Medetomidine Premedication for Caesarean Section in the Bitch

By K G M De Cramer, BVSc, MMedVet(Gyn), PhD & K E. Joubert  B.V.Sc M.Med Vet (Anaes)

The ideal anaesthetic protocol for caesarean section (CS) should provide adequate muscle relaxation, analgesia and narcosis for optimal operating conditions, be safe for the bitch 2 and should not affect the viability and survival of the puppies 3,4. Additional recommendations include using drugs with a short duration of action 5 and using drugs which are reversible 6. Induction using propofol and maintenance of anaesthesia using isoflurane is widely accepted and associated with good outcome 4,7-12. The use of alpha-2-adrenergic agonists before anaesthesia for CS, is however, controversial. The alpha-2-adrenergic agonist xylazine, is not recommended in patients undergoing CS because it was identified as a risk factor for increased puppy mortality 12,13, associated with increased risk of death in the dog 14,15 and caused severe maternal and neonatal cardiovascular depression 16.

The greatest objection to the use of the alpha-2-adrenergic agonists are the cardiopulmonary effects which include transient hypertension followed by mild hypotension, bradycardia, increased systemic vascular resistance, reduced cardiac output, and respiratory depression. In recent surveys of anaesthetic mortality in the dog however, premedication with medetomidine prior to anaesthesia for routine surgery, was not identified as an increased risk factor for mortality 17 and data from human literature has shown that dexmedetomidine is associated with a reduction in all causes of mortality when used for non-cardiac, cardiac and vascular surgery 18,19.

No studies in veterinary medicine have been conducted on the use of medetomidine for CS.

This study assessed puppy vigour and survival following medetomidine, propofol and sevoflurane anaesthesia for elective CS.

Materials and Methods

This is a retrospective, descriptive study that included 292 CSs in 256 privately owned bitches which underwent a CS and were selected from the general obstetric population because of increased obstetric risk. High-risk pregnancies in the current study were considered those occurring in bitches from breeds with a high risk of complicated parturition 20, with a history of dystocia 21, or with known very large litters 22. All the bitches were admitted 3 to 4d prior to the predicted parturition date calculated as 57d following D0**.

**D1 was redefined as D0 because it chronically caused the error of vets starting to count from the old d1 and the next day d2 whereas it should be d1 = 0 and the next day is d1, this led to intervals between cytological dioestrus and parturition being wrong (out with 24 hours) which can be devastating in the bitch when timing CS

During these days, the bitches were observed for signs of impending parturition (panting, inappetence, nesting behaviour, tenesmus) and by 6 hourly vaginal speculum examinations to assess the cervix. The decision on when to perform a CS was based upon the first appearance of any degree of dilatation of the cervix. Once the decision to perform a CS was made, an ultrasound examination of the abdomen was performed to establish if there were any dead foetuses (absence of detectable heartbeat).

Starting at induction, fluid therapy commenced and the PCV before and after CS determined as previously described 23. All bitches weighed more than  of 20 kg. The anaesthetic protocol used in the current study included the alpha-2-adrenergic agonist medetomidine hydrochloride (Domitor®, Zoetis Animal Health, Sandton, South Africa) at 7 µg/kg iv as a  premedicant, followed one minute later by propofol (Fresenius propoven®1%, Fresenius Kabi, Midrand, South Africa) at 1 to 2 mg/kg iv as an induction agent. The propofol was administered as follows, the calculated dose of 2 mg/kg was drawn up in syringe and 1 mg/kg/iv was administered as a bolus. The remaining propofol was used as top-up if required. This was followed by immediate intubation and inhalation of room air. Following surgical preparation (averaging 3 to 5 min), the bitch was connected to a closed-circuit anaesthetic machine with 2% sevoflurane (Sevoflo®, Safeline Pharmaceuticals, North Cliff, South Africa) in oxygen for maintenance of anaesthesia.

The CS was performed in standard fashion as described 24. Meloxicam (Metacam®, Boehringer Ingelheim, Randburg, SA) was administered iv (0.1 mg/kg) intra-operatively, as previously proposed 25, immediately after delivery of the last puppy. The processing of puppies following delivery involved immediate administration of atipamezole hydrochloride (Antisedan®, Zoetis Animal Health, Sandton, South Africa) at the dose of 50 µg/puppy sc, tying off of the umbilicus and applying 10% povidone iodine thereto, drying the puppies, gently shaking fluids from their airways and placing them in an air-heated incubator set at 35°C. No oxygen support was offered to the puppies after delivery.

Also, immediately after surgery, atipamezole hydrochloride at the dose of 20 µg/kg was administered iv (extra label route of admin) to the bitch. The bitch was observed until the coughing reflex returned and was then extubated and observed until she was breathing comfortably whilst in sternal recumbency, sitting upright or standing. It was recorded if the bitches were fully ambulatory within 15 minutes following extubation. After delivery of the puppies the following records were made; total number of puppies delivered, live puppies, dead puppies, deformed puppies and puppies euthanased. The Apgar scores were assessed starting with the first puppy 15 minutes after the last puppy was delivered according to the methods published 26 and later used by 27. The bitch and puppies were discharged usually 2 to 3/h following surgery.

The puppy survival rate was established immediately after delivery, at 2 hours post CS and 7d post CS and the maternal survival rate was established after delivery of the last puppy, at 2h and at 7d post CS. The Glasgow pain scale evaluation was performed at the time of discharging the bitch according to Glasgow Composite Measure Pain Scale to ensure adequacy of pain management 28.

Results

A total of 292 CS were performed, 148 on 133 Boerboel bitches, 84 on 68 English bulldog bitches and 60 on 55 other purebred bitches, which resulted in a total of 2232 puppies (1378, 541 and 313, respectively, per breed). Thirty-six bitches underwent more than one CS in this study. The percentage of puppies live at delivery for the Boerboel, English bulldog and other purebreds puppies respectively was; 97.39%, 96.67% and 91.69%.

The 2h survival rate was 95.43%, 88.35% and 89.78% and the  7-day survival rate was 89.19%, 79.11% and 84.03% respectively. The numbers of puppies euthanased due to malformation were 16/1378 (1.16%), 32/541 (5.59%) and 4/313 (1.28%) respectively for Boerboels, English bulldogs and other purebred dogs.

Thirty-five, 18 and 26 Boerboel-, English bulldog and other purebred puppies were stillborn. Of these, 12 (34%), 9 (50%) and 15 (58%) had, respectively, been found dead on ultrasound examination immediately before the CS was performed. After correction for foetuses discovered dead on ultrasound and malformed euthanased puppies, the survival rates for Boerboel-, English bulldog and other purebred puppies were 98.21%, 95.60% and 94.30%, respectively, at 2 hours and 91.78%, 87.17% and 88.26% at 7 days.

The Apgar scores averaged 9.66 for all the breeds combined, and 9.77, 9.35 and 9.68 for Boerboel, English bulldogs and other purebred breeds respectively. The maternal survival rate was 291/292. One Boerboel bitch died from gastric dilatation and volvulus 2 days following surgery. The average Glasgow pain scale for bitches at discharge was 6.4 (S.D. 0.65, with a minimum of 5 and a maximum of 8, n = 292). No bitch had a PCV of below 30% after surgery and all bitches were fully ambulatory 15 minutes after extubation.

Discussion

The prime objectives for using premedication in any patient prior to surgery and CS is to reduce the induction dose of anaesthesia 5, reduce the minimum alveolar concentration of inhalation anaesthetics 6, decrease maternal stress and anxiety 29, provide analgesia and providing chemical restraint allowing preoperative preparation in cases where this is required. The use of alpha-2-adrenergic agonists meet all these requirements as they are potent sedatives, may induce narcosis at high doses, act as analgesics 30, vastly reduce induction doses of propofol 31 and reduces minimum alveolar concentration of isoflurane and sevoflurane 32-35.

Analgesics and non-steroidal anti-inflammatory agents in pregnant animals and humans are problematic for CS 36 -49. Opioids provide analgesia but cross the placenta and can cause significant central nervous system and respiratory depression in neonates 49 which may take 2 to 6d in canine neonates to eliminate 25. There is consensus that a single intravenous administration of meloxicam at a dosage of 0.1 mg/kg immediately after delivery of the puppies is safe 25. Failing to administer analgesics for obstetric surgery has become unacceptable in the veterinary profession and its legislative bodies. The analgesic properties of medetomidine intra-operatively and post-operatively have been demonstrated 50 and reviewed in the dog 51 and demonstrated in the pregnant woman before and after CS without adverse neonatal effects 52. However, reversal by atipamezole will also reverse the analgesic effects and therefore the administration of other analgesics in the recovery and immediate post-operative period are indicated.

Although both maternal and neonatal cardiovascular depression associated with the use of medetomidine may have been present in the current study, our results show that it did not appear to affect puppy survival rate, Apgar score or maternal survival rate. The route of administration of medetomidine is important as the iv route requires a much smaller dose to achieve the same effect as opposed to the IM route 53.

In the absence of premedication, the dose of propofol required to induce and intubate pregnant bitches approaches 6 mg/kg 27. A significant reduction of the induction dose of propofol, reducing it to 1 mg/kg  was recorded when medetomidine is used as premedicant at doses of 20 to 40 µg/kg 31. Despite the use of a lower dose of medetomidine in the current study, it was possible to intubate and surgically prepare all bitches never exceeding a total dose of 2 mg/kg of propofol.

The rate of stillbirth and neonatal death is known to be relatively high in dogs with mortality ranging from 17% to 30% within the first 8 weeks of life 54 and with asphyxia being the leading cause of death 55,56. Stillbirths and deaths within the first week were responsible for the majority of these losses. In a large study involving 10,810 litters, the perinatal mortality was present in 24.6% of litters. In the same study, 4.3% of puppies were stillborn and another 3.7% died within a week 57. Survival was 92% in the first week, and only another 1% dying from 8 days to 8 weeks of age 57.

The puppy survival rate in other studies varied from 92.4% to 88.1% 58-61. In a study by Moon et al. (1988), survival rates immediately, 2 hours and 7d after delivery were 92%, 87%, and 80%, respectively, for puppies delivered by CS (n = 3,410) and 86%, 83%, and 75%, respectively, for 498 puppies born naturally. Moon et al. (1998) reported that in 76% of litters delivered by CS, all the puppies were born alive. The maternal mortality rate was 1% 62. Our results show that medetomidine appeared not to have affected puppy survival rate and maternal survival rate.

In a study involving 37 CSs performed on English Bulldogs, 14.9% of the puppies were stillborn, 8.2% of the puppies alive, were deformed; mainly cleft palate (palatoschisis) and anasarca, and 10% of the puppies died before the age of weaning 20. This is similar to the 6.9% birth defects and 15.91%, 24h mortality that Batista et al. (2014) reported for elective CS in English bulldogs. The results from the current study in English bulldogs were similar with respect to the number of deformed puppies (5.59%) but better for percent live birth (96.67%). In 193 puppies from 42 litters, 65 born by spontaneous delivery, 66 by assisted delivery and 62 by caesarean section, the percentage of stillbirths were 14%, 20% and 8% respectively 26.

Puppy survival rates at birth and at 2h after birth supposedly reflect the effects of perioperative conditions more specifically than at 7d after delivery 12. This is because deaths within the first week are often associated with factors relating to maternal care, agalactia, undetected birth defects and infectious causes 54.

It is important to establish prior to a treatment or procedure whether all the foetuses are alive and well prior to the intervention 63 to avoid a false overestimation of the effect of the intervention on puppy mortality. Pre-operative ultrasound enables one to account for at least some foetuses that died before and therefore those stillbirths may be considered independent of the intervention. The current study identified 45.6% (36/79) of stillborn puppies on ultrasound examination immediately before the CS was performed.

Because the aim of the current study was to evaluate the effect of our anaesthetic protocol on puppy survival rates, puppy vigour and maternal survival rates, attempts were made to exclude confounders which may have affected outcome not related to anaesthetic protocol. These were foetuses detected dead on ultrasound examination prior to administering any drugs included in the anaesthetic protocol and puppies euthanased because they were deformed. This correction is particularly useful when comparing puppy survival rates of the English bulldog breed to those of others. English Bulldog puppies not only have poorer survival rates at 2h and at weaning age but also an increased risk of being stillborn or being born with defects requiring euthanasia 20,64.

The maternal survival rate in this study was good as all but one of the 292 bitches anaesthetised were alive at 7d post CS. A single Boerboel bitch died from gastric dilatation and volvulus two days after discharge. Gastric dilatation and volvulus is a well-recognized cause of sudden death in large breeds 65. Therefore, our results support the finding by another worker that premedication with medetomidine was not identified as an increased risk factor for anaesthesia in the dog 17. In contrast, xylazine has safety concerns for both dam and neonate when used for CS in the bitch 12-16. The demonstrated safety recorded in the current study may in part be explained by; improved alpha 1 : alpha 2 specificity exhibited by medetomidine compared to xylazine, greater awareness of the physiologic effects of alpha2-adrenergic agonists 17 and the use of a relative low dose (7 µg/kg) of medetomidine. Although dexmedetomidine and medetomidine induced similar clinical effects 66, there are theoretical pharmacological advantages of the non-racemic alpha2-adrenergic agonist, dexmedetomidine, over medetomidine 67. The primary author observed no differences in clinical effects when medetomidine is replaced by dexmedetomidine.

Apgar scores have been shown to be influenced by anaesthetic agents. Alfaxalone (1 to 2 mg/kg) has been shown to be slightly superior to propofol (2 to 6 mg/kg) in this respect 27,68. Overall, the Apgar scores achieved in the current study were higher than those reported in other studies irrespective of method of delivery or anaesthetic protocol used 26,27, 68, 69. The time at which Apgar scores are measured after birth is important. Although in early studies in man, a score at one minute was used 70, it was later demonstrated that the score at five minutes is more predictive of survival 71. Similarly, in puppies, evaluations performed at five minutes after birth were less predictive than those done at 15 minutes and 60 minutes after delivery 26,27,69. This may be because more time was allowed to elapse for removal of depressive effects of the anaesthetic agents before an Apgar score was evaluated. In the current study the delay from delivery till Apgar score evaluation may have allowed for complete reversal of medetomidine in the puppy and for the concentration of propofol in the central nervous system to diminish by redistribution which is reported to take 15 to 20 min 8,72. Furthermore, the low induction dose of propofol used and atipamezole administration, may also have favourably influenced Apgar scores.

In agreement with the studies by others 27,26, the current study shows that Apgar scores correlate with mortality, with lower scores being associated with higher mortality rates. Although another study has shown that English Bulldog puppies were at increased risk compared to other breeds for suffering from poor vigour after CS 11, our study showed only slightly reduced Apgar scores when compared to other purebreds and Boerboel puppies. The method of delivery may impact puppy vigour. In contrast to findings by others, we recorded higher Apgar scores for puppies from caesarean deliveries than those of the eutocic vaginally delivered puppies reported in the literature 73,74. These differences may reflect the different anaesthetic protocols used in the studies and the timing of obstetric intervention. Our results show that medetomidine appeared not to have affected the Apgar score.

The effect of early intervention by CS (at the first observation of a dilated cervix), prior to foetal compromise may in part explain the good results obtained in our study. This agrees with the literature reporting positive correlation between timeous intervention and puppy survival 12,21,75-81

Conclusions

This study shows that anaesthetic protocol using medetomidine hydrochloride at 7 µg/kg iv as premedicant combined with 1 to 2 mg/kg propofol as induction agent and 2% sevoflurane in oxygen as maintenance, is safe for CS in the bitch. The Apgar scores 15 min after delivery, as well as the puppy and maternal survival rates at delivery, 2h and 7d compare favourably to those reported in publications using other anaesthetic protocols. The use of medetomidine as premedicant permits the use of less than half the induction dose of propofol usually required when no premedicant is used.

Practical hints

The primary author has been using medetomidine in more than 2000 CSs and in bitches of all sizes. The weight (real mass not lean body mass) of the bitch requires discussion.

In this study, all anaesthetic subjects were 20kg or more. It is however the experience of the primary author when using medetomidine at 7 µg/kg for CSs in toy breeds, the dose of propofol required to allow intubation approaches 2-3 mg/kg as opposed to 1-2 mg/kg. Furthermore, the time allowed following induction and intubation in the prep room whilst not on gas inhalation, is reduced when using medetomidine at the low dose of 7 µg/kg. Therefore, the author uses 10 µg/kg medetomidine premedication for CS in dogs under 20 kg with no observable differences in outcome. Not all practices perform CSs frequently and many may not have sufficient experienced staff to ensure rapid surgical preparation and neither do all prep rooms have access to gas-inhalation anaesthesia. Therefore, if delays between intubation and connection to gas-inhalation maintenance of anaesthesia are expected, the author advises using medetomidine premedication at 10 µg/kg irrespective of the weight of the bitch and has observed no differences in outcome when doing so. Practitioners are reminded that propofol is available in 1% and 2% concentrations and should make sure that they adjust their volumes accordingly.

The question whether a bitch should be surgically prepped prior to premedication, after premedication or after premedication, induction and intubation is not straight forward. Some bitches are combative and will resist the restraint required to perform surgical preparation. Some heavily pregnant bitches of any breed that are placed in dorsal recumbency desaturate whilst all English bulldogs suffering from brachycephalic obstructive airway syndrome (BOAS) severely desaturate when doing so (unpublished data, de Cramer). Furthermore, in heavily pregnant bitches suffering from BOAS, the relaxation of the airway induced by premedication causes severe desaturation (visible cyanosis) even in the patient that is not handled and remains in the standing or sitting position (unpublished date, de Cramer). Because it cannot always be predicted how a bitch will respond to restraint and because a substantial proportion of all CSs are performed on brachycephalic bitches, the prime author prefers to surgically prepare all bitches following induction and intubation.

The volume of the medetomidine at the doses described in this paper is small and therefore accurate measurement of the required volume can more reliably made when using a 1 ml syringe. Also, when using very small volumes in toy breeds, a significant volume of medetomidine may be lost in the hub of the intravenous catheter and in the tip of the syringe and therefore extending the medetomidine volume with ringers or saline may ensure administration of the full required volume. Practitioners may be tempted to use the medetomidine intramuscularly in aggressive bitches to allow for the placement of an intravenous catheter. Very large doses of medetomidine are required intramuscularly to achieve this goal and this practice cannot be advised as its safety has not been established.

REFERENCES:  Available on www.vet360.vetlink.co.za

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