Comparison of Performance of a Conventional Blood Gas Analyser With the Proxima System

Inclusion Criteria: * Male and female patients ≥18 years old * Patients who have (or will have) an arterial line, which was (will be) inserted for clinical need, will be considered for inclusion in this study * Patients who give informed consent (or their personal/nominated consultee) to participate in the study * Patients who are likely to have an arterial line for at least 48 hours. Exclusion Criteria: * Patients not indicated or contraindicated for an arterial line * Refusal of consent by a patient or their personal/nominated consultee to participate * Not for use with patients with uncorrected hyperphosphataemia, hypocalcaemia or hypercalcaemia. * The patient is considered by the investigator to be unsuitable for the study Arterial line location/placement is not dictated by this study, however the following criteria should be considered when deciding if arterial catheter placement is suitable, including but not limited to: * peripheral vascular disease, * history of placement site neuropathy or chronic pain, * history of placement extremity coagulopathy or clot formation, * history of vascular surgery on same extremity as catheter placement (and/or vascular grafts) * patients with a plan for perioperative anticoagulation. These patients are at increased risk of known complications such as bleeding and arteriospasm and should be excluded in order to mitigate patient study risk.