Inclusion Criteria:  Adults (≥18 years) due to undergo elective surgery who are likely to have an arterial line sited as part of their standard intraoperative care. Recruitment after booking for surgery with sufficient time to read, understand and question study patient information prior to attending for surgery. Ability and willingness to provide informed consent Expected duration of anaesthesia > 60 minutes Intra-arterial monitoring is part of routine clinical care  Exclusion Criteria:  Inability to provide informed consent No arterial line planned as part of their intraoperative care Aged < 18 years of age Patients with aortic regurgitation Patients fitted with an intra-aortic balloon pump (IABP) No other intra-arterial pressure management system in use Concurrent participation in another experimental intervention or drug study