Evaluation of the Recovery of the Sympathetic Block After Spinal Anesthesia by Thermographic Image

Spinal anesthesia is a locoregional perimedullary anesthesia which consists of temporary interruption of nerve transmission within the subarachnoid space.

This anesthesia is produced, via a lumbar puncture, by injecting a solution of local anesthetics into cerebrospinal fluid.

Spinal anesthesia generates installation of three consecutive blocks: (i) a sympathetic block (vasodilation, increased skin heat in the sleeping area, arterial hypotension); (ii) a sensory block (i.e. a sensory paralysis: no pain or sensations at the incision), the level of which is assessed by the "touch" test and (iii) a motor block (a motor paralysis : immobility of the limb at the incision) measured by the Bromage score.

Spinal anesthesia is a common method of anesthesia in orthopedic surgery of the lower limbs, as demonstrated by field practice at the Surgical Center Emile Gallé (CCEG). This mode of anesthesia is an alternative to general anesthesia, especially because of a higher benefit / risk ratio. Patients at high-risk will be referred preferentially to a spinal anesthesia to prevent complications related to general anesthesia.

If the regulatory conditions are met (formalized expert recommendations 2009), some orthopedic procedures can be performed in the context of day hospitalization. The constraints of day hospitalization involve the use of anesthetic agents or doses compatible with a short duration of action equivalent to the duration of the act of surgery. They must allow a duration of action of maximum 4 hours, a predictable extension of the block at the T10 level (at the sensory level of the umbilicus), a short period of autonomy in ambulation, a rapid recovery from the dysfunction bladder and absence of hemodynamic effects. The return to normal motor function is the sine qua non of fitness for the street.

The discharge from the post-intervention monitoring room of a patient operated on under spinal anesthesia is validated once the recovery of the 3 blocks has been objectified : the motor block (bromage score), the sensory block (touch test) and the sympathetic block ( hemodynamic parameters).

One of the side effects associated with spinal anesthesia is the installation of a sympathetic block causing more or less significant hemodynamic variations. In current practice, its recovery is objectified when the hemodynamic parameters (FC, PNI) postoperatively (measured every 10 minutes) differ by less than 15% from the basic parameters pre-locoregional anesthesia (LRA). This evolution (with a threshold value of 15%) of the hemodynamic parameters constitutes the reference test (Gold Standard) to objectify a recovery of the sympathetic block. In reality, the criteria on which the recovery of the sympathetic block is based are imperfect because side effects (low blood pressure, nausea, vomiting) can be observed when returning to the outpatient surgery sector's, indicating the presence of a residual block.

Data from literature suggest that thermography would be an interesting alternative tool for evaluating the effectiveness of a nerve block after locoregional anesthesia (LRA). Installation of a nerve block is objectified by the change in temperature of the blocked body areas. In fact, the sympathetic block causes vasodilation in these areas and therefore an increase in local temperatures visible on thermography.

Concretely, performed before locoregional anesthesia (LRA), thermography gives a basal body mapping of local temperatures. These are the reference temperatures.

After performing locoregional anesthesia, the sympathetic block sets in and causes vasodilation which is itself responsible for an increase in local temperatures (appearance of warm colors on the body map). Then the return to the initial temperatures (i.e. those observed at the preoperative time) is done gradually as the sympathetic block recovers.

Use of the thermographic method to detect the recovery of sympathetic block after spinal anesthesia has never been evaluated. However, by comparison to these preoperative images, thermography could allow the detection of areas of local temperatures that are still high postoperatively (persistence of hot colors on the body map), while a return to basic local temperatures is expected. (neutral or cool colors).

This could alert to the persistence of a residual sympathetic block, with areas of the body where vasodilation persists despite the return to normal of hemodynamic parameters, and prevent complications in outpatient surgery sector's.

Main research hypothesis of the investigators is that thermographic image evaluation has very good sensitivity for the recovery of sympathetic block after spinal anesthesia compared to the reference method based on hemodynamic parameters. In addition, second research hypothesis of the investigators is that the recovery of the residual sympathetic block evaluated by thermographic image can reduce the occurrence of subsequent adverse events.