A comparison of arterial and blood gas analyses in sleep studies

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Patients having sleep/CPAP/ventilation (SCV) studies for chronic respiratory failure often require arterial blood to be taken for measurement of blood oxygen, pH and carbon dioxide measurement - so called arterial blood gases (ABGs). These measurements are used to assist in 1. identifying inadequate breathing 2. determining need for assisted ventilation in patients with known or possible ventilatory disorders (eg chronic respiratory or neuromuscular disease or reduced ventilatory capacity associated with obesity ( “obesity hypoventilation syndrome”) . 3. providing information to assist in modification of ventilator settings in such patients. An arterial blood sample is somewhat more difficult to obtain and generally believed to be more painful than a venous sample (venous blood gases (VBGs)), but there is currently no evidence that VBGs can adequately replace the role of ABGs in this setting. The project will involve taking a venous blood sample for VBGs at the same time as the standard arterial blood sample (for ABGs) from patients already having SCV studies already including ABGs. The results from the arterial and venous samples will be compared. The study will involve an additional paired blood test for each ABG (estimated two per patient), during the same visit and should take minimal extra time (estimated 10 minutes total). At the end of the study we will compare the ABG and VBG measurements to see if we can use venous gases as a reliable alternative to ABGs in future. The data may also be used to see if we can use an equation to better approximate the arterial values from VBGs and if there is a consistent relationship between VBGs and transcutaneous measurements (across skin) of carbon dioxide, which are already being measured.