Median Local Anaesthetic dose (MLAD) of Intrathecal Bupivacaine in anterior supine intermuscular total hip arthroplasty

Total hip arthroplasty (THA) has become a common elective orthopedic procedure
surgical orthopedic procedure. Surgical approaches for THA have evolved over
the years leading to lees postoperative pain and reduction of length of stay.
Moreover, the peri-operative management has developed in order to improve
outcomes. As a result fast track programs have evolved as a multimodal
procedure to achieve faster rehabilitation, reduction of organ dysfunction,
morbidity and mortality. In fast-track total hip arthroplasty, spinal
anesthesia with bupivacaine is preferred over general anesthesia due to less
nausea, vomitting and less blood loss.

Mean surgery time for Total Hip arthroplasty with the anterior supine
intermuscular approach at RdGG is 69,7 minutes and other studies found a mean
surgery time of 68,5 minutes. Provided that administration of 8mg 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
THA.

Current dosages of thecal bupivacaine are adequate for allowing surgery with
total analgesia but do not provide optimization for direct post-operation
mobilization and often leave patients unable to mobilize for several hours
post-surgery prolonging a patients hospital stay.

The objective of this study is to find the median local anesthetic dose
(MLAD/ED50) of bupivacaine that allow direct post-operative mobilization and
will accommodate sufficient anesthesia during surgery.

Results from this study might lead to the justification of lower doses of
bupivacaine used for anesthesia in THA. Henceforth, the fast track program can
be optimized and rehabilitation can start immediate postoperatively.

This is an intervention study designed to find the MLAD/ED50 of bupivacaine at
a certain threshold.

In this study we use 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 anesthesia. For the cut-off point or threshhold point needed with
this model we use the 95th percentile of the mean surgery time for THA-ASI in
order to expose less patients to inadequate anesthesia during surgery.

At the cut-off point patients must be able to fully recover from the sensory-
and motor blockade induced by bupivacaine. Therefore, at the cut-off point (119
minutes after spinal anesthesia is administered) patients much reach;

- Full recovery of the motor block, measured with the modified bromide scale
(Bromage 0)
- Full recovery of the sensory block (positive pinprick test at the S1 and L5
Dermatome)

Possible test outcomes;
1. Patient reach the cut-off parameters before 119 minutes -> inadequate
bupivacaine
2. patient reach the cut-off parameters after 119 minutes -> adequate
bupivacaine
If a patient is documented with inadequate anesthesia (possible outcome 1) the
succeeding patient receives a dose of bupivacaine Xy+0,5mg
If a patient is documented with adequate anesthesia (possible outcome 2) the
succeeding patient receives a dose of bupivacaine Xy-0,5mg

During surgery sensory block height is assesed using a pinprick test. The
patient is asked to score pain on the NRS for pain. Testing will be perfomed in
10 minute intervals starting from the injection of bupivacaine.
After surgery sensory block recovery is assesed using a pinprick test at
dermatome L5 and S1. Suplementery recovery from motor block is assed by
assesing the modiefied 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 THA-ASI protocol. The
intervention targets the spinal anesthesia protocol. The current protocol
imposes a dose of 8mg of Bupivacaine administered intrathecally 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 up until it*s recovery from the nerve blockade.

Patients will receive their scheduled THA 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 with 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.