Ventilator Beats Midazolam: Safer, Faster Apnoea Induction in Canine CT

A Comparison of Two Different Methods for Inducing Apnoea During Thoracic Computed Tomography of Dogs

Thomas Hordle, Maria Navarro-Carrillo, Imogen Schofield, Mark Plested, Maria Chie Niimura del Barrio

Background:
To minimize motion artefacts during thoracic CT in dogs, controlled apnoea is often induced. Two methods are commonly used: administration of midazolam for pharmacologic respiratory depression and interruption of mechanical ventilation. This study aimed to compare the efficacy, onset, duration, and side effects of these methods. The hypothesis was that ventilator interruption would more reliably induce apnoea with fewer side effects and better image quality.

Methods:
Sixty-one dogs undergoing general anaesthesia for thoracic CT were randomly assigned to receive either intravenous midazolam (0.2 mg/kg; group M, n=30) or to be mechanically ventilated until apnoea was induced by stopping the ventilator (group V, n=31). Onset and duration of apnoea were recorded, along with cardiorespiratory parameters and CT-derived lung aeration metrics. Image quality and atelectasis were assessed using semi-automated segmentation and reviewed by blinded radiologists.

Results:
Apnoea was successfully induced in 93.6% of dogs in group V and 76.7% in group M, though the difference was not statistically significant (p=0.08). Onset of apnoea was immediate in group V and averaged 30 seconds in group M (p<0.0001). Apnoea duration was significantly longer in group V (median 120s vs. 69s, p<0.001). Group M exhibited higher heart rates, greater hypercapnia, and more frequent hypotension. CT scans from group V showed significantly greater lung aeration and less atelectasis (p=0.01), particularly in the right middle lung lobe. No dogs desaturated below SpO₂ 90%.

Limitations:
Although randomised, the study's modest sample size may have been insufficient to detect significant differences in primary outcome measures. The primary observer was not blinded to the intervention group. Premedication protocols were not standardized, and some variability in anaesthetic depth and monitoring may have occurred. The study did not include long-term outcome or recovery data.

Conclusions:
Interruption of mechanical ventilation is a more predictable and controllable method of inducing apnoea in dogs undergoing thoracic CT than midazolam administration. It leads to better image quality and fewer cardiorespiratory complications, making it the preferred technique when resources permit. Midazolam remains a viable alternative when mechanical ventilation is not available but carries a higher risk of cardiovascular side effects and less predictable apnoea.

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