Some cardiopulmonary effects of midazolam premedication in clenbuterol-treated bitches during surgical endoscopic examination of the uterus and ovariohysterectomy

INTRODUCTION It has been suggested that the intraoperative administration of clenbuterol may facilitate the endoscopic examination of the canine uterus for diagnostic purposes. Clenbuterol is a selective 2-adrenergic agonist that is administered to induce bronchial di lat ion and myometrial relaxation. Possible adverse effects that may occur are systemic hypotension and cardiac arrhythmias when used in conjunction with inhalation anaesthetic agents, and in particular in animals with pre-existing cardiac disease. The use of this agent during anaesthesia is not recommended by the manufacturer, and its use during anaesthesia is listed as contra-indicated. Midazolam minimally effects arterial blood pressure, and is recommended for animals with compromised cardiovascular function, but may decrease minute ventilation in humans and animals. Isoflurane is suitable for animals with compromised cardiovascular function, but results in a decrease in peripheral vascular resistance, similar to midazolam and propofol. The combined use of midazolam and isoflurane may therefore decrease systemic arterial blood pressure and possibly also minute ventilation. Investigations in anaesthetised horses indicated possible adverse cardiopulmonary effects such as hypoxaemia when clenbuterol was administered intravenously. The purpose of the investigation was to examine the effects of midazolam premedication on some cardiopulmonary variables after: (1) propofol induction at 2 different dose rates, and (2) intravenous clenbuterol in isofluraneanaesthetised bitches during endoscopic examination of the uterus and ovariohysterectomy.


INTRODUCTION
It has been suggested that the intraoperative administration of clenbuterol may facilitate the endoscopic examination of the canine uterus for diagnostic purposes.Clenbuterol is a selective $2-adrenergic agonist that is administered to induce bronchial dilation and myometrial relaxation.Possible adverse effects that may occur are systemic hypotension and cardiac arrhythmias when used in conjunction with inhalation anaesthetic agents, and in particular in animals with pre-existing cardiac disease 13 .The use of this agent during anaesthesia is not recommended by the manufacturer, and its use during anaesthesia is listed as contra-indicated.
Midazolam minimally effects arterial blood pressure, and is recommended for animals with compromised cardiovascular function, 9 but may decrease minute ventilation in humans 2 and animals 17 .Isoflurane is suitable for animals with compromised cardiovascular function 10 , but results in a decrease in peripheral vascular resistance 6 , similar to midazolam 3 and propofol 14 .The combined use of midazolam and isoflurane may therefore decrease systemic arterial blood pressure and possibly also minute ventilation.Investigations in anaesthetised horses indicated possible adverse cardiopulmonary effects such as hypoxaemia when clenbuterol was administered intravenously 5 .The purpose of the investigation was to examine the effects of midazolam premedication on some cardiopulmonary variables after: (1) propofol induction at 2 different dose rates, and (2)   intravenous clenbuterol in isofluraneanaesthetised bitches during endoscopic examination of the uterus and ovariohysterectomy.

MATERIALS AND METHODS
Eight healthy German shepherd bitches with a mean body mass of 26.9 kg, ranging between 25 and 31 kg, and 9-31 months old, were scheduled for endoscopic examination of the uterus, and ovariohysterectomy.The animals were randomly allocated to 2 groups of 4 each.A 20 G catheter (Jelco, Johnson & Johnson) was introduced percutaneously into the dorsal pedal artery for direct arterial blood pressure measurement, and collection of arterial blood for blood-gas analysis.A similar catheter was introduced into the cephalic vein for the administration of propofol and Ringer Lactate.The latter was administered at a rate of 10 m /kg/ min during surgery.Arterial blood samples were anaerobically collected in 3-m heparinised plastic syringes, stored in iced water, and analysed within 2 h of collection with a blood-gas analyser (ABL300 Radiometer).The samples were corrected for body temperature.Blood samples were collected before induction, after incremental propofol, and at the end of the endoscopic examination.
Group A was premedicated 2 min before induction with intravenous midazolam (Dormicum, Roche) at a dose of 0.2 mg/kg, and Group B treated with saline placebo before induction.The observer was blinded to the treatment administered.Both groups were induced with intravenous propofol (Diprivan, Zeneca) at 4 mg/kg, administered as a bolus over 60 sec.The trachea was intubated with a cuffed silicon endotracheal tube.Incremental propofol was added at a dose of 1 mg/kg until loss of the pedal reflex was achieved.Anaesthesia was maintained with a circle anaesthetic machine with carbon dioxide absorption, and an out-of-circuit precision isoflurane vaporiser (Fortec MK III, Cyprane).Fresh gas flow rate was set at 30 m /kg/min after induction of anaesthesia, and decreased to 10 m /kg/min when the pedal reflex disappeared.Ventilation was controlled after induction of anaesthesia with a mechanical ventilator (Penlon AV800 Intermed) with an inspiratory pressure of 20 cmH20, and an inspiratory:expiratory time ratio of 1:2.The ventilation rate was set at a rate of 20-30 breaths/min to maintain an end-tidal carbon dioxide concentration (ETC02) between 5 % and 6 %.The inspired isoflurane (Forane, Abbott) concentration was adjusted to the minimum concentration required to maintain absence of the pedal reflex during surgery.At the start of the abdominal skin incision, clenbuterol hydrochloride (Planipart, Janssen) was administered intravenously at a dose of 1.2 µg/kg.
Multiparameter physiological monitors were used to measure the cardiopulmonary variables.The Cardiocap II (Datex) was used to monitor the electrocardiogram, with a 3-limb lead and disposable electrodes attached to the limbs.Arterial blood pressure was measured with a calibrated strain gauge pressure transducer connected to the pedal artery.The Capnomac Ultima (Datex) with side-stream spirometer was attached to the endotracheal tube to measure tidal volume, minute volume, ventilation rate and ETC02 concentration.The cardiopulmonary variables were recorded at similar intervals to those between arterial blood collection during induction, and at 5 min intervals during inhalation maintenance of anaesthesia.The values were recorded on a standard anaesthetic monitoring sheet

Data analysis
Data are reported as mean (±SD).Twoway analysis of variance for repeated measures was used to analyse the data for treatment and time.When significant differences were found, the Tukey test was applied.Analysis of data was performed on a personal computer using SigmaStat (Jandel Scientific).Significance was accepted at P < 0.05.The investigation was approved by the Ethics and Research Committees of the University of Pretoria.

RESULTS
The intra-operative cardiopulmonary and blood-gas variables are reported in Table 1 as the mean and standard deviation (±SD).A non-significant decrease in the mean arterial blood pressure occurred in both groups after induction with propofol.In the midazolam group, the pressure was unchanged after the incremental dose of propofol; however, in the placebo group, the mean arterial pressure increased from 12.7 (2.0) to 16.4 (4.4) kPa during the same period (Fig. 1).The mean incremental dose for propofol to obtain loss of the pedal reflex was 3.4 and 5.5 mg/kg for the midazolam and control group respectively 18 .The administration of clenbuterol resulted in a decrease in the mean arterial pressure from 11.8 (2.1) to 8.7 (0.5) kPa, and 10.3 (1.3) to 7.7 (0.7) kPa respectively in the midazolam (26) and placebo (25 %) groups (Fig. 1).An increase in blood pressure was observed in both groups within 10 min of clenbuterol administration.The heart rate increased after induction with propofol and during maintenance with isoflurane in both groups.No cardiac arrhythmias were observed after clenbuterol administration during maintenance of anaesthesia.
The mean minute volume decreased significantly (P < 0.05) from 7.4 (1.8) to 1.7 (2.0) /min after incremental propofol administration to obtain loss of the pedal reflex in the midazolam group, but remained unaltered in the placebo group (Table 1

DISCUSSION
The purpose of this investigation was to examine the effects of midazolam premedication on some cardiopulmonary variables after propofol induction, and in clenbuterol-treated dogs during surgery and isoflurane maintenance of anaesthesia.Large variation in the variables of some parameters may have prevented statistically significant differences despite apparent large differences in mean values.
As the magnitude of the decrease in mean arterial blood pressure was the same in both groups (24 and 23 %) after induction with propofol at a dose of 4 mg/kg, it appeared that the blood pressure was minimally affected by midazolam.Minimal changes in blood pressure may occur after midazolam administration as result of peripheral vasodilation 3 .In addition, propofol decreases arterial blood pressure as result of venodilation 8 and myocardial depression 4 .During this investigation, the blood pressure increased in the placebo group after incremental propofol, compared to the pressure that remained unaltered in the midazolam group.The increase in blood pressure was probably the result of endotracheal intubation, as the dogs were intubated before the incremental propofol administration.This may indicate that midazolam prevented the reflex sympathetic increase in blood pressure associated with endotracheal intubation 18 .However, the heart rate remained unaltered in both groups between the propofol administrations.
The decrease in the arterial blood pressure observed in this investigation after clenbuterol administration (26 %), was similar to the decrease observed in halothane-anaesthetised horses (28 %) treated with clenbuterol 5,15 .In addition, isoflurane also causes peripheral vasodilation 6 , and could potentiate the hypotensive effects of midazolam and clenbuterol.Unfortunately, the design of this investigation did not allow comparison with a control group not treated with clenbuterol.The decrease in blood pressure was of short duration, as an increase in pressure was observed in both groups within 10 min after clenbuterol administration (Fig. 1).The mean arterial blood pressure values from dogs premedicated with acepromazine, induced and maintained with propofol during deep anaesthesia 16 , were similar (11.2) compared to the midazolam group (11.0 kPa) in this investigation.As healthy animals were used in this investigation, it may be assumed that the homeostatic mechanisms were minimally affected.However, the administration of clenbuterol to anaesthetised and cardiovascularly compromised animals during clinical treatment may result in intra-operative hypotension, and can therefore not be recommended.
Clenbuterol was administered during this investigation to facilitate surgical endoscopic examination of the uterus for possible future use as a clinical diagnostic aid.This agent is a selective $2-adrenergic agonist that relaxes smooth muscle, and therefore results in systemic vasodilation 13 .It may also have minimal $1-adrenergic activity that can cause tachycardia and ventricular arrhythmias 1,13 .Its administration during anaesthesia in horses 5,7,12,15 has been reported previously, but not its intravenous administration during anaesthesia in dogs.Intra-operative duodenal administration in dogs resulted in a 15 % decrease in arterial blood pressure 11 , and a decrease of 31 % in horses anaesthetised with intravenous agents 5 .In this investigation, it appeared that mean arterial blood pressure was only transiently decreased in midazolam-premedicated dogs after clenbuterol administration during isoflurane anaesthesia.
Midazolam in humans 2 and goats 17 decreases ventilation.The decrease in minute volume after incremental propofol in the midazolam group was statistically significant (P < 0.05), and resulted in apnoea in 2 dogs.As the ventilatory variables were not measured before induction, minute volume after incremental propofol administration was compared with a quasi-equipotent isoflurane dose at the start of skin suturing.As the duration of anaesthesia was approximately 80 min in both groups, it can be assumed that the effects of propofol were negligible at this stage.The differences in minute ventilation for both groups were statistically significant (P < 0.05), and therefore probably the result of propofol administration.The total dose of propofol administered in the midazolam group was 7.  9.5 mg/kg in the control group.The ventilatory depression from midazolam was apparent from the significant decrease in minute ventilation (Table 1) despite a decrease in the propofol dose.Values obtained for the PaCO2 and ETCO 2 concentrations remained within accepted physiological limits immediately after induction of anaesthesia, and were maintained at the same levels during maintenance of anaesthesia as result of positive-pressure ventilation.The hypoventilation and hypoxaemia observed in both treatment groups, and the apnoea that occurred in 50 % (2 of 4) of dogs in the midazolam-premedicated group, were clinically significant.Apnoea from propofol has only been reported after a deliberate overdose in dogs 16 .Midazolam premedication may therefore increase ventilatory depression as evidenced by the apnoea, and the statistically significant (P < 0.05) decrease in the PaO2 and oxyhaemoglobin saturation (Table 1).Hypoxaemia occurred after induction of anaesthesia as the PaO2 decreased to 6.6 and 5.2 kpa for the placebo and midazolam groups respectively.Increased inspired oxygen concentration and increased minute ventilation from controlled ventilation improved the oxygen tension during inhalation maintenance of anaesthesia in both groups.Aanaesthesia of dogs with propofol to a deep stable plane resulted in PaO2 tensions of 8.8 kPa 16 , higher than the tensions recorded in this investigation (6.6 kPa).Oxyhaemoglobin saturation decreased to 88 %, 16 higher than the values obtained (59.3 %) in this investigation.The PaCO2 was less in this investigation (5.4 kPa) compared to the dogs (6.7 kPa) 16 .In horses anaesthetised with intravenous agents, hypoxaemia was potentiated by clenbuterol through increased pulmonary shunting 5 .The high arterial oxygen tension (60.5 kPa) and oxyhaemoglobin saturation (99.9 %) observed after positive-pressure ventilation with isoflurane-in-oxygen may indicate that pulmonary shunting was not of clinical significance in this investigation.
It is concluded that adverse effects such as hypoventilation and hypoxaemia may occur after induction of anaesthesia with propofol.In addition, midazolam premedication may result in apnoea after propofol administration to obtain surgical anaesthesia (loss of the pedal reflex).To avoid apnoea, it is recommended that the dose of propofol be reduced during the co-administration of agents that may depress ventilation such as midazolam and propofol.Clenbuterol administration during isoflurane maintenance of anaesthesia resulted in a transient decrease in arterial blood pressure, and midazolam premedication significantly influenced the decrease in arterial blood pressure.

Fig 1 :
Fig 1: Mean (±SD) arterial blood pressure in dogs before and after propofol induction, and intravenous clenbuterol administration during isoflurane maintenance of anaesthesia for endoscopic examination and ovariohysterectomy.