Chemical restraint is often necessary to facilitate diagnostic imaging procedures, like sonography. The use of sedative drugs decreases the risk of injury to both the cat and the handler; decreases stress in the cat, thereby improving the patient experience; and may help the sonographer capture better-quality images.
Ultrasound is a popular modality used for imaging the feline spleen. It is more accessible and affordable compared to CT and MRI. Ultrasound allows for the evaluation of splenic size, contour, parenchymal echogenicity, echotexture, and blood flow.1,2 It aids in the diagnosis of diseases that include both infectious and neoplastic processes, including but not limited to lymphoid hyperplasia, extramedullary hematopoiesis, histoplasmosis, lymphoma, other round cell neoplasms, and more.3 The decision to carry out fine-needle aspirates or biopsies is often based on abnormal splenic appearance as there are no specific bloodwork parameters that are diagnostic for splenic disease. Changes in gross splenic appearance are more likely to be associated with neoplasia in cats than dogs; this may lead practitioners to take on a more proactive approach in splenic disease diagnosis.4,5 Spangler and Culbertson4 showed that 33% of feline specimens submitted for pathologic evaluation were diagnosed with neoplasia. The same authors showed in a separate study5 that in canine spleens derived from surgical or necropsy specimens, 78.5% were non-neoplastic. Therefore, drug-induced abnormalities may cause patients to be unnecessarily subjected to further diagnostic procedures. Changes that have been linked to round cell neoplasia include splenomegaly, irregular margins, diffuse hypoechogenicity, mottling, and the presence of hypoechoic nodules.3 Hanson et al6 showed that 83% of subjects tested that had been diagnosed with lymphoma had splenomegaly, and 53% displayed a large, smoothly marginated, hypoechoic and nodular spleen. Lamb et al7 described splenic changes in dogs and cats associated with lymphoma. This included diffuse hypoechogenicity with multiple hypoechoic foci, cavitary masses, and irregular margins. Abdominal sonography also requires the patient to be positioned in dorsal recumbency and the application of ultrasound gel or alcohol to ensure adequate probe contact. These processes are potentially stressful for the patient.
Despite the advantages that sedative drugs confer, many drugs have also been shown to cause changes in the appearance of the spleen, thus interfering with accurate evaluation and diagnosis. Splenic changes have been demonstrated on radiographs, ultrasound, and CT in both dogs and cats.8–11 Baldo et al10 demonstrated in dogs that IV acepromazine, thiopental, and propofol resulted in splenomegaly, whereas dexmedetomidine did not change splenic volume, and hydromorphone decreased it. Some of the most common sedative drugs used in cats include dexmedetomidine, acepromazine, ketamine, and alfaxalone, often combined with an opioid.11,12 Auger et al13 showed that acepromazine increased splenic measurements on both radiographs and ultrasound and that this was accompanied by a decrease in Hct. In the same study, butorphanol did not increase splenic measurements. In another study by Short et al,11 dexmedetomidine with morphine did not affect splenic size on ultrasound, but both acepromazine with morphine and alfaxalone with morphine did. The results of a separate study by Finck et al14 agreed with these findings, and it was also shown that dexmedetomidine increased splenic size in a significant manner on radiographs but not on ultrasound.
Alfaxalone (3α-hydroxy-5α-pregnane-11,20-dione) is a neuroactive steroid that causes a sedative effect by binding to the GABA subtype A receptors in the CNS, ultimately inhibiting the pathways for awareness.15 Alfaxalone is a popular choice as it can be administered IM, causes reliable short-acting sedation,16 and is considered to have a less severe effect on the cardiovascular system compared to dexmedetomidine, acepromazine, and ketamine. Despite its increasing popularity, there is limited literature documenting the effect that alfaxalone has on splenic appearance. A study17 conducted on dogs showed that alfaxalone caused an increase in splenic volume on CT, with an accompanying decrease in Hct. Finck et al14 showed that alfaxalone with butorphanol increased splenic thickness in cats, whereas dexmedetomidine with butorphanol did not affect splenic size when assessed by ultrasound. Alfaxalone administered alone was not evaluated in this study. Apart from this study, the authors were not able to find any other published studies investigating the effect of alfaxalone on the feline spleen when imaged by ultrasound at the time of writing.
The aim of this study was to evaluate the effects of alfaxalone, alone and in combination with butorphanol, on the appearance of the feline spleen on ultrasound. Based on splenic changes that are commonly associated with disease, the appearance parameters assessed were margins, size as determined by the height of the splenic body, echogenicity, echotexture, and blood flow. In addition, we examined the effect of the combinations on sedation, heart rate (HR), and respiratory rate (RR). We hypothesized that alfaxalone administration would be associated with increased sedation scores, decreased HRs and RRs, splenic rounding, increased splenic size shown by an increase in the height of the body, a nonhomogenous echotexture, and increased blood flow. A secondary hypothesis was that alfaxalone with butorphanol would cause the most profound sedation, demonstrated by significantly higher feline multiparametric sedation scores (FMSS) scores.
Methods
This study was a randomized, prospective crossover experiment. This protocol received ethical approval from the Mississippi State University IACUC (IACUC-21-379). Informed owner consent was conducted through the use of a signed form.
Animals
Sample size analyses were conducted using G*Power (version 3.1.9.7, Heinrich Heine Universitat Dusseldorf Matematisch-Naturwissenschaftlichen Fakultat website)18 assuming a paired t test, 2-tailed test, α level of 0.05, power of 0.95, correlation between groups of 0.5, and mean (SD) feline spleen body thickness of 8.71 mm (0.95) and 10.24 mm (0.95) of control and alfaxalone-butorphanol–treated groups, respectively, using parameters derived from Finck et al.14 A minimum number of 8 cats were determined to be necessary for this study.
All cats recruited were domestic shorthair cats. There were 4 neutered males and 4 neutered females, aged between 1 year 3 months and 4 years 11 months (mean, 3 years; SD, 1 year 4 months). They weighed between 3.3 and 5.4 kg (mean, 4.6 kg; SD, 0.7 kg). The cats were owned by private individuals who either worked for or studied at Mississippi State University College of Veterinary Medicine. Exclusion criteria included age (< 1 year old or > 5 years old), being outdoors cats and therefore at an increased risk of parasitic disease, having preexisting diseases (including hypertrophic cardiomyopathy, food allergies or inflammatory bowel disease, and FIV), or having required anxiolytic drugs at prior veterinary visits. Physical examinations and FMSS19 were carried out on 20 cats. Five cats were excluded at this point—1 due to a previously unknown heart murmur and 4 because of fractious temperament. Bloodwork was then carried out on 15 cats. This included a CBC, biochemistry, a Fever of Unknown Origin Comprehensive Panel (3540 Fever of Unknown Origin Real PCR Comprehensive: Feline; Idexx), and urinalysis. One cat was excluded due to testing positive for Mycoplasma haemofelis. Fourteen cats underwent a full abdominal ultrasound. Four cats were excluded due to a heterogeneously hyperechoic spleen prior to drug administration. Two cats were excluded due to their intolerance toward restraint in dorsal recumbency. Ultimately, 8 cats were included in this experiment. All participants were between 1 and 5 years old, weighed greater than 3 kg, were indoor only, had normal bloodwork and urinalysis results, had a normal presedation abdominal ultrasound, and were amenable to handling. The decision to include or exclude a cat was determined by 4 of the author: a second-year anesthesia resident (GM), an American College of Veterinary Anesthesia and Analgesia–boarded veterinary anesthesiologist (KMF), and 2 American College of Veterinary Radiology–boarded veterinary radiologists (AML and MS).
Blood sampling, urine sampling, and analysis
Prior to the commencement of the study, 7 mL of blood was collected from each cat from the right or left medial saphenous vein using a 21-gauge (ga) butterfly collection set. Five milliliters of urine was collected via ultrasound-guided cystocentesis using a 21-ga, 1.5-inch hypodermic needle and placed into a plain glass vacutainer. Complete blood count, biochemistry, and urinalysis were performed in the Mississippi State College of Veterinary Medicine Diagnostic Laboratory. The Feline Fever of Unknown Origin Comprehensive Panel was carried out by Idexx Reference Laboratories. This was a PCR test screening for Anaplasma subsp, Bartonella subsp, Cryptococcus var, Cytauxzoon felis, Ehrlichia subsp, feline calicivirus, feline coronavirus, feline hemotropic mycoplasma, feline panleukopenia virus, FeLV, FIV, Salmonella subsp, and Toxoplasma gondii.
Drugs
The 3 drug protocols used for sedation were:
Protocol A: 2.0 mg/kg alfaxalone (Alfaxan multidose), IM
Protocol B: 0.2 mg/kg butorphanol (Torbugesic or Torphadine), IM
Protocol AB: 2.0 mg/kg alfaxalone (Alfaxan multidose) and 0.2 mg/kg butorphanol (Torbugesic or Torphadine), IM.
For protocol AB, the drugs were mixed in the same syringe before administration. For protocols A and B, 0.9% NaCl was added in order to achieve a consistent volume across all 3 protocols. No precipitation was visually observed in the syringes after mixing. There was no delay in mixing the drugs after they had individually been drawn up and no delay in administering the drugs after they had been combined.
All protocols were drawn up into a 3-mL syringe (Terumo Corp) and administered IM into the lumbar spinal epaxial muscles or the quadriceps muscles using a 21-ga, 1-inch-long needle.
Each cat received each protocol once, with a minimum washout period of 7 days between treatments. Cats were not administered any other drugs or medications that could have affected their splenic appearance during the time period that they were enrolled in this study. Three drug sheets were created for all cats with each protocol on separate sheets. On the morning of a scan, the cat would be weighed, then the drug sheets would be placed face down, and 1 would be randomly selected by technician Zachary Spangler. The technician would then calculate the volume of drug to be given based on the most recent weight obtained that morning and the doses outlined above. The drugs were drawn up and mixed by the same technician. The syringes were labeled with the name of the cat and the treatment number but not the drug name(s). The technician was not involved in sedation scoring or in the process of carrying out and interpreting the ultrasounds. All other personnel involved in the study (GM, KMF, AML, MS, and sonographer Jamie Simross) were blinded to the order of the treatments.
Sedation score
The level of sedation was scored on a scale from 0 to 12 using a validated feline sedation score, the FMSS.19 An FMSS was carried out and recorded at the initial physical examination to establish a baseline for each cat. For each subsequent presentation, the FMSS was carried out at least 1 hour prior to the administration of drugs, then at 0, 15, 30, 45, and 60 minutes after drugs were given IM. All sedation scores were carried out by the principal investigator (GM) to ensure consistency. The criteria of the FMSS can be found in Table 1.
The feline multiparametric sedation score (FMSS) allows an observer to assess the level of sedation of a cat based on posture, behavior, response to sound, and response to restraint and/or IM injection or IV catheter placement.
| FMSS |
|---|
Posture (observe from a distance first)
Behavior
Response to sound
Response to restraint and/or IM injection/IV catheter (if responses to restraint and needle correlate with different numbers, circle the lowest value)
Final score (0–12) Select one: none, mild, moderate, profound Additional behaviors observed (notes): |
(Reprinted with permission from Rutherford AA, Sanchez A, Monteith G, Tisotti T, Aguilera R, Valverde A. Description and validation of a new descriptive and multiparametric numeric rating scale to assess sedation in cats. Can Vet J. 2022;63[6]:603–608.)
Heart rate and RR
Heart rate was obtained via chest auscultation. Respiratory rate was obtained by observing chest excursions. These were obtained and recorded on baseline physical examination and every 5 minutes after drug administration for 60 minutes.
Ultrasound image acquisition
Each cat was scanned on 4 different occasions. On first presentation, a baseline scan was conducted without any drugs to evaluate all abdominal structures and ensure normalcy. On each subsequent presentation, only the spleen was scanned prior to sedation (time −1), then immediately after IM drug administration (time 0) and every 15 minutes for 60 minutes (times 15, 30, 45, and 60). All scans were carried out by the same certified sonographer (JS) using the same ultrasound machine.
Ultrasound scan interpretation
All images and videos captured by the sonographer were then reviewed and analyzed by 2 board-certified veterinary radiologists that were blinded to the treatments. The criteria for assessing splenic appearance can be found in Table 2. This scoring system has not been validated. It was developed by the authors of this paper to quantify splenic changes using simple numeric values. It is based on common splenic changes seen on ultrasound caused by drugs or disease.6
The criteria used for the assessment of the splenic images.
| Evaluation of splenic appearance | ||||
|---|---|---|---|---|
| Parameter | Description | Change | Score | |
| 1 | Margins | Thin, smooth, sharp, well-defined margins | Less sharp and well defined | 0 |
| No change from baseline | 1 | |||
| Sharper and well defined | 2 | |||
| 2 | Splenic size | Hb: perpendicular measurement across spleen at central third in the transverse plane | Measurement with calipers | mm |
| 3 | Splenic shape | Slender head and body. Wider tail | Flattening | 0 |
| No change from baseline | 1 | |||
| Rounding | 2 | |||
| 4 | Echogenicity | Dense, fine, granular appearance. More hyperechoic than liver. More hypoechoic than kidney | More hypoechoic | 0 |
| No change from baseline | 1 | |||
| More hyperechoic | 2 | |||
| 5 | Echotexture | Uniformity of the echopattern | No change from baseline | 1 |
| Nonhomogenous or mottled | 2 | |||
| 6 | Blood flow | Splenic artery and vein size. Flow | Decreased vessel size/blood flow | 0 |
| No change from baseline | 1 | |||
| Increased vessel size/blood flow | 2 | |||
Hb = Height at the body.
Splenic size was determined by measuring the Hb using digital calipers and is expressed in millimeters. All other parameters were scored from 0 to 2. The score of 0 indicated a decrease from baseline, 1 indicated no change from baseline, and 2 indicated an increase from baseline.
Images and videos from 16 time points for each of the 8 cats were captured and analyzed. Each radiologist evaluated 128 sets of images and videos and used the scoring system detailed above to document any changes to the spleen compared to baseline in the hour after treatment administration. The radiologists first reviewed the images separately, then differences were reviewed to consensus.
The average measurement of splenic body height was calculated from the 2 sets of results. Any discrepancies in the margins, shape, echogenicity, echotexture, and blood flow were discussed, and a consensus was reached regarding interpretation.
Statistical analysis
Statistical analysis was performed by a board-certified veterinary epidemiologist with extensive statistical analysis experience (RW). As this was a crossover study, each cat received each drug protocol once and acted as their own control. Heart rate, RR, FMSS, and splenic appearance parameters were compared to the individual’s baseline obtained before the administration of any drugs. The association of drug protocol on splenic appearance parameters, including margins, shape, echotexture, echogenicity, and blood flow, were evaluated with mixed-model logistic regression using PROC GLIMMIX with SAS for Windows, version 9.4 (SAS Institute Inc). Cat identity was included as a random effect. When mixed-model logistic regression was attempted for echogenicity, the model did not converge, and a logistic regression model using PROC LOGISTIC was fitted, with the Firth penalized likelihood approach option utilized. The association of drug protocol with splenic body height, FMSS, HR, and RR was evaluated as linear mixed models using PROC MIXED, with drug protocol, time, and their interaction as fixed effects. Cat identity was included as a random effect and cat identity within drug protocol as the subject in a repeated statement with an autoregressive covariance structure to account for the repeated measures over time. An LSMESTIMATE statement with the simulate option for the adjustment of P values was used to compare the effect of time within each drug protocol and the effect of the drug protocol within each sampling time point. If the interaction term was not significant but either drug protocol, time, or both were found to have a significant effect on the outcome, comparison of least squares means using an LSMEANS statement with Dunnett adjustment of P values was used to make statistical inferences. The assumptions of normality and homoscedasticity were ensured by visually assessing conditional residual plots for the linear mixed models. The Pearson correlation for sedation score, HR, and RR was assessed using PROC CORR. An α level of 5% was used to determine statistical significance for all statistical tests.
Results
Association between drug protocol and splenic margins, shape, echogenicity, echotexture, and blood flow
None of the protocols was associated with a statistically significant change in splenic margins, shape, echogenicity, echotexture, or blood flow. Odds ratio estimates showed that none of the drug protocols was more likely than another to be associated with a change in shape. There was a trend toward splenic shape rounding for all 3 protocols; however this was not statistically significant (P = .53) among protocols, shown using a type III test of fixed effects. As shown in Table 3, drug protocol A was associated with a rounding in the shape of the spleen in 6 of the 8 cats, with 2 cats showing no change from baseline. Protocol AB was associated with a rounding of the spleen in 7 of the 8 cats, and protocol B was associated with splenic rounding in 5 of the 8 cats.
The number of cats assigned each score for categorical variables margins, shape, echogenicity, echotexture, and blood flow.
| Number of cats assigned each score for all parameters | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Margins | Shape | Echogenicity | Echotexture | Blood flow | ||||||||||
| Protocol | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 1 | 2 | 0 | 1 | 2 |
| A | 0 | 8 | 0 | 0 | 2 | 6 | 0 | 7 | 1 | 8 | 0 | 0 | 7 | 1 |
| AB | 0 | 8 | 0 | 0 | 1 | 7 | 0 | 7 | 1 | 8 | 0 | 0 | 6 | 2 |
| B | 0 | 8 | 0 | 0 | 3 | 5 | 0 | 8 | 0 | 8 | 0 | 0 | 7 | 1 |
A = Alfaxalone. AB = Alfaxalone plus butorphanol. B = Butorphanol.
The maximum score for each cat was recorded (ie, if a cat scored 1 at one time point but 2 at another, the score of 2 was counted).
For both protocols A and AB, 1 cat showed an increase in echogenicity. There was no change from baseline in the other 7 cats, and no cats showed any change from baseline with protocol B. Two different individuals displayed increases in echogenicity for protocol A and protocol AB. This finding was not statistically significant (P = .77).
Echotexture remained the same as baseline for all cats with all 3 drug protocols.
Two individuals showed an increase in splenic blood flow with protocol AB. For protocols A and B, 1 individual showed an increase in blood flow in each group. This was not statistically significant (P = .69).
Association between drug protocol and time with splenic size
There was a statistically significant increase (P < .01) in the height of the body of the spleen over time, peaking at time 30 for protocol AB and time 45 for all protocols A and B. The mean increase in the height of the body was the greatest for protocol AB at 0.93 mm ± 0.16 compared to 0.89 mm ± 0.17 for protocol A and 0.87 mm ± 0.15 for protocol B. However, these differences were not statistically significant (P < .21). This shows that while there was a significant increase in the size of the spleen for all 3 protocols, there was not a significant difference detected between them. Figure 1 shows the similar changes in splenic size over time for all 3 protocols.
This chart compares the height of the splenic body over time for the 3 protocols. The average measurement of the 8 subjects was calculated at each time point: −1, 0, 15, 30, 45, and 60 minutes. This illustrates a similar change in splenic size for all 3 protocols. A = Alfaxalone. AB = Alfaxalone plus butorphanol. B = Butorphanol.
Citation: American Journal of Veterinary Research 86, 3; 10.2460/ajvr.24.09.0250
Association between drug protocol and time with FMSS, HR, and RR
Table 4 illustrates the changes in FMSS, HR, and RR over time. Protocol AB provided the greatest degree of sedation, with all subjects experiencing moderate or profound sedation in the hour after administration. There was a significant increase in FMSS score, indicating increased sedation, for both protocol A and AB compared to baseline at times 15, 30, and 45 (P < .01). At time 60, FMSS was significantly increased from baseline for protocol A (P < .01), but it was not for protocol AB (P < .11), suggesting a faster recovery from sedation. There was no significant change from baseline for protocol B at any time point (P < .31), indicating no sedation.
The mean heart rate (HR), respiration rate (RR), and FMSS at all time points for each drug protocol.
| HR, RR, and FMSS for each protocol over time | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Time (min) | ||||||||||||||||||
| −1 | 0 | 15 | 30 | 45 | 60 | |||||||||||||
| Protocol | HR | RR | FMSS | HR | RR | FMSS | HR | RR | FMSS | HR | RR | FMSS | HR | RR | FMSS | HR | RR | FMSS |
| A | 163 | 41 | 0 | 153 | 44 | 0 | 148 | 36 | 8 | 138 | 28 | 6 | 158 | 38 | 7 | 177 | 39 | 5 |
| AB | 170 | 37 | 0 | 158 | 37 | 0 | 124 | 26 | 8 | 134 | 28 | 8 | 161 | 33 | 6 | 182 | 36 | 4 |
| B | 177 | 36 | 0 | 164 | 41 | 0 | 124 | 35 | 3 | 139 | 41 | 3 | 141 | 43 | 3 | 147 | 41 | 1 |
Values were rounded to the nearest whole number.
The highest FMSS scores occurred at times 15 and 30 for both protocols A and AB. At time 15, the mean FMSS score for protocol A was 8.1 ± 2.5, and for protocol AB it was 7.5 ± 2.9. At time 30, the mean FMSS score for protocol A was 5.3 ± 4.7, and for protocol AB it was 8.3 ± 2.9. Although protocol AB achieved the highest FMSS scores, it was not significantly different from protocol A. In 3 of the 8 cats, protocol A was associated with the onset of paddling, ataxia, hyperkinesia, and aggressive behavior starting between 15 and 30 minutes after administration. In 1 cat, behavior included scratching, biting, and attempts to escape, such that data collection had to be stopped after 15 minutes. No other complications were detected following the administration of sedation.
Heart rate decreased significantly with all 3 drug protocols (P < .0001). There was no significant difference detected between protocols (P < .66). Heart rate decreased progressively from time 0 to time 20, where the greatest decrease occurred. The mean difference in HR at time 20 was less than the baseline (least squares mean baseline HR, 170; least squares mean HR at time 20, 132). From time 25 to time 60, HR progressively increased and was not significantly different from baseline at times 45, 55, and 60.
Although the average RR for each protocol decreased over time, this was not statistically significant (P > .10). Protocol B appeared to be associated with the smallest decrease in RR, whereas protocol AB appeared to have the greatest decrease in HR.
Association between FMSS and HR and FMSS and RR
There was a significant linear negative association between FMSS and HR (P < .001) and FMSS and RR (P < .001). As FMSS increased, indicating a greater level of sedation, HR and RR both decreased.
Discussion
The results showed that alfaxalone did cause an increase in splenic size, partially supporting our hypothesis. Contradictory to our hypothesis, we did not detect a significant change in the margins, shape, echogenicity, echotexture, or blood flow in association with any protocol. An increase in splenic size occurred over time, with no significant difference between protocols. The finding of increased splenic size with the administration of butorphanol alone is contradictory to previous studies,13,14 whose results showed no change in splenic size after the administration of butorphanol alone or with dexmedetomidine. In fact, it has been theorized that a decrease in splenic size occurs secondary to opioid administration due to their sympatholytic effects, leading to a reduction in splenic blood flow.10,14 It is possible that in this present study, the stress level in the participants was high enough to counteract this mechanism and lead to the opposite effect.
The findings of increased splenic size associated with alfaxalone are in alignment with existing literature.14 There are several proposed mechanisms for the increase in splenic size associated with alfaxalone administration. This includes relaxation of the splenic capsule leading to an increase in size, blood flow redistribution secondary to a drug-induced decrease in arterial blood pressure,20 red blood cell sequestration in the spleen,17 and histamine release leading to peripheral vasodilation and portal hypertension. It seems unlikely that the splenomegaly seen in the present study is due to blood flow redistribution or portal hypertension as we did not detect concurrent changes in splenic blood flow or vessel size. The increase in splenic size was mild and transient, with values returning to normal or near normal by time 60. Sayre and Spaulding2 defined splenomegaly as a splenic height of greater than 9.1 mm. The same 4 individuals repeatedly displayed splenomegaly for every protocol. In addition, their splenic body height was already at the high end of the normal range prior to receiving any drugs. Therefore, a mild increase in splenic size can be expected, but marked splenomegaly or progressively increasing splenic size beyond 60 minutes is unlikely to be caused by the sedative drugs administered, and investigation into alternative mechanisms can be considered.
According to the FMSS results, protocol AB caused the greatest and most reliable sedation effect. At some time points, protocol A and AB caused a similar degree of sedation. However, the administration of alfaxalone alone resulted in hyperkinesia, trembling, and ataxia in multiple cats and aggressive behavior in 1 cat. Therefore, the authors cannot recommend the administration of alfaxalone as a sole agent for sedation. Protocol B resulted in no-to-mild sedation in the subjects in this study. The authors do not recommend butorphanol alone when the desired effect is moderate-to-profound sedation. Peak sedation effects occurred for protocols A and AB at time 15 and time 30. The greatest change in splenic size occurred at time 45. This suggests that the best time to image the spleen following administration of IM sedation could be 15 to 30 minutes after drug administration when sedative effects would be at their peak but change in splenic size would not yet have reached its maximum effect.
A decrease in HR and RR was observed for all protocols and was an expected finding.21,22 The decreases in HR and RR were not significantly different between the protocols. Despite a comparable decrease in HR and RR, protocol AB caused a significantly more profound sedation effect based on the FMSS compared with protocol B. AB also did not cause the undesirable side effects associated with protocol A. The statistically significant decrease in HR and RR over time irrespective of protocol emphasizes the importance of monitoring vital parameters, especially for patients that are suffering from cardiovascular or respiratory disease. While not quantified in this study, practitioners may consider monitoring blood pressure in conjunction with HRs and RRs when using alfaxalone combined with butorphanol for sedation
The limitations of this study include a small sample size with the inclusion of only young, healthy cats. Although a priori sample size calculations based on available information indicated that 8 cats would provide sufficient statistical power, the study may have been underpowered to make statistical inferences for the observed changes. Perhaps if more cats had been scanned, more changes or a greater degree of change could have been detected. Splenic shape showed a trend toward rounding, and RR showed a trend toward a decrease over time for all 3 protocols. These trends may become statistically significant if a larger population is studied. A second limitation was the lack of a control group. Although we scanned each of the cats’ spleen prior to the administration of drugs, using this as our baseline and control, we did not inject cats with saline to assess the effects that the action of injecting might have had on splenic appearance. The stress of an IM injection could have caused splenic contracture to varying degrees in different individuals. A third limitation was that the splenic appearance scoring system used has not been validated. It is therefore unclear if it is sensitive enough to detect more subtle changes that might have occurred. Additionally, the evaluation of echogenicity and echotexture is somewhat subjective, even when using a defined scoring system. A fourth limitation is the lack of comparison between different routes of administration of the drugs. The authors chose to administer the drugs IM as they felt that this best reflects the reality of sedation in a clinical setting. Individual variation in body fat and muscle composition could have resulted in differences in the rate of absorption. IV and SC administration of drugs were not investigated in this study, and it is not known whether a faster or slower rate of absorption would affect splenic appearance. Although not observed, the combination of alfaxalone and butorphanol in a single syringe prior to administration could have resulted in precipitation or an interaction that was not accounted for. A fifth limitation was that the spleen was imaged at set time intervals of 0, 15, 30, 45, and 60 minutes after the administration of the drugs. Therefore, the appearance of the spleen in between these time intervals is not known. A sixth limitation was that the mechanism for splenic enlargement was not investigated. For example, Hct measurements were not repeated after the administration of the drugs. Therefore, a change in splenic size was not able to be correlated with changes in Hct. Lastly, the development of aggressive behavior in 1 cat, which included hissing, scratching, and biting, following the administration of protocol A precluded it from receiving ultrasounds and sedation scores at times 30, 45, and 60.
This study found that a statistically significant increase in splenic size occurred with alfaxalone alone, alfaxalone with butorphanol, and butorphanol alone. There was no significant difference between the groups. The increase in splenic size was mild and transient in all groups. Marked or persistent splenomegaly or changes in margins, shape, echogenicity, or echotexture were not detected and are unlikely to be confounding factors leading to misdiagnosis.
Alfaxalone with butorphanol resulted in the best sedation without any additional decreases in HR or RR when compared to alfaxalone alone and butorphanol alone. The IM administration of alfaxalone with butorphanol may be a better option when moderate-to-profound sedation is required to facilitate an accurate assessment of the spleen when using ultrasound imaging in cats. The authors advise against using alfaxalone alone for IM sedation because of unpredictable aggressive behavior as well as the hyperkinesia, trembling, and ataxia observed in the participants of the study.
Acknowledgments
The authors would like to thank Jamie Simross, who performed all ultrasounds; Zachary Spangler for randomizing the protocol and preparing the drugs; and Andrea Krauss for assisting in cat handling and restraint.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
Funding was supported in part by the Mississippi State University College of Veterinary Medicine House Officer Clinical Research Grant Program.
ORCID
A. M. Lee https://orcid.org/0000-0003-4452-1935
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