Cardiovascular effects of romifidine in the standing horse

Measurement of Tissue Oximetry in Standing Unsedated and Sedated Horses

Near infrared spectroscopy (NIRS) noninvasively measures peripheral tissue oxygen saturation (StO₂) and may be useful to detect early changes in StO₂ in anaesthetized and critically ill horses. This study aimed to identify the muscle belly that provided the highest percentage of successful StO₂ readings and the highest mean StO₂ value. Fifty adult horses were enrolled in a prospective controlled study. StO₂ was measured at six different muscles in each horse, for each intervention: hair overlying the muscle was clipped (post clipping: PC), clipped skin was cleaned with chlorhexidine (post-surgical prepping: PP) and medetomidine was administered intravenously (post medetomidine: PM). Mean StO₂ values were calculated for each muscle, and a linear effects model was used to assess the effect of muscle group and intervention on StO₂. The sartorius muscle gave the highest percentage of successful StO₂ values (p < 0.001) and the highest mean (90% CI) StO₂ values for the PC, PP and PM interventions. Surgical prepping of the skin increased the success for measurement of StO₂ values. For all muscles, administration of medetomidine was associated with lower StO₂ values (p < 0.001). In conclusion, of the muscles examined, the sartorius muscle may be the preferred muscle to measure StO₂ in horses, and clipping and cleaning of the probe placement site is recommended.

Analgesic and Cardiopulmonary Effects of Epidural Romifidine and Morphine Combination in Horses

The study aim is to compare the effects of epidural administration of two different doses of romifidine combined with morphine in horses. A prospective crossover blinded experimental design was used. Five adult healthy horses two males and three females with a mean body weight of 380 ± 45 Kg (335-425 kg), were studied. Treatments consisted of romifidine 30 μg/kg (R30) or 60 μg/kg (R60) combined with morphine 0.1 mg/kg with a washout interval of 72 hours, administered through an epidural catheter placed at the first intercoccygeal space. Heart rate (HR) and respiratory rate (f_R), pH, blood gases, arterial blood pressures (mmHg), and threshold for electrical noxious stimulation was evaluated for 120 minutes and after 240 minutes of epidural injection. Data were collected before injections and every 15 minutes for 120 minutes, and at 240 minutes of epidural administration. Significant sedation occurred in both treatments with no statistically significant difference between them. There were significant changes in f_R and HR from baseline but no difference between treatments. Arterial blood pressures were significantly lower in R60 treatment from 75 up to 120 minutes post epidural injection. Analgesia was considered moderate for both treatments lasting longer with romifidine at 60 μg/kg. Epidurally administered romifidine and morphine combination in horses produces dose-dependent sedation, arterial hypotension, and antinociceptive effects.

Assessment of a multimodal analgesia protocol in goats undergoing claw amputation

Objective: To evaluate the anti-nociceptive, sedative and behavioral effect of multimodal analgesic drugs in goat's undergoing unilateral claw amputation. Design: A randomized experimental design. Animals: Sixteen goats were randomly assigned into 4 groups (4 bucks per group). Procedure: Animals used in this study were randomly assigned into 4 groups, four animals for each group. All goats were anaesthetized by intravenous regional anesthesia (IVRA) of the right forelimb cephalic vein with 2% lidocaine in a dose of 3mg/kg body weight (BW) before surgery. Administration of carprofen (50 mg/ml) at a dose 0.1 mg/kg BW took place before surgery on day 0 and was then repeated once daily for another 3 consecutive days postoperatively in groups Lidocaine (L), carprofen_lidocaine (CL), carprofen – romifidine - lidocaine (CRL), and carprofen – romifidine - butorphenol - lidocaine (CRBL). Heart rate (HR), respiratory rate (RR), rectal temperature (RT), serum IL-6 level, sedation score, and lameness score were determined at preset time intervals and the animals’ behavior was recorded. Results: The mean average of HR, RR and RT in IVRA treated animals was significantly increased until the end of surgery followed by gradual decrease up to 90 min post-surgery. In CRL and CRBL it showed a significant decrease compared to controls. There was a significant decrease in lameness score in CRL and CRBL treated animals from 30 min until 2 hours and for 3 days post-surgery. After injection of romifidine it showed a significant decrease of IL-6 levels compared to lidocaine and CL treated animals. Conclusion and clinical relevance: It was concluded that, the use of romifidine and buterphanol in the analgesic protocol during claw amputation in goats, alleviate pain response and potentiate the analgesic effects of IVRA. Addition of carprofen to the analgesic protocol can be used for long- term treatment of orthopedic pain in goats without complications.

Sedative and electrocardiographic effects of low dose dexmedetomidine in healthy cats

ABSTRACT: In feline veterinary practice sedation is often needed to perform diagnostic or minimally invasive procedures, minimize stress, and facilitate handling. The mortality rate of cats undergoing sedation is significantly higher than dogs, so it is fundamental that the sedatives provide good cardiovascular stability. Dexmedetomidine (DEX) is an α2-adrenergic receptor agonist utilized in cats to provide sedation and analgesia, although studies have been utilized high doses, and markedly hemodynamic impairments were reported. The aim of this study was to prospectively investigate how the sedative and electrocardiographic effects of a low dose of DEX performing in cats. Eleven healthy cats were recruited; baseline sedative score, systolic arterial pressure, electrocardiography, and vasovagal tonus index (VVTI) were assessed, and repeated after ten minutes of DEX 5μg/kg intramuscularly (IM). A smooth sedation was noticed, and emesis and sialorrhea were common adverse effects, observed on average seven minutes after IM injection. Furthermore, electrocardiographic effects of a low dose of DEX mainly include decreases on heart rate, and increases on T-wave amplitude. The augmentation on VVTI and appearance of respiratory sinus arrhythmia, as well as sinus bradycardia in some cats, suggesting that DEX enhances parasympathetic tonus in healthy cats, and therefore will be best avoid in patients at risk for bradycardia.

Pharmacokinetic profile and partitioning in red blood cells of romifidine after single intravenous administration in the horse

The aims of this study were to assess the plasma concentrations of romifidine in horses after intravenous injection, to evaluate the red blood cell (RBC) partitioning of the anaesthetic drug, and to improve knowledge regarding its sedative effect in horses describing the pharmacokinetic model. Eight adult Standardbred horses received a single bolus of romifidine at a dosage of 100 μg/kg. Blood samples (5 mL) were collected immediately before romifidine administration (t0), and at 2, 5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 90, 105, 120, 150 and 180 min after injection. A sedation score was recorded at the same time. The romifidine concentrations in plasma and red blood cells were determined by high performance liquid chromatography (HPLC). The plasma and red blood cell concentrations were correlated with the sedation at each time point. Romifidine produced a satisfactory level of sedation in all animals. The sedation was detectable in all horses for up to 105 min. All the animals returned to normal without any behavioural changes at 180 min. The romifidine concentrations in the red blood cells were significantly higher (P < 0.01) at all time points than those in the plasma. The T1/2β was 148.67 ± 61.59 min and body clearance was 22.55 ± 6.67 mL/kg per min. The results showed that after a single bolus administration of romifidine, a partitioning in the RBCs was detected.

Sedative and cardiopulmonary effects of dexmedetomidine infusions randomly receiving, or not, butorphanol in standing horses

Dexmedetomidine (DEX) alone, or combined with butorphanol (BUT), may be administered by constant rate infusions (CRIs) in standing horses. This blinded, randomised, crossover study in six healthy adult horses aimed to determine the sedative and cardiopulmonary effects of DEX (dexmedetomidine (3.5 µg/kg+5 µg/kg/hour CRI) and DEX/BUT (dexmedetomidine (3.5 µg/kg+3.5 µg/kg/hour CRI) and butorphanol (20 µg/kg+24 µg/kg/hour CRI)). Head height above ground (HHAG), ataxia, responses to tactile/auditory stimuli and cardiopulmonary variables were recorded before, at 5/15/30/60/90 minutes and after CRIs terminated (15/30/60 minutes). Repeated measures analysis of variance with Tukey-Kramer test were used for cardiopulmonary values (mean±SD) and HHAG reduction (per cent), and Friedman's and Dunn's for non-parametric data (P<0.05). Maximum HHAG reductions of 54 per cent (DEX) and 58 per cent (DEX/BUT) occurred at 15 minutes, with ataxia for 15 minutes in both treatments. Responses to stimuli were reduced for 30 minutes in both treatments, and auditory up to 60 minutes in DEX. Cardiopulmonary effects typical of α₂-agonists were observed, with no differences between treatments. At the doses and rates reported here, both regimens provided clinically sufficient sedation for only 30 minutes.

Pharmacokinetics and selected pharmacodynamics of romifidine following low-dose intravenous administration in combination with exercise to quarter horses

Romifidine is an alpha-2 adrenergic agonist used for sedation and analgesia in horses. As it is a prohibited substance, its purported use at low doses in performance horses necessitates further study. The primary goal of the study reported here was to describe the serum concentrations and pharmacokinetics of romifidine following low-dose administration immediately prior to exercise, utilizing a highly sensitive liquid chromatography-tandem mass spectrometry assay that is currently employed in many drug testing laboratories. An additional objective was to describe changes in heart rate and rhythm following intravenous administration of romifidine followed by exercise. Eight adult Quarter Horses received a single intravenous dose of 5 mg (0.01 mg/kg) romifidine followed by 1 h of exercise. Blood samples were collected and drug concentrations measured at time 0 and at various times up to 72 h. Mean ± SD systemic clearance, steady-state volume of distribution and terminal elimination half-life were 34.1 ± 6.06 mL/min/kg and 4.89 ± 1.31 L/kg and 3.09 ± 1.18 h, respectively. Romifidine serum concentrations fell below the LOQ (0.01 ng/mL) and the LOD (0.005 ng/mL) by 24 h postadministration. Heart rate and rhythm appeared unaffected when a low dose of romifidine was administered immediately prior to exercise.

Comparison of midazolam and diazepam as co-induction agents with ketamine for anaesthesia in sedated ponies undergoing field castration

OBJECTIVE

To compare intravenous (IV) midazolam and diazepam administered with ketamine for induction of anaesthesia in ponies, already sedated with detomidine, undergoing field castration.

STUDY DESIGN

Prospective, randomised, 'blinded', clinical study.

ANIMALS

Twenty Welsh pony yearlings.

METHODS

After IV injection of detomidine (20 μg kg(-1) ) and phenylbutazone (4.4 mg kg(-1) ) ponies were allocated to receive either IV midazolam (group M) or diazepam (group D) (both 0.06 mg kg(-1) ) with ketamine (2.2 mg kg(-1) ) for induction of anaesthesia. Using simple descriptive scales, quality of sedation, induction, endotracheal intubation, surgical conditions and recovery were scored by observers blinded to treatment. Time from sedation to induction of anaesthesia, IV injection to lateral recumbency, induction to start of surgery, induction to first head lift and to standing, and total surgical time were measured. Cardiorespiratory function was assessed every 5 minutes. Time, number and total quantity of additional IV ketamine as well as any adverse effects were documented. Data were tested for normality and analysed using two-way anova with Bonferroni post hoc tests, unpaired t-tests and Mann-Whitney U tests as appropriate. Significance was set at p < 0.05.

RESULTS

There were no significant group differences in any of the measured variables except bodyweight (mean ± SD: group M 163 ± 12 kg; group D 150 ± 7 kg; p = 0.01). One pony in group M required ketamine 15 minutes after induction of anaesthesia. Surgical conditions were good in all cases; time from induction to standing was 50 ± 11 minutes in group M and 48 ± 12 minutes in group D. There were no adverse effects. Recoveries were uneventful with minimal ataxia.

CONCLUSIONS AND CLINICAL RELEVANCE

Midazolam and diazepam at 0.06 mg kg(-1) can be used interchangeably in combination with ketamine for IV induction of short term anaesthesia in ponies.

Evaluation of a romifidine constant rate infusion protocol with or without butorphanol for dentistry and ophthalmologic procedures in standing horses

OBJECTIVE

To compare the clinical usefulness of constant rate infusion (CRI) protocols of romifidine with or without butorphanol for sedation of horses.

STUDY DESIGN

Prospective 'blinded' controlled trial using block randomization.

ANIMALS

Forty healthy Freiberger stallions.

METHODS

The horses received either intravenous (IV) romifidine (loading dose: 80 μg kg(-1) ; infusion: 30 μg kg(-1)  hour(-1) ) (treatment R, n = 20) or romifidine combined with butorphanol (romifidine loading: 80 μg kg(-1) ; infusion: 29 μg kg(-1)  hour(-1) , and butorphanol loading: 18 μg kg(-1) ; infusion: 25 μg kg(-1)  hour(-1) ) (treatment RB, n = 20). Twenty-one horses underwent dentistry and ophthalmic procedures, while 19 horses underwent only ophthalmologic procedure and buccal examination. During the procedure, physiologic parameters and occurrence of head/muzzle shaking or twitching and forward movement were recorded. Whenever sedation was insufficient, additional romifidine (20 μg kg(-1) ) was administered IV. Recovery time was evaluated by assessing head height above ground. At the end of the procedure, overall quality of sedation for the procedure was scored by the dentist and anaesthetist using a visual analogue scale. Statistical analyses used two-way anova or linear mixed models as relevant.

RESULTS

Sedation quality scores as assessed by the anaesthetist were R: median 7.55, range: 4.9-9.0 cm, RB: 8.8, 4.7-10.0 cm, and by the dentist R: 6.6, 3.0-8.2 cm, RB: 7.9, 6.6-8.8 cm. Horses receiving RB showed clinically more effective sedation as demonstrated by fewer poor scores and a tendency to reduced additional drug requirements. More horses showed forward movement and head shaking in treatment RB than treatment R. Three horses (two RB, one R) had symptoms of colic following sedation.

CONCLUSIONS AND CLINICAL RELEVANCE

The described protocols provide effective sedation under clinical conditions but for dentistry procedures, the addition of butorphanol is advantageous.

The effects of intravenous romifidine on intraocular pressure in clinically normal horses and horses with incidental ophthalmic findings

DESIGN

Original study.

OBJECTIVE

To evaluate the effect of sedation with romifidine hydrochloride 1% (Sedivet: Boehringer-Ingelheim) on intraocular pressure (IOP) in the normal horse and horses with incidental ophthalmic findings as measured by applanation tonometry.

ANIMALS

Nineteen clinically normal horses (13 geldings, six mares) and eight horses (three geldings, five mares) with incidental ophthalmic findings were included in this study.

PROCEDURES

All horses underwent complete ophthalmic examination with pharmacologic mydriasis a minimum of 2 weeks prior to IOP evaluation. Baseline intraocular pressure values were obtained following auriculopalpebral nerve block and topical anesthetic. Immediately thereafter, romifidine was administered intravenously (75 µg/kg) and the IOP recorded at 5, 15, 30, 45 and 60 min postsedation in both eyes. Five successive readings were obtained at each time point, the low and high value discarded, and three remaining readings averaged for a mean.

RESULTS

The changes with time were consistent between eyes and OD and OS results were pooled. The mean IOP at baseline was 26.35 ± 5.57 mmHg. Mean IOP values were significantly lower than baseline at 5 (P < 0.0001), 15 (P < 0.0001), 30 (P = 0.0003), 45 (P < 0.0001) and 60 (P = 0.0005) minutes. The largest change from baseline (16.7%) was noted at t = 15 min.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of romifidine significantly decreased the IOP from baseline at all time points measured. The greatest decline in IOP was noted at 15 min postsedation. Results are consistent with other studies noting a decline in IOP with administration of α-2 agonists.

Effects on cardiopulmonary function and oxygen delivery of doses of romifidine and xylazine followed by constant rate infusions in standing horses

The objective of this study was to compare the cardiopulmonary effects of a xylazine or romifidine loading-dose, followed by a constant rate infusion (CRI) of the same α(2)-agonist. Nine research horses were treated in a randomized, blinded, crossover design with xylazine or romifidine. After instrumentation, a loading dose of intravenous xylazine (1mg/kg) or romifidine (80μg/kg) was administered, immediately followed by a CRI of xylazine (0.69mg/kg/h) or romifidine (30μg/kg/h) for a duration of 2h. Cardiopulmonary variables were recorded before bolus administration, during CRI, and for 1h after discontinuing drug administration. A significant decrease in haemoglobin concentration (tHb), arterial oxygen content (CaO(2)), oxygen delivery (D˙O(2)), mixed venous partial pressure of oxygen, heart rate, and cardiac output (Q˙t) followed the loading dose with both treatments. Carotid arterial blood pressure (ABP), systemic vascular resistance, and right atrial pressure (RAP) increased significantly. The increased ABP was followed by a significant decrease compared to baseline. Mean pulmonary arterial pressure increased significantly with romifidine only. No significant changes in stroke volume, arterial partial pressure of oxygen, and oxygen consumption were observed. Changes in Q˙t and RAP were more pronounced with romifidine. During CRI, tHb, and CaO(2) were significantly higher with romifidine, whereas D˙O(2) did not differ between treatments. Overall, cardiopulmonary effects were more pronounced and lasted longer with romifidine compared to xylazine. However, during CRI, there was no difference in D˙O(2) between drugs. With both α(2)-agonists, cardiovascular effects were most pronounced after loading dose administration and tended to stabilize during CRI.

Preliminary investigation of concurrent administration of phenylbutazone and romifidine in healthy horses

OBJECTIVE

To characterize the cardiorespiratory and electrocardiographic effects of the combined administration of phenylbutazone and romifidine.

STUDY DESIGN

Prospective four-period, four-treatment, blinded, randomized, crossover trial.

ANIMALS

Five, healthy, mixed breed horses.

METHODS

Prior to treatment administration, a catheter was introduced into the intra-thoracic cranial vena cava via the jugular vein and a subcutaneously located carotid artery was catheterised. All treatments were administered intravenously (IV) and consisted of saline placebo (PLC), phenylbutazone (PBZ, 4.4 mg kg(-1) ) romifidine (ROM, 80 μg kg(-1) ) and a combination of phenylbutazone (4.4 mg kg(-1) ) and romifidine (80 μg kg(-1) ). There was at least a 1 week washout period between treatments. Heart rate (HR), respiratory rate (f(R) ), systolic (SAP), diastolic (DAP) and mean (MAP) arterial pressures and central venous pressure (CVP) were recorded for baseline (prior to drug administration) and at 5 minute intervals thereafter for 30 minutes. Electrocardiographic abnormalities were recorded. Data were analyzed by anova.

RESULTS

For the cardiovascular variables there were no statistically significant (p>0.05) differences between horses treated with ROM and PBZ_ROM. Statistically significant (p<0.05) differences only occurred between treatments with romifidine (ROM and PBZ_ROM) and without romifidine (PLC and PBZ). Within treatments, for ROM, changes over time were statistically significant (p<0.05) for HR, SAP, DAP, MAP and CVP. For PBZ_ROM, changes over time were statistically significant (p<0.05) for CVP. Sino-atrial and atrio-ventricular blocks occurred in horses treated with ROM and PBZ_ROM.

CONCLUSIONS AND CLINICAL RELEVANCE

The combined IV administration of phenylbutazone and romifidine had no statistically significant effect on cardiorespiratory variables. These limited data suggest no evidence why both agents should not be included in a preoperative medication protocol for healthy horses but do not exclude the possibility of interactions occurring in a larger population.

Post-anesthetic pulmonary edema in two horses

CASE 1: A two-year old, 462 kg Standard bred horse was anesthetized for arthroscopy and castration. During anesthesia, hyperemia of the mucosal membranes and urticaria were noticed. During 5 hours of anesthesia subcutaneous edema of the eyelids and neck region developed. In the recovery box, the orotracheal (OT) tube was left in situ and secured in place with tape. Following initial attempts to stand, the horse became highly agitated and signs consistent with pulmonary edema developed subsequently. Arterial hypoxemia (PaO(2): 3.7 kPa [28 mmHg]) and hypocapnia (PaCO(2): 3.1 kPa [23 mmHg]) were confirmed. Oxygen and furosemide were administered. The horse was assisted to standing with a sling. Therapy continued with bilateral intra-nasal oxygen insufflation. Ancillary medical therapy included flunixin meglumine, penicillin, gentamycin and dimethylsulfoxide. Following 7 hours of treatment the arterial oxygen tensions began to increase towards normal values. CASE 2: An 11-year old, 528 kg Paint horse was anesthetized for surgery of a submandibular mass. The 4-hour anesthetic period was unremarkable. The OT tube was left in situ for the recovery. During recovery, the horse was slightly agitated and stood after three attempts. Clinical signs consistent with pulmonary edema and arterial hypoxemia (PaO(2): 5 kPa [37.5 mmHg]) subsequently developed following extubation. Respiratory signs resolved with medical therapy, including unilateral nasal oxygen insufflation, furosemide, flunixin meglumine and dimethylsulfoxide. The diagnosis of pulmonary edema in these horses was made by clinical signs and arterial blood-gas analysis. While pulmonary radiographs were not taken to confirm the diagnosis, the clinical signs following anesthesia support the diagnosis in both cases. The etiology of pulmonary edema was most likely multifactorial.

Sedative and cardiorespiratory effects of three doses of romifidine in comparison with medetomidine in five cats

This study was designed to compare the effects of three doses of romifidine (200, 400 and 600 microg/kg) with medetomidine (80 microg/kg) administered intramuscularly to five cats. The quality of sedation and the cardiovascular and respiratory effects of each treatment were evaluated, and the onset and duration of the sedation, and the cats' recovery times, were measured. Cardiorespiratory variables were also analysed. The dose of 200 microg/kg romifidine was clinically superior to the other doses of romifidine, providing moderate sedation, with minor cardiorespiratory and other adverse effects. However none of the doses of romifidine induced as deep and reliable sedation as the dose of medetomidine.

Cardiovascular effects of enoximone in isoflurane anaesthetized ponies

OBJECTIVE

Enoximone is a phosphodiesterase III inhibitor frequently used to improve cardiac output (CO) in man. As the use of enoximone has not been reported in horses, the effects of this inodilator were examined in isoflurane anaesthetized ponies.

STUDY DESIGN

Prospective, randomised, experimental study.

ANIMALS

Six healthy ponies, weighing 286 (212-367) +/- 52 kg, aged 5.0 +/- 1.6 years (4-6.5).

METHODS

After sedation with romifidine [80 microg kg(-1) intravenously (IV)], general anaesthesia was induced with midazolam (0.06 mg kg(-1) IV) and ketamine (2.2 mg kg(-1) IV) and maintained with isoflurane in oxygen (Et Iso 1.7%). The ponies were ventilated to maintain eucapnia (PaCO(2) 4.66-6.00 kPa). Each pony was anaesthetized twice with an interval of 3 weeks; receiving enoximone 0.5 mg kg(-1) IV (E) or saline (S) 90 minutes post-induction. Heart rate (HR), arterial (AP) and right atrial pressure (RAP) were measured before treatment, every 5 minutes between T0 (treatment) and T30 and then every 10 minutes until T120. Cardiac output measurements (lithium dilution technique) and blood gas analysis (arterial and central venous samples) were performed before T0 and at T5, T10, T20, T40, T60, T80, T100 and T120. Stroke volume (SV), systemic vascular resistance (SVR), venous admixture (Qs/Qt) and oxygen delivery (DO(2)) were calculated.

RESULTS

Enoximone induced significant increases in HR, CO, SV, Qs/Qt and DO(2) and a significant decrease in RAP. No significant differences were detected for AP, SVR and blood gases. No cardiac arrhythmias or other side effects were observed.

CONCLUSIONS AND CLINICAL RELEVANCE

The present results suggest that in isoflurane anaesthetized ponies, enoximone has beneficial effects on CO and SV without producing significant changes in blood pressure. Despite an increase in Qs/Qt, DO(2) to the tissues was improved.

Dissociative anaesthesia during field and hospital conditions for castration of colts

The principal aim of this study was to evaluate dissociative anaesthesia for castration of colts during field conditions. Three dissociative anaesthetic protocols were evaluated during castration of colts in an animal hospital. The protocol considered to be the most suitable was thereafter evaluated during castration of colts under field conditions. Respiratory and haemodynamic parameters and the response to surgery were determined during anaesthesia. All horses breathed air spontaneously during anaesthesia. Under hospital conditions 26 colts were randomised to receive one of three anaesthetic protocols: Romifidine and tiletamine-zolazepam (RZ); acepromazine, romifidine and tiletamine-zolazepam (ARZ); or acepromazine, romifidine, butorphanol and tiletamine-zolazepam (ARBZ). The surgeon was blinded to the anaesthetic protocol used and decided whether supplemental anaesthesia was needed to complete surgery. Under field conditions 31 colts were castrated during anaesthesia with the ARBZ protocol. All inductions, anaesthesia and recoveries were calm and without excitation under both hospital and field conditions. Surgery was performed within 5–20 minutes after the horses had assumed lateral recumbency during both hospital and field castrations. Under hospital conditions some horses needed supplemental anaesthesia with all three anaesthetic protocols to complete surgery. Interestingly, none of the horses castrated with protocol ARBZ under field conditions needed additional anaesthesia. Cardiorespiratory changes were within acceptable limits in these clinically healthy colts.

High inspired oxygen concentrations increase intrapulmonary shunt in anaesthetized horses

OBJECTIVES

To compare pulmonary function and gas exchange in anaesthetized horses during and after breathing either O2-rich gas mixtures or air.

ANIMALS

Six healthy standard bred trotters (age range 3-12 years; mass range 423-520 kg), four geldings and two mares. Study design Randomized, cross-over experimental study.

METHODS

Horses were anaesthetized on two occasions with tiletamine-zolazepam after pre-anaesthetic medication with acepromazine, romifidine and butorphanol. After endotracheal intubation and positioning in left lateral recumbency, animals were allowed to breathe spontaneously. One of two, randomly allocated inspired gas treatments was provided: either i) room air (fractional concentration of inspired O2 [FIO2] = 0.21) provided throughout anaesthesia; or ii) an O2-rich gas mixture (FIO2 = >0.95) for 15 minutes, followed by room air. The alternative treatment was delivered at the second anaesthetic. Respiratory and haemodynamic variables and the distribution of ventilation-perfusion (VA/Q) ratios (using the multiple inert gas elimination technique) were determined in the standing conscious horse (baseline) after sedation and during anaesthesia.

RESULTS

Breathing O2-rich gas was associated with a decreased respiratory rate (p = 0.015) increased PaCO2 (p < 0.001) and increased PaO2 (p = 0.004) compared with breathing air. All horses developed intrapulmonary shunt during anaesthesia, but shunt was significantly greater (13 +/- 5%) when O2-rich gas was delivered compared with air breathing (5 +/- 2%; p = 0.013). Ten minutes after O2-rich gas was replaced by air, shunt remained larger in horses that had initially received oxygen compared with those breathing air (p = 0.042). Mixed venous oxygen tensions were significantly lower during sedation than at baseline (p < 0.001) and during anaesthesia (p < 0.001).

CONCLUSIONS

During dissociative anaesthesia, arterial oxygenation was greater when horses breathed gas containing more than 95% oxygen, compared with when they breathed air. However, breathing O2-rich gas increased intrapulmonary shunt and caused hypoventilation. The intrapulmonary shunt created during anaesthesia by high inspired O2 concentrations remained larger when FIO2 was reduced to 0.21, indicating that absorption atelectasis produced during O2-rich gas breathing persisted throughout anaesthesia.

CLINICAL RELEVANCE

In healthy horses undergoing short-term dissociative anaesthesia, air breathing ensures a level of oxygen delivery that meets tissue demand. There is no benefit to horses in breathing O2-rich gas after the gas supply is discontinued. On the contrary, the degree of shunt induced by breathing O2-rich gas persists. The clinical relevance of this during recovery requires investigation.

Effects of acepromazine on pulmonary gas exchange and circulation during sedation and dissociative anaesthesia in horses

OBJECTIVE

To study pulmonary gas exchange and cardiovascular responses to sedation achieved with romifidine and butorphanol (RB) alone, or combined with acepromazine, and during subsequent tiletamine-zolazepam anaesthesia in horses.

ANIMALS

Six (four males and two females) healthy Standardbred trotters aged 3-12 years; mass 423-520 kg.

STUDY DESIGN

Randomized, cross-over, experimental study.

MATERIALS AND METHODS

Horses were anaesthetized on two occasions (with a minimum interval of 1 week) with intravenous (IV) tiletamine-zolazepam (Z; 1.4 mg kg(-1)) after pre-anaesthetic medication with IV romifidine (R; 0.1 mg kg(-1)) and butorphanol (B; 25 microg kg(-1) IV). At the first trial, horses were randomly allocated to receive (protocol ARBZ) or not to receive (protocol RBZ) acepromazine (A; 35 microg kg(-1)) intramuscularly (IM) 35 minutes before induction of anaesthesia. Each horse was placed in left lateral recumbency and, after tracheal intubation, allowed to breathe room air spontaneously. Respiratory and haemodynamic variables and ventilation-perfusion (; multiple inert gas elimination technique) ratios were determined in the conscious horse, after sedation and during anaesthesia. One- and two-way repeated-measures anova were used to identify within- and between-technique differences, respectively.

RESULTS

During sedation with RB, arterial oxygen tension (PaO(2)) decreased compared to baseline and increased mismatch was evident; there was no O(2) diffusion limitation or increase in intrapulmonary shunt fraction identified. With ARB, PaO(2) and remained unaffected. During anaesthesia, intrapulmonary shunt occurred to the same extent in both protocols, and mismatching increased. This was less in the ARBZ group. Arterial O(2) tension decreased in both protocols, but was lower at 25 and 35 minutes of anaesthesia in RBZ than in ARBZ. During sedation, heart rate (HR) and cardiac output (Qt) were lower while arterial-mixed venous oxygen content differences and haemoglobin concentrations were higher in RBZ compared with ARBZ. Total systemic vascular resistance, mean systemic, and mean pulmonary arterial pressures were higher during anaesthesia with RBZ compared to ARBZ.

CONCLUSIONS AND CLINICAL RELEVANCE

Acepromazine added to RB generally improved haemodynamic variables and arterial oxygenation during sedation and anaesthesia. Arterial oxygenation was impaired as a result of increased shunt and mismatch during anaesthesia, although acepromazine treatment reduced disturbances and falls in PaO(2) to some extent. Haemodynamic variables were closer to baseline during sedation and anaesthesia when horses received acepromazine. Acepromazine may confer advantages in healthy normovolaemic horses.