Caesarean section in the bitch is a commonly seen emergency procedure often performed at night when there are limited staff members on hand. Anaesthesia of the pregnant bitch requires careful consideration of a number of factors that may often be overlooked due to the urgent nature of the surgery. Furthermore, the nursing role comprises not only monitoring and maintenance of the anaesthetic, but also expeditious preparation of the bitch, resuscitation of the neonates and provision of consumables and surgical equipment to the veterinary surgeon, all simultaneously. An underlying knowledge of the physiological effects of anaesthetic drugs and appropriate anaesthetic technique are essential to maximize the survival rates of the bitch and neonates and promote a successful outcome for caesarean surgery.
Considerations
Complicating factors in anaesthesia of the peri-parturient bitch include altered physiology of the bitch during pregnancy, immature physiology of the foetuses, and elective versus emergency presentation. Further developments such as dehydration, hypovolaemia, hypotension, exhaustion, hypothermia, toxaemia, hypoxia, haemorrhage and shock, may also be present if dystocia (slow or difficult labour/delivery) has been in progress for some time (Ryan and Wagner, 2006a). All of these will result in changes in the pharmacokinetics and pharmacodynamics of injectable and inhalant anaesthetic drugs.
When considering anaesthesia for caesarean section, there should be four main aims:
- Delivery of viable neonates with minimal depression
- Production of minimal post-operative maternal depression
- Provision of sufficient analgesia, muscle relaxation and narcosis to enable surgery to be performed
- The return of the bitch and neonates to their own environment as soon as possible (Figure 1).
In order to achieve these aims, it is necessary to understand and meet the unique anaesthetic requirements of the bitch and neonates.
Physiological changes in the peri-parturient bitch
Haemodynamic changes include an increase in maternal blood volume, by approximately 40%, and cardiac output in order to provide adequate blood flow to the foetuses. As a consequence, blood pressure and cerebral perfusion are increased, which results in a more rapid induction (Paddleford, 1992). This increase in plasma volume is not proportional to the increase in erythrocytes therefore haemodilution and relative anaemia develop, which is thought to increase with the number of foetuses (Ryan and Wagner, 2006a). This anaemia further decreases induction time, which must be borne in mind to avoid excessive anaesthetic depth immediately following induction (Rawstron, 1968).
Ventilatory changes include an increased pressure on the diaphragm by the gravid uterus resulting in a reduction in functional residual capacity by 20% and a decrease in total lung volume (Paddleford, 1992). In addition, the pregnant bitch has an increased oxygen consumption of 20%. Tidal volume and minute ventilation are, therefore, increased to compensate for the greater oxygen demand and to prevent desaturation and hypoxaemia (Meyer, 2007). This results in a more rapid induction with inhalant anaesthetics due to a faster equilibration between inspired and alveolar volatile agent concentration (Ryan and Wagner, 2006a). The increased progesterone levels in advanced pregnancy are thought to affect sensitivity of the respiratory centre to carbon dioxide resulting in tachypnoea, thereby causing an increase in alveolar ventilation, and a further rise in the alveolar concentration of inhalation agent (Muir et al, 2007). Finally, if the procedure is non-elective, the bitch's respiratory rate may be increased due to pain or discomfort, which will further exacerbate the effect due to a proportional increase in anaesthetic gas intake with increased respiratory rate (Paddleford, 1992).
In relation to effects on the central nervous system (CNS), Pascoe and Moon (2001) propose an increase in potency of as much as 25% for halothane and 28–40% for isoflurane due to increased CNS sensitivity to volatile agents secondary to elevated levels of progesterone and β-endorphins during pregnancy. This, therefore, reduces the required volume of inhalation anaesthetic further still.
With regards to gastrointestinal considerations, the bitch experiences delayed gastric emptying during late pregnancy. She will also have decreased lower oesophageal sphincter tone and raised intragastric pressure due to cranial pressure from the gravid uterus, therefore increasing the likelihood of regurgitation or emesis (Marti, 2008). Furthermore, placental gastrin secretion results in an increase in gastric acidity, which will augment the possibility of regurgitation, therefore placing the bitch at risk of aspiration pneumonia. It is, therefore, essential that the airway is protected (Muir et al, 2007).
Physiology of the neonate
Drugs generally last longer in the neonate than they do in the bitch (Muir et al, 2007). Lipid soluble drugs diffuse across biological membranes such as the blood–brain barrier and the placenta with little difficulty. Non-ionized drug forms are more lipophilic than their ionized counterparts and, therefore, cross membranes more easily (Meyer, 2007). However, in the hypoxaemic, acidotic foetus, opioids and local anaesthetics tend to revert to their ionized form. Consequently, ion-trapping can occur, where these drugs cannot diffuse back across the placenta and accumulate in the foetus resulting in a greater degree of foetal CNS and cardiorespiratory depression and a far greater resuscitation risk in the neonate following delivery (D'Alessio and Ramanathan, 1998). Furthermore, the hepatic enzyme systems involved in drug metabolism are immature or absent in neonates, therefore drugs requiring hepatic metabolism have a longer duration of effect and as a result should be used sparingly (Ryan and Wagner, 2006a).
Respiratory and CNS depression of the neonate as a result of anaesthetic drug administration also requires careful consideration. It is essential that minimal quantities of respiratory and CNS depressant drugs are present in the neonate at the time of delivery in order to maximize successful resuscitation efforts. Furthermore, neonates have poor haemodynamic stability (ability to maintain organ perfusion and function through adequate blood pressure), and cardiac output is predominantly heart rate dependent leading to a reduced capacity to compensate for significant changes in blood pressure (Dugdale, 2010). It is, therefore, favourable to avoid the use of anaesthetic drugs causing significant cardiovascular depression.
Anaesthetic technique
Epidural anaesthesia with lignocaine and morphine has the advantage of minimizing exposure of the foetus to anaesthetic drugs, thereby increasing neonatal vigour at delivery (Moon-Massat and Erb, 2002), while providing good regional anaesthesia and abdominal muscle relaxation in the bitch. However, while the use of epidural anaesthesia is advantageous in theory, there are a number of potential complications that should be taken into account. Depending on the condition of the bitch prior to surgery, the use of heavy sedation may be required in order to facilitate the administration of the epidural, which will subsequently affect the neonates. Vasodilation and subsequent hypotension, and lack of airway protection to guard against aspiration pneumonia in the case of regurgitation should also be considered (Ryan and Wagner, 2006b).
In comparison, the disadvantages of general anaesthesia include greater neonatal depression, however, through the use of the correct combination of anaesthetic drugs, this risk can be markedly reduced.
Pre-medication
Pre-medication of caesarean patients is considered to be beneficial for a number of reasons. It can decrease cortisol and catecholamine levels associated with the maternal stress response, thereby preventing constriction of uterine vessels and aiding uterine perfusion to the neonates (Pascoe and Moon, 2001). It also increases the bitch's tolerance of pre-oxygenation by face mask (Figure 2), and allows the reduction of drug dosages for induction and maintenance agents, thereby reducing exposure of the neonates to high drug volumes. The use of short acting agents that can be reversed, such as opioids and benzodiazepines, is recommended as they provide sedation and analgesia with minimal cardiac and respiratory effects, but can be reversed in the neonate if respiratory depression is experienced (Ryan and Wagner, 2006a). However, benzodiazepines, such as diazepam and midazolam, should be used with caution as they are lipophyllic and therefore cross the placenta rapidly, but are slow to be eliminated from the neonate causing mild sedation, hypotonia, and apnoeic spells (Meyer, 2007). In addition, when used in combination with opiates the effects of benzodiazepines are potentiated, which must be taken into account when considering dosages. The use of midazolam is preferable over diazepam due to its shorter duration of action. Pethidine, morphine, oxymorphone and fentanyl are the opioids of choice due to their good sedative and analgesic properties alongside minimal cardiovascular effects (Meyer, 2007; Muir et al, 2007).
Acepromazine should be avoided due to its long duration of action, requirement for hepatic metabolism by the neonate's immature hepatic system, and adverse effects on thermoregulatory and respiratory abilities in the neonate, which are already compromised (Ryan and Wagner, 2006a).
Alpha-2-agonists, such as xylazine and medetomidine, should also be avoided as they have been found to increase neonatal mortality, it is proposed that this is due to a reduction in uterine perfusion causing foetal hypoxia and acidosis (Meyer, 2007).
Induction
The ideal induction will achieve rapid unconsciousness to facilitate intubation, thereby providing airway protection and enabling ventilatory support in the cases of regurgitation or induction apnoea. In order to reduce the negative effects of potential induction apnoea, pre-oxygenation of the bitch with flow-by oxygen for 3–5 minutes prior to induction is recommended (Pascoe and Moon, 2001; Ryan and Wagner, 2006b; Raffe and Carpenter, 2007; Robertson, 2007)
Propofol/isoflurane induction/maintenance has been linked with a reduced neonatal mortality rate when compared with thiopentone/isoflurane. This is thought to be due to the faster metabolism of propofol by the liver thereby reducing the amount of agent remaining in the neonate at the time of delivery. Furthermore, a more rapid recovery following propofol induction means that bitches are able to resume the provision of care for the pups more quickly than those that have received thiopentone (Funquist et al, 1997).
The use of alfaxalone can be considered as it is both short acting and produces minimal cardiorespiratory effects (Raffe and Carpenter, 2007).
Inhalation induction is advantageous in that it is less likely to have a depressive effect on the neonates and is likely to have a fairly rapid onset in the periparturient bitch due to the aforementioned physiological changes. However, there are a number of disadvantages, including stress caused to the bitch, resulting in catecholamine release and subsequent hypoxia and acidosis of the neonates. Furthermore, it increases the length of time that the patient's airway is left unprotected, thereby increasing the risk of aspiration and preventing the provision of ventilatory support if required (Ryan and Wagner, 2006b), therefore is not ideal.
Ketamine is thought to cause foetal hypoxia as a result of increased uterine tone and decreased uterine perfusion (Muir et al, 2007) and has also been shown to increase the risk of neonatal mortality when used in combination with midazolam (Luna et al, 2004), therefore its use is not recommended in caesarean surgery.
Maintenance
Of the inhalation agents, halothane, isoflurane and sevoflurane, that are commonly used in veterinary practice, all have a relatively low molecular weight and high lipid solubility and, therefore, cross the placenta readily. The effect on the respiratory function of the foetus can be minimized by using as little agent as possible for as short a time as possible (Muir et al, 2007). Concurrent administration of fentanyl will allow a reduction in the amount of agent used and thus help to maintain blood pressure and cardiovascular function (Robertson, 2009).
Isoflurane has a much lower solubility than halothane and is therefore eliminated from the body more rapidly, which is advantageous to both bitch and neonates. Hepatic metabolism of halothane is also far greater than that of isoflurane, 20–25% and 1% respectively, with the remainder eliminated by expiration. Halothane is, therefore, less appropriate for use involving neonates with an immature hepatic function (Matthews, 2007). The minimum alveolar concentration (MAC) of inhalation agents is reduced during pregnancy by 25% for halothane and 28–40% for isoflurane, therefore care must be taken with both agents not to inadvertently overdose the bitch and neonates (Ryan and Wagner, 2006b).
Studies have yet to be carried out on the use of desflurane and sevoflurane for caesarean anaesthesia, however, their cardiopulmonary and respiratory depressant effects are similar to those of isoflurane so, theoretically, similar results should be expected (Ryan and Wagner, 2006b). However, on account of their lower solubility, desflurane and sevoflurane would cross the placenta more rapidly than isoflurane, exerting a greater depressant effect on the neonate. Therefore, despite the rapid excretion of all inhalation agents once spontaneous respiration occurs, this may be harder to initiate in the more depressed neonate, potentially making isoflurane a better option (Littleford, 2004).
Suggested anaesthetic protocol
Pre-medication with fentanyl, or morphine if fentanyl is not available (Table 1), should be administered to produce sufficient relaxation in the bitch. This enables administration of oxygen by mask for 5 minutes prior to induction as well as clipping and general preparation of the bitch. Naloxone should be available to be administered to the neonates on delivery if required to counteract opioid-induced respiratory depression Table 1). Alternatively, the opioid may be combined with a low dose of midazolam titrated to effect, provided its reversal agent flumazenil is available, due to its potential for reversal and shorter duration of action than diazepam. However, it should be noted that benzodiazepines are less effective in stressed animals (Monsey, 2008).
Table 1. Suggested drug doses
| Premedication | fentanyl | 3 µg/kg IV (Muir et al, 2007) |
| morphine | 0.1–0.5 mg/kg IM/IV | |
| Reversal agents | naloxone | 0.04 mg/kg IV/IM (Plumb, 2002) |
| flumazenil | 0.01–0.02 mg/kg IV (Harvey, 2009) | |
| Induction | propofol | 2–5 mg/kg IV |
| Epidural | 2% lidocaine (without adrenaline) morphine (preservative free) | 0.3 ml/kg (up to a maximum of 6 ml) |
| 0.2 ml/kg (up to a maximum of 6 ml) |
IV, intravenous; IM, intramuscular
General anaesthesia should be induced with propofol intravenously (Table 1), titrated to effect. Close attention should be paid to the risk of induction apnoea in the bitch and intermittent positive pressure ventilation started if necessary to prevent development of maternal and foetal hypoxaemia.
The bitch should then be maintained on isoflurane and oxygen, not forgetting that the MAC is reduced by 28–40% for isoflurane in pregnant bitches. Care should be taken with the vapouriser setting and the bitch should be monitored closely for signs of anaesthetic overdose. The aim should be to maintain the bitch on as light a surgical plane of anaesthesia as possible. It may be beneficial to provide the bitch with intermittent sigh breaths throughout the procedure at a pressure of 15–20 cmH2O for 1–2 seconds' duration to minimize the severity of atelectasis (collapse of the lung tissue) and to re-expand the lungs (Robertson, 2009).
To maximize neonatal survival, epidural anaesthesia produces the least foetal CNS and cardiorespiratory depression. However, in the vast majority of cases, this will not provide sufficient restraint of the bitch to enable surgery to be performed, therefore general anaesthesia would be necessary. If the bitch presents as debilitated and the anaesthetist has sufficient experience, an epidural can be administered with 2% lidocaine or morphine at 0.3 ml/kg or 0.2 mg/kg bodyweight respectively, up to a maximum of 6 ml (Muir et al, 2007).
Throughout the procedure careful monitoring of cardiovascular and respiratory parameters should be carried out through the use of a stethoscope, capnography, pulse oximetry, electrocardiography and blood pressure recordings.
Above all it should be remembered that each patient must be treated as an individual, and while the above recommendations have been shown to be advantageous in the caesarean patient, the anaesthetic protocol should be reviewed and adapted to suit each patient according to their presenting signs and requirements.
Further study
There are a number of anaesthetic agents that have become more widely used in the veterinary world in recent years such as sevoflurane, desflurane and alfaxalone. As yet, there have been limited studies carried out on their use in the peri-parturient bitch despite them, theoretically, being potentially well suited to the situation. Studies that have been carried out have either been in the human field, and therefore their validity is questionable in veterinary application, or have been sponsored by the manufacturer, and so potentially have reliability constraints. Further research into the efficacy and safety of alfaxalone in comparison with propofol, and sevoflurane/desflurane in comparison with isoflurane would be beneficial.
Conclusion
When anaesthetizing the peri-parturient bitch it is essential to be aware of the altered physiology associated with pregnancy and the subsequent effect this will have on the anaesthetic. The bitch will have an increased potential for oxygen desaturation and hypoxaemia, an increased sensitivity to drugs, and will require lower drug dosages. She will be more prone to vomiting or regurgitation, which poses an increased risk of developing complicating factors such as aspiration pneumonia.
In an ideal world, epidural anaesthesia would be the best option for both bitch and neonates, however, this requires an experienced anaesthetist and a very well behaved, or collapsed patient, and therefore is unsuitable in the majority of cases.
When selecting anaesthetic drugs it is important to bear in mind that due to the bitch's increased sensitivity for many drugs, all should be titrated to effect and the bitch closely monitored throughout for signs of anaesthetic overdose. Drugs with a short duration of action, are rapidly metabolized by the bitch, and require minimal hepatic neonatal metabolism, and are, therefore, desirable. Drugs that are reversible are also advantageous so that their undesirable effects can be antagonized in the bitch or neonates if required. The length of use and the dose of inhalant anaesthetic agent should be kept to a minimum where possible. This can be facilitated by the implementation of strategies such as local nerve blocks or epidural anaesthesia to enable the volatile agent to be used at much lower doses. In principle opioid/benzodiazepine premedication, propofol induction and isoflurane maintenance is an ideal combination, drugs such as phenothiazines, alpha-2-adrenoreceptor agonists and barbiturates should be avoided. However, it is important to remember that no two patients are the same; breed, age, underlying systemic disease, emergency versus elective presentation, and duration of dystocia all affect the pharmacological action of the drugs used, therefore an anaesthetic plan should be tailored to the individual patient following initial examination and assessment.
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Key Points
- The peri-parturient bitch has unique anaesthetic requirements due a number of physiological changes that affect the action and effect of drugs.
- Anaesthetic drugs can cause marked neonatal depression and thus affect neonatal survival rates.
- It is essential that the veterinary nurse is familiar with the anaesthetic requirements of the bitch and neonates in order to ensure a successful outcome.