Today, surgery in an ambulatory setting is preferred for many reasons. Total hip and knee arthroplasty (THA and TKA) are now procedures where patients can be sent home on the same day of the surgical procedure. However, this requires substantial knowledge of factors affecting the length of hospital stay. In this prospective, observational cohort study, we will investigate factors that affect the duration of spinal anaesthesia which is often used in THA and TKA. The aim is to gather enough data to be able to determine how local anaesthetic volume and dose affect the duration of spinal anaesthesia. This will enable us to decide whether spinal anaesthesia is a good option when performing TKA and THA in an ambulatory setting.

With the increasing health care cost and the scarcity of nursing staff, there is considerable focus on providing surgery in an ambulatory setting. Total hip and knee arthroplasty (THA and TKA) are now procedures where selected patients are sent home on the same day after the procedure. Therefore, knowledge of key factors affecting the length of stay is important. One factor is the method of anaesthesia used for the operative procedure.

Spinal anaesthesia is often used for THA and TKA and has several advantages compared to general anaesthesia. However, spinal anaesthesia also has drawbacks, because it reduces the force in the legs, hindering early mobilisation. There are no good data on the duration of spinal anaesthesia. Compared to the past, there has been a shift towards using lower doses of local anaesthetic (LA) injected into the subarachnoid space. Theoretically, this should result in a shorter duration of anaesthesia, and this would be of value in an ambulatory setting. However, if the variation in the duration is high and therefore not predictable for the individual patient, spinal anaesthesia has limitations in the ambulatory setting.

The objective of the present study is to prospectively collect data on the type and dose of LA injected into the subarachnoid space and the duration of spinal anaesthesia in participants scheduled for orthopaedic surgery in spinal anaesthesia. We hypothesise that the mean duration of spinal anaesthesia is positively correlated to the dose of LA injected into the subarachnoid space.

Inclusion Criteria:

• Scheduled for elective orthopaedic surgical procedures performed in spinal anaesthesia
• ASA 1-4.

Exclusion Criteria:

• Age < 18 years
• Anticoagulant medication therapy not paused according ESAIC/ESRA 2022 guidelines
• Local infection at the lumbar puncture site
• Bleeding disorder
• Unable to cooperate with the spinal anaesthesia procedure.

Adult participants scheduled for elective surgical procedures performed in spinal anaesthesia