Evaluation of a method for converting venous values of acid-base and oxygenation status to arterial values

Abstract

Objective: This paper evaluates a method in which arterial values of pH, carbon dioxide tension (Pco₂) and oxygen tension (Po₂) calculated from venous values and pulse oximetry are compared with simultaneously measured arterial values.

Methods: 103 adult patients from three departments (pulmonary medicine, thoracic intensive care and multidisciplinary intensive care) were studied. The patients belonged to three groups: (1) 31 haemodynamically stable patients with a diagnosis of chronic obstructive lung disease (COLD); (2) 49 haemodynamically stable patients without COLD; and (3) 23 haemodynamically unstable patients without COLD. Arterial and venous (peripheral and, where possible, central and mixed) blood samples were taken simultaneously and anaerobically. Peripheral arterial oxygen saturation was measured with a pulse oximeter. The principle of the method is to simulate the transport of venous blood back through the tissues using the respiratory quotient (adding oxygen and removing carbon dioxide) until simulated arterial oxygenation matches that measured by pulse oximetry.

Results: Calculated values of arterial pH and Pco₂ had very small bias and standard deviations regardless of the venous sampling site. In all cases these errors were within those considered acceptable for the performance of laboratory equipment, and well within the limits of error acceptable in clinical practice. In addition, the standard deviation (SD) of calculated values of pH and Pco₂ was similar to the variability between consecutive arterial samples. For peripheral oxygen saturation values ≤96%, the method can calculate Po₂ with an SD of 0.93, which may be useful in clinical practice. Calculations made from peripheral venous blood were significantly more accurate than those from central venous blood.

Conclusion: Arterial pH and Pco₂ can be calculated precisely from peripheral venous blood in a broad patient population. The method has potential for use as a screening tool in emergency medical departments and in medical and surgical wards to assess a patient’s acid-base and oxygenation status prior to sampling arterial blood or to help in the decision to refer the patient to the ICU. In departments where arterial blood gas values are used to monitor patients (eg, pulmonary medicine), the method might reduce the number of arterial samples taken by replacing them with peripheral venous blood samples, thus reducing the need for painful arterial punctures.