) are different [Reference], strictly they cannot be subtracted from one another… However, the value of the difference is clinically useful so the problem is usually overlooked!
1 2 3 4 5 Rule
Simple table to calculate metabolic compensation in respiratory acidosis and alkalosis (aka the 1-2-3-4-5 rule)
- Respiratory Acidosis: Increased PaCO2. For every 10mmHg rise should have INCREASE in HCO3 by 1 (acute) or 4 (chronic) as compensation
- Respiratory Alkalosis: Decreased PaCO2. For every 10mmHg rise should have DECREASE in HCO3 by 2 (acute) or 5 (chronic) as compensation,
References and Links
- Acid-Base: ABG analysis – Anion Gap – SID – NAGMA
- Metabolic acidosis: Overview – evaluation – DDx
- Metabolic alkalosis: Overview – evaluation – DDx
- Respiratory acidosis: Overview
- Respiratory alkalosis: Overview
BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Associate Professor Curtin Medical School, Curtin University. Emergency physician, Sir Charles Gairdner Hospital. Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |