Associations Between Anesthetic Variables and Functional Outcome in Dogs With Thoracolumbar Intervertebral Disk Extrusion Undergoing Decompressive Hemilaminectomy

Abstract

Background: Outcome of acute experimental spinal cord injury is strongly associated with tissue perfusion and oxygenation. Cardiopulmonary depression could affect outcome in dogs undergoing general anesthesia for surgical treatment of thoracolumbar intervertebral disk extrusion (IVDE).

Hypothesis/objectives: To evaluate the effects of general anesthesia on functional outcome in dogs undergoing surgery to treat thoracolumbar IVDE.

Animals: Eighty-four client-owned dogs with acute thoracolumbar IVDE treated by decompressive hemilaminectomy.

Methods: Exploratory, retrospective observational study. Medical records were reviewed for clinical presentation and anesthetic monitoring variables, including duration of anesthesia and surgery, hypotension, bradycardia, temperature, and respiratory parameters. Multivariable regression tree analysis was performed to explore associations between anesthetic variables and functional outcome scores after 6 weeks, as well as return to ambulatory status.

Results: Episodes of bradycardia (69%) and hypotension (57%) were frequent. Across all outcome measures, regression tree analysis highlighted functional grade at presentation as the primary determining factor, and among pain perception negative dogs, there was a possible association between increased duration of surgery and poorer outcome. In dogs with intact pain perception, duration of bradycardia, mean body temperature, and mean end-tidal carbon dioxide were highlighted.

Conclusions and clinical importance: Exploratory statistical methods can facilitate hypothesis-generating studies to inform prospective investigations in veterinary medicine. Although the mechanism is uncertain, increased duration of surgery might be associated with poorer outcome in pain perception negative dogs with thoracolumbar IVDE.

Keywords: Anesthesia; Canine; Intervertebral disk extrusion; Prognosis; Spinal cord injury.

Figure 1

An estimated regression tree displaying associations between recorded variables and outcome, as measured by improvement in open field score (OFS: 0–12) 6 weeks after hemilaminectomy to treat intervertebral disk extrusion (IVDE). Each gray box represents a variable that subdivides the population—branches to the left correspond to a lower OFS and therefore poorer outcome while branches to the right correspond to subgroups with better outcome, with numbers in the terminal white boxes representing the estimated average OFS improvement achieved, and the number of dogs for that subgroup (n). The hierarchical nature of the tree suggests that the primary discriminating factor in predicting OFS improvement is functional grade at presentation (Grade), with Grade 5 dogs having poorer OFS improvement. Of the grade 5 dogs, those undergoing surgery of duration >3.8 hours had the poorest average OFS improvement of 3.4. In this way, each further branching describes the interactions between these variables and OFS improvement for different subgroups of dogs. This regression tree therefore suggests that there might be a relationship between OFS improvement and total duration of surgery, duration of general anesthesia (GA), and duration of bradycardia, and that these are therefore candidate variables for fitting a general linear model (Table 3).

Figure 2

An estimated regression tree displaying associations between recorded variables and outcome, as measured by regularity index (RI: 0–100) 6 weeks after hemilaminectomy to treat intervertebral disk extrusion (IVDE). The branching of the tree follows the same rules as outlined for Figure 1. This regression tree therefore suggests that the primary discriminatory factor in predicting RI is functional grade at presentation (Grade), and that there might be a relationship between RI outcome and mean temperature and mean end‐tidal carbon dioxide (ETCO₂) in grade 3 and grade 4 dogs. These candidate variables are therefore selected for fitting a general linear model to predict RI (Table 4).

Figure 3

An estimated regression tree displaying associations between recorded variables and outcome, as measured by ambulatory status 6 weeks after hemilaminectomy to treat intervertebral disk extrusion (IVDE). The branching of the tree follows the same rules as outlined for Figure 1. This tree suggests that the primary discriminatory factor in predicting ambulatory status improvement is functional grade at presentation (Grade), and that of grade 5 dogs those undergoing surgery duration >3.8 hours might have a lower probability of regaining ambulatory status. There might also be a relationship between duration of low systolic blood pressure in a subgroup of dogs. These candidate variables are therefore selected for fitting a ridge regression model to predict ambulatory status (Table 5).