Effects of medetomidine and ketamine on the regional cerebral blood flow in cats: A SPECT study
Introduction
Measurements of regional cerebral blood flow (rCBF), an indirect tool for determination of neuronal functionality, can be performed by using tools based on functional magnetic resonance imaging (fMRI), positron emission tomography (PET) or single photon emission computed tomography (SPECT) (Moretti et al., 1995, Hagen et al., 1999, Feng et al., 2004).
SPECT using 99mtechnetium ethylcysteinate dimer (99mTc-ECD) is a functional neuroimaging technique that has become a routine procedure in human nuclear medicine for evaluation of rCBF (Catafau, 2001). In veterinary medicine, the tracer has been used in behaviour disorders and in epileptic dogs (Peremans et al., 2003, Martle et al., 2009, Vermeire et al., 2009). ECD is a neutral lipophilic complex which is rapidly taken up by the human brain (Vallabhajosula et al., 1989). Following intravenous (IV) injection, ECD crosses the blood brain barrier and becomes rapidly trapped in the brain in proportion to the rCBF (Ishizu et al., 1995, Ichise et al., 1997). The tracer exhibits prolonged retention in the brain due to its intracellular conversion into a hydrophilic compound. The lack of redistribution makes it possible to inject the tracer several hours before acquisition, creating an image that represents the cerebral blood flow distribution pattern at the time of injection (Leveille et al., 1992, Ichise et al., 1997, Catafau, 2001, Peremans et al., 2001).
Sedation or anaesthesia is required in veterinary nuclear medicine during image acquisition and can cause changes in the cerebral blood flow (Zornow et al., 1990, McPherson et al., 1997, Prielipp et al., 2002). This is comparable with the situation in children or in patients with severe cognitive dysfunction, dementia or behavioural disorders. In those patients it is recommended that the tracer is administered prior to sedation to avoid sedation-induced metabolism or blood flow changes (Catafau, 2001). It is generally accepted that the influence of sedatives or anaesthetics is negligible and that tracer displacement is very unlikely due to the rapid intra-neuronal trapping mechanism of the tracer (Walovitch et al., 1994).
In cats, intramuscular (IM) sedation or general anaesthesia is often required before IV catheter placement and tracer administration, and this may affect the distribution pattern of the tracer. Medetomidine, a highly selective α2-agonist, induces a reliable sedative effect following IM administration. Ketamine, a dissociative anaesthetic and N-methyl-d-aspartate (NMDA) antagonist, is frequently used in cats in combination with medetomidine (as an IM injection) for general anaesthesia (Lin, 2007).
The purpose of the present study was to investigate the influence of IM sedation with medetomidine alone or in combination with ketamine on the movement of 99mTc-ECD to the brain and on the semi-quantification of rCBF in domestic cats. Our hypothesis was that sedation or anaesthesia before tracer injection may change the uptake of this tracer within the whole brain and, more importantly, that this would also influence the semi-quantification of rCBF.
Section snippets
Subjects
Six healthy, drug naïve, domestic shorthair neutered female cats, 6 years of age, weighing 5.34 ± 0.51 kg (mean ± SD), were included in this study. The animals were used to being handled for IV catheter placement and all procedures were performed according to good animal practice in order not to provoke excitation or aggression. In every study condition, a 22G IV catheter was placed in the cephalic vein. After catheter placement, the cats were allowed to relax for 10 min in a quiet room with dimmed
Results
Comparing the total counts in the different conditions revealed that tracer uptake in conditions M and MK were significantly higher than in condition A in all brain regions (Table 1). Although significance could not be reached, tracer uptake was lower in all brain regions in the MK-condition when compared to condition M (Table 1).
Statistical analysis of the perfusion indices (Table 2) showed increased perfusion indices in the cingulate gyrus, the olfactory bulb and the right parietal cortex,
Discussion
The results of this study confirmed our hypothesis that significant differences in the uptake of 99mTc-ECD occur when tracer injection was preceded by the administration of medetomidine alone (M) or in combination with ketamine (MK). Table 1 indicates that medetomidine alone caused the highest tracer uptake in all brain regions, while the combination of medetomidine and ketamine caused a significant higher tracer uptake in all brain regions, compared to non-sedated cats.
Cardiovascular changes,
Conclusions
Sedation or anaesthesia with medetomidine or the combination of medetomidine and ketamine is sometimes required before IV tracer injection in cats. In these cases, caution is needed when rCBF is measured. Medetomidine alone or in combination with ketamine will induce an increased 99mTc-ECD supply to the brain and the semi-quantification, mainly of the sub-cortical brain regions and the cingulate gyrus, can be influenced as well. It was clear from this work that sedation or anaesthesia before
Conflict of interest statement
None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
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