Safe General Anaesthesia for Mums

Safe General Anaesthesia for Mums

  • issue 78

Marc Van de Velde, MD, PhD, EDRA
Marc.vandevelde@uzleuven.be

Since the seminal publications by Mendelsohn (on gastric content aspiration) and Hodges et al. in 1959 (on thiopental and succhinylcholine), general anaesthesia (GA) for Caesarean section (CS) is induced using a “modified” technique of rapid sequence induction (RSI) which is different from a conventional RSI induction for procedures in non-pregnant patients. The technique is further modified because of fear of transplacental passage of anaesthetic agents (especially opioids) and postnatal depression of the neonate. Typically, a RSI in pregnancy includes:

  • Skin disinfection and surgical preparation is performed prior to induction of anaesthesia to shorten the induction to delivery interval.
  • Administration of acid aspiration prophylaxis to reduce gastric content and increase gastric pH.
  • Reduced induction dose of hypnotic drug (thiopental usually).
  • No opioid prior to delivery of the baby because of fear of neonatal depression.
  • Rapid acting muscle relaxant (typically in textbooks succhinylcholine).
  • Cricoid pressure is applied. No mask ventilation is performed.
  • Following intubation, surgery is immediately started.
  • Following delivery of the baby, anaesthesia is deepened using opioids and higher volatile concentrations.

This technique is associated with potential problems:

  • Increased risk of maternal mortality: In recent reports, the relative risk of dying with GA compared to regional anaesthesia is 1.7. Airway problems (aspiration, esophageal intubation and/or failed intubation) remain a constant and significant problem
  • Maternal morbidity and mortality also occur as a result of hypertension and intracranial haemorrhage occurring at the time of delivery, especially in patients suffering from preeclampsia.
  • The National Audit Project (NAP) 5 report has focused recently on the incidence of awareness during surgery performed under GA. The overall incidence of awareness was 1/20.000 anaesthetics performed. Risk factors were opioid free anaesthesia, emergency/unplanned procedures and procedures in which muscle relaxants were administered. Therefore, it comes as no surprise that the incidence of awareness was highest in obstetric patients undergoing GA for Caesarean Delivery (1/670).

In view of these issues, several modifications to this “traditional” RSI technique for CS have been proposed in recent years so that safety for mums is increased.

Opioids at induction.

The conventional RSI technique of induction of general anaesthesia withholds opioids prior to delivery of the fetus because of fears it might induce post-delivery depression of the neonate. In recent years therefore several authors suggest to induce anaesthesia also with an induction dose of opioid. Most likely short acting, lipophylic opioids with rapid clearance, also in neonates, are preferred based on theoretical considerations.

A growing number of studies and case reports have used remifentanil at induction of GA for RSI in pregnancy and noted significantly better haemodynamic stability. Remifentanil has the highest transplacental passage of all opioids. Van de Velde et al. were the first to prospectively report a series of 10 C-section patients undergoing GA with remifentanil at induction and noted that 50% of neonates were briefly (1-5 minutes) respiratory depressed. Tactile stimulation and mask ventilation were required in these infants; however endotracheal intubation was not performed. Hence it can be concluded that short-lived, minimal respiratory depression of the neonates is reported. However, overall the neonatal effects are minimal.

Pre-oxygenation

Denitrogenation and adequate pre-oxygenation prior to induction of GA are common practices to avoid rapid desaturation upon induction of anaesthesia and before securing the airway. Using succhinylcholine as the muscle relaxant has been shown to induce more rapid desaturation then non-depolarising muscle relaxants such as rocuronium in non pregnant individuals. Positioning the patient at induction can probably also improve oxygenation. Hignett et al demonstrated in term parturients that a 30° head-up position increased the functional residual capacity with 188 mL from 1.51 L to 1.70 L. Russell et al. studied the effect of the fresh oxygen flow rate (5 or 10 or 15 L/min) for pre-oxygenation during tidal volume breathing using a circle system. Higher flow rates were associated with higher end tidal oxygen concentrations after 3 minutes of pre-oxygenation. In a computer simulated physiological modelling system, McClelland et al. demonstrated that 2 minutes of tidal volume breathing results in the most optimal maternal oxygen content. Vital capacity breathing is far less effective then in non-pregnant individuals.

A more recent development might also be useful to maintain oxygen saturation during apnoea at induction or during intubation efforts. In a non-pregnant population, with difficult airways, the so-called transnasal humidified rapid-insufflation ventilator exchange (THRIVE) was used to prolong apnoea times. During pre-oxygenation in the 40° Head-up position a high flow of oxygen (70 L/min) was administered using the Optiflow nasal cannula for 10 minutes. Oxygen saturation could be maintained at levels above 90% for up to 65 minutes in some patients. Recently, THRIVE was studied in a pregnant population presenting for emergency C-section and the results presented as an abstract during the most recent annual meeting of the OAA (McMaster). No adverse effects were noted. It therefore seems a potential tool that can be used during obstetric airway management, however more study is required.

Cricoid pressure

The use of cricoid pressure during RSI has been questioned in recent years. Several studies have indicated that cricoid pressure might be unsuccessful in achieving its primary goal which is to seal the oesophagus. In various volunteer studies using MRI imaging, cricoid pressure proved to deviate the oesophagus but did not provide sealing. Additionally, cricoid pressure might worsen the view at laryngoscopy making intubation problematic. In a large retrospective study by Fenton and Reynolds, cricoid pressure was actually associated with higher mortality and produced no protection against aspiration of gastric contents. Therefore, many experts and guidelines now advocate to use cricoid pressure but when intubation is unsuccessful at a first attempt to consider releasing it and perform an intubation attempt without the application of cricoid pressure.

 

LMA.

It is now very well accepted that GA can be performed in CS using a laryngeal mask airway (LMA). Second generation LMAs are considered safe in pregnancy and guidelines indicate that they should be used when normal intubation is not possible after one or two attempts.

 

Maintenance of anaesthesia.

Inhalational anaesthesia together with post-delivery opioids are considered the gold standard of care. However, inhalational anaesthesia has been shown to reduce uterine tone possibly resulting in more blood loss. Also, the current technique of thiopenthal, succhinyl choline and low MAC values of inhalational anaesthetic is causing potentially a short period of inadequate anaesthetic levels promoting awareness. In recent years the use of total intravenous anaesthesia (TIVA or TCI) has been investigated. Van de Velde et al. were the first to use this technique with good outcome. Although many aspects of propofol pharmacokinetics in pregnancy are not known and although propofol has potentially negative effects on the neonate (similar to those of inhalational anesthestics), TIVA or TCI anaesthesia offers the advantage to avoid a “gap” in anaesthetic delivery to the brain thus improving the quality of anaesthesia. This potentially comes with depressed neonatal conditions. However, based on case reports and several studies, the effects of a TCI technique (using propofol) or the use of a technique using opioids seems minimal and short-lived.

 

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