What is the normal Arterial Blood Gas (ABG) pH range?

Normal Arterial Blood Gas (ABG) pH Range

The normal arterial blood gas pH range is 7.35 to 7.45, with acidosis defined as pH < 7.35 and alkalosis as pH > 7.45. 1

Understanding the pH Range

The normal pH range reflects the tightly regulated hydrogen ion concentration in blood, maintained between 35 and 45 nmol/L 1. This narrow physiological window is critical for:

• Protein function and enzymatic activity - deviations outside the compatible range (generally 6.8 to 7.8) can lead to protein denaturation, though survival has been documented at pH values as low as 6.7 in severe metabolic acidosis cases 2
• Cellular metabolism and oxygen delivery - pH directly affects hemoglobin-oxygen binding and tissue oxygen utilization 1

Clinical Significance of pH Deviations

Acidosis (pH < 7.35)

Respiratory acidosis occurs when pH falls below 7.35 in the presence of elevated PaCO₂ (> 6.0 kPa or 45 mmHg), indicating inadequate ventilation and carbon dioxide retention 3, 1. Key thresholds include:

• pH < 7.35 with PaCO₂ > 6.0-6.5 kPa - consider non-invasive ventilation, especially if respiratory rate > 23 breaths/min 3
• pH < 7.26 - predictive of poor outcome in COPD exacerbations and indicates severe respiratory compromise 1
• pH < 7.25 - threshold for considering invasive mechanical ventilation in acute respiratory failure 1

Metabolic acidosis presents with pH < 7.35 but with low bicarbonate levels rather than elevated CO₂, requiring investigation for causes such as diabetic ketoacidosis, renal failure, or lactic acidosis 1.

Alkalosis (pH > 7.45)

Alkalosis is defined as pH exceeding 7.45, though the provided evidence focuses primarily on acidotic states in acute care settings 1.

Practical Considerations for ABG Interpretation

When obtaining blood gases:

• Arterial samples are preferred for critically ill patients, those with shock, or systolic blood pressure < 90 mmHg to ensure accurate oxygenation assessment 1
• Local anesthesia should be used for all ABG specimens except emergencies 1
• Venous blood gas pH averages 0.035 units lower than arterial pH (mean arterial pH 7.39 vs venous pH 7.35), though only 72% of paired samples fall within clinically equivalent ranges 4

Target oxygen saturation varies by clinical context:

• 94-98% for most acutely ill patients 1
• 88-92% for patients at risk of hypercapnic respiratory failure (COPD, neuromuscular disease, obesity hypoventilation) to avoid worsening CO₂ retention 1

Common Pitfalls

Avoid these errors when interpreting pH:

• Don't rely solely on pulse oximetry - normal SpO₂ does not exclude abnormal pH or PaCO₂, particularly in patients receiving supplemental oxygen 1
• Recognize compensated states - chronic respiratory acidosis may show elevated PaCO₂ with normal pH due to renal bicarbonate retention, but acute decompensation can still occur 1, 3
• Account for spontaneous variability - ABG values can vary with a coefficient of variation of 4.7% for PCO₂ over short time periods in stable patients 5