Critical Values for Venous Blood Gas Analysis
In adult patients, critical venous blood gas values requiring immediate intervention include pH <7.35 with pCO2 >6.0-6.5 kPa (45-49 mmHg), as these thresholds indicate acute respiratory acidosis necessitating urgent clinical action including consideration for non-invasive ventilation. 1, 2
Critical Lower Limits
pH
- pH <7.35 is the established threshold for acute respiratory acidosis requiring immediate senior review and potential ventilatory support 1, 2
- pH <7.26 represents severe acidemia indicating acute-on-chronic respiratory failure and strongly warrants non-invasive ventilation 2
- pH <7.1 (arterial equivalent) has been cited as severe acidosis requiring buffer therapy in cardiac arrest situations, though this specifically references arterial values 1
pCO2 (Upper Critical Limit for Hypercapnia)
- pCO2 >6.5 kPa (49 mmHg) combined with pH <7.35 is the threshold for initiating non-invasive ventilation after one hour of optimal medical therapy 1
- pCO2 between 6.0-6.5 kPa (45-49 mmHg) with pH <7.35 should prompt consideration for NIV, though this range may not contribute substantially to acidosis alone 1
- pCO2 of 65 mmHg represents marked hypercapnia indicating severe acute-on-chronic respiratory failure 2
Oxygen Parameters
- pO2 <42 mmHg with O2 saturation 73% indicates severe hypoxemia requiring immediate oxygen therapy 2
- Note: Venous pO2 values have poor agreement with arterial values (95% limits of agreement 145.3 to -32.9) and should not be used for clinical decision-making 3
Critical Upper Limits
pH
- pH >7.45 defines alkalemia requiring investigation, though specific critical upper thresholds for venous blood gas are not well-established in guidelines 4
Glucose (when measured on blood gas analyzer)
- Glucose ≤2.2 mmol/L (≤40 mg/dL) represents severe hypoglycemia requiring immediate treatment 1
- Glucose 2.3-3.9 mmol/L (41-70 mg/dL) represents moderate hypoglycemia 1
Important Clinical Caveats
Venous vs. Arterial Correlation
- Venous pH correlates well with arterial pH (mean difference 0.027-0.035 units), with 72-80% of paired samples falling within clinically acceptable limits 5, 6, 3
- Venous pCO2 is less reliable than pH, with mean arterio-venous difference of 3.8-5.7 mmHg but 95% limits of agreement up to ±20 mmHg 6, 7
- The v-TAC conversion method is unreliable in severe metabolic acidemia (pH <7.1), where it may falsely normalize calculated values 4
When Arterial Sampling is Mandatory
- All critically ill patients with shock or hypotension (systolic BP <90 mmHg) require arterial blood gas measurement initially 1
- Patients with severe acid-base disturbances where accurate values are essential for management decisions 5
- When venous pO2 assessment is needed, as venous values show no correlation with arterial pO2 4, 3