Abstract
Key Implications
Key Facts
Definition Failed intubation is the inability to successfully intubate the trachea after two attempts despite optimal positioning and the use of adjunctive techniques, with a third attempt undertaken only by an additional experienced anaesthetist.
Incidence 1 in 224 [1]. Almost 10 times higher in the obstetric population than in the general surgical population.
Analysis of the Confidential Enquiry into Maternal Deaths and Morbidity from 2009 to 2011 revealed two deaths due to airway problems: one due to potentially unrecognised bronchospasm with failure to ventilate and one after hypoventilation after extubation. Reassuringly, there have been no maternal deaths following airway complications reported in the UK since the publication of this report.
The incidence of general anaesthesia continues to decline, as regional techniques are now performed for complex obstetric indications such as placenta praevia and maternal cardiac conditions which were previously indications for general anaesthesia. At the author’s institution only 2% of all caesarean sections are performed under general anaesthesia.
General anaesthesia may be indicated in cases of urgency, maternal refusal of regional technique, inadequate regional technique or when regional technique is contraindicated such as in cases of coagulopathy or the concurrent use of anticoagulants.
According to the NICE Clinical Guideline for Caesarean Section, 41% of categor1 caesarean sections were under general anaesthesia in the UK in 2004.
Key Implications
Maternal
Major complications of general anaesthesia include failed intubation, aspiration of gastric contents, increased blood loss and awareness.
Anatomical changes in pregnancy such as increased adipose tissue, pharyngeal and laryngeal oedema, large tongue and enlarged breasts result in decreased Mallampati scores and contribute to the likelihood of difficult laryngoscopy [1]. Mallampati scores have also been shown to deteriorate during labour [2].
In pregnancy there is an increase in volume and acidity of gastric contents, a reduction in lower oesophageal sphincter tone and raised intra-abdominal pressure due to the gravid uterus. The incidence of gastric aspiration has been demonstrated to be 5.6 per 1,000,000 maternities in the UK, of which 75% of cases are associated with general anaesthesia [3]. Morbidity is reduced by the use of prokinetics, head up tilt, rapid sequence induction, antacids and proton pump inhibitors [4].
The estimated blood loss for caesarean section under general anaesthesia has been shown to be greater than under neuraxial blockade. A systematic review estimates blood loss to be 100–200 mL greater with general anaesthesia compared to regional techniques [5].
There is a higher incidence of awareness due to a fear of volatile agents causing oversedation of the fetus and reduced contractility of the uterus. The 5th National Audit Project of the Royal College of Anaesthesia estimates the incidence of accidental awareness under general anaesthesia for caesarean section as 1 in 670, compared to 1 in 19,000 in the non-obstetric population[6].
Neonatal
Placental transfer of general anaesthetic drugs has traditionally been implicated in causing lower 1- and 5-minute APGAR scores in neonates [7]. Importantly, more recent studies into the impact of general anaesthesia on the short-term outcomes of neonates delivered by caesarean section have been equivocal, some demonstrating no difference, while in line with traditional thinking [8, 9].
Database analysis of caesarean section under general anaesthesia demonstrated a difference in the rate of neonatal resuscitation based on urgency of delivery. While only 3% of neonates delivered via elective caesarean section required endotracheal intubation and ventilation, 12.3% of those delivered as an emergency required such intervention. Given that both groups received a general anaesthetic, this difference is likely to represent a degree fetal distress, where the fetus may already be compromised to some extent, rather than a direct effect of placental transfer of general anaesthesia [10].
Opioids are not used routinely as part of the anaesthetic induction but in cases of maternal cardiac disease or hypertensive disorders of pregnancy opioids may be given in order to attenuate the hypertensive response to laryngoscopy and intubation. These drugs often result in neonatal respiratory depression and the neonatal team must be made aware of their use so that appropriate support may be given.
Similarly, it should be noted that pethidine administered for maternal analgesia during labour has an elimination half-life of 2–4 hours in the mother and 7–34 hours in the neonate and therefore may have an impact following delivery [11].
Periods of prolonged maternal hypoxia which can occur during a failed intubation may adversely affect neonatal neurological outcome.
Key Pointers
General anaesthesia is often administered in emergency situations and out of hours by trainees whose experience may be limited. It is more commonly required in category 1 caesarean sections when there is a considerable amount of pressure on the anaesthetist to act quickly. The recent reduction in hours worked by junior doctors in the UK due to the European Working Time Directive has compounded this situation.
Physiological and anatomical changes of pregnancy result in reduced functional residual capacity which is worsened by the supine position. In addition, oxygen requirements and cardiac output are increased, resulting in a tendency to rapid oxygen desaturation during periods of apnoea.
Key Diagnostic Signs for General Anaesthesia
Consent for general anaesthesia is difficult, as it is frequently administered in emergency situations. High-risk patients should be identified on the delivery suite, allowing earlier identification of problems.
Where possible, high-risk patients should be identified antenatally, allowing formal review in a High Risk Obstetric Anaesthesia Clinic. Here operative risk can be stratified and appropriate plans made in a timely fashion.
A full preoperative assessment including evaluation of the airway must be performed prior to administering general anaesthesia, especially as the obstetric population is becoming more medically complex. Even in the category 1 emergency caesarean section there is time to ascertain a succinct medical and drug history and examine the airway.
Cardiovascular history is particularly relevant, as cardiac disease remains the most common cause of indirect maternal death in the UK [12].
Current haemoglobin and confirmation a valid group and save sample has been processed are equally important.
Obesity poses specific challenges for general anaesthesia. Regional anaesthetic techniques are preferable but technically more difficult [13, 14]. These patients also have a higher incidence of surgical delivery [15]. Anaesthetic problems relating to obesity include
◦ Increased likelihood of difficult airway management.
◦ Reduced chest wall compliance results in high inflation pressures required for ventilation.
◦ Anatomical and physiological changes which predispose to oxygen desaturation are exaggerated in the obese parturient.
A pertinent obstetric history should be elicited to ascertain specific anaesthetic concerns such as risk of postpartum haemorrhage, evidence of preeclampsia, multiple surgical deliveries and fetal condition.
An anaesthetic history is required to identify issues with previous general anaesthetics or a family history of problems. There are two specific conditions of note:
◦ Plasma cholinesterase deficiency in the form of suxamethonium apnoea is an autosomal recessive inherited condition which results in a reduced level of the enzyme which metabolises suxamethonium. This causes prolonged muscle paralysis and the need for mechanical ventilation and sedation on an intensive care unit. The incidence of clinically significant suxamethonium apnoea has been estimated at 1 in 2800 patients; however, pregnancy is thought to induce a relative deficiency of plasma cholinesterase [16].
◦ Malignant hyperthermia is an autosomal dominant inherited condition, with an estimated incidence of 1 in 5000 to 50 000 patients. It is a life-threatening condition caused by abnormal skeletal muscle contraction and increased metabolism which results in severe metabolic acidosis, hyperkalaemia, severe pyrexia and rhabdomyolysis. It is triggered by suxamethonium and volatile inhalational anaesthetic agents; consequently these patients must receive total intravenous anaesthesia and suxamethonium should be avoided.
A thorough airway evaluation must be performed on all patients presenting for general anaesthesia to identify those with anticipated airway difficulties. The following are a selection of bedside tests which can be performed in less than a minute and can identify potential difficulties with intubation:
Mouth opening; patient opens mouth as wide as possible. Distance of less than three finger breadths between incisors predicts reduced airway access
Mallampati assessment; ability to visualise different structures within the oropharynx (Figure 32.1). Grades 3 and 4 are predictors of difficulty.
Range of neck movement, reduced flexion significantly restricts views at laryngoscopy.
Figure 32.1 Grading of predictors of difficulty for intubation (Mallampati assessment).
Thyromental distance; distance between the chin and thyroid cartilage of less than 7 cm is a predictor of short neck and difficulty with intubation.
Jaw protrusion; assesses temporomandibular joint movement. The patient attempts to bite their top lip with their lower teeth. Lower incisors should move further forward than the upper incisors. The inability to perform this manoeuvre can also indicate difficulties.
Prominent dentition; protruding incisors can cause difficulty with instrumenting the airway. Loose teeth pose an aspiration risk if dislodged into the airway.
Facial trauma or tongue swelling.
Hoarseness, stridor or change in voice character predict oedema of the larynx and may indicate the need for a smaller endotracheal tube.
General Anaesthesia Key Actions
Equipment and Preparation
Most general anaesthetics are administered in emergency situations; therefore drugs and equipment must be prepared in advance. Every 24 hours the anaesthetic machine and airway equipment must be checked and emergency drugs prepared ready for immediate use.
A size 7.0 cuffed endotracheal tube is most commonly used; however, smaller and bigger tubes must be present on the airway trolley. Different laryngoscopy blades, in addition to the standard Macintosh, are necessary; short-handle, McCoy and polio laryngoscopes can aid intubation in the patient with limited mouth opening and large breasts.
The use of video laryngoscopes in the obstetric population has increased in recent years [17]. A video laryngoscope, such as the GlideScope, C-MAC or McGrath laryngoscopes should be considered.
The gum elastic bougie is a useful adjunct in difficult intubation and is easy and cheap to use. Rescue supraglottic airway devices, most commonly a laryngeal mask airway/i-gel airway, must be readily available on the airway trolley.
Starvation and Antacid Prophylaxis Premedication
For elective cases patients should fast for 6 hours for solids. The author’s institution has recently relaxed its fasting guidance for clear fluids to 1 hour.
Elective patients should also receive preoperative antacid prophylaxis and metoclopramide to increase gastric emptying. Sodium citrate antacid is also administered orally to all patients just prior to induction due to its short duration of action.
Women in labour at high risk of surgical intervention should receive regular prophylactic antacid and prokinetic medication, such as ranitidine and metoclopramide.
All pregnant women beyond the first trimester presenting for general anaesthesia are at risk of aspiration and should receive a rapid sequence induction. Analysis data collected from the South West Thames Region Hospitals of the UK between 1987 and 2016 demonstrated that 2% of obstetric patients with failed intubation experienced gastric aspiration.
Rapid Sequence Induction
The purpose of rapid sequence induction is to secure a definitive airway with a cuffed endotracheal tube as quickly as possible in patients at high risk of aspiration. A force of 10 Newtons of cricoid pressure is applied to the cricoid cartilage to occlude the oesophagus and prevent aspiration of gastric contents, while awake, prior to induction. This should be increased to 30 Newtons following induction. It must be performed by a skilled operator familiar with the technique and can be removed only once the cuff is inflated and the tube position confirmed.
The patient must be appropriately positioned on the operating table to aid intubation, and lateral tilt applied until the baby is delivered to relieve aorto-caval compression. Large-bore intravenous access must be secured with intravenous fluids attached and running at an appropriate rate. Mandatory standards of monitoring must be attached [18] – non-invasive blood pressure, pulse oximetry, capnography and three-lead ECG – and present until the patient is fully awake. Suction must be turned on and easily accessible in case of aspiration at induction.
Strict pre-oxygenation is fundamental to a safe rapid sequence induction. The purpose is to ensure an adequate oxygen reservoir to prevent the patient profoundly desaturating when apnoeic. The functional residual capacity is de-nitrogenised by administering 100% oxygen for 3 minutes, or if time is limited by asking the patient to take 4 vital capacity breaths. Ideally the expired oxygen should be greater than 90% and a capnography trace should be seen to ensure there is an adequate seal of the anaesthetic circuit. This is performed while the surgeon is scrubbing and prepping the patient.
The patient must be reassured that surgery will not commence until anaesthesia has been induced but all preparations are made before starting to minimise placental transfer to the fetus. Additionally, they should be warned about the application of cricoid pressure.
Induction of anaesthesia should begin only once the entire theatre team are fully prepared and ready to make the first skin incision.
Traditionally the following drugs have been used in rapid sequence induction in obstetrics.
◦ Thiopentone is a barbiturate used to induce anaesthesia. The dose is 3–5 mg/kg, it has a quick onset of action causing unconsciousness within one arm to brain circulation time and the half-life is 3 minutes.
◦ Suxamethonium is a depolarising muscle relaxant. The dose is 1–2 mg/kg, the time of onset is 30 seconds, leading to widespread fasciculations followed by rapid paralysis which facilitates intubation. It has a half-life of 3 minutes.
In recent years, the drugs considered appropriate for use in a modified rapid sequence induction in obstetrics has expanded, in line with common practice in non-obstetric anaesthesia.
◦ Propofol is a phenol derivative used to induce anaesthesia, with a bolus induction dose of 1.5–2 mg/kg. Consciousness is lost after 30–40 seconds, with a distribution half-life of 1.3–4.1 minutes.
◦ Rocuronium is an aminosteroid non-depolarising muscle relaxant. In the context of a rapid sequence induction, a dose of 1 mg/kg is recommended, bringing about optimum intubating conditions within 60 seconds. It has a longer duration of action than suxamethonium, with recovery of muscular function after approximately 1 hour following a 1 mg/kg dose.
If the decision is made to modify the rapid sequence induction by including rocuronium in place of suxamethonium, the resulting longer duration of neuromuscular block should be recognised in the event of a failed intubation. Consequently, simply waiting for the neuromuscular function to return and waking the patient up is not an option. It may be prudent to have an appropriate dose of the modified gamma cyclodextrin sugammadex available. A larger, 16 mg/kg dose is recommended for the immediate reversal of an intubating dose of rocuronium.
Commencement of surgery should occur only once intubation has been confirmed with capnography, bilateral chest movement and audible bilateral air entry.
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