Sage Journals: Discover world-class research

Abstract

In this study, the investigation of the intraoperative effects of dipyrone (metamizol) on heart rate (HR), mean arterial pressure (MAP) and analgesic efficacy in rabbits is described for the first time. This was carried out to evaluate the cardiovascular stability achieved using dipyrone compared with fentanyl. In this prospective study, 17 female New Zealand White rabbits were randomly allocated to either one of two groups: dipyrone/propofol (DP) or fentanyl/propofol (FP). Anaesthesia was induced in both groups using propofol to effect (4.0–8.0 mg/kg intravenously) until the swallowing reflex was lost for intubation. After induction, anaesthesia was maintained with continuous infusion of propofol 1.5–1.7 mg/kg/min intravenously. Analgesics were then injected in defined boluses of either dipyrone 65 mg/kg or fentanyl 0.0053 mg/kg. After surgical tolerance, defined as loss of the ear pinch reflex and loss of the anterior and posterior pedal withdrawal reflex, was achieved, two surgical procedures were performed. The surgical procedures (implantation of either a pacemaker or an electrocardiogram transmitter), both require a comparable level of analgesic depth. During and after surgery, clinical variables, such as MAP, HR, peripheral arterial oxygen saturation (SpO₂) and end-tidal CO₂ (Pe′CO₂) were recorded simultaneously every 2 min. Eight time points were chosen for comparison: baseline, surgical tolerance (ST), values at 10, 20 and 30 min after reaching ST, values at the end of propofol infusion (EI) and data at 10 and 20 min after EI. Both FP and DP combinations provided effective anaesthesia and analgesia in rabbits. In both groups a significant decrease of HR and MAP was measured. The results of this study indicate that the non-opioid drug dipyrone produces similar analgesic and even better cardiovascular effects by trend in rabbits. Therefore we conclude that dipyrone in combination with propofol can be used as an alternative to FP for intraoperative analgesia.

Keywords

Dipyrone metamizol fentanyl analgesia cardiovascular rabbits propofol anaesthesia

Propofol is a short-acting hypnotic and has been used in rabbits for short-lasting anaesthesia as well as for the induction or maintenance of general anaesthesia but does not provide a clinically relevant degree of analgesia.^1,2 Due to its short half-life and the fact that propofol has only slight cumulative effects, anaesthesia with propofol is easy to control.^3,4 But according to Mayer et al. ⁵ propofol produces a reduction in both cardiac index and mean arterial pressure (MAP).^6,7

Fentanyl is a highly potent, centrally acting, morphine-like analgesic, widely used in anaesthesia,^8,9 and particularly in combination with propofol.² The rapid onset and short duration of its effects have been regarded as particularly advantageous.^10–12 However, Hess et al. ¹³ showed that the analgesic effect of fentanyl (0.01 mg/kg) is accompanied by a delayed decrease in blood pressure, which is due to a reduction in total peripheral resistance and a decrease in cardiac output. This also corresponds to Mayer et al.,⁵ who suggested an accumulation of the cardio-depressant action when used in the combination of propofol and fentanyl in humans. Accordingly, Liehmann et al. ¹¹ suggested that fentanyl might contribute to hypotension in propofol anaesthetized cats.

Dipyrone (metamizol), a potent also centrally acting non-opioid analgesic and antipyretic agent with an additional spasmolytic effect, is widely used in humans to provide postoperative pain relief on its own and in combination with opioids.¹⁴ The primary advantage of non-opioid analgesic agents is related to the absence of respiratory depression and major side-effects on the central nervous system.^15,16 Accordingly, results from clinical studies of postoperative pain in man indicate that intravenous infusion of dipyrone produces marked postoperative pain relief, but is not associated with a clinically relevant impairment in haemodynamic function.¹⁵ Although it is not available in all countries (e.g. in the USA or Sweden), there is a wide experience in the use of dipyrone, particularly in the management of postoperative pain in animals.¹⁶ However, the cardiovascular properties of dipyrone in rabbits, particularly when used as the sole analgesic in combination with propofol infusion, have not been studied in detail. Therefore, the study should provide a comparative description of the clinical alterations that occur during anaesthesia with the combination of fentanyl/propofol (FP) and dipyrone/propofol (DP) in patients undergoing pacemaker and electrocardiogram (ECG) transmitter implantation. These procedures can be compared by having equivalent analgesic need. Further on, the study should contribute to improve and refine the knowledge about dipyrone, particularly to clarify the intraoperative analgesic efficacy in rabbits.

Material and methods

Animals

Experiments were approved by the local animal care committee and were in accordance with the German Animal Welfare Act.

Seventeen healthy female rabbits of the New Zealand White strain, obtained from a colony free of respiratory pathogens (NZW; Asamhof, Bad Kissingen, Germany), were used in this study. Rabbits were randomly allocated to one of two groups: DP, n = 9, or FP, n = 8. The mean body mass (mean ± SD) was 2.85 ± 0.2 kg in the DP group and 3.06 ± 0.4 kg in the FP group and the animals' age ranged between 10 and 16 weeks. They were housed individually in cages on dust-free wooden shavings. Mean room temperature was maintained at 19 ± 2°C, relative humidity was maintained between 50% and 60% and a light cycle (12 h of light and 12 h of darkness) was induced. Rabbits were fed a commercial pelleted diet (rabbit maintenance diet #2123, pellets 4.5 mm, Altromin, Lage, Germany); autoclaved hay and water were available ad libitum. As enrichment factors, the animals were supplied with commercially available compressed hay barrels, oblong pure wood for gnawing and play dumb-bells (Sniff, Soest, Germany). All animals were allowed to acclimatize to their environment for at least seven days before the onset of the experiment. Due to species-specific risk of hypoglycaemia and decreasing intestinal motility, rabbits were not starved prior to anaesthesia.

Anaesthesia

On the day of the experiment, each rabbit was weighed and clinically examined for behaviour, respiration and cardiovascular variables. Experiments were conducted between 09:00 and 12:00 h. A local anaesthetic (lidocaine/prilocaine, EMLA^® cream; AstraZeneca GmbH, Wedel, Germany) was put on the skin of the left ear of each rabbit, and a catheter (Vasofix^®; 20 SWG [1.1 × 33 mm], B Braun Melsungen AG, Melsungen, Germany) was inserted in the median auricular artery for arterial blood pressure measurements and another catheter (Vasofix^®; 22 SWG [0.9 × 25 mm], B Braun Melsungen AG) was inserted in the lateral auricular vein. Anaesthesia was induced by intravenous administration of propofol 1% (Propofol; Fresenius Kabi, Bad Homburg, Germany) to effect (4.0–8.0 mg/kg) until the swallowing reflex was lost and the trachea could be intubated (inner diameter of endotracheal tube, 2.5–3.0 mm) without direct laryngeal observation (blind technique¹⁷). Each rabbit was shaved in preparation for the surgical procedure and body temperature was maintained during the whole intervention between 37 and 38°C by a heating pad. All rabbits received a continuous intravenous infusion of Ringer's lactate solution at 10 mL/kg/h during the operation. Anaesthesia was maintained with a continuous infusion of propofol 2% (Propofol; Fresenius Kabi) 1.5–1.7 mg/kg/min intravenously. After that induction of state of hypnosis/immobilization by propofol, analgesics had to be added before painful surgical interventions could be started. Therefore, either dipyrone (Novaminsulfon-ratiopharm^® 2.5, Ratiopharm GmbH, Ulm, Germany) or fentanyl (Fentanyl^® B Braun 0.5 mg, B Braun Melsungen AG) was given as titrating bolus injections (dipyrone: 65 mg/kg per bolus; fentanyl: 0.0053 mg/kg per bolus) until the ear pinch reflex and anterior and posterior pedal withdrawal reflexes were lost. To investigate the drug dosages for reaching surgical tolerance (ST), both dipyrone and fentanyl were injected as smaller single bolus applications (DP 65 mg/kg per bolus and FP 0.0053 mg/kg per bolus).

The bolus doses were determined in the case of fentanyl as one-third of a published dosage of fentanyl 0.01–0.02 mg/kg. Regarding fentanyl, the decrease in blood pressure, that could possibly be caused by the analgesic, should be diminished by titrating the total dose (0.01–0.02 mg/kg) over several bolus injections (0.0053 mg/kg).¹⁸

In case of dipyrone no total dosage was known in rabbits for the sole intraoperative use. Therefore, we took the recommended postoperative dose for one bolus (65 mg/kg), assuming that a higher dose in total is required for acute intraoperative pain.¹⁶

Regarding the different pharmacokinetic profile of the analgesics (e.g. approximate start of action), the time between the bolus injections was 4 min (fentanyl) and 8 min (dipyrone), respectively. After each bolus application, reflexes were always tested by one person; the fore and hind limb pedal withdrawal reflexes and the ear pinch reflex were tested by firmly pinching the ear base and the skin between the toes and the pinna, respectively, with fingernails to define the drug dosage inducing ST.^19,20

The injection volume per bolus was adjusted with 0.9% sodium chloride solution to 0.6 mL and the injection speed was 20 s per bolus. During surgical anaesthesia, status of hypnosis and analgesia were judged on the basis of autonomic nervous reactions (heart rate [HR] or blood pressure increase) to surgical stimulation, the degree of muscle relaxation and status of reflexes (ear pinch, anterior and posterior pedal withdrawal reflexes, palpebral reflex). If for example strong palpebral reflex or central rotation of the eyeball occurred at any time during anaesthesia, hypnosis was considered inadequate and further aliquots of propofol (4 mg/kg) were injected. Furthermore, the efficiency of analgesia induced by fentanyl or dipyrone was continuously controlled during surgery by monitoring HR, blood pressure and status of reflexes. In order to induce a fast recovery of the animals, the infusion of propofol was continuously decreased at the end of surgical intervention (only after time point: 30 min after reaching ST).

Rabbits were ventilated (Anaesthesia Workstation; Hallowell EMC, Pittsfield, MA, USA) with 100% oxygen at a rate of 30 breaths per minute, peak ventilation pressure of 10 cmH₂O and an average tidal volume of 10 mL/kg. Monitoring of MAP, HR, end-tidal CO₂ (Pe′CO₂) and peripheral arterial oxygen saturation (SpO₂) was conducted by use of a patient monitor (Datex Ohmeda S/5, Type F-CM1.00, Helsinki, Finland, pressure transducers: Hellige Type 4-327-I).

Data were collected every 2 min. For comparison of DP and FP groups, eight time points were chosen: baseline (before the first bolus application of the analgesics, HR, MAP, SpO₂, Pe′CO₂ with no obvious fluctuation [±5% baseline for 5 min]), ST (when no reaction to incision of the skin could be elicited, after the absence of ear pinch reflex and anterior and posterior pedal withdrawal reflexes), values at 10, 20 and 30 min after reaching ST, values at the end of propofol infusion (EI) and data at 10 and 20 min after EI (10/20 min after EI, only HR and MAP). After the end of propofol infusion additionally the time (minutes) until the recovery of spontaneous breathing (termination of artificial ventilation) and until extubation (recovery of swallowing reflex) were recorded in all animals.

Experimental protocol

The evaluation of the analgesics was carried out during two different surgical interventions, either pacemaker (n = 10: DP, 6 animals; FP, 4 animals) or ECG transmitter implantation (n = 7: DP, 3 animals; FP, 4 animals). The two interventions were part of studies carried out within cardiovascular research. However, as the animals only received the test drugs of these studies after total recovery from pacemaker or ECG transmitter implantation, there was an opportunity to investigate the hypothesis of the present study without the need for additional animal use. This was approved by the local animal care committee and was in accordance with the German Animal Welfare Act.

The surgery time for both interventions was approximately 30 min and surgery was performed by two different experienced surgeons.

For pacemaker implantation, the right jugular vein was dissected free from surrounding tissue after midline incision of the skin at the ventral neck. Thereafter, a central venous catheter (CVC; Cavafix^®, 1.1 × 1.7 mm/16G, REF 4173589, B Braun Melsungen AG) was placed into the vessel. Under X-ray guidance, the CVC was placed into the right ventricle. Then, a pacemaker probe (Lead 2F; Medtronic, Duesseldorf, Germany) was implanted into the right ventricle using the CVC as a guidance catheter. Afterwards, the pacemaker (Vita 2 DDD model 710; Vitatron, Cologne, Germany) was positioned under the skin muscle in the ventral region of the abdomen and connected to the pacemaker probe, which was placed subcutaneously (SC) between the right jugular vein and the device. Except to a short test period (10 s) HR was not influenced by the pacemaker during surgery as the device was deactivated after implantation. For ECG transmitter (TA 11 CA-F40, Transoma Medical, St Paul, MN, USA) implantation, a pocket was prepared under the skin muscle caudal to the left costal arch. The negative transmitter electrode was positioned via a hollow needle and fixed SC in the right cranial sternum area, and the positive electrode was implanted dorsal and caudal to the left costal arch.

For postoperative analgesia rabbits were given buprenorphine (Temgesic^®, Essex Pharma, Munich, Germany) 0.01 mg/kg SC once and carprofen (Rimadyl^®, Pfizer GmbH, Karlsruhe, Germany) 4 mg/kg SC on three days postoperatively as well as sulphadoxine/trimethoprim (Borgal^® 24%, Intervet Deutschland GmbH) 10 mg/kg for antibiosis.

Statistical evaluation

Descriptive statistics (mean ± SD) were calculated for normally distributed data. Where the normal assumption was considerably violated, we report median and interquartile range (from the 25th to 75th percentile).

Two-way analysis of variance (ANOVA for repeated measurements) was employed to investigate within-subjects effect (time trend), between-subjects effect (difference between the DP and the FP group) and related interaction effects. For statistical compatibility in the ANOVA, values of Pe′CO₂ had to be transformed (normalized) by natural logarithm (ln). Due to the lack of normality of SpO₂, the Friedman test was used to analyse changes in time in this parameter. Group comparisons regarding SpO₂ were made by the generalized estimation equation approach, comparing the odds of having full saturation (SpO₂ = 100%) between treatment groups in a logistic (logit) model framework. Statistical comparisons were performed two-sided at a 0.05 level of significance. No correction of alpha error level was conducted. All statistical analyses were conducted with commercially available software (SPSS^®, version 15.0, SPSS Inc, Chicago, IL, USA).

Results

No significant differences were identified between the animals' body masses in group DP (2.85 ± 0.2 kg) and FP (3.06 ± 0.4 kg).

In all animals, propofol produced a rapid and smooth induction of anaesthesia. Furthermore, a continuous infusion of the anaesthetic induced adequate hypnosis and relaxation for the operations. Additional aliquots of propofol to adjust the individual hypnotic state during anaesthesia were similar in both groups; total mean dosage of further propofol aliquots was 8.5 ± 7.1 mg/kg in the DP group versus 8.0 ± 4.2 mg/kg in the FP group.

Before painful surgical interventions in propofol immobilized rabbits started, analgesics (dipyrone or fentanyl) were injected using titrating boluses of the drugs until ST was reached (indicated by the loss of pedal withdrawal and ear pinch reflexes). Status of reflexes was determined after each titrating dipyrone or fentanyl bolus injection (see Table 1). After the first dipyrone bolus, ear pinch and pedal withdrawal reflexes could still be elicited in all animals. Only after the third dipyrone bolus were the pedal withdrawal reflexes absent in all animals. In one animal the ear pinch reflex was still present after the third dipyrone bolus. Hence, this animal received a fourth dipyrone bolus injection. The mean dipyrone dosage for the loss of anterior and posterior pedal withdrawal reflexes in all animals was 151.7 ± 32.5 mg/kg (2–3 bolus injections of dipyrone 65 mg/kg). For the loss of ear pinch reflex in all animals 158.9 ± 34.3 mg/kg of dipyrone had to be used (2–4 bolus injections of dipyrone 65 mg/kg).

Table 1
Status of reflexes after the different dipyrone (65 mg/kg) and fentanyl (0.0053 mg/kg) bolus injections in propofol anaesthetized rabbits for induction of surgical tolerance

Group Bolus no. Ear pinch reflex Anterior pedal withdrawal reflex Posterior pedal withdrawal reflex

DP 1 +(9/9) +(9/9) +(9/9)

DP 2 +(4/9) +(3/9) +(3/9)

DP 3 +(1/9) − −

DP 4 − − −

FP 1 +(8/8) +(4/8) +(4/8)

FP 2 +(5/8) +(2/8) +(1/8)

FP 3 − − −

DP: dipyrone and propofol group; FP: fentanyl and propofol group; (x/n): number of animals per group

+ Indicates reflex elicitable after intravenous drug bolus injection

− Indicates reflex not elicitable after intravenous drug bolus injection

The mean dipyrone dosage of 158.9 ± 34.3 mg/kg did induce sufficient analgesia (ST) during surgery (efficiency of analgesia was continuously controlled during surgery by monitoring HR, blood pressure and status of reflexes); therefore animals did not need further dipyrone injections during intervention (duration: approximately 30 min).

By using fentanyl for induction of intraoperative analgesia, 50% of the animals lost anterior and posterior pedal withdrawal reflexes after the first bolus injection, but the absence of ear pinch and pedal withdrawal reflexes in all animals could only be reached after injection of the third bolus. In the fentanyl group, anterior and posterior pedal withdrawal reflexes were lost using a mean dosage of fentanyl 0.0093 ± 0.0048 mg/kg (1–2 single bolus injections of fentanyl 0.0053 mg/kg). The ear pinch reflex was lost in all animals of the fentanyl group using a mean dosage of 0.014 ± 0.002 mg/kg of fentanyl (2–3 single bolus injections of fentanyl 0.0053 mg/kg). The mean fentanyl dosage of 0.014 ± 0.002 mg/kg did induce sufficient analgesia (ST) during surgery; therefore, animals did not need further fentanyl injections during intervention (duration: approximately 30 min).

Clinical haemodynamic parameters

During surgical intervention, HR, MAP, SpO₂ and Pe′CO₂ were simultaneously recorded every 2 min. Data were determined for baseline, after reaching ST, at 10, 20 and 30 min during ST, at the end of propofol infusion (EI) and at 10 and 20 min after EI (see Table 2). Baseline values of HR, MAP, SpO₂ and Pe′CO₂ did not significantly differ between the two groups. After application of the analgesics for induction of ST, values of HR and MAP were significantly decreased in both groups compared with baseline. However, comparing HR and MAP of both groups, no significant difference was measured between DP and FP anaesthesia. During the early recovery, at 10 and 20 min after EI, MAP was re-elevated in both groups (significant quadratic effect). The SpO₂ values did not significantly differ between groups. Furthermore, Pe′CO₂ was elevated in the DP and the FP group (significant linear effect in the DP group). Due to recovery of spontaneous breathing, artificial ventilation could be terminated at 4.7 ± 4.5 min after EI in the DP group and at 6.3 ± 5.1 min (mean ± SD) in the FP group. Thereafter, animals of the DP group were extubated at 12.9 ± 8.8 min and of the FP group at 11.9 ± 6.3 min after EI.

Table 2
Intraoperative clinical variables of dipyrone/propofol (n = 9) and fentanyl/propofol (n = 8) anaesthesia

Mean ± SD or median (IQR: 25th–75th percentile)

Group Time point HR* (bpm) MAP* (mmHg) SpO₂ (%) P_E′CO₂ ^,† (kPa)

DP Baseline 217 ± 25 69 ± 13 99 (98–100) 4.93 (4.93–5.07)

DP ST 183 ± 17 50 ± 15 100 (98–100) 5.07 (4.8–5.13)

DP 10 min after ST 182 ± 32 53 ± 27 100 (98–100) 5.07 (4.87–5.13)

DP 20 min after ST 180 ± 31 47 ± 24 100 (98–100) 5.07 (4.93–5.20)

DP 30 min after ST 188 ± 18 55 ± 23 98 (96–100) 4.93 (4.93–5.20)

DP EI 193 ± 14 67 ± 20 99 (96–100) 5.20 (5.20–5.33)

DP 10 min after EI 221 ± 14 80 ± 13 99 (94–100) −

DP 20 min after EI 238 ± 17 87 ± 11 − −

P value overall comparison: Within subjects effect (time) <0.001^‡ 0.010^‡ 0.178^§ 0.009

FP Baseline 239 ± 25 71 ± 10 99 (99–100) 5.07 (4.97–5.30)

FP ST 178 ± 15 32 ± 9 98 (98–99) 5.07 (5.07–5.17)

FP 10 min after ST 179 ± 25 35 ± 9 98 (97–99) 5.13 (4.97–5.40)

FP 20 min after ST 193 ± 24 45 ± 16 98 (98–100) 5.07 (5.07–5.20)

FP 30 min after ST 195 ± 24 48 ± 16 99 (97–99) 5.07 (4.83–5.20)

FP EI 191 ± 23 57 ± 21 99 (99–100) 5.27 (5.10–5.43)

FP 10 min after EI 219 ± 24 77 ± 19 98 (98–99) −

FP 20 min after EI 244 ± 23 83 ± 15 − −

P value overall comparison: Within-subjects effect (time) <0.001^‡ 0.003^‡ 0.312^§ 0.362

DP versus FP Between-subjects effect (group) 0.550 0.185 0.294^†† 0.067

DP versus FP Interaction (time × group) 0.527 0.532 0.101^†† 0.169

DP: dipyrone and propofol group; FP: fentanyl and propofol group; ST: surgical tolerance; EI: end of infusion; HR: heart rate; MAP: mean arterial pressure; SpO₂: arterial oxygen saturation; P_E′CO₂: end-tidal carbon dioxide

ANOVA: ^†normalized by ln-transformation; ^‡transient (quadratic) effect; ^§Friedman test; **linear (monotonous) effect; ^††GEE (generalized estimation equation: logit model); SpO₂-values were summarized to: 1 if SpO₂ = 100% and 0 if SpO₂ < 100%

− Indicates no measurement at this time point

Discussion

In the present study, we investigated the analgesic efficacy and clinical cardiovascular stability of DP and FP in rabbits undergoing pacemaker or ECG transmitter implantation.

Non-opioid analgesics play an important role in the treatment of postoperative pain; in mild and moderate pain they can be the only analgesic utilized and thereby provide the chance to avoid adverse effects of opioids completely. More importantly, in severe pain they are a relevant component of multimodal analgesia. In such a setting they are not only opioid-sparing, but can also improve analgesia and carry the potential to reduce adverse effects of opioids.¹⁶ Dipyrone (metamizol) is a water-soluble pyrazolone derivate available in oral, rectal and injectable forms. Since its introduction in 1922 it has been recognized as an effective analgesic, antipyretic, antispasmodic and slight anti-inflammatory drug. It is indicated for severe pain and, particularly, for pain associated with smooth muscle spasm or colic affecting the gastrointestinal, biliary and urinary tracts. The drug is widely used in some countries, while in others (i.e. the USA and Sweden) it has been banned or restricted because of the risk of adverse reactions in humans, notably agranulocytosis, which has not been observed in animals so far.^21,22 The pharmacological mechanism of action of dipyrone has remained uncertain. There is the assumption that it might be an inhibitor of the cyclooxygenase (COX) isoenzymes, particularly of the COX-3 isoenzyme, thereby reducing prostaglandin synthesis in the dorsal horn of the spinal cord.²³ Shimada et al. ²⁴ suggest that dipyrone treatment not only inhibits prostaglandin synthesis centrally but it can inhibit prostaglandin synthesis outside the blood–brain barrier as well. However, it has been reported to exert a greater inhibitory effect on cyclooxygenases from the central nervous system than on those from other tissues.²⁵ Furthermore, the effect of dipyrone has been associated with the endogenous opioid system, because its antinociceptive effect can be blocked by naloxone.²⁶ Accordingly, it was shown that dipyrone is able to induce a significant antinociceptive effect in the absence of an anti-inflammatory response. Therefore, an important participation of central mechanisms in the analgesic action of the drug has been suggested.²⁷

Based on this knowledge, we stated the hypothesis that dipyrone (metamizol) could be an intraoperative analgesic alternative for opioids in patients with low cardiovascular reserve because it could produce better cardiovascular stability (mainly blood pressure values) than fentanyl. In dogs undergoing hip replacement, the use of dipyrone had a marked opioid-sparing or even opioid-replacing effect.²⁸

In the current study a continuous infusion of propofol 1.5–1.7 mg/kg/min intravenously combined with a mean total dosage of dipyrone 158.9 ± 34.3 mg/kg intravenously produced sufficient analgesia for ST (absence of the ear pinch and pedal withdrawal reflexes, no reaction to skin incision, no increase of HR and blood pressure) without the need of additional opioid rescue analgesics. Traditional reflexes used in the monitoring of rabbit anaesthesia include righting, palpebral, corneal, pedal withdrawal and ear pinch reflex (ear movement in response to a compressive force).¹⁷ The latter is the most accurate measure of depth of anaesthesia, followed by the pedal withdrawal, corneal and palpebral reflexes.²⁹ Hence, ear pinch and pedal withdrawal reflexes were investigated in combination with HR and blood pressure reaction in the present study to determine ST. The required dosage in the present study was higher compared with those described by Erhardt et al. ¹: dipyrone 25–100 mg/kg. However, it has to be considered that particularly the pacemaker and the ECG transmitter interventions started at the ventral neck area, which is highly sensitive to surgical stimuli in rabbits. Therefore, intraoperative analgesia had to be highly efficient, explaining the difference in dosing comparing the two studies. Richter et al. ²⁸ utilized 75 mg/kg in dogs undergoing hip replacement for intraoperative analgesia, but 45% (10 of 22 dogs) of the animals required additional fentanyl in that study. Furthermore, Clemm³⁰ successfully used 40 mg/kg of dipyrone for intraoperative analgesia in rabbits for male castrations, but this was in combination with ketamine–medetomidine anaesthesia which already has analgesic properties itself. However, there is not much known about the use of dipyrone as a sole intraoperative analgesic in rabbits. Accordingly, there is nothing known about a cumulative effect or adverse reactions in rabbits after using higher concentrations than 100 mg/kg (LD50 in mouse and rat 4160/4300 mg/kg).³¹ In our study, the dose of dipyrone and the mode of application were well tolerated, inducing neither acute nor deferred occurring adverse reaction during anaesthesia and during recovery period in the animals. Hence, according to Hoigné et al. ³² and Erhardt et al.,¹⁶ very fast parenteral administration can be accompanied by sudden hypotension in rare cases, induced by a decrease of vascular resistance. Therefore, a low injection speed is recommended for intravenous application of the drug.¹⁶ In comparison to dipyrone, a mean total fentanyl dosage of 0.014 ± 0.02 mg/kg (2–3 bolus injections of fentanyl 0.0053 mg/kg) was equipotent to induce ST in propofol anaesthetized rabbits in the current study. This corresponds to the recommended total dosage of 0.01–0.02 mg/kg published by Haberstroh and Henke² inducing sufficient analgesia for approximately 30 min.⁹

In the current study, a significant decrease in HR was measured at reaching ST and during the period of ST in both groups. Deutschman et al. ⁷ demonstrated that propofol anaesthesia reduces parasympathetic tone to a lesser degree than sympathetic tone, and may predispose to bradycardia and bradyarrhythmia during parasympathomimetic stimulation. Furthermore, propofol induces a significant and sustained resetting of baroreflex set point, allowing unchanged HR at lower arterial pressure.³³ According to Egan et al. ³⁴ and Liehmann et al. ¹¹ fentanyl might contribute to bradycardia, whereas a centrally mediated increase in parasympathetic tone, direct negative chronotropic action at the sinus node, potentiation of vagally released acetylcholine at the sinus node and reduction in sympathetic activity implicated for several opioids. However, observed alterations during ST and during recovery after EI (10 and 20 min) were similar in both study groups, suggesting a high hypnotic-related impact on HR during anaesthesia. MAP was better maintained at all time points in the DP group compared with the FP group but changes were not significantly different, indicating only a slight improvement of haemodynamic stability during propofol anaesthesia. Regarding the FP combination, these findings are in accordance to Hess et al.,¹³ who showed that the analgesic effect of fentanyl (0.01 mg/kg) is accompanied by a delayed decrease of blood pressure, which has been related to a reduction in total peripheral resistance and a decrease in cardiac output. Therefore, the dose of fentanyl might contribute to propofol-related hypotension.^5,11 Particularly during ST the measured MAP values were low, especially in the FP group. However, all animals showed a fast and complete recovery after anaesthesia. According to Erguen et al. ³⁵ dipyrone might also exert a hypotensive effect, possibly induced by a smooth muscle relaxing effect which is produced by one of dipyrone's degradation products. As the respiratory rate and tidal volume were held constant during anaesthesia, collected Pe′CO₂ data showed an increase of the parameter in both groups. Due to lower baseline data this increase was significantly different in the DP group. However, the values stayed within physiological limits (4.6–6 kPa).² Possible reasons for increases in Pe′CO₂ are insufficient ventilation or decrease of anaesthetic depth. The continuous reduction of propofol infusion rate at the end of surgical intervention (only after time point: 30 min after reaching ST) which was similarly performed in both groups, in order to induce fast recovery of the animals and the associated elevation of HR and MAP, might be the reason for Pe′CO₂ increase.

The analgesic activity of fentanyl in rabbits after intravenous application is terminated at about 30 min, whereas both redistribution and metabolism might be regarded as primarily responsible for its limited duration of action.⁹ Due to the short duration of surgical intervention (approximately 30 min) and as the animals showed no significant signs of insufficient analgesia (e.g. HR or MAP elevation more than 20% over baseline, movements, increase of muscle tonus), neither in the DP nor in the FP group additional injections of the analgesics were administered during the intervention.^11,36 Pe′CO₂ values were not compared during recovery as artificial ventilation could be terminated at 4.7 ± 4.5 min after EI in the DP group and at 6.3 ± 5.1 min (mean ± SD) in the FP group and as animals were extubated (recovery of swallowing reflex) in the DP group at 12.9 ± 8.8 min and in the FP group at 11.9 ± 6.3 min (mean ± SD). Regarding HR and MAP, both groups showed a similar elevation of the parameters after end of propofol infusion. SpO₂ values stayed within the range of physiological data (>95%) at 10 min after end of propofol infusion with no significant difference between the groups. Values at 20 min after EI were not compared as most of the animals did not tolerate the pulse oxymeter probe anymore. HR and MAP continuously increased at 10 and 20 min after end of propofol infusion, similarly in both groups. This confirms data of Cockshott³⁷ and Beller et al.,³⁸ who showed that propofol has little or no cumulative effects inducing a fast recovery after anaesthesia. Furthermore, the short duration of action and rapid elimination of fentanyl might be regarded advantageous for a fast recovery of haemodynamic parameters.¹⁰ In contrast, the analgesia induced by dipyrone lasts approximately for 4–6 h.¹⁶ In accordance to Avellaneda et al. ¹⁵ our study confirms that this effect is not associated with a significant impairment and re-elevation of haemodynamic parameters during early recovery of the animals immediately after anaesthesia.

The principal limitation of the study stems from the fact that neither flow nor loading conditions of the heart were investigated. A further limitation is the fact that the used rabbits were only 10–16 weeks old. Therefore, regarding the difference in pharmacokinetic variables between premature and mature animals, results need to be confirmed in mature animals for generalization.

Furthermore, postoperative data collection including different pain scores should be performed in further studies, as the objective of this study was to evaluate the unknown intraoperative opioid replacing use of dipyrone.

In summary, as the animals showed no signs of evident pain during surgical intervention, dipyrone also provided effective intraoperative analgesia. In both groups a significant decrease of HR and MAP was measured, but comparing the arterial blood pressure and SpO₂ data of the DP group to those of the FP group, values were better maintained in group DP by trend. Additionally, the early recovery of the clinical haemodynamic parameters after propofol infusion was similar in both study groups.

In conclusion, the study indicates that the non-opioid drug dipyrone produced similar analgesic and even better cardiovascular effects by trend in rabbits and can be used alternatively to fentanyl for intraoperative analgesia in combination with propofol.

Group	Bolus no.	Ear pinch reflex	Anterior pedal withdrawal reflex	Posterior pedal withdrawal reflex
DP	1	+(9/9)	+(9/9)	+(9/9)
DP	2	+(4/9)	+(3/9)	+(3/9)
DP	3	+(1/9)	−	−
DP	4	−	−	−
FP	1	+(8/8)	+(4/8)	+(4/8)
FP	2	+(5/8)	+(2/8)	+(1/8)
FP	3	−	−	−

	Mean ± SD or median (IQR: 25th–75th percentile)
DP	Baseline	217 ± 25	69 ± 13	99 (98–100)	4.93 (4.93–5.07)
DP	ST	183 ± 17	50 ± 15	100 (98–100)	5.07 (4.8–5.13)
DP	10 min after ST	182 ± 32	53 ± 27	100 (98–100)	5.07 (4.87–5.13)
DP	20 min after ST	180 ± 31	47 ± 24	100 (98–100)	5.07 (4.93–5.20)
DP	30 min after ST	188 ± 18	55 ± 23	98 (96–100)	4.93 (4.93–5.20)
DP	EI	193 ± 14	67 ± 20	99 (96–100)	5.20 (5.20–5.33)
DP	10 min after EI	221 ± 14	80 ± 13	99 (94–100)	−
DP	20 min after EI	238 ± 17	87 ± 11	−	−
P value overall comparison:	Within subjects effect (time)	<0.001^‡	0.010^‡	0.178^§	0.009**
FP	Baseline	239 ± 25	71 ± 10	99 (99–100)	5.07 (4.97–5.30)
FP	ST	178 ± 15	32 ± 9	98 (98–99)	5.07 (5.07–5.17)
FP	10 min after ST	179 ± 25	35 ± 9	98 (97–99)	5.13 (4.97–5.40)
FP	20 min after ST	193 ± 24	45 ± 16	98 (98–100)	5.07 (5.07–5.20)
FP	30 min after ST	195 ± 24	48 ± 16	99 (97–99)	5.07 (4.83–5.20)
FP	EI	191 ± 23	57 ± 21	99 (99–100)	5.27 (5.10–5.43)
FP	10 min after EI	219 ± 24	77 ± 19	98 (98–99)	−
FP	20 min after EI	244 ± 23	83 ± 15	−	−
P value overall comparison:	Within-subjects effect (time)	<0.001^‡	0.003^‡	0.312^§	0.362**
DP versus FP	Between-subjects effect (group)	0.550	0.185	0.294^††	0.067
DP versus FP	Interaction (time × group)	0.527	0.532	0.101^††	0.169

References

Erhardt

, Henke

, Kroker

. Allgemeinanaesthetika. In: Erhardt

, Henke

, Haberstroh

, eds. Anaesthesie & Analgesie beim Klein- und Heimtier. Stuttgart: Schattauer, 2004:31–62

Haberstroh

, Henke

. Kaninchen. In: Erhardt

, Henke

, Haberstroh

, eds. Anaesthesie & Analgesie beim Klein- und Heimtier. Stuttgart: Schattauer, 2004:629–40

Smith

, White

, Nathanson

, Gouldson

. Propofol. An update on its clinical use. Anesthesiology 1994;81:1005–43

Fish

. Pharmacology of injectable anesthetics. In: Kohn

, Wixson

, White

, Benson

, eds. Anaesthesia and Analgesia in Laboratory Animals. London: Academic Press, 1997:8–9

Mayer

, Ochmann

, Doenicke

, Angster

, Suttmann

. Einfluss einer propofol-ketamin-narkose auf kreislaufverhalten und analgesie im vergleich mit propofol-fentanyl. Anaesthesist 1990;39:609–16

Aeschbacher

, Webb

. Prolonged anaesthesia with propofol in rabbits. Vet Surg 1992;21:159

Deutschman

, Harris

, Fleisher

. Changes in heart rate variability under propofol anaesthesia: a possible explanation for propofol-induced bradycardia. Anesth Analg 1994;79:373–7

Van Wijngaarden

, Soudijn

. The metabolism of the analgesic fentanyl (R 4263) by Wistar rats. Life Sci 1968;7:1239–44

Hess

, Herz

, Friedel

. Pharmacokinetics of fentanyl in rabbits in view of the importance for limiting the effect. J Pharmacol Exp Ther 1971;179:474–84

10.

Hess

, Stiebler

, Herz

. Pharmacokinetics of fentanyl in man and the rabbits. Eur J Clin Pharmacol 1972;4:137–41

11.

Liehmann

, Mosing

, Auer

. A comparison of cardiorespiratory variables during isoflurane-fentanyl and propofol-fentanyl anaesthesia for surgery in injured cats. Vet Anaesth Analg 2006;33:158–68

12.

Mastronardi

, Cafiero

. Rational use of opioids. Minerva Anesthesiol 2001;67:332–7

13.

Hess

, Tarnow

, Patschke

, Passian

, Brueckner

. Die wirkung von fentanyl und althesin auf die haemodynamik, die herzinotopie und den myokardialen sauerstoffverbrauch des menschen. Anaesthesist 1976;25:10–18

14.

Schug

, Manopas

. Update on the role of non-opioids for postoperative pain treatment. Best Pract Res Clin Anaesthesiol 2007;21:15–30

15.

Avellaneda

, Gómez

, Martos

, Rubio

, Sarmiento

, de la Cuesta

. The effect of a single intravenous dose of dipyrone 2 g, ketorolac 30 mg and propacetamol 1 g on haemodynamic parameters and postoperative pain after heart surgery. Eur J Anaesthesiol 2000;17:85–90

16.

Erhardt

, Henke

, Kroker

. Pharmaka im rahmen der anaesthesie und der perioperativen schmerzlinderung. In: Erhardt

, Henke

, Haberstroh

, eds. Anaesthesie & Analgesie beim Klein- und Heimtier. Stuttgart: Schattauer, 2004:125–6

17.

Lipman

, Marini

, Flecknell

. Anaesthesia and analgesia in rabbits. In: Kohn

, Wixson

, White

, Benson

, eds. Anaesthesia and Analgesia in Laboratory Animals. London: Academic Press, 1997:209–11 and 19

18.

Freye

. Unterschiedliche pharmakokinetik der opioide und ihre bedeutung für den praktischen einsatz. In: Freye

, ed. Opioide in der Medizin. Berlin: Springer Verlag, 1991:108–12

19.

Whelan

, Flecknell

. Anaesthesia of laboratory rabbits using etorphine/methotrimeprazine and midazolam. Lab Anim 1995;29:83–9

20.

Murphy

, Roughan

, Baxter

, Flecknell

. Anaesthesia with a combination of ketamine and medetomidine in the rabbit: effect of premedication with buprenorphine. Vet Anaesth Analg 2010;37:222–9

21.

Maier

. Medikamentöse schmerztherapie. In: Diener

, Maier

, eds. Das Schmerztherapie-Buch. Baltimore, MD: Urban & Schwarzenberg, 1997:307–52

22.

Erhardt

. Tiere mit bestehenden kardiovaskulaeren problemen. In: Erhardt

, Henke

, Haberstroh

, eds. Anaesthesie & Analgesie beim Klein- und Heimtier. Stuttgart: Schattauer, 2004:432–46

23.

Chandrasekharan

, Dai

, Roos

, COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: cloning, structure, and expression. Proc Natl Acad Sci USA 2002;99:13926–31

24.

Shimada

, Otterness

, Stitt

. A study of the mechanism of action of the mild analgesic dipyrone. Agents Actions 1994;41:188–92

25.

Dembinska-Kieĉ

, Zmuda

, Krupinska

. Inhibition of prostaglandin synthetase by aspirin-like drugs in different microsomal preparations. Adv Prostaglandin Thromboxane Res 1976;1:99–103

26.

Tortorici

, Vasquez

, Vanegas

. Naloxone partial reversal of the antinociception produced by dipyrone microinjected into the periaqueductal gray of rats. Possible involvement of medullary off- and on-cells. Brain Res 1996;24:106–10

27.

Aguirre-Bañuelos

, Granados-Soto

. Evidence for a peripheral mechanism of action for the potentiation of the antinociceptive effect of morphine by dipyrone. J Pharmacol Toxicol Methods 1999;42:79–85

28.

Richter

, Pieper

, Henke

, Erhardt

, Matis

. Intraoperative analgesia in dogs with metamizole and/or fentanyl for hip replacement. In: Proceedings of the Autumn Meeting of the Association of Veterinary Anaesthetists and the European College of Veterinary Anaesthesia and Analgesia. Leipzig: Germany, 2007;19–21:53

29.

Borkowski

, Danneman

, Russel

, Lang

. An evaluation of three intravenous anesthetic regimens in New Zealand White rabbits. Lab Anim Sci 1990;40:270–6

30.

Clemm

. Vergleichsuntersuchungen zur Intraoperativen Analgetischen Wirksamkeit von Metamizol, Carprofen und Fentanyl bei der Orchiektomie des Kaninchens. PhD dissertation. Munich: Ludwig-Maximilians-University of Munich, 2008

31.

Kramer

. The toxicology of non-narcotic analgesics. Agents Actions Suppl 1986;19:225–35

32.

Hoigné

, Zoppi

, Sollberger

, Hess

, Fritschy

. Fall in systolic blood pressure due to metamizol (dipyrone, noramidopyrine, novaminsulfone). Results from the Comprehensive Hospital Drug Monitoring Berne (CHDMB). Agents Actions Suppl 1986;19:189–95

33.

Samain

, Marty

, Gauzit

, Effects of propofol on baroreflex control of heart rate and on plasma noradrenaline levels. Eur J Anaesthesiol 1989;6:321–6

34.

Egan

, Brock-Utne

. Asystole after anaesthesia induction with a fentanyl, propofol, and succinylcholine sequence. Anesth Analg 1991;73:818–20

35.

Erguen

, Ayhan

, Tulunay

. Pharmacological characterization of metamizol-induced relaxation in phenylephrine-precontracted rabbit thoracic aorta smooth muscle. Gen Pharmacol 1999;33:237–41

36.

Henke

, Erhardt

. Analgesie. In: Erhardt

, Henke

, Haberstroh

, eds. Anaesthesie & Analgesie beim Klein- und Heimtier. Stuttgart: Schattauer, 2004:370–81

37.

Cockshott

. Propofol (‘Diprivan’) pharmacokinetics and metabolism – an overview. Postgrad Med J 1985;3 (suppl.):45–50

38.

Beller

, Pottecher

, Lugnier

, Mangin

, Otteni

. Prolonged sedation with propofol in ICU patients: recovery and blood concentration changes during periodic interruptions in infusion. Br J Anaesth 1988;61:583–8

	Mean ± SD or median (IQR: 25th–75th percentile)
Group	Time point	HR* (bpm)	MAP* (mmHg)	SpO₂ (%)	P_E′CO₂ ^*,† (kPa)
DP	Baseline	217 ± 25	69 ± 13	99 (98–100)	4.93 (4.93–5.07)
DP	ST	183 ± 17	50 ± 15	100 (98–100)	5.07 (4.8–5.13)
DP	10 min after ST	182 ± 32	53 ± 27	100 (98–100)	5.07 (4.87–5.13)
DP	20 min after ST	180 ± 31	47 ± 24	100 (98–100)	5.07 (4.93–5.20)
DP	30 min after ST	188 ± 18	55 ± 23	98 (96–100)	4.93 (4.93–5.20)
DP	EI	193 ± 14	67 ± 20	99 (96–100)	5.20 (5.20–5.33)
DP	10 min after EI	221 ± 14	80 ± 13	99 (94–100)	−
DP	20 min after EI	238 ± 17	87 ± 11	−	−
P value overall comparison:	Within subjects effect (time)	<0.001^‡	0.010^‡	0.178^§	0.009**
FP	Baseline	239 ± 25	71 ± 10	99 (99–100)	5.07 (4.97–5.30)
FP	ST	178 ± 15	32 ± 9	98 (98–99)	5.07 (5.07–5.17)
FP	10 min after ST	179 ± 25	35 ± 9	98 (97–99)	5.13 (4.97–5.40)
FP	20 min after ST	193 ± 24	45 ± 16	98 (98–100)	5.07 (5.07–5.20)
FP	30 min after ST	195 ± 24	48 ± 16	99 (97–99)	5.07 (4.83–5.20)
FP	EI	191 ± 23	57 ± 21	99 (99–100)	5.27 (5.10–5.43)
FP	10 min after EI	219 ± 24	77 ± 19	98 (98–99)	−
FP	20 min after EI	244 ± 23	83 ± 15	−	−
P value overall comparison:	Within-subjects effect (time)	<0.001^‡	0.003^‡	0.312^§	0.362**
DP versus FP	Between-subjects effect (group)	0.550	0.185	0.294^††	0.067
DP versus FP	Interaction (time × group)	0.527	0.532	0.101^††	0.169

Comparison of dipyrone/propofol versus fentanyl/propofol anaesthesia during surgery in rabbits

Abstract

Keywords

Material and methods

Animals

Anaesthesia

Experimental protocol

Statistical evaluation

Results

Clinical haemodynamic parameters

Discussion

References