How do you interpret and treat an abnormal Arterial Blood Gas (ABG) reading?

How to Interpret and Treat an Abnormal Arterial Blood Gas (ABG) Reading

Use a systematic three-step approach: evaluate pH first to determine acidemia (pH <7.35) or alkalemia (pH >7.45), then examine PaCO2 to identify the respiratory component (>45 mmHg indicates respiratory acidosis, <35 mmHg indicates respiratory alkalosis), and finally evaluate bicarbonate/base excess to identify the metabolic component (HCO3 <22 or base excess <-2 indicates metabolic acidosis, HCO3 >26 or base excess >+2 indicates metabolic alkalosis). 1, 2

Systematic Interpretation Algorithm

Step 1: Assess pH

• pH <7.35 = acidemia 1
• pH >7.45 = alkalemia 1
• This determines the primary direction of the acid-base disturbance 1, 2

Step 2: Identify Respiratory Component

• PaCO2 >45 mmHg with low pH = respiratory acidosis 1
• PaCO2 <35 mmHg with high pH = respiratory alkalosis 1
• The respiratory component moves in the opposite direction to pH (the "Ro" in RoMe technique) 3

Step 3: Identify Metabolic Component

• Base excess <-2 or HCO3 <22 = metabolic acidosis 1
• Base excess >+2 or HCO3 >26 = metabolic alkalosis 1
• The metabolic component moves in the same direction as pH (the "Me" in RoMe technique) 3

Step 4: Calculate Delta Ratio for Mixed Disorders (When Applicable)

• Delta ratio = (Anion Gap - 12) / (24 - HCO3⁻) 2
• This calculation identifies mixed acid-base disorders in critically ill patients where multiple pathophysiologic processes coexist 2
• Critical caveat: Delta ratio has limitations in chronic conditions where baseline bicarbonate differs significantly from 24 mmol/L 2

Primary Indications for ABG Testing

Critical Situations Requiring ABG

• All critically ill patients to assess oxygenation, ventilation, and acid-base status 1, 4, 2
• Shock or hypotension (initial sample must be arterial) 1, 4
• SpO2 fall below 94% on room air or supplemental oxygen 1, 4
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 4

Metabolic Conditions Requiring ABG

• Suspected diabetic ketoacidosis 1
• Metabolic acidosis from renal failure, trauma, shock, or sepsis 1
• Any breathlessness with risk of metabolic disturbances 4

Timing of ABG Monitoring

• Within 60 minutes of starting oxygen therapy in COPD patients 4
• Within 60 minutes of any change in inspired oxygen concentration 4
• After each titration of oxygen flow rate in patients with baseline hypercapnia 1, 4, 2

Treatment Based on ABG Abnormalities

Acute Hypercapnic Respiratory Failure

Initiate non-invasive ventilation (NIV) for pH <7.35 and PaCO2 >6.5 kPa (49 mmHg) despite optimal medical therapy. 1, 2

• Start with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 1
• Target SpO2 88-92% in COPD or hypercapnic respiratory failure 1, 2
• Administer NIV in ICU setting with intubation readily available for severe acidosis 1

Intubation Criteria on NIV

• Worsening ABG/pH in 1-2 hours on NIV 1
• Lack of improvement after 4 hours of NIV 1
• Respiratory rate >35 breaths/min 1
• PaCO2 rise >1 kPa (7.5 mmHg) despite NIV 1

Controlled Oxygen Therapy for Hypercapnic Risk

Start with low-flow oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90%, then confirm with repeat ABG. 4, 2

• Target SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 2
• Repeat ABG after each titration to monitor for worsening hypercapnia 1, 4, 2
• PaCO2 rise >1 kPa (7.5 mmHg) during oxygen therapy indicates clinically unstable disease requiring further medical optimization 4

Persistent Respiratory Acidosis Despite Optimization

• Consider nocturnal ventilatory support for patients with persistent respiratory acidosis despite medical optimization 4
• Reassess after 4 weeks of medical optimization 4

Special Population Considerations

COPD and Chronic Hypercapnia

• Check ABG when starting oxygen therapy, especially with known CO2 retention 1, 4
• Use controlled oxygen therapy with frequent ABG monitoring 1, 2
• Patients with baseline hypercapnia require ABG after each flow rate adjustment 4, 2

Hepatopulmonary Syndrome

• Use P(A-a)O2 ≥20 mmHg cutoff (instead of ≥15 mmHg) for diagnosis in patients aged ≥65 years 1
• PaO2 <60 mmHg warrants evaluation for liver transplantation 5
• Severe hypoxemia (PaO2 <45-50 mmHg) is associated with increased post-transplant mortality, requiring ABG analysis every 6 months 5

Critical Pitfalls to Avoid

Common Interpretation Errors

• Normal oxygen saturation does NOT rule out significant acid-base disturbances or hypercapnia, especially in patients on supplemental oxygen 4, 2
• Pulse oximetry will be normal in patients with abnormal acid-base status, ventilation problems, or low oxygen content from anemia 4
• Failing to repeat ABG after changes in oxygen therapy in patients at risk for CO2 retention 4, 2

Management Errors

• Overlooking the need for ABG in patients with metabolic conditions causing acid-base disturbances 4
• Relying solely on pulse oximetry when acid-base status and ventilation assessment are needed 4
• Not recognizing mixed acid-base disorders through delta ratio calculation, which directly influences treatment priorities 2

Technical Considerations for ABG Sampling

Pre-Procedure Requirements

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 2
• Obtain informed consent with discussion of possible risks 2
• Use local anesthesia for all ABG specimens except in emergencies 4

Alternative Sampling Methods

• Capillary blood gases (CBG) can replace ABG for re-measuring PaCO2 and pH during oxygen titration 2
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 2
• Arterialized earlobe blood gases may be used for non-critical patients to measure acid-base status and ventilation 4, 2