From the Guidelines
Venous pH is a reliable and practical alternative to arterial pH in most clinical situations, with a typical difference of 0.03 units, as noted in the management of diabetic ketoacidosis 1. When comparing venous pH to arterial pH, it's essential to consider the underlying physiological differences. Venous blood has a higher carbon dioxide content due to the metabolic waste produced by tissues, which lowers its pH compared to arterial blood.
- The normal pH range for arterial blood is 7.35-7.45.
- Venous blood pH ranges from 7.32-7.42, reflecting the higher CO2 levels.
- In the context of diabetic ketoacidosis (DKA), venous pH is used as a diagnostic criterion, with a threshold of <7.3 1. Key points to consider when using venous pH include:
- The venous-arterial difference remains relatively constant in most cases, allowing for reliable interpretation of venous pH values.
- Venous samples are easier to obtain and can be useful for screening or monitoring trends.
- However, in critically ill patients or those with severe circulatory compromise, the venous-arterial pH difference may increase, making venous samples less reliable 1. In clinical practice, venous pH can be followed to monitor resolution of acidosis, with the understanding that it is typically 0.03 units lower than arterial pH 1.
From the Research
Comparison of Venous and Arterial pH
- The comparison of venous and arterial pH is a topic of interest in medical research, with studies aiming to determine the extent of correlation between the two [(2,3,4)].
- A study published in the Emergency Medicine Journal found that venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department, with a high correlation (r=0.92) and narrow 95% limits of agreement (-0.11 to +0.04 units) between the two 2.
- Another study published in Anesthesia and Analgesia found that central venous blood gas parameters of pH and Pco2 are potentially good surrogates for determining arterial pH and Pco2 in stable patients without severe acid-base disturbances 3.
Clinical Equivalence of Venous and Arterial Samples
- A study published in Diabetic Medicine found that venous samples are clinically equivalent to arterial samples for the estimation of pH, serum bicarbonate, and potassium concentration in critically ill patients 4.
- The study found that the bias (95% limits of agreement) between arterial and venous samples was 0.03 (-0.02 to 0.08) for pH, 0 (-1.3 to 1.3) mmol/l for bicarbonate, and 0.1 (-0.4 to 0.6) mmol/l for potassium 4.
Bicarbonate Therapy and pH
- A study published in the Journal of the American Society of Nephrology discussed the pros and cons of bicarbonate therapy for patients with severe metabolic acidosis, and found that bicarbonate should be given at an arterial blood pH of ≤7.0 5.
- Another study published in The Scientific World Journal found that replacement of sodium bicarbonate is useful in patients with sodium bicarbonate loss due to diarrhea or renal proximal tubular acidosis, but there is no definite evidence that sodium bicarbonate administration is beneficial in patients with acute metabolic acidosis 6.