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

ABG Interpretation and Management

Systematic Interpretation Approach

Use a three-step systematic method to interpret all ABG results: first evaluate pH to determine acidemia (pH < 7.35) or alkalemia (pH > 7.45), then examine PaCO2 to identify the respiratory component, and finally evaluate base excess/bicarbonate to identify the metabolic component. 1, 2

Step 1: Assess pH Status

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

Step 2: Identify Respiratory Component

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
• Remember the "Respiratory opposite" rule: in respiratory disorders, pH and PaCO2 move in opposite directions 3

Step 3: Identify Metabolic Component

• Base excess < -2 or HCO3 < 22 indicates metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 indicates metabolic alkalosis 1
• Remember the "Metabolic equal" rule: in metabolic disorders, pH and HCO3 move in the same direction 3

Detecting Mixed Disorders

• A systematic approach identifies mixed acid-base disorders in approximately 50% of cases, compared to only 12.9% with bedside methods 4
• Mixed disorders are common in critically ill patients and require careful analysis of all three parameters 4

Primary Indications for ABG Testing

Obtain ABG analysis in all critically ill patients to assess oxygenation, ventilation, and acid-base status. 1, 2, 5

Critical Situations Requiring ABG

• Shock or hypotension: Initial blood gas measurement must be from an arterial sample 2, 5
• Oxygen saturation fall below 94% on room air or supplemental oxygen 1, 5
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 5
• Suspected metabolic conditions: diabetic ketoacidosis, metabolic acidosis from renal failure, trauma, shock, or sepsis 1, 5

Common Pitfall to Avoid

• A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 2, 5
• Pulse oximetry will be normal in patients with normal oxygen levels but abnormal acid-base status, ventilation problems, or low blood oxygen content due to anemia 5

Management of Respiratory Acidosis

For acute hypercapnic respiratory failure with pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg), initiate non-invasive ventilation (NIV) despite optimal medical therapy. 1, 2

Oxygen Therapy in Hypercapnic Patients

• Start with controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 2
• Begin at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 6, 5
• Perform ABG after each titration to monitor for worsening hypercapnia 6, 1, 5

NIV Parameters and Monitoring

• Start with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 1
• Administer NIV in an ICU setting with intubation readily available for severe acidosis 1
• Monitor ABG/pH in 1-2 hours on NIV 1

Intubation Criteria

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

Critical Management Pitfall

• Patients who develop respiratory acidosis and/or rise in PaCO2 >1 kPa (7.5 mmHg) during oxygen titration may have clinically unstable disease and should undergo further medical optimization and reassessment after 4 weeks 6, 5
• If this occurs on two repeated occasions while apparently clinically stable, only order domiciliary oxygen in conjunction with nocturnal ventilatory support 6

Management of Metabolic Acidosis

In cardiac arrest, administer one to two 50 mL vials (44.6 to 100 mEq) of sodium bicarbonate initially and continue at 50 mL every 5 to 10 minutes as indicated by arterial pH and blood gas monitoring. 7

Non-Emergency Metabolic Acidosis

• For less urgent metabolic acidosis, infuse 2 to 5 mEq/kg body weight over 4 to 8 hours 7
• Monitor therapy by measuring blood gases, plasma osmolarity, arterial blood lactate, hemodynamics, and cardiac rhythm in metabolic acidosis associated with shock 7
• Plan bicarbonate therapy in a stepwise fashion since the degree of response from a given dose is not precisely predictable 7

Target Goals and Timing

• Aim for total CO2 content of about 20 mEq/liter at the end of the first day of therapy 7
• It is unwise to attempt full correction of low total CO2 content during the first 24 hours, as this may be accompanied by unrecognized alkalosis due to delayed readjustment of ventilation 7
• Values brought to normal or above normal within the first day are very likely associated with grossly alkaline blood pH with ensuing undesired side effects 7

Special Population Considerations

COPD and Hypercapnic Risk Patients

• Check ABG when starting oxygen in COPD patients, especially with known CO2 retention 1, 2, 5
• Perform ABG analysis within 60 minutes of starting oxygen therapy and within 60 minutes of a change in inspired oxygen concentration 5
• Non-hypercapnic patients on long-term oxygen therapy should increase flow rate by 1 L/min during sleep in the absence of contraindications 6

Long-Term Oxygen Therapy (LTOT) Assessment

• Patients being assessed for LTOT should undergo initial assessment using arterial blood gases sampling 6
• During clinical stability, obtain two ABG measurements at least 3 weeks apart before confirming need for LTOT 6, 2
• After oxygen titration is complete, reassess with ABG to determine whether adequate oxygenation (target PaO2 ≥8 kPa or 60 mm Hg) has been achieved without precipitating respiratory acidosis 6, 2

Hepatopulmonary Syndrome

• Use PaO2 < 80 mmHg or P(A-a)O2 ≥ 15 mmHg for diagnosis in patients under 65 years 1
• For patients aged ≥ 65 years, use P(A-a)O2 ≥ 20 mmHg cutoff instead 1
• Perform ABG analysis every six months in patients with hepatopulmonary syndrome to facilitate prioritization for liver transplantation, as severe hypoxemia (PaO2 <45-50 mmHg) is associated with increased post-transplant mortality 6

Technical Considerations

Pre-Procedure Assessment

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 6, 2, 5
• Obtain consent with discussion of possible risks 6, 5
• Use local anesthesia for all ABG specimens except in emergencies 2, 5

Alternative Sampling Methods

• For oxygen titration during LTOT assessment, capillary blood gases (CBG) can replace ABG for re-measuring PaCO2 and pH at different oxygen flow rates 6
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 6
• In community settings where ABG is not practical, a combination of CBGs and oximetry (but not capnography alone) can be used as an alternative for initial LTOT assessment 6

Critical Pitfalls to Avoid

• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention 2, 5
• Relying solely on pulse oximetry when acid-base status and ventilation need assessment 5
• Overlooking the need for ABG in patients with metabolic conditions that may cause acid-base disturbances 5
• Attempting full correction of metabolic acidosis within 24 hours, risking rebound alkalosis 7
• Not recognizing that most previously stable patients who deteriorate clinically and require increased FiO2 to maintain constant oxygen saturation should undergo ABG analysis 5