Respiratory function and extraocular muscle paralysis following administration of pancuronium bromide in dogs

Pancuronium bromide, a neuromuscular blocking agent, was evaluated in canine cataract surgical patients under general anesthesia to determine its effects on respiratory function and globe position. Two paralytic, anesthetic regimes were studied: one using a standard dosage of 0.066 mg kg⁻¹ pancuronium bromide, given intravenously while providing the patient with ventilatory support, and one using a dosage of 0.022 mg kg⁻¹ in which no ventilatory support was provided. Eye position and anterior vitreal position/displacement were recorded by a surgeon who was blinded as to treatment group. Physiological parameters indicative of respiratory function were monitored. Both dosages of pancuronium produced comparable, neutral globe position within 30 s following administration which lasted for 20–30 min. All patients in the standard dose group experienced uneventful anesthetic episodes with physiological parameters well within the normal ranges. Within 5 min after administration, all patients in the low-dose group developed a pronounced respiratory acidosis (mean arterial pH = 7.07 ± 0.08; mean PaCO₂ = 79.8 ± 10.7 mmHg), which exceeded a set of predetermined safety limits, and subsequently these dogs received ventilatory support. We conclude that 0.022 mg kg⁻¹ pancuronium rapidly produces an unacceptable level of respiratory acidosis and, as a result, patients receiving neuromuscular blocking agents should routinely receive ventilatory support.     

The effect of low dose rocuronium on globe position, muscle relaxation and ventilation in dogs: a clinical study

OBJECTIVE: The purpose of this study was to evaluate globe position, muscle relaxation and changes in ventilatory parameters after intravenous administration of 0.1 mg/kg rocuronium. STUDY DESIGN: Prospective clinical study. ANIMAL STUDIED: Sixteen dogs of different breeds, with a body weight of 22.1 +/- 13 kg and age of 5.6 +/- 2.8 years (mean +/- SD), were anesthetized for a short ophthalmic examination requiring central position of the globe. PROCEDURES: All dogs were premedicated with 0.005 mg/kg medetomidine and 0.1 mg/kg methadone IV. Anesthesia was induced with propofol to effect and maintained with 10 mg/kg/h propofol by continuous rate infusion. Following endotracheal intubation all dogs breathed 100% oxygen via an anesthetic circle system. Neuromuscular function was assessed with an acceleromyograph (TOF-Guard, Organon Teknika NV, Turnhout, Belgium) and by stimulation of the nervus peroneus superficialis. The ventilation parameters were measured using spirometry and capnography. After baseline measurements 0.1 mg/kg rocuronium was administered IV. Minute volume (MV), tidal volume (Vt), respiratory rate (RR), end expiratory carbon dioxide concentration (PE'CO(2)) and maximal depression of the response of the first twitch (T1) of train-of-four (TOF) stimulation and train-of-four ratio (TOFR) was measured. The change in the position of the globe was recorded. RESULTS: T1 decreased to 61 +/- 18% and the TOF ratio to 45 +/- 21% of baseline values. Both parameters returned to baseline after 9 min. There was no significant reduction in MV, TV and RR and no increase in PE'CO(2). The globe rotated to a central position of 45 +/- 7.7 s after administration of rocuronium and remained there for 23 +/- 10.8 min in all dogs. CONCLUSION: Rocuronium administered intravenously at a dose of 0.1 mg/kg to dogs causes a central position of the globe but minimal impairment of ventilation parameters.     

The effect of low dose rocuronium bromide on eyeball position, muscle relaxation, and ventilation in dogs

A central eyeball position is often required during sedation or anaesthesia to facilitate examination of the eye. However, use of neuromuscular blockade to produce a central eye position may result in depressed ventilation. This study evaluated the eyeball position, muscle relaxation and changes in ventilation during general anaesthesia after the IV administration of 0.1 mg kg^?1 rocuronium. With client consent, 12 dogs of different breeds, body mass 27.2 ± 11.8 kg, aged 5.6 ± 2.8 years (mean ± SD) were anaesthetized for ocular examination. Pre‐anaesthetic medication was 0.01 mg kg^?1 medetomidine and 0.2 mg kg^?1 butorphanol IV. Anaesthesia was induced with propofol to effect and maintained with 10 mg kg^?1 hour^?1 propofol by infusion. The dogs were placed in left lateral recumbency, their trachea intubated and connected to a circle breathing system (Fi O₂ = 1.0). All dogs breathed spontaneously. The superficial peroneal nerve of the right hind leg was stimulated every 15 seconds with a train‐of‐four (TOF) stimulation pattern and neuromuscular function was assessed with an acceleromyograph (TOF‐Guard). Adequacy of ventilation was measured with the Ventrak 1550. After 10 minutes of anaesthesia to allow stabilisation of baseline values, 0.1 mg kg^?1 rocuronium was administered IV. Minute volume (Vm ), tidal volume (Vt ), respiratory rate (RR), Pe ′CO₂ and maximal depression of T1 and TOF ratio were measured. Data were analysed using a paired t‐test. The changes in the eyeball position were recorded. A total of 100 ± 33 seconds after the injection of rocuronium, T1 was maximally depressed to 62 ± 21% and the TOF ratio to 42 ± 18% of baseline values. Both variables returned to baseline after 366 ± 132 seconds (T1) and 478 ± 111 seconds (TOF). There was no significant reduction in Vm (2.32 ± 1.1 L minute^?1), Vt (124.1 ± 69.3 mL) and RR (10 ± 3.8 breaths minute^?1) and no increase in Pe ′CO₂ (6.5 ± 2.1 kPa (48.8 ± 16.1 mm Hg)) throughout the procedure. The eyeball rotated to a central position 35 ± 7 seconds after rocuronium IV and remained there for a minimum of 20 ± 7 minutes in all dogs. We conclude that rocuronium at a dose of 0.1 mg kg^?1 can be administered to dogs IV with minimal changes in ventilatory variables. The eyeball is fixed in a central position for at least 20 minutes, which greatly facilitates clinical examination.     

Neuromuscular Effects of Doxacurium Chloride in Isoflurane-Anesthetized Dogs

Objective—To determine the neuromuscular effects of doxacurium chloride and to construct a dose-response curve for the drug in isoflurane-anesthetized dogs. Design—Randomized, controlled trial. Animals—Six healthy, adult, mixed-breed dogs (five female, one male) weighing 24.8 ° 2.8 kg. Methods—Anesthesia was induced with isoflurane in oxygen and maintained with 1.9% to 2.3% end-tidal isoflurane concentration. Paco₂ was maintained between 35 and 45 mm Hg with mechanical ventilation. Mechanomyography was used to quantitate the evoked twitch response of the paw after supramaximal train-of-four stimulation of the superficial peroneal nerve. After baseline values were recorded, the dogs received one of three doses of doxacurium (2.0, 3.5, 4.5 μg/kg of body weight) or a saline placebo intravenously in random order. All dogs received all treatments with at least 7 days between studies. After drug administration, the degree of maximal first twitch depression compared with baseline (T,%) was recorded. Dose-response relations of doxacurium were plotted in log dose-probit format and analyzed by linear regression to determine effective dose (ED₅₀ and ED₉₀) values for doxacurium. Results—The median log dose-probit response curve showed good data correlation (r= .999) with estimates of the ED₅₀ (2.1 μg/kg) and ED₉₀ (3.5 μg/kg) for doxacurium in isoflurane-anesthetized dogs. Mean ± SD values for T₁% (first twitch tension compared with baseline) at maximal depression after drug administration, onset (time from drug administration to maximal depression of T₁%), duration (time from maximal depression of T₁% to 25% recovery of T₁%), and recovery (time from 25% to 75% recovery of T₁%) times were 92%± 4%, 40 ± 5 minutes, 108 ± 31 minutes, and 42 ± 11 minutes for dogs treated with 3.5 μg/kg of doxacurium and 94%± 7%, 41 ± 8 minutes, 111 ± 33 minutes, and 37 ± 10 minutes for dogs treated with 4.5 μg/kg of doxacurium. Conclusion and Clinical Relevance—We conclude that doxacurium is a long-acting neuromuscular blocking agent with a slow onset of action. Doxacurium can be used to provide muscle relaxation for long surgical procedures in isoflurane-anesthetized dogs. Interpatient variability, particularly of duration of drug action, may exist in the neuromuscular response to the administration of doxacurium in dogs.     

Recovery from neuromuscular block in dogs: restoration of spontaneous ventilation does not exclude residual blockade

ObjectiveTo evaluate if return of spontaneous ventilation to pre-relaxation values indicates complete recovery from neuromuscular blockade.Study designProspective, with each individual acting as its own control.AnimalsTen healthy adult female Beagle dogs weighing 6.2–9.4 kg.MethodsDogs were anesthetized with propofol, dexemedetomidine and isoflurane. Spontaneous ventilation was assessed by measuring end-tidal CO₂, expired tidal volume, peak inspiratory flow, respiratory rate and minute ventilation. Vecuronium 25 μg kg^?1 IV was administered and neuromuscular block was evaluated by measuring the train-of-four (TOF) ratio with acceleromyography in the hind limb. During spontaneous recovery from neuromuscular block, the TOF ratio when each ventilatory variable returned to baseline was recorded.ResultsThis dose of vecuronium produced moderate neuromuscular block in all dogs, with TOF ratio values of 0–18% at maximal block. Expired tidal volume, peak inspiratory flow and minute ventilation returned to pre-relaxation values when the median TOF ratio was ≤ 20%. The median TOF ratio was 42% when the end-tidal CO₂ returned to pre-relaxation values.Conclusions and clinical relevanceSignificant residual neuromuscular block could be measured at the hind limb with acceleromyography when ventilation had spontaneously returned to pre-vecuronium values. Monitoring spontaneous ventilation, including end-tidal CO₂, expired tidal volume, peak inspiratory flow or minute ventilation cannot be used as a surrogate for objective neuromuscular monitoring, and this practice may increase the risk of postoperative residual paralysis.     

Effects of Brimonidine Ingestion on Cardiovascular Responses and Renal Function in Conscious Dogs

The effects of brimonidine, an α₂-adrenoceptor agonist, on blood pressure, heart rate, respiratory rate, renal function and some blood parameters were investigated in 10 dogs. Dogs were divided into two groups, low dose (LD; 0.2 mg/kg) and high dose (HD; 0.5 mg/kg) of brimonidine given orally. The α₂-adrenergic antagonist yohimbine hydrochloride was injected to dogs at a dose of 0.1 mg/kg in both groups at the fifth hour after brimonidine administration. The results demonstrated that after administration of brimonidine, mean arterial blood pressure decreased dramatically at 2 h by 23% and 20% in LD and HD groups, respectively. Heart rate was decreased in a similar manner and both remained low at 5 h after brimonidine administration. Respiratory rate was decreased by 50%, while the electrocardiogram showed prolongation of the PR interval. Glomerular filtration rate (GFR) and effective renal blood flow were reduced when measured at 4 h after brimonidine ingestion in both groups, but the effect was more pronounced in the LD group. Brimonidine caused natriuresis and kaliuresis in both LD and HD groups. The packed cell volume was decreased and hyperglycaemia was detected. Most of the effects can be reversed completely after administration of yohimbine. However, yohombine can restore GFR only partially. These data suggest that brimonidine caused cardiovascular and respiratory depression. The adverse effects of this drug can be antagonized by yohimbine, suggesting that these effects were mediated via the α₂-adrenoceptor.     

Neuromuscular and cardiovascular effects of pancuronium bromide in calves anesthetized with halothane

Pancuronium bromide was administered to calves to define the dosage level necessary to produce surgical relaxation (90% to 99% reduction of base-line evoked, hindlimb digital-extensor muscle twitch tension). Initial dosage level requirement was 43 +/- 9 micrograms/kg of body weight. Calves with this degree of relaxation required 26 +/- 14 minutes to achieve 50% recovery and 43 +/- 19 minutes to achieve complete return of base-line muscle twitch. Calves given a repeat injection of pancuronium at base-line muscle twitch required 27 +/- 9 micrograms/kg to achieve relaxation similar to that of the 1st dose. The 2nd dose did not last as long as the 1st, with complete recovery occurring in 37 +/- 12 minutes. Maximum evoked tension occurred at 200- to 400-g resting tension on the hoof. There was an absence of heart rate or blood pressure changes after injection of relaxant and a variable and inconsistent fade response to train-of-four and tetanic stimulus of the facial muscles. Acid-base values were alkalemic (pHa 7.5 +/- 0.08) when ventilation was controlled at eucapnia (PaCO2, 25 to 45 mm of Hg).     

Observations on the neuromuscular blocking action of pipecuronium in the dog

Pipecuronium bromide is a nondepolarising muscle relaxant which is an analogue of pancuronium. Doses of 0.025 and 0.05 mg kg-1 produced neuromuscular block in the anaesthetised dog. Previous work would suggest that the drug has minimal effects on the cardiovascular system. However, a dose of 0.05 mg kg-1 produced a profound hypotension in one dog on one occasion. The neuromuscular blocking action of the drug would appear to be extremely variable as indicated by the wide range of the results. The neuromuscular block was readily reversible with neostigmine preceded by atropine.     

Cardiovascular effects of pancuronium bromide in mongrel dogs.

The cardiovascular effects of a new nondepolarizing muscle relaxant, pancuronium bromide, were studied in mongrel dogs. Small, but significant, increases in mean arterial blood pressure were observed after each of 2 intravenous doses (0.01 mg/kg and 0.1 mg/kg) were given. Heart rate increased significantly in dogs administered the larger dosage, and indexes of ventricular functions demonstrated a trend toward positive cardiac inotrophy after either the large or the small dose.     

Comparison of a sidestream capnograph and a mainstream capnograph in mechanically ventilated dogs

OBJECTIVE: To compare the ability of a sidestream capnograph and a mainstream capnograph to measure end-tidal CO2 (ETCO2) and provide accurate estimates of PaCO2 in mechanically ventilated dogs. DESIGN: Randomized, double Latin square. ANIMALS: 6 healthy adult dogs. PROCEDURE: Anesthesia was induced and neuromuscular blockade achieved by IV administration of pancuronium bromide. Mechanical ventilation was used to induce conditions of standard ventilation, hyperventilation, and hypoventilation. While tidal volume was held constant, changes in minute volume ventilation and PaCO2 were made by changing the respiratory rate. Arterial blood gas analysis was performed and ETCO2 measurements were obtained by use of either a mainstream or a sidestream capnographic analyzer. RESULTS: A linear regression model and bias analysis were used to compare PaCO2 and ETCO2 measurements; ETCO2 measurements obtained by both capnographs correlated well with PaCO2. Compared with PaCO2, mainstream ETCO2 values differed by 3.15 +/- 4.89 mm Hg (mean bias +/- SD), whereas the bias observed with the sidestream ETCO2 system was significantly higher (5.65 +/- 5.57 mm Hg). Regardless of the device used to measure ETCO2, bias increased as PaCO2 exceeded 60 mm Hg. CONCLUSIONS AND CLINICAL RELEVANCE: RelevancehAlthough the mainstream cas slightly more accurate, both methods of ETCO2 measurement correlated well with PaCO2 and reflected changes in the ventilatory status. However, ETCO2 values > 45 mm Hg may inaccurately reflect the severity of hypoventilation as PaCO2 may be underestimated during conditions of hypercapnia (PaCO2 > 60 mm Hg).     

Use of rocuronium administered by continuous infusion in dogs

OBJECTIVE: A clinical trial to determine whether continuous infusion administration technique was suitable for maintaining neuromuscular blockade with rocuronium bromide in dogs. ANIMALS: Twenty-two dogs, 10 males and 12 females, median age 2 years 4 months, median weight 32 kg undergoing elective surgical procedures under general anaesthesia: ASA classification I or II. MATERIALS AND METHODS: After induction of anaesthesia, neuromuscular function was evaluated using train-of-four (TOF) stimulation of the dorsal buccal branch of the facial nerve. A bolus dose of 0.5 mg kg(-1) rocuronium was administered intravenously and an infusion of 0.2 mg kg(-1) hour(-1) was started immediately. Neuromuscular blockade was assessed visually by counting the number of twitches observed during TOF stimulation repeated at 10-second intervals. RESULTS: The bolus dose of rocuronium abolished the response to TOF stimulation in 21 of the 22 dogs. The median onset time of neuromuscular blockade (complete loss of all four twitches) was 82 seconds (range 38-184 seconds). Median infusion duration was 76 minutes (range 20.3-146 minutes). CONCLUSIONS AND CLINICAL RELEVANCE: This protocol of rocuronium administration was considered to be effective in dogs. Constant infusion of rocuronium is easily applicable to clinical practice and further work is required on infusion titration.     

Neuromuscular blocking action of vecuronium in the dog and its reversal by neostigmine

Vecuronium bromide is one of a new series of competitive or nondepolarising muscle relaxants which is closely related chemically to pancuronium. Doses of 0.06, 0.1 and 0.2 mg kg-1 produced neuromuscular block in the anaesthetised dog. There were no observable effects on arterial blood pressure. The neuromuscular block was readily reversible with neostigmine preceded by atropine.     

Experimental investigation of the cardiovascular and respiratory effects of increasing concentrations of isoflurane in the dog

The cardiovascular and respiratory effects of the new inhalational anaesthetic agent isoflurane were investigated in dogs. Anaesthesia was induced with thiopentone after premedication with acepromazine. Isoflurane was administered with nitrous oxide and oxygen by spontaneous ventilation after base line values had been determined. Arterial blood pressure decreased as the concentration of administered isoflurane increased. Isoflurane produced a profound and dose related respiratory depression as measured by the increase in end tidal carbon dioxide levels. Isoflurane administration did not produce any visible muscle twitching.     

Common tiger snake envenomation in dogs and mice—relationship between the amount of venom injected and the onset of clinical signs

SUMMARY Common tiger snake (Notechis scutatus) venom was injected into mice and dogs at various dose rates calculated on the known lethal dose (LD) for each species. The larger the dose of venom, the earlier was the onset of clinical signs and the more rapid and severe the course of the disease in both species. In dogs injected with 32 LD of venom, there was sudden collapse and death in about one hour from the time of injection without recovery from premonitory depression and before mydriasis occurred. Dogs given 5 to 16 LD of venom developed preparalytic signs (vomition, salivation or defaecation) in 5 to 30 min, mydriasis in 2 to 4h, became paralysed and died in about 2.5 to 5 h. When doses of venom of about 1 LD were injected, vomition and salivation occurred within 2 h and mydriasis in about 4 h. The dogs were unable to close the mouth completely despite retention of jaw muscle tone. At the site of injection of venom there was occasional but slight erythema and oedema. Sublethally envenomed dogs did not show preparalytic signs nor did they have general skeletal muscle paralysis. Even at the lowest dose tested (0.25 LD), however, they developed mydriasis and photophobia, which persisted for several days.     

Vecuronium infusion in the dog

The non-depolarising muscle relaxant vecuronium bromide was administered to 20 dogs undergoing a variety of surgical procedures under general anaesthesia. An initial dose of 0–1 mg/kg was administered and followed by an infusion of 0–1 mg/kg/hour. Reversal of the neuromuscular block was carried out with neostigmine and atropine.     

Neuromuscular blocking properties of atracurium during sevoflurane or propofol anaesthesia in dogs

OBJECTIVE: To quantify the neuromuscular blockade (NMB) produced by atracurium in either sevoflurane or propofol-anaesthetized dogs. ANIMALS: Twelve healthy, female adult mixed-breed dogs weighing 13 +/- 3 kg (range 10-22 kg). MATERIALS AND METHODS: Three doses of atracurium (0.1, 0.2 and 0.3 mg kg(-1)) were tested at 1-week intervals. Anaesthesia was induced with inhaled sevoflurane or intravenous propofol and maintained with end-tidal sevoflurane concentrations of 1.95% (1.25 x MAC) or propofol 0.6 mg kg(-1) minute(-1) respectively. Acceleromyography and train-of-four stimulation of the fibular nerve were used for the assessment of NMB. The percentage depression of the first twitch (T1) and the fourth to the first twitch ratio (T4/T1), the maximum degree of neuromuscular block achieved and surgical muscle relaxation were recorded. Before and during neuro muscular blockade (at 10 minute intervals) body temperature, ECG, arterial blood pressure, inspired and expired CO2 concentrations and SpO2 were recorded. RESULTS: Atracurium produced a dose-dependent duration of NMB in both propofol and sevoflurane-anaesthetized dogs. Duration of block was longer in dogs anaesthetized with sevoflurane. All studied doses of atracurium caused twitch depression > or =95% with little or no cardiovascular changes. CONCLUSIONS: Sevoflurane produces a clinically relevant potentiation of atracurium-induced NMB in dogs compared with propofol. CLINICAL RELEVANCE: Significant differences in the potentiation of NMB drugs are encountered with commonly used anaesthetics in the dog.     

Evaluation of neoadjuvant prednisone administration and surgical excision in treatment of cutaneous mast cell tumors in dogs

OBJECTIVE: To determine response rate and reduction in tumor burden and effect of dose on tumor response in dogs treated with neoadjuvant prednisone for cutaneous mast cell tumors (MCTs). DESIGN: Combined prospective clinical study and retrospective case series. ANIMALS: 49 dogs with MCT. PROCEDURES: Medical records were retrospectively reviewed for dogs with primary untreated cutaneous MCT managed with neoadjuvant prednisone administration and surgery. Tumor characteristics and response to treatment were recorded. A subset of dogs assigned to low-dose (LD) treatment with neoadjuvant prednisone (1.0 mg/kg [0.45 mg/lb], PO, q 24 h) or high-dose (HD) treatment (2.2 mg/kg [1.0 mg/lb], PO, q 24 h) was used to determine the effects of dose. RESULTS: The overall objective response rate was 70% for dogs treated with neoadjuvant prednisone; prednisone dose was not significantly associated with response. Prospectively, the median sum maximal diameter (MaxD) reduction was 45.2%, and reduction in tumor volume was 80.6%. In both treatment groups, the mean percentage MaxD reduction and tumor volume reduction were significant. The difference in response between the LD and HD groups was not significant. The LD group had mean MaxD and tumor volume reductions of 35.4% and 52.5%, respectively, compared with mean reductions of 48.8% in MaxD and 78% in tumor volume in the HD group. CONCLUSIONS AND CLINICAL RELEVANCE: Treatment with neoadjuvant prednisone appears to be useful for inducing reduction of MCTs and may facilitate resection when adequate surgical margins cannot be confidently attained because of mass location or size or both.     

Effect of Ketorolac Tromethamine on Atracurium-Induced Neuromuscular Blockade in Anesthetized Dogs

Objective —The purpose of this study was to determine the effect of ketorolac tromethamine or placebo on the neuromuscular blockade induced by an infusion of atracurium in isoflurane-anesthetized dogs. Design —Randomized, controlled trial. Animals —Six healthy, adult mixed-breed dogs (five female, one male) weighing 24.8 ± 2.8 kg. Methods —Dogs were studied on two occasions with a minimum of 7 days between studies. Dogs were induced with 5% isoflurane in oxygen and maintained with 1.6 ± minimum alveolar concentration (MAC) end-tidal isoflurane. Neuromuscular blockade was assessed using the train of-four response. Once 50% depression of the first twitch (T₁) was achieved, the atracurium infusion rate was held constant for 30 minutes. Then ketorolac, 0.5 mg/kg, or the same volume of placebo (0.9% sodium chloride solution) was administered intravenously and the atracurium infusion maintained for an additional 60 minutes. Before and at 2, 5, 10, 15, 30, and 60 minutes after ketorolac or placebo, the percent depression of T₁ and the fourth twitch to the first twitch (T₄/T₁) ratio were recorded. The atracurium infusion was discontinued and the time for T₁ to recover from 50% to 75% of its original value was recorded. At 75% T₁, edrophonium, 0.5 mg/kg intravenously, was administered to antagonize the residual blockade. Results —There was no significant difference in T₁%, T₄/T₁ ratio, or recovery time after ketorolac administration compared with placebo. Conclusions —Ketorolac, 0.5 mg/kg intravenously, has no significant effect on either atracuriuminduced neuromuscular blockade or recovery time for T₁ in isoflurane-anesthetized dogs. Clinical Relevance —The concurrent use of atracurium should not be a contraindication for the administration of ketorolac for intraoperative or postoperative analgesia.     

The cardiovascular effects of dexmedetomidine given by continuous infusion during isoflurane anesthesia in dogs

In a previous study we showed that the MAC of isoflurane was decreased by 18 ± 12% and 59 ± 7% by constant rate infusions of dexmedetomidine at 0.5 and 3 μg kg^–1 hour^–1. The purpose of this study was to document the cardiovascular changes associated with these infusions of dexmedetomidine at 1.3 MAC isoflurane/ dexmedetomidine. Dogs were anesthetized with isoflurane in oxygen given by mask. A cephalic venous catheter, a dorsal pedal arterial catheter and a balloon tipped, Swan–Ganz, pulmonary arterial catheter were placed percutaneously. After instrumentation the dogs were maintained at 1.3 MAC isoflurane for 60 minutes. At this time a set of measurements was made including, heart rate, respiratory rate, core body temperature, pulmonary and systemic arterial blood pressures (SAP, MAP, DAP, CVP, SPAP, MPAP, DPAP and PAOP), cardiac output and arterial and mixed venous blood samples were collected for the measurement of blood gases, pH, hemoglobin concentration, PCV and total protein. Calculated variables included base excess (BE), (HCO₃^?), cardiac index, systemic and pulmonary vascular resistance indices, oxygen delivery, oxygen consumption, oxygen utilization ratio and shunt fraction. After these measurements to dogs were randomly assigned to receive a loading dose of 0.5 or 3 μg kg^–1 of dexmedetomidine given over 6 minutes followed by an infusion of 0.5 (LD) or 3 μg kg^–1 hour^–1 (HD), respectively. The concentration of isoflurane was reduced by the above percentages, respectively, to maintain 1.3 MAC. Full sets of measurements were repeated at 10, 30, 60, 90, 120, 150 and 180 minutes after the start of the loading dose. Measured and calculated variables were compared with baseline using an anova and a post‐hoc Tukey's test. Significance was set at p = 0.05 and results are given as mean ± SD. The initial concentration of isoflurane was 1.73 ± 0.02% and was reduced to 1.41 ± 0.02 and 0.72 ± 0.09% for the LD and HD, respectively. Heart rate decreased with both doses but no other parameter changed significantly with the LD. With the HD there were significant changes in SAP, MAP, DAP, CVP, MPAP, PAOP, CI, SVRI, PCV, DO₂ and shunt fraction. The LD appeared to have minimal effect on the cardiopulmonary values measured, whereas the HD caused typical changes expected with an alpha‐2 agonist.     

Interactions between vecuronium and suxamethonium in the dog

The non-depolarising muscle relaxant vecuronium (0.2 mg kg-1) was administered to four dogs. At 50 per cent return of neuromuscular activity, as measured by the train-of-four technique, the depolarising muscle relaxant suxamethonium (0.3 mg kg-1) was injected intravenously. At 50 per cent return of neuromuscular activity reversal of the block was achieved with atropine and neostigmine. The duration of action of suxamethonium was reduced by the prior administration of vecuronium. In the second series of experiments the order of administration of the suxamethonium and vecuronium was reversed. Suxamethonium (0.3 mg kg-1) was administered first and at 50 per cent recovery vecuronium (0.2 mg kg-1) was given. At 50 per cent recovery of the twitch response after vecuronium administration the block was reversed with atropine and neostigmine. The previous administration of suxamethonium prolonged the duration of the vecuronium induced neuromuscular block.