From the Guidelines
The normal values for PaCO2, PaO2, pH, and HCO3 in humans are approximately 4.6–6.1 kPa (34–46 mm Hg) for PaCO2, with the other values not explicitly stated in the most recent and highest quality study available 1. However, based on general medical knowledge and previous studies, the normal ranges for these parameters are typically considered to be:
- PaCO2: 35-45 mmHg
- PaO2: 80-100 mmHg
- pH: 7.35-7.45
- HCO3: 22-26 mEq/L These values are crucial for evaluating respiratory and metabolic function, with PaCO2 levels above 6.1 kPa (46 mm Hg) indicating hypercapnia and potential type 2 respiratory failure, as noted in the BTS guideline for oxygen use in adults 1. Key points to consider include:
- PaCO2 reflects the balance between CO2 production and elimination by the lungs
- PaO2 measures oxygen content in arterial blood
- pH represents the acid-base balance in the body
- HCO3 reflects the metabolic component of acid-base balance It is essential to interpret these values in the context of individual patient circumstances and clinical presentation, as hypercapnia was found to be more common than pure hypoxaemia in various hospital settings, including surgical wards and critical care areas 1.
From the Research
Normal Values for Arterial Blood Gases
The normal values for arterial blood gases, including PaCO2, PaO2, pH, and HCO3, can be summarized as follows:
- PaCO2: 35-45 mmHg or 4.7-6.0 kPa 2, 3, 4
- PaO2: 75-100 mmHg or 10-13.3 kPa 3, 5, 4
- pH: 7.35-7.45 6, 3
- HCO3: 22-28 mmol/L 6, 3
Factors Affecting Arterial Blood Gas Values
Several factors can affect arterial blood gas values, including:
- Age: PaO2 values may decrease with age, but this decrease is not as significant as previously thought 4
- Respiratory disease: Conditions such as chronic obstructive pulmonary disease (COPD) can affect PaO2 and PaCO2 values 3, 4
- Metabolic state: Changes in metabolic state can affect pH and HCO3 values 2, 6
Prediction of Arterial Blood Gas Values
Venous blood gas values can be used to predict arterial blood gas values, including pH, PaCO2, and HCO3, with reasonable accuracy 6
- The equations for predicting arterial blood gas values from venous blood gas values are:
- Arterial pH = 0.45 + 0.94 x venous pH
- PaCO2 = 3.06 + 0.76 x venous CO2
- Arterial HCO3 = 2.34 + 0.82 x venous HCO3 6