Interpreting Venous Blood Gas (VBG) for Acid-Base Status and Ventilation

Systematic Approach to VBG Interpretation

A pH < 7.30 in a venous blood gas sample indicates acidemia, while a pH > 7.43 indicates alkalemia, according to clinical guidelines 1
Elevated pCO2 (>58 mmHg) in a venous blood gas sample suggests respiratory acidosis, and low pCO2 (<38 mmHg) suggests respiratory alkalosis, as recommended by clinical guidelines 1
Elevated HCO3- (>30 mmol/L) in a venous blood gas sample suggests metabolic alkalosis or compensation for respiratory acidosis, while low HCO3- (<22 mmol/L) suggests metabolic acidosis or compensation for respiratory alkalosis, based on evidence from clinical studies 2
A negative base excess (<-1.9) in a venous blood gas sample indicates metabolic acidosis, according to clinical guidelines 1

In critically ill patients, especially those with shock or hypotension, the arterio-venous difference may be greater than normal, making VBG less reliable, as noted in the Thorax journal 3
For patients with respiratory failure, especially when precise oxygenation assessment is needed, arterial samples are preferred, as recommended by the Thorax journal 3
For patients with COPD at risk of hypercapnic respiratory failure, a target oxygen saturation of 88-92% is recommended when using pulse oximetry alongside VBG, based on guidelines from the Thorax journal and Praxis Medical Insights 3, 1
Proper sample handling is crucial, as air bubbles, delayed analysis, or improper storage can significantly affect VBG results, as highlighted in the Thorax journal 3

The American Thoracic Society recommends obtaining an arterial blood gas (ABG) in all critically ill patients with shock or hypotension (systolic BP <90 mmHg) 4
The World Journal of Emergency Surgery suggests that when clinical deterioration occurs despite "acceptable" VBG values, an ABG should be obtained 5

The World Journal of Emergency Surgery strongly recommends early blood gas (arterial or venous) for baseline base-deficit or lactate assessment in elderly trauma patients, with serial base deficit and lactate levels serving as markers of occult hypoperfusion alongside vital signs monitoring 5
In elderly trauma patients, a lower threshold for obtaining ABG is recommended given their increased risk of poor outcomes and the need for precise assessment 5

In patients with elevated venous PCO₂ suggesting hypoventilation, common etiologies include chronic obstructive pulmonary disease exacerbation, neuromuscular weakness, oversedation, obesity‑related hypoventilation, and restrictive lung disease. 6

Hyperventilation is associated with the production of additional CO₂ from body stores, resulting in a gas‑exchange ratio that exceeds the normal respiratory quotient. 7