Median Local Anaesthetic Dose (MLAD) of intrathecal Bupivacaine in primary total knee arthroplasty

Total knee arthroplasty (TKA) has become a common elective orthopaedic surgical
procedure. The perioperative management has been developed in order to improve
the outcomes of TKA. As a result fast-track protocols have evolved as a
multimodal procedure to achieve faster rehabilitation and reduction of
complications. In fast-track TKA, spinal anaesthesia with Bupivacaine is
preferred over general anaesthesia due to less nausea and vomiting.
Mean surgery time for TKA at the RdGG is 71 minutes, in another study of Lozano
et al. a mean surgery time of 83 minutes was found. Provided that
administration of 8 mg of Bupivacaine intrathecal at level L3-L4 will lead to a
motor block of 100-225 minutes according to Gautier et al. it might be possible
to reduce the amount of Bupivacaine used during surgery in order to optimize
rehabilitation after TKA.
Current dosage of thecal Bupivacaine are adequate for allowing surgery with
total anaesthesia, however this does not provide optimization for direct
postoperative mobilisation and often leave patients unable to mobilize for
several hours postoperative prolonging a patients hospital stay.

The objective of this study is to find the Median Local Anaesthetic Dose
(MLAD/ED50) of Bupivacaine that allow direct postoperative mobilization and
will accommodate sufficient anaesthesia during surgery.
Results of this study might lead to justification of lower doses of Bupivacaine
used for anaesthesia in TKA. Henceforth, the fast-track program can be
optimized and rehabilitation can start immediate postoperative.

This is an intervention study designed to find the MLAD/ED50 of Bupivacaine at
a certain threshold.
In this study we used the Up-and-Down method as described by Dixon and Massey.
This is a sequential allocation model where patients receive a dose of
Bupivacaine according to the outcome of the preceding patient. With the
Up-and-Down method we approach the MLAD/ED50 from above, leaving less patients
with inadequate anaesthesia. For the cut-off point or threshold point needed in
this model we use the 95th percentile of the mean surgery time for TKA, in
order to expose less patients to inadequate anaesthesia during surgery.
At the cut-off point patients must be able to fully recover from sensory and
motor blockage induced by Bupivacaine. Therefore, at the cut-off point (116
minutes after spinal anaesthesia is administrated) patients must reach:
- Full recovery of the motor block; measured with the modified Bromage scale
(Bromage 0)
- Full recovery of sensory block; positive pinprick test at the S1 and L5
dermatome

Possible test outcome:
1. Patient reach the cut-off parameters before 116 minutes -> inadequate
Bupivacaine dose
2. Patient reach the cut-off parameters after 116 minutes -> adequate
Bupivacaine dose
If a patient is documented with inadequate anaesthesia (possible outcome 1) the
succeeding patient receives a dose of Bupivacaine Xy+0.5 mg
If a patient is documented with adequate anaesthesia (possible outcome 2) the
succeeding patient receives a dose of Bupivacaine Xy-0.5 mg

During surgery sensory block height is assessed using a pinprick test. The
patient is asked to score pain on the NRS for pain. Testing will be performed
in 10 minute intervals starting from the injection of Bupivacaine.
After surgery sensory block recovery is assessed using a pinprick test at
dermatome L5 and S1. Supplementary, recovery from motor block is assessed by
the modified Bromage scale. After surgery the testing will be performed in 10
minute intervals until the cut-off parameters are reached.

The patients will undergo surgery according to the TKA protocol. The
intervention targets the spinal anaesthesia protocol. The current protocol
imposes a dose of 6 mg Bupivacaine administrated intrathecal at the L3-L4
intervertebral space. In this study we lower the dose for the succeeding
patient according to the outcome of the preceding patient.
The patient will undergo testing and monitoring in 10 minute intervals starting
from the start of injection until it is recovered from the nerve blockade.

Patients will receive their scheduled TKA according to the regular planned fast
track surgery programme. The Up-and-Down sequential allocation technique,
rather than random allocation, is chosen due to the ease which it estimates the
mean of a sample. By starting from known effective concentration and
approaching the ED50 from above the number of patients subjected to potentially
inadequate analgesia is minimized.