Anaesthesia of the caesarean section patient is complicated because the anaesthetist must consider the unique physiology of not only the dam but the foetus(es) as well. There are surprisingly few large prospective trials comparing anaesthetic protocols for caesarean sections in veterinary medicine. Therefore, practice is often influenced by theory, dogma and human medical practices in addition to veterinary evidence.
What is the ideal anaesthetic agent for caesarean sections in veterinary practice?
|· Provides anxiolysis to reduce dam’s stress
· Provides pre-emptive analgesia
· Preserves uterine and foetal blood supply
· Has minimal adverse effects on the dam
· Has minimal effects on the foetus/newborn
· Reduces risk of aspiration for the dam
Finding the ideal anaesthetic for a caesarean section (Box 1) is challenging because all anaesthetic agents cross the blood–placenta barrier, albeit at varying speeds.
Before and during a caesarean section, it is essential to preserve the blood pressure of the dam as placental blood flow is unregulated and directly related to systemic blood pressure. Excessive pain and stress can cause uterine vasoconstriction, reducing placental blood flow. Anaesthetic plans should guard against this (Self, 2019). Regardless of the anaesthetic agent, the aim should be to use the lowest possible dose to achieve the desired effect.
One should also be aware that hormonal changes make the periparturient dam particularly sensitive to anaesthetic agents. Therefore, anaesthesia for caesarean sections requires a cautious approach to administration, using more conservative doses than in non-pregnant patients. Towards the end of pregnancy, body weight increases by 25 percent in dogs and 38 percent in cats, due to maternal fluid and foetal size. So dosages should be calculated based on an estimate of the patient’s physiological body weight (Fontaine, 2012). When taking these factors into account, overall reductions of anaesthetic doses by 25 to 40 percent have been suggested (Meyer, 2007).
An opioid analgesic and a sedative agent are usually provided as premedication to provide pre-emptive pain relief and anxiolysis and reduce the necessary dose for the induction and maintenance of anaesthetic agents. While these attributes are highly desirable in caesarean patients, premedication is controversial due to concerns about adverse effects in the dam and litter. This means induction and performing a caesarean section without premedication is not uncommon.
Acepromazine is frequently used in veterinary anaesthesia but is usually avoided in caesarean patients. This is because the periparturient dam has a decrease in vascular resistance and a 20 to 50 percent increase in cardiac output to compensate. Adverse effects of acepromazine include non-antagonisable vasodilation and a reduced cardiac output, which the dam can ill afford (Tranquilli et al., 2007). Furthermore, acepromazine passes to the foetus during anaesthesia, causing prolonged sedation after delivery. However, some studies advocate low doses for excessively anxious dogs, with no effect on neonatal mortality (Luna, 2004).
Alpha-2 agonists change vascular tone, reduce cardiac output and cause respiratory depression. Therefore, they are seldom used in unstable or emergent patients with significant cardiovascular or respiratory dysfunction (Lemke, 2004). They do, however, provide excellent sedation and analgesia, and their effects are reversible using atipamezole in the dam and newborn. Xylazine is not recommended due to poor outcomes in dams and newborns. However, low doses (7μg/kg) of medetomidine (Domitor, Vetoquinol) or dexmedetomidine (2μg/kg) have recently been advocated in dogs, resulting in puppies with good Apgar scores and normal neonatal survival rates (Groppetti et al., 2019; De Cramer et al., 2017).
Benzodiazepines (eg midazolam and diazepam) are readily transferred to the foetus, causing prolonged sedation in the newborn human (referred to as “floppy infant syndrome”) and poorer outcomes when used in some drug combinations in dogs (Luna, 2004). However, benzodiazepines are reversible with flumazenil. This may mean these drugs could have a role as an anxiolytic in extremely nervous or fractious patients with cardiovascular disturbances that prevent acepromazine or alpha-2 agonist use.
Opioids are the mainstay of analgesia in veterinary anaesthesia, and when to use them is arguably the most controversial aspect of anaesthesia for caesarean sections.
Opioids are the mainstay of analgesia in veterinary anaesthesia, and when to use them is arguably the most controversial aspect of anaesthesia for caesarean sections
It is counterintuitive to perform major abdominal surgery without pre-emptive analgesia, yet this is commonplace for caesarean patients. Opioids provide an effective and reliable means of analgesia for the dam but cross the placenta to the foetus, causing respiratory and central nervous system depression and potentially poor vigour upon delivery. However, fears of poor Apgar scores and subsequent increased neonatal mortality are not necessarily documented in recent studies (Schmidt et al., 2021).
The concern surrounding preoperative opioid use often leads to the omission of opioids until after the last neonate is delivered. While this tactic is understandable, a lack of pre-emptive analgesia and sedation may increase stress and pain in the dam, causing catecholamine release, uterine vasoconstriction and a subsequent requirement for higher concentrations of inhalant doses. Collectively, these effects alter foetal blood supply and neonatal vitality (Raffe, 2015; Self, 2019). Insufficient analgesia also has welfare impacts, as well as leaving the dam prone to rapidly changing depths of anaesthesia, which can leave them vulnerable to regurgitation and aspiration – a major cause of maternal mortality (Moon et al., 1998).
A lack of pre-emptive analgesia and sedation may increase stress and pain in the dam, causing catecholamine release, uterine vasoconstriction and a subsequent requirement for higher concentrations of inhalant doses
Modest preoperative doses of opioids are now widely advocated in an attempt to usher a paradigm shift in veterinary practice (Cain and Davidson, 2023; Robertson, 2016; Self 2019).
What opioids should be used for analgesia of the caesarean patient?
Despite being the slowest to cross the placental barrier, morphine is usually avoided due to its propensity to cause preoperative vomiting. Methadone (UK) or hydromorphone (North America) are common alternatives. If “top-up” doses are given following delivery of the last newborn, one should be wary of apnoea following intravenous opioid administration while the patient is anaesthetised. Slow intravenous or intramuscular administration can mitigate this risk.
Various opioids are licensed for dogs and cats, although some, such as buprenorphine, specifically state in their datasheet not to use them preoperatively in cases of caesarean section; others, such as methadone, state not to use during pregnancy or lactation. The use of these opioids would, therefore, be off-licence and clear permission would be required from the patient’s caregiver, given the litigious nature of reproductive cases.
Naloxone antagonises and reverses the effects of opioids. It can be given intravenously or sublingually to all newborns or those that are bradycardic or responding poorly to resuscitative efforts. Not every veterinary practice stocks naloxone, and the author considers the agent’s availability a prerequisite for preoperative opioid use.
Local anaesthesia can be an excellent source of pain control for caesarean patients and has minimal systemic effects on the dam or offspring. It can also be used in lieu of or in addition to other analgesic agents.
Under Schedule 3 guidance, a simple “line block” can be performed by a vet or veterinary nurse with ease. This procedure involves the instillation of a local anaesthetic where the midline abdominal incision will be made to allow for lower inhalant gas concentrations during anaesthesia maintenance. More complex local anaesthetic techniques such as epidurals and transverse abdominal plane (TAP) and quadratus lumborum (QL) blocks can provide further analgesia but are rarely used in veterinary medicine.
Local anaesthesia can be an excellent source of pain control for caesarean patients and has minimal systemic effects on the dam or offspring
Lidocaine has a rapid onset of action (under five minutes) but would ideally be administered before induction (if tolerated) to reduce the time from induction to newborn delivery. Lidocaine is significantly more acidic than the subcutis, but administration can be made more comfortable in the conscious patient by diluting with saline. Another alternative would be buffering with 8.4 percent sodium bicarbonate at a volume ratio of 1 part bicarbonate to 9 parts lidocaine (Frank and Lalonde, 2012).
Lidocaine has a short duration of action (one to two hours) and therefore does not provide analgesia in the post-operative patient. Although bupivacaine (eg Marcain, Aspen Pharma) has a longer time to onset (around 30 minutes), it also has a longer duration of action (four to five hours) and can be applied when the surgeon is closing the midline to provide more prolonged post-operative analgesia (Grubb and Lobprise, 2020). Liposome-encapsulated bupivacaine (Nocita, Elanco) is not yet available in the UK but provides up to three days of local anaesthesia, which is desirable while the dam nurses her offspring.
Admixtures of local anaesthetics are not universally recommended because they shorten the duration of action of agents (Lawal and Adetunji, 2009).
Lastly, lidocaine administration into (or onto) the ovarian pedicles can be considered an effective analgesic when exteriorising the uterus (Cicirelli et al., 2022). This block may have the additional benefit of blunting reflex hypotension, which can occur with uterine exteriorisation (Onclin and Verstegen, 2008).
Miscellaneous preoperative anaesthesia
Caesarean patients are at high risk of regurgitation and aspiration (known as “Mandelson syndrome”) due to the pressure of a gravid uterus on the stomach and lower oesophageal sphincter. This risk is compounded by the over-representation of regurgitation-prone brachycephalic breeds requiring caesarean sections (Evans and Adams, 2010). Maropitant, a dual antiemetic and visceral analgesic, can be used in non-pregnant/lactating animals but has been linked to bone marrow hypoplasia in neonates and paediatric patients up to 11 weeks old, so off-licence use would not be recommended by the author (Boscan et al., 2011; Hay Kraus, 2017). Metoclopramide may help avoid Mandelson syndrome and has the additional benefit of stimulating prolactin and milk production in the post-operative period (Cain and Davidson, 2023).
Preoxygenation for five minutes prior to induction is recommended to replenish oxygen reserves before surgery (McNally et al., 2009). Oxygen demand and consumption are 20 percent higher at parturition due to the effects of pregnancy and the demand of the litter (Robertson, 2016). A large litter also impedes thoracic expansion and causes atelectasis of the lung, decreasing functional lung capacity by up to 20 percent (Kushnir and Epstein, 2012). This leaves the dam prone to desaturation and hypoxaemia, which directly affects the litter.
Intravenous fluid therapy is often required following prolonged periods of dystocia, with the dam vulnerable to shock due to blunted compensatory mechanisms
Intravenous fluid therapy is often required following prolonged periods of dystocia, with the dam vulnerable to shock due to blunted compensatory mechanisms, such as the already elevated cardiac output. Fluid boluses may be required if the dam is hypovolaemic and 5 to 10ml/kg/hr fluid rates are appropriate intraoperatively.
Anaesthesia for veterinary caesarean sections
Propofol and alfaxalone are common induction agents and are deemed appropriate in the pregnant dam as they allow for rapid intubation and airway protection. Some studies suggest that alfaxalone results in better neonatal Apgar scores; however, the strength of this evidence has been questioned in a recent analysis (Sofyan and Martinez-Taboada, 2021).
The higher induction doses usually required to induce a non-premedicated animal may not be necessary in pregnant dams due to the sensitivity of periparturient patients to anaesthetic agents (Meyer, 2007).
Some practitioners advocate for a minimum of 15 to 20 minutes between induction and delivery to allow the induction agent to circulate from the foetus back to the dam and be processed by her mature liver (Groppetti et al., 2019). Such a delay is already realistic in most settings.
Isoflurane and sevoflurane gases are considered appropriate for maintaining anaesthesia but not for mask induction in the caesarean patient (Broadbelt et al., 2008). While these drugs rapidly cross the blood–placenta barrier, they are readily cleared from the newborn (providing they breathe effectively after delivery). Furthermore, the central nervous system of the dam may be up to 40 percent more sensitive to isoflurane due to the effects of progesterone and oestrogen. Diligent monitoring is, therefore, necessary to prevent relative overdose, particularly in patients where preoperative sedatives or opioids have been used (Ryan and Wagner, 2006).
Inhalant gases have a dose-dependent cardio-depressive and vasodilatory effect. Therefore, they should be titrated carefully to effect in the hypotension-prone dam.
Prolonged or inappropriately deep anaesthesia maintained via inhalant agents is associated with poor newborn vigour (Antończyk et al., 2023; Raffe, 2015). Total intravenous anaesthesia techniques could be considered but have been linked to poorer recovery for the dam and offspring (Conde Ruiz et al., 2016). Therefore, they are not universally recommended.
A consensus has long existed that one dose of NSAIDs is appropriate following a caesarean section (Gilson, 2003). The timing of administration is variable, with some clinicians opting not to administer NSAIDs pre- or intraoperatively for fear of possible hypotension during the procedure, leading to renal insult. Longer post-operative courses have previously been avoided for fear of adverse effects on suckling newborns. However, recent studies have suggested that such fears are unfounded when using carprofen in a bitch without mastitis (Ferrari et al., 2022)
Longer post-operative courses have previously been avoided for fear of adverse effects on suckling newborns. However, recent studies have suggested that such fears are unfounded
Paracetamol is widely used in human medicine following caesarean section yet there is minimal data associated with its use in veterinary medicine despite its popularity. Extrapolating from this safety profile in humans means it is likely appropriate in post-partum canine patients (note paracetamol is toxic to cats). Proprietary formulations of paracetamol with codeine (Pardale V, Dechra) should be used with caution as codeine use in breastfeeding women is a topic of some controversy (Zipursky et al., 2023).
A suggested anaesthetic protocol for veterinary caesarean section
The author based this suggested protocol on research by Robertson (2016), Self (2019) and Cain and Davidson (2023).
- Ensure the environment and staff are calm to reduce maternal catecholamine levels and preserve uterine blood flow (Gilroy and DeYoung, 1986)
- Prepare the operating theatre and newborn resuscitation area and brief staff on the case
- Get intravenous access
- Obtain emergency database bloods to ensure adequate electrolyte, glucose and haematocrit values
- Begin intravenous fluid therapy – “surgical rates” of 5 to 10ml/kg/hr due to propensity for intraoperative fluid loss. Use intravenous boluses as necessary
- Consider preoperative opioid use (eg methadone at 0.1 to 0.2mg/kg IV) administered 15 minutes prior to induction. Dexmedetomidine at 2μg/kg could be considered in a very fractious patient in lieu of a preoperative opioid
- Consider metoclopramide (0.1 to 0.5mg/kg IV) for brachycephalic dogs
- Pre-clip abdomen (if tolerated)
- Preoxygenate for two to five minutes (if tolerated)
- Perform a line block with lidocaine 1 to 2mg/kg (before induction, if tolerated; if not, afterwards)
- Induce patient (propofol 2 to 4mg/kg IV titrated to effect, or alfaxalone 1.6 to 3mg/kg IV)
- Intubate rapidly with a cuffed endotracheal tube
- Conduct the final aseptic preparation of the abdomen
- Set isoflurane or sevoflurane according to anaesthetic depth
- Aim for time from induction to the delivery of the first newborn to be around 15 minutes
- Provide a top-up (if required) or first use of opioids after the last newborn is delivered
- Use a bupivacaine splash block on the midline during closure
- Resuscitate the litter with use of appropriate reversal agents if required (eg naloxone, atipamezole or flumazenil)
- Use NSAIDs and maropitant with or without paracetamol (for dogs only) in recovery