How do you interpret and manage arterial blood gas (ABG) results?

ABG Interpretation and Management

Systematic Interpretation Approach

Use a three-step systematic approach: evaluate pH first to determine acidemia versus alkalemia, then examine PaCO2 for the respiratory component, and finally evaluate bicarbonate/base excess for the metabolic component. 1, 2

Step 1: Evaluate pH

• pH < 7.35 indicates acidemia 1, 2
• pH > 7.45 indicates alkalemia 1, 2

Step 2: Identify Respiratory Component

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1, 2
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1, 2

Step 3: Identify Metabolic Component

• Base excess < -2 or HCO3 < 22 mmol/L indicates metabolic acidosis 1, 2
• Base excess > +2 or HCO3 > 26 mmol/L indicates metabolic alkalosis 1, 2

Determine Compensation Status

• Fully compensated: pH normalized but PaCO2 and HCO3 both abnormal 2
• Partially compensated: pH abnormal with both PaCO2 and HCO3 abnormal, moving in opposite directions 2
• Uncompensated: pH abnormal with only one system (respiratory or metabolic) abnormal 2

Primary Indications for ABG Testing

Obtain ABG in all critically ill patients to assess oxygenation, ventilation, and acid-base status. 1, 3

Specific Clinical Scenarios Requiring ABG

• Shock or hypotension 1, 3
• Oxygen saturation fall below 94% on room air or supplemental oxygen 1, 3
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 3
• Suspected diabetic ketoacidosis 1
• Metabolic acidosis from renal failure, trauma, shock, or sepsis 1
• COPD patients when starting oxygen therapy, especially with known CO2 retention 1, 3

Management of Abnormal ABG Results

Acute Hypercapnic Respiratory Failure

Initiate non-invasive ventilation (NIV) when pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) persist despite optimal medical therapy. 4, 1

Oxygen Therapy Protocol

• Target SpO2 88-92% in all causes of acute hypercapnic respiratory failure, including COPD 4, 1
• Start with low flow oxygen (1 L/min) in patients at risk for hypercapnic respiratory failure 3
• Titrate up in 1 L/min increments until SpO2 >90% 3
• Repeat ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration 3

NIV Initiation and Monitoring

• Start with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 1
• Obtain ABG prior to and following starting NIV 4
• Maximize time on NIV in the first 24 hours depending on patient tolerance 4
• Monitor for worsening physiological parameters, particularly pH and respiratory rate, which indicate need to change management strategy 4

Criteria for Intubation

• 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, 2
• Severe acidosis alone does not preclude a trial of NIV in an appropriate area with ready access to intubation 4

Discontinuation of NIV

• Discontinue when pH and pCO2 normalize with general improvement in patient condition 4
• Taper daytime NIV use over 2-3 days depending on pCO2 self-ventilating before discontinuing overnight 4

Respiratory Acidosis Management

• Initiate oxygen therapy cautiously in COPD or other risk factors for hypercapnic respiratory failure 3
• Monitor for worsening hypercapnia after each oxygen titration in patients with baseline hypercapnia 1, 3
• Patients developing respiratory acidosis (PaCO2 rise >1 kPa or 7.5 mmHg) during oxygen therapy require further medical optimization 3
• Consider nocturnal ventilatory support for persistent respiratory acidosis despite optimization 3

Critical Pitfalls to Avoid

• Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 2, 3
• Pulse oximetry will be normal in patients with normal oxygen levels but abnormal acid-base status, ventilation issues, or low blood oxygen content from anemia 3
• Failing to repeat ABG measurements after oxygen therapy changes in patients at risk for CO2 retention is a critical management error 2, 3
• Chest radiography is recommended but should not delay initiation of NIV in severe acidosis 4
• NIV use should not delay escalation to invasive mechanical ventilation when more appropriate 4

Special Population Considerations

COPD Patients

• Check ABG when starting oxygen in COPD patients, especially with known CO2 retention 1, 3
• Chronic CO2 retention leads to metabolic compensation with elevated HCO3 2
• Base excess changes to compensate in chronic respiratory disorders, whereas in acute disorders it remains initially normal 2

Hepatopulmonary Syndrome

• Use PaO2 < 80 mmHg or P(A-a)O2 ≥ 15 mmHg for diagnosis (≥ 20 mmHg if age ≥ 65 years) 1
• Patients with PaO2 <60 mmHg should be evaluated for liver transplantation 1
• Perform ABG analysis every 6 months as severe hypoxemia (PaO2 <45-50 mmHg) is associated with increased post-transplant mortality 1