From the Guidelines
The venous blood gas results indicate a metabolic acidosis with partial respiratory compensation, requiring immediate evaluation and treatment of the underlying cause. The pH of 7.33 is slightly below the normal range (7.35-7.45), suggesting acidemia, as defined by the BTS guideline for oxygen use in adults in healthcare and emergency settings 1. The low bicarbonate (vHCO3) of 13 mEq/L confirms a metabolic acidosis as the primary disturbance. The low venous PCO2 of 26 mmHg represents respiratory compensation through increased ventilation to eliminate CO2 and raise the pH toward normal. Key considerations in the management of this patient include:
- Identifying and treating the underlying cause of metabolic acidosis, such as diabetic ketoacidosis, lactic acidosis, renal failure, toxic ingestions, or severe diarrhea
- Monitoring electrolytes, particularly potassium which often rises in acidosis
- Fluid resuscitation, especially in conditions like diabetic ketoacidosis or severe diarrhea
- Serial blood gas measurements to monitor response to treatment
- Consideration of sodium bicarbonate administration in severe acidosis (pH < 7.2), although this is controversial and should be used cautiously, as stated in the guideline by O’Driscoll BR, et al. 1.
From the Research
Analysis of Venous Blood Gas
The given venous blood gas values are pH 7.33, pCO2 26, and HCO3 13. To analyze these values, we need to consider the agreement between venous and arterial blood gas analysis.
- The study by 2 found that venous blood gases are suitable for initial evaluation of acid-base status in critically ill patients, with a bias of 0.044 for pH, -6.2 mmHg for pCO2, and -0.07 mEq·L-1 for lactate.
- The study by 3 derived regression equations to predict arterial pH, pCO2, and HCO3- values from venous blood gas values, with a mean percentage difference of 0.50 +/- 0.45 for pH, 17.09 +/- 9.60 for pCO2, and 9.72 +/- 7.73 for HCO3-.
- The review article by 4 found that venous pH, bicarbonate, and base excess have sufficient agreement to be clinically interchangeable with arterial values, but the agreement between arterial and venous pCO2 is too poor to be clinically useful as a one-off test.
Comparison with Arterial Blood Gas Values
- The study by 5 found that central venous pH, pCO2, and bicarbonate may be an acceptable substitute for arterial blood gas analysis in ICU patients, with a mean difference of 0.04 units for pH, -5.84 mmHg for pCO2, and 0.89 mmol/L for bicarbonate.
- The systematic review and meta-analysis by 6 found that venous and arterial pH and bicarbonate agree reasonably, but the agreement is highest at normal values, and arteriovenous agreement for pCO2 is poor.
Interpretation of Results
- Based on the study by 3, the predicted arterial pH value would be approximately 7.33 x 1.005 = 7.37, the predicted arterial pCO2 value would be approximately 26 x 0.83 = 21.58, and the predicted arterial HCO3- value would be approximately 13 x 0.90 = 11.7.
- However, it is essential to note that the agreement between venous and arterial blood gas values can vary depending on the patient's condition and the specific laboratory methods used. Therefore, these predictions should be interpreted with caution and in the context of the patient's overall clinical picture.