How do you interpret an arterial blood gas (ABG) result?

ABG Interpretation: A Systematic Approach

Interpret arterial blood gas results using a three-step systematic method: 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

Step-by-Step Interpretation Algorithm

Step 1: Assess the pH

• pH < 7.35 = Acidemia 1
• pH > 7.45 = Alkalemia 1
• pH 7.35-7.45 = Normal (but may still have compensated disorders) 1

Step 2: Identify the Respiratory Component

• PaCO2 > 45 mmHg with low pH = Respiratory acidosis 1, 2
• PaCO2 < 35 mmHg with high pH = Respiratory alkalosis 1, 2
• PaCO2 35-45 mmHg = Normal respiratory component 1

Step 3: Identify the Metabolic Component

• Base excess < -2 or HCO3 < 22 = Metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 = Metabolic alkalosis 1
• Base excess -2 to +2 or HCO3 22-26 = Normal metabolic component 1

Primary Indications for ABG Testing

Critical Care Settings

• All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status 1, 2, 3
• Patients with shock or hypotension must have initial blood gas measurement from an arterial sample 2, 3
• Patients on vasopressor therapy require arterial rather than venous sampling 3

Respiratory Compromise

• Oxygen saturation fall below 94% on room air or supplemental oxygen 1, 2
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 2
• Most previously stable patients who deteriorate clinically and require increased FiO2 to maintain constant oxygen saturation 2

Metabolic Disturbances

• Suspected diabetic ketoacidosis 1, 2
• Metabolic acidosis from renal failure, trauma, shock, and sepsis 1, 2
• Patients with breathlessness at risk for metabolic conditions 2

Post-Resuscitation

• After return of spontaneous circulation following cardiopulmonary resuscitation to guide ongoing oxygen therapy 2, 3

Management of Abnormal ABG Results

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
• Use controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1
• Repeat ABG after each titration to monitor for worsening hypercapnia 1, 2

Oxygen Therapy Titration in At-Risk Patients

• Start with low flow oxygen (1 L/min) and titrate up in 1 L/min increments until SpO2 >90%, then confirm with repeat ABG 2
• Perform ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration 2, 3
• Patients who develop respiratory acidosis (rise in PaCO2 >1 kPa or 7.5 mmHg) during oxygen therapy have clinically unstable disease and require further medical optimization 2

Severe Respiratory Acidosis

• Consider NIV or mechanical ventilation for severe respiratory acidosis 2
• For persistent respiratory acidosis despite optimization, consider nocturnal ventilatory support 2

Special Populations

COPD and Chronic Hypercapnia

• Check ABG when starting oxygen in COPD patients, especially with known CO2 retention 1, 2, 3
• After each titration of oxygen flow rate in patients with baseline hypercapnia, perform ABG analysis 2

Hepatopulmonary Syndrome

• Use P(A-a)O2 ≥ 20 mmHg cutoff instead of ≥ 15 mmHg for diagnosis in patients aged ≥ 65 years 1
• PaO2 < 80 mmHg or P(A-a)O2 ≥ 15 mmHg (≥ 20 mmHg if age ≥ 65) confirms diagnosis 1

ECMO Patients

• ABG samples should come from a right radial arterial line as this best represents cerebral perfusion 3
• ABG helps identify "Harlequin syndrome" or "North-South syndrome" in VA-ECMO patients where differential oxygenation occurs 3

Cardiogenic Shock

• ABG analysis identifies metabolic acidosis in cardiogenic shock, which is associated with poor outcomes 3

Technical Considerations

Sampling Technique

• Use local anesthesia for all ABG specimens except in emergencies 2, 3
• Perform Allen's test before radial ABG to ensure dual blood supply to the hand 2, 3
• Arterial samples are preferred over capillary samples in critically ill patients 3

Timing of Measurements

• Pressure measurements must be taken at end expiration if the patient is breathing spontaneously and at end inspiration if ventilated 4
• Review arterial blood gases frequently during cardiac catheterization studies 4

Critical Pitfalls to Avoid

Common Interpretation Errors

• A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 2, 3
• Pulse oximetry will appear normal in patients with normal PO2 but abnormal pH or PCO2 2, 3
• Normal oxygen saturation does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy 2

Management Pitfalls

• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention 2
• Overlooking the need for ABG in patients with metabolic conditions that may cause acid-base disturbances 2
• Relying solely on pulse oximetry when acid-base status and ventilation need assessment 2

Physiologic Considerations During Testing

• Blood pH has a potent effect on pulmonary vascular tone—acidosis causes vasoconstriction while alkalosis causes vasodilation 4
• Hypoxia causes pulmonary vasoconstriction, whereas hyperoxia causes vasodilation 4
• Awareness of arterial blood gas measurements during procedures is critical for accurate interpretation of baseline hemodynamics 4