5 General anaesthesia for caesarean section revisited

5 General anaesthesia for caesarean section revisited M A R K U S C. S C H N E I D E R

There is no doubt that the many risks associated with the administration of anaesthesia for obstetrics continue to contribute to maternal morbidity and mortality. In the past, ignorance of the physiological changes associated with pregnancy has resulted in rather high maternal morbidity and mortality rates linked to anaesthetic practice (Morgan, 1987; Sachs et al, 1989; Department of Health and Social Security, 1991, 1994; Chadwick et al, 1991). According to the last Report on Confidential Enquiries into Maternal Deaths in the United Kingdom, problems and complications related to anaesthesia were responsible for four early and one late maternal deaths and a contributory factor in ten additional deaths reported between 1988 and 1990 (Department of Health and Social Security, 1994). Retrospective analysis of the circumstances resulting in such a catastrophic anaesthetic outcome revealed that the majority of these maternal deaths were caused by difficulties and complications encountered during the administration of general anaesthesia. As in previous years, general anaesthesia was much more likely to contribute to maternal mortality than regional anaesthesia (Department of Health and Social Security, 1991, 1994). In order to avoid some of the problems associated with obstetric anaesthesia and to reduce the burden of impending litigation, a remarkable shift in current practice has occurred as evidenced by increasing rates of regional anaesthetics performed in parturients presenting for caesarean deliveries (Gibbs et al, 1986; Hawkins et al, 1994; Glosten, 1994). Therefore, part of the decline achieved in anaesthesia-related maternal mortality over the last decades may not only be the result of improved safety standards implemented in obstetric anaesthesia practice but also appears to be a consequence of profound changes in common policies in favour of regional anaesthetic techniques (Table 1). Results obtained from an audience survey performed at the 1993 annual meeting of the Society of Obstetric Anesthesia and Perinatology (SOAP) clearly substantiated this trend away from general anaesthesia for caesarean delivery, as only 11 out of 227 responding participants (5%) indicated that more than 25% of their caesarean sections were performed under general anaesthesia (Glosten, 1994). Batlhdre "s Chmcal Anaesthestology649 Vol 9, No 4, December 1995 ISBN 0-7020-2071-0

Copyright © 1995, by Baillirre Tindall All nghts of reproducUon in any form reserved

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M. C. SCHNEIDER Table 1. Anaesthesia for caesarean section USA* USA* Baselt Baselt

Period 1981 1992 1982 1994

General (%) 41 (35--46) 17 (12-20) 86 11

Epldural (%) 21 (12-29) 44 (32-56) 14 51

Spinal (%) 34 (33-37) 40 (33-48) none 38

* From Hawkins et al (1994, Anesthestology 81: A1128) with permxsslon ~ Data courtesy of Dr Nenad Pavlc, Department of Obstetrics and Gynaeeology, Umverslty Women's Hospital, CH-4031 Basel, Switzerland

In the long term, such a shift in management of obstetric cases presenting for caesarean delivery paradoxically may again increase the hazards related to this risky 'business' of our professional activities since lack of practice has repeatedly been held as an important factor contributing to adverse anaesthetic outcome (Department of Health and Social Security, 1991, 1994). Under such circumstances, some prerequisites deemed necessary for achieving professional performance levels might not only be missing in routine procedures but also in emergency situations for which general anaesthesia should be immediately available (Lussos and Datta, 1993). Obstetric emergencies such as life-threatening antepartum haemorrhage related to placenta praevia or abruption and severe fetal distress due to umbilical cord prolapse or shoulder dystocia often occur quite unpredictably, be it in the middle of the night or at places without trained anaesthesia personnel readily available for support. Such conditions can be life-threatening and the parturient as well as her baby may be at risk. As a consequence, emotional pressure and mental stress may impinge tremendously on decision-making and the professional performance of both the obstetrician and the anaesthetist. Under such circumstances, lack of training or practice and failure of communication or co-ordination may easily result in maternal and fetal morbidity or even mortality (McDonald and Jacoby, 1995). SOME IMPORTANT CHANGES OF ANATOMY AND PHYSIOLOGY DURING PREGNANCY Cardiovascular alterations

Pregnancy is associated with major changes in anatomy and is characterized by a far-reaching adaptation of most physiological functions to the increased metabolic and respiratory demands (Moir, 1980a; Camann and Ostheimer, 1990). Thus, in response to enhanced oxygen consumption, there is a continuous rise in cardiac output reaching a maximum by about 28 to 32 weeks of gestation as a result of an increase in heart rate and a concomitant decrease in systemic vascular resistance. Under the influence of gestational hormones, total blood volume increases by 35% during pregnancy and results in dilutional anaemia as the increase in plasma volume exceeds the increase in red blood cell volume. Extravascular fluid

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volume also increases during pregnancy reaching a peak 1500 ml above non-pregnant levels at term (Moir, 1980a). The gravid uterus receives about 20% of cardiac output at term which corresponds to 500 ml per minute. Approximately 80% of uteroplacental blood flow goes to the placenta, whereas 20% supplies the uterus (Moir, 1980a). One of the most important changes in maternal physiology is brought about by the enlarged uterus during the second half of pregnancy. In the supine position, the gravid uterus puts pressure on both major intraabdominal vessels, aorta and inferior vena cava, involving a considerable risk of inducing the so-called aorta-caval occlusion syndrome by mechanical compression (Eckstein and Marx, 1974). As a consequence, venous return to the heart, stroke volume and cardiac output are reduced and acute hypotension may ensue; these changes not only affect maternal well-being but may also cause fetal distress as uteroplacental perfusion is pressuredependent. Maternal supine hypotensive syndrome indicates the end-point of a process which may remain concealed as long as compensatory mechanisms such as vasoconstriction and tachycardia are effective in maintaining systemic blood pressure (Crawford et al, 1972). Nausea and fainting may occur and appear to be related to reduced cerebral perfusion pressure.

Respiratory alterations Increasing metabolic demands imposed by the growing fetus and hormonal changes induce a marked change in alveolar ventilation which reaches 70% above baseline at term (Prowse and Gaensler, 1965). This is achieved by an almost 40% increase of tidal volume due to changes of rib cage conformation and a somewhat smaller rise in respiratory rate (Camann and Ostheimer, 1990). The marked increase in oxgen consumption is acccompanied by a partially compensated respiratory alkalosis with maternal PaCO2 values ranging between 3.8 and 4.2 kPa (Prowse and Gaensler, 1965). During the course of pregnancy, the expanding uterus displaces the diaphragm more and more cephalad, thus reducing functional residual capacity, which consists of expiratory reserve volume plus residual volume, by almost 20% (Knuttgen and Emerson, 1975). Physiological dead space is decreased at term, probably in response to improved matching of ventilation and perfusion (Camann and Ostheimer, 1990). As pregnancy progresses, the closing volume increases and may exceed the functional residual capacity in about 50% of pregnant women so that airway closure may occur within the normal tidal volume range (Moir, 1980a). Important anatomical changes of the oropharynx may occur which contribute to an eightfold increase in frequency of difficult intubations seen in pregnant as compared with nonpregnant patients. According to a retrospective analysis by Samsoon and Young (1987), failure of laryngoscopic orotracheal intubation occurred in one out of 280 obstetric patients, whereas this only happened in one out of 2230 non-obstetric cases. As total body water increases during pregnancy, the soft tissues of the oral, nasal and pharyngeal cavities may undergo oedematous changes sometimes resulting in significant swelling and increased vulnerability to mechanical manipulation. These changes can be

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accompanied by engorgement of the larnygeal mucous membranes in parturients with pre-eclampsia (Brock-Utne et al, 1977). Gastrointestinal alterations Although the issue of gastrointestinal changes associated with pregnancy remains controversial (Macfie et al, 1991), it seems to be prudent to assume that gastric emptying may be unpredictably prolonged and that, therefore, the risk of a full stomach should always be considered irrespective of the time between the last meal and the onset of labour (Roberts and Shirley, 1974; O'Sullivan et al, 1987). Placental production of gastrin adds to the likelihood that a high volume of low pH gastric juice may be present (Roberts and Shirley, 1974). In addition, the rise in pressure within the stomach may not be offset by an equal rise in lower oesophageal sphincter pressure thereby lowering the important barrier pressure (lower oesophageal sphincter pressure minus gastric pressure). Therefore, pregnant women frequently complain of heartburn indicating lower oesophageal sphincter incompetence and gastro-oesophageal reflux (BrockUtne et al, 1981). A hiatus hernia was detected by X-ray in 27% of pregnant women suffering from heartburn (Mixson and Woloshin, 1956), whereas reflux of gastric acid was demonstrated in 8 out of 10 pregnant women with heartburn versus 3 out of 10 without heartburn (Hey et al, 1977). Nervous system alterations Pregnancy is associated with an increased sensitivity to the depressant effects of volatile inhalation anaesthetics (Palahniuk et al, 1974). This is evidenced by a decrease in minimal alveolar concentration (MAC) values averaging up to 40%, a phenomenon which may be partly explained by elevated plasma and cerebro-spinal fluid progesterone concentrations (Merryman et al, 1954; Datta et al, 1986; Gin and Chan, 1994). In ovariectomized rabbits treated with progesterone, there is an inverse correlation between halothane MAC and progesterone levels (Datta et al, 1989). Furthermore, elevated maternal blood levels of [~-endorphin seem also to contribute to an increased threshold for nociception during pregnancy (Gintzler, 1980). In pregnant rats, nociceptive thresholds were increased for somatic as well as visceral painful stimulation compared with their non-pregnant controls (Gintzler, 1980; Iwasaki et al, 1991). In both experimental settings, pregnancy-induced analgesia could be antagonized by administration of naloxone. IMPACT OF PREGNANCY ON THE SAFETY OF GENERAL ANAESTHESIA Cardiovascular alterations The cardiovascular changes seen during pregnancy have a direct bearing on the management of general anaesthesia. In the healthy parturient, the rise in

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cardiac output and blood volume is well tolerated; in case of pre-existing cardiac disease, serious troubles may arise and even result in maternal and fetal morbidity or mortality. Healthy parturients generally are effectively protected against the risks of blood loss at delivery due to their hypervolaemic state. Blood loss at a lower segment caesarean section under general anaesthesia may be substantial and easily reach 750 to 1000 ml (Moir, 1980b), almost double that encountered during epidural anaesthetic (Moir 1970). Recent years have witnessed a dramatic decline in transfusion practice because of the fear that a blood-borne infectious disease such as hepatitis or AIDS might be transmitted. In line with these considerations, transfusion of blood or blood components is not advocated following caesarean section unless the seriousness of anaemia or concomitant coagulation disorder demands immediate corrective therapy. Consequently, even a blood loss of 1.5 to 2 litres resulting in haemoglobin levels between 60 and 80 g/litre may be solely replaced by equivalent volumes of balanced crystalloid solutions and plasma expanders such as starch, gelatin or albumin. In the event of such a policy being adopted on a large scale, the rate of transfusion would be likely to fall below the 3-7% rate reported in a recent review article on contemporary obstetric blood transfusion practice (Glosten, 1994). One of the most important issues in caring for pregnant patients is to avoid aorto-caval compression associated with the supine position. Parturients should be encouraged to rest in the lateral position and uterine displacement, achieved by placing a wedge under the left or preferably the right hip, should be imperative. It is interesting to note that the vast majority of pregnant women admitted to an antenatal ward adopted a sleeping position which reduced the likelihood of aorto-caval compression (Mills and Chaffe, 1994). For caesarean section, a 30 ° left-lateral tilt is recommended to prevent matemal hypotension and concomitant fetal compromise as a result of reduced uteroplacental blood supply. Though general anaesthesia is less likely to interfere with the intrinsic sympathetic innervation than regional anaesthesia, there is still a considerable risk of worsening the effects of aorto-caval compression by two mechanisms: --compensatory peripheral vasoconstriction may be abolished by the pharmacologic effects of anaesthetics; ---cardiac filling may be further reduced during positive pressure ventilation as the intrapleural pressure rises. The uptake, distribution and elimination of inhalational anaesthetics tend to be more rapid, which is in keeping with the accelerated cardiac pump performance. The rapid uptake of the anaesthetic slows down the rise in its alveolar concentration towards the inspired concentration, an effect which applies much more to compounds with a high than those with a low blood solubility (Eger, 1974). On the other hand, hyperventllation partially counteracts these pharmacokinetie changes and accelerates the rise in alveolar concentration towards the inspired concentration. Therefore, the impact of both the circulatory and ventilatory changes on the kinetics of

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inhalational anaesthetics tends to be rather minor. In contrast, the impact of the various haemodynamic and metabolic changes on oxygen uptake and consumption is crucial as such changes predispose to early desaturation during apnoea (Archer and Marx, 1974).

Respiratory alterations Difficulties in upper airway management continue to contribute substantially to maternal morbidity and mortality. The association of an increase in oxygen consumption and a 20% reduction in functional residual capacity predisposes even healthy parturients to rapid desaturation following short periods of apnoea (Archer and Marx, 1974; Norris et al, 1989). In the presence of morbid obesity, the risk of failed intubation is even further enhanced (Rocke et al, 1992). According to data from a survey on anaesthesia-related maternal deaths in Michigan, obesity was a significant risk factor in 80% of the deaths (Endler et al, 1988). Difficulties of laryngoscopy may be increased by large breasts unless short handled larnygoscopes are used (Datta and Briwa, 1981). Another problem may arise from hyperaemia and swelling of the soft tissue within the upper airway as bleeding may occur impairing visualization of the larynx. Occasionally, as in pre-eclampsia, passage of a normal size endotracheal tube through the vocal cords may be impossible because of laryngeal oedema (Brock-Utne et al, 1977).

Gastrointestinal alterations Delay in gastric emptying and increase in residual gastric volume expose pregnant women to the hazards associated with regurgitation and pulmonary aspiration of gastric contents as soon as the protective airway reflexes are abolished. Since Mendelson (1946) called attention to aspiration pneumonia in 66 parturients who received general anaesthesia with mask ventilation, fears of this looming complication have never subsided within the anaesthetists' community. Although general agreement indicates that volume, acidity and nature of the material aspirated into the lungs would correlate with the severity and outcome of aspiration pneumonia, all available data are derived from animal work, hence the extrapolation to humans may be misleading (Roberts and Shirley, 1974). Conditions characterized by a gastric pH less than 2.5 and volume in excess of 50 ml or 0.8 ml/kg have been indicated as critically dangerous (Raidoo et al, 1990).

Nervous system alterations In pregnant women, the decrease in MAC observed in a variety of animal studies (Datta et al, 1989; Palahniuk et al, 1974; Strout and Nahrwold, 1981) was not confirmed until recently for ethical and practical reasons. At present, there is one human study assessing isoflurane MAC requirements in patients presenting for termination of pregnancy at 8-12 weeks'

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gestation. In this study, MAC, determined by tetanic stimulation, was reduced by 28% compared with MAC values in non-pregnant women undergoing elective gynaecologic surgery (Gin and Chan, 1994). This result indicates that there may be the potential for overdosage. In another study, induction of general anaesthesia for caesarean section was associated with a marked response of the endorphin system; maternal plasma ~-endorphin levels were almost tripled compared with baseline values before induction (Abboud et al, 1983). Whether this finding only represents a hormonal stress reaction or indicates a hypothetical mechanism involved in decreasing anaesthetic requirements has not been clarified. G E N E R A L ANAESTHESIA F O R CAESAREAN SECTION Preoperative evaluation and care Standards

As Curtis L. Mendelson (1946) concluded in his key paper on aspiration, the 'dangers of this complication as an obstetric hazard may be avoided by: (a) withholding oral feeding during labour and substituting parenteral administration where necessary; (b) wider use of local anaesthesia where indicated and feasible; (c) alkalinization of, and emptying the stomach contents prior to the administration of a general anaesthetic; (d) competent administration of general anaesthesia with full appreciation of the dangers of aspiration during induction and recovery; (e) adequate delivery-room equipment, including transparent anaesthetic masks, tiltable delivery table, suction, laryngoscope, and b r o n c h o s c o p e . . . ' . Fifty years later, there is nothing new to be added to these statements. In order to be able to provide a satisfactory level of anaesthetic care in obstetric units, standards of care should be defined and institutionalized to ensure the best possible outcome for both the mother and her baby. Despite impressive advances in obstetrics, neonatology and anaesthesia; despite impressive gains in knowledge, technology and safety over the years; pregnancy still remains a 'risky business' (Grimes, 1994). Therefore, rounds with obstetricians should be implemented into everyday practice allowing anaesthetists to be involved in the management of high risk pregnancies at an early stage and to be offered antenatal opportunities for counselling of the mother. In this way, management difficulties may be avoided provided that they are predictable; otherwise, if unexpected problems should occur, communication within the team will facilitate competent handling and minimize any struggle for leadership. Decision-making on anaesthetic method

During the preoperative visit, the parturient should be carefully evaluated for the presence of pre-existing or pregnancy-induced disease. Some clinical conditions may preclude the administration of a regional

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anaesthetic such as a coagulation disorder, severe hypovolaemia, maternal sepsis, progressive central nervous system disease or the patient's absolute refusal. The patient should get all necessary information on the procedure and on the different anaesthetic options. This is the basis for choosing the most appropriate method of anaesthesia which should combine the least risks to the mother with her personal preferences and the individual judgement of the anaesthetist in charge. Is there still a place for general anaesthesia in obstetric practice? Although there is no doubt that general anaesthesia is second choice for caesarean section in most healthy parturients, a general anaesthetic offers several advantages over regional techniques such as speed of onset, reliability of technique, controllability of depth of anaesthesia, relative haemodynamic stability by avoiding neuraxial sympathetic blockade and, last but not least, the possibility of 100% oxygen administration to the mother to improve fetal oxygenation (Marx and Mateo, 1971). These benefits may be offset by some important disadvantages which are related to the above-mentioned changes in physiology. On one hand, there are the risks of a difficult airway management with the concomitant complications of cerebral hypoxaemia and/or aspiration of gastric contents, while on the other hand, there are hazards related to the impossibility of determining exactly the depth of general anaesthesia. Problems that need to be addressed may arise out of the conflict between maternal and fetal interests. The risks of maternal awareness because of a superficial level of anaesthesia and neonatal depression as a result of too deep a level of anaesthesia must be taken into account. Obstetric anaesthesia is the only field of activity where the anaesthetists have to assume the responsibility for more than just one living being at a given moment.

Evaluation of the upper airway During the preoperative examination, a thorough assessment of the upper airway should be performed. The relative risk of experiencing a difficult tracheal intubation in pregnant subjects may be determined by using the Mallampati criteria to record oropharyngeal structures (Mallampati et al, 1985) supplemented by an appraisal of additional risk factors such as obesity, short neck, receding mandible and protruding maxillary incisors (Rocke et al, 1992). Despite some methodological shortcomings in study design (Lewis et al, 1994), the relative risk analysis proposed by Rocke and colleagues clearly indicated that there was a very low probability of difficult intubation in parturients presenting as Mallampati class I or II. By classifying the view at laryngoscopy according to Cormack and Lehane (1984), most of the glottis was visible (grade A) in 98.1% and 86.7% of class I and II, respectively. In contrast, patients in whom the visibility of the glottis was limited (grade B) or non-existent (grade C and D) were found with increasing frequencies in Mallampati classes II to IV (Table 2). Failed intubation occurred in two patients (Mallampati class II and III) giving an overall incidence of 1 in 750 cases. As expected, the intubation difficulties increased exponentially in the presence of the following risk factors

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(relative risk [confidence interval of 95%]): short neck (5.0 [2.4; 10.4]), protruding maxillary incisors (8.0 [1.5; 42.5]), receding mandible (9.7 [1.9; 49.3]). Another review on the best way to determine oropharyngeal classification in non-obstetric adult patients compared the impact of body, head and tongue positions and phonation on predicting difficult laryngoscopy (Lewis et al, 1994). It was concluded that the sitting position with the head in full extension and the tongue out for phonation was the most predictive and reproducible method in assessing Mallampati classification. Table 2. Correlation between degree of vmlbility of oropharyngeal structures as assessed according to Mallampati et al (1985) with the modificauon by Samsoon and Young (1987) and subsequent view at laryngoscopy (Cormack and Lehane, 1984) in an obstetric population (Rocke et al, 1992), with permission. Mallampatl classification uvula tonslllar pillars fauces soft palate View at laryngoscopy most of glottm posterior part of glottis glottis invisible epiglottis invisible

Class I

Class II

Class III

Class IV

visible visible visible visible

tip obscured not visible barely vxsxble visible

base visible not visible not visible visible

not visible not visible not visible not visible

98 1% 1 7% 0 2% 0%

86.7% 11 7% 1.4% 0 2%

75 1% 21 5% 3 4% 0%

71% 22 4% 6.6% 0%

Reducing the risk of pulmonary aspiration of gastric contents Although the assumption that all parturients are potentially at risk for aspiration of gastric contents may not stand up to scrutiny (O'Sullivan et al, 1987; Macfie et al, 1991), it is still one of the central tenets of obstetric anaesthesia. The bottom line of a controversial body of literature on gastric emptying is that candidates for a general anaesthetic should not be allowed to eat solid food for at least 4 hours beforehand (Lewis and Crawford, 1987) but that drinking of clear liquids may be permissible (Elkington, 1991). In obstetric patients offered a light breakfast of tea and toast within 4 hours of elective caesarean section, the volume of gastric contents was double that of the fasted group, the pH of the aspirate from the stomach was significantly lower and in 2 out of 11 patients recognizable particles of toast could be identified (Lewis and Crawford, 1987). This is in accord with ultrasound findings showing that with the onset of active labour and institution of epidural analgesia, solid food may be detected in the stomach for 8-24 hours (Carp et al, 1992). Preoperative administration of 0.3 M sodium citrate (20-30 ml) as a nonparticulate oral antacid is now a common and, in most places, compulsory practice and has replaced particulate antacids which may themselves cause severe lung damage when aspirated (Gibbs et al, 1979). When given from 10 minutes to 1 hour before induction of general anaesthesia, the gastric pH was raised to a range considered to be safe in all subjects studied and

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continued to offer protection until the emergence from anaesthesia and extubation in almost all patients (Gibbs et al, 1979; Dewan et al, 1985). Sodium citrate (30 ml) did not significantly increase the volume of gastric contents (Dewan et al, 1985). The dopaminergic antagonist metoclopramide is used to accelerate gastric emptying and to increase the lower oesophageal pressure, especially in parturients complaining of heartburn (Hey and Ostick, 1978). Although gastric pH is not influenced and gastric stasis associated with labour and opioids only partially reversed (Murphy et al, 1984), metoclopramide is recommended as beneficial in improving the overall safety of general anaesthesia. Furthermore, its anti-emetic efficacy, as evidenced by a substantial reduction in incidence of perioperative nausea and vomiting, seems to be free of any adverse maternal or neonatal effect (Lussos et al, 1992). Hrreceptor antagonists, such as cimetidine and ranitidine, are effective in reducing the volume of gastric contents and the acidity of gastric secretion (Okasha et al, 1983; Rout et al, 1993). Neonatal safety of both drugs has been well documented (Howe et al, 1981; McAuley et al, 1983), whereas parenteral cimetidine, in contrast to ranitidine, has been charged with interfering with maternal haemodynamic stability and liver function (Ramanathan, 1988). As Hrreceptor antagonists require some time to be effective, they should be given orally at least 2-6 hours prior to surgery; in the case of intravenous administration, 30 minutes before surgery may suffice (Rout et al, 1993). Omeprazole reduces gastric acid secretion by specifically blocking the H÷K+ ATPase proton pump. Its use in obstetrics seems to be safe for both mother and neonate (Moore et al, 1989). When given in combination with metoclopramide on the evening and in the morning before caesarean section, the aspiration risk was lowered (Orr et al, 1993). One issue remains to be answered: is there any evidence that omeprazole is superior to classical Hrreceptor antagonists to justify the increase in financial costs resulting from using a more expensive drug for aspiration prophylaxis? Nowadays, there is no indication left for anticholinergic drugs to be administered preoperatively, since gastric secretion is reduced at the cost of a decrease in the tone of the gastro-oesophageal sphincter which predisposes to regurgitation (Brock-Utne et al, 1978). Furthermore, salivation is rarely troublesome during a general anaesthetic, since diethyl ether has largely been abandoned in favour of more appropriate volatile anaesthetics.

Preoperative management Position of the patient In order to avoid aorto-caval compression, the supine position should be avoided. Therefore, a left lateral tilt of 15°-30 ° is recommended and can be achieved by tipping the operating table to the left side and/or placing a wedge under the fight hip of the patient (Crawford et al, 1972). Proper positioning of the head is of paramount importance to accomplish straight-

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forward endotracheal intubation. A pillow placed under the head, allowing it to extend at the atlanto-occipital joint, combined with flexion of the lower portion of the cervical spine may be important in achieving alignment of the axes of the mouth, pharynx and larynx thus improving the view of the vocal cords (King and Adams, 1990).

Patient monitoring The physical presence and competence of the person administering a general anaesthetic remains the most important factor ensuring the safety of the patient. Minimal monitoring includes intermittent or continuous recording of blood pressure and pulse rate, continuous display of electrocardiographic signal, pulse oximetry throughout the administration of the anaesthetic and, if available, the post-operative recovery period, and, finally, capnography to ensure proper placement of the endotracheal tube and, subsequently, control of normoventilation.

Preoxygenation Efficient denitrogenation by preoxygenation is critical in parturients receiving general anaesthesia for caesarean seetion. The speed of arterial desaturation during apnoea is markedly increased and, according to an early study, is more than twice as fast in pregnant than in non-pregnant women after 1 minute of apnoea (Archer and Marx, 1974). In obstetric emergencies, the 3 minutes of breathing 100% oxygen before proceeding to rapid sequence induction may not be possible. In these cases, four deep breaths of 100% oxygen provides almost the same oxygen reserve as three minutes of tidal oxygen breathing as evidenced by end-tidal nitrogen concentrations of 5.1 + 1.7% versus 1.0 + 0.2%, respectively (Norris et al, 1989).

Induction of general anaesthesia

Standards A rapid sequence induction still seems to represent the common standard of obstetric anaesthesia. In patients in whom a general anaesthetic is required in spite of a suspected difficult intubation anatomy, this approach should be avoided. Alternative methods comprise awake intubation following topical anaesthesia or oral fibreoptic intubation (Malan and Johnson, 1988). Attempts at blind or fibreoptic nasal intubation are discouraged, since iatrogenic bleeding may hamper subsequent intubation attempts (Lussos and Datta, 1993). As soon as the patient loses consciousness, Sellick's manoeuvre should be performed. This technique, which consists of firm pressure applied on the cricoid cartilage by an assistant until correct positioning of the endotracheal tube is confirmed (Sellick, 1961), ensures occlusion of the oesophagus thus preventing regurgitation and aspiration of stomach

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contents until the upper airway is secured by a cuffed endotracheal tube. If applied incorrectly, such as at the wrong place or with too little pressure, Sellick's manoeuvre is not effective. More recent work suggests that cricoid pressure should be established before the induction of anaesthesia as a decrease in upper oesophageal sphincter pressure usually precedes the loss of consciousness (Vanner, 1993). In order to increase the tolerance of Sellick's manoeuvre, the level of pressure should be limited to 20 N (approximately 2 kg) in awake patients which can be raised to 30 to 40 N after loss of consciousness (Vanner, 1993). The free hand may be used for neck support to prevent anteflexion of the head while exerting cricoid pressure as such a positional change would increase the difficulties at intubation. A central tenet of general obstetric anaesthesia consists of always utilizing an endotracheal tube for airway protection.

Drugs used for induction of anaesthesia Various intravenous agents have been used for the induction of anaesthesia (Lussos and Datta, 1993). Barbiturates, which have been the gold standard for almost half a century, are still the most widely used induction agents, but alternatives such as propofol, ketamine, etomidate and midazolam are becoming more popular as alternatives.

Barbiturates. Sodium

thiopentone (Pentothal ®) and methohexitone (Brevital ®) are two short-acting compounds which are suitable for obstetric anaesthesia. These drugs rapidly cross the placenta. Thiopentone can be detected in the fetal circulation within 30 seconds following maternal administration and peak concentrations are reached after 2-4 minutes (Capogna and Celleno, 1993). The umbilical vein to maternal vein concentration ratio (UV/MV) and the umbilical artery to umbilical vein concentration ratio (UA/UV) increase with longer induction to delivery intervals, are dosedependent and may range between 0.42-1.8 and 0.3-0.87, respectively (Schepens and Heyndrickx, 1975; Capogna and Celleno, 1993). As thiopentone in excess of 8 mg/kg may result in neonatal depression, it is probably best to limit maternal dose to 4-7 mg/kg (Kosaka et al, 1969). Paying attention to these recommendations does not mean the elimination of all influences on neonatal adaption to extra-uterine life as subtle changes of neurological behaviour, excitability and muscular tone have been observed to persist for the first day of life in the presence of a normal Apgar score and acid-base status at delivery (Capogna and Celleno, 1993). We have abandoned using methohexitone for induction because its shorter elimination half-life does not lead to improved neonatal outcome compared with thiopentone (Capogna and Celleno, 1993) and, hypothetically, the risk of maternal awareness might be increased by too rapid recovery.

Propofol. Although propofol has not yet been approved for obstetric anaesthesia, much investigational data has accumulated regarding its potential in this field of anaesthesia. It is a very lipophilic drug with rapid placental transfer and fetal tissue uptake (Capogna and Celleno, 1993).

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Propofol has a very short elimination half-life, allowing the patient to be less sedated on emergence from anaesthesia (Gregory et al, 1990). Using a single bolus of 2 mg/kg for induction followed by 50% nitrous oxide (N20) in oxygen in halothane 0.2-0.5%, low neonatal neurobehavioural scores were observed in 70% of all babies 30 minutes after delivery (Dailland et al, 1989). This may be explained by a higher percentage of unbound propofol in umbilical plasma (mean + sD 3.1 +0.7) compared with maternal plasma (1.7+0.4) at delivery (Gin et al, 1991). In this study, neonatal propofol concentrations were significantly higher than the corresponding maternal values starting at 2 hours and persisting until 4 hours after delivery without apparently affecting fetal well-being. Reducing the dosage of propofol in an attempt to avoid neonatal depression may be complicated by an increase in the frequency of intraoperative awareness without being beneficial to the neonate (Dailland et al, 1989; Gregory et al, 1990). So far, the superiority of propofol over thiopentone has not been shown in parturients. Ketamine. This agent induces a state referred to as 'dissociative anaesthesia' and, therefore, may be associated with hallucination and delirium during emergence from anaesthesia unless benzodiazepines are coadministered, to reduce the intensity and incidence of these side effects and to induce amnesia (Dich-Nielsen and Holasek, 1982). Because of its powerful sympathomimetic effects, ketamine is indicated when haemodynamic stability is compromised, such as massive maternal haemorrhage, and in patients with asthma. Using an induction dose of 2 mg/kg, intraoperative dreaming was reported by almost 50% of patients and, moreover, 16% of neonates suffered from depression (Meer et al, 1973). There is some controversy regarding neonatal outcome following 1 mg/kg ketamine compared with thiopentone 3-4 mg/kg for induction of anaesthesia. While some authors observed better neuro-behavioural scores following ketamine (Hodgkinson et al, 1978), others noted that habituation responses as assessed by the Scanlon test were impaired for the the first day of life (Jones et al, 1985). Such adverse effects are likely to be counterbalanced by clear benefits in situations where maternal safety is at stake. There is no advantage in combining low-dose ketamine (0.5 mg/kg) with a reduced dosage of thiopentone (2 mg/kg) supplemented by 70% of N20 in oxygen in terms of maternal side effects; using such a combination, the incidence of intra-operative awareness was reported to be 46% compared with 8.3% in patients receiving only ketamine at 1 mg/kg (Schultetus et al, 1986). Etomidate. Etomidate is another induction agent causing minimal cardiovascular depression, and may be indicated for anaesthesia in pregnant women with pre-existing cardiac disease who would not tolerate the haemodynamic effects associated with regional anaesthesia. A comparison of neonatal outcome following etomidate 0.3 mg/kg versus thiopentone 3.5 mg/kg followed by general anaesthesia using enflurane 0.5-0.85% and 50% N20 in oxygen supported the use of etomidate because the time to sustained respiration was almost halved without altering Apgar scores

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(Downing et al, 1979). As the placenta is rich in cholinesterase, placental transfer of etomidate may be limited due to partial hydrolysis to inactive components and this may explain a mean UV/MV ratio of 0.5 (Suresh et al, 1986). As in patients receiving etomidate directly, this drug may also transiently suppress adrenocortical activity in neonates, as evidenced by significantly lower cortisol levels 1 hour post-delivery than seen following the use of thiopentone (Reddy et al, 1988). The significance of this finding is unclear.

Midazolam. If benzodiazepines are used at all, midazolam seems to be the best choice for induction of general anaesthesia in pregnant women as it has the shortest plasma half-life and does not cause venous irritation. However, midazolam's slow onset of action increases the risk of aspiration and an induction dose of 0.2 mg/kg has been demonstrated to increase the time to sustained respiration and caused neonatal respiratory depression necessitating intubation in more than 50% of neonates (Bland et al, 1987). Neuromuscular blocking agents Neuromuscular blocking agents are commonly used to facilitate endotracheal intubation. Because of low lipid solubility and a high degree of ionization in physiologic conditions, placental transfer of all agents used is minimal and negligible with respect to neonatal condition if administered in weight adjusted dosages.

Suxamethonium. To achieve a rapid sequence induction, suxamethonium is the drug of choice using dosages of 1.0-1.5 mg/kg which provide complete muscular relaxation and optimal intubation conditions within 90 seconds and last for an average of 6.2 minutes (Baraka et al, 1986). Maintenance of muscular relaxation throughout the surgical procedure can be assured by either a continuous infusion of suxamethonium, or by administration of a non-depolarizing compound as soon as there is evidence of recovery from suxamethonium block as indicated by muscular response to peripheral nerve stimulation. Although a small dose of a non-depolarizing muscle relaxant (e.g. atracurium or D-tubocurarine 2-4 mg) may be administered prior to suxamethonium, in order to prevent muscular fasciculations and a concomitant rise in intra-gastric pressure, this is not the author's practice because: --gastro-oesophageal barrier pressure has been shown to remain almost unchanged as there is also a rise of lower oesophageal sphincter pressure (Cotton and Smith, 1984); --partial muscle relaxation may result from an inadvertent overdose (Conklin, 1984); --fasciculations causing postoperative muscle pain are much less of a problem in parturients than in the general population (Crawford, 1971).

Atracurium and vecuronium. Both of these agents have become very popular because of their relatively short duration of action, and have

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663

replaced D-tubocurarine and pancuronium in many places. As placental transfer is negligible, there are no adverse effects on neonatal outcome as measured by Apgar scores, umbilical cord acid-base values and neurobehavioural scores (Flynn et al, 1984; Dailey et al, 1984). Neuromuscular block induced by either of these agents is easily reversible as soon as twitch height reaches more than 20% of control or, when using a train-of-four stimulation, 1 to 2 twitches can be elicited. Neostigmine or edrophonium should then be administered in combination with atropine or glycopyrrolate as appropriate.

Mivacurium and rocuronium. Mivacurium is a new short-acting benzylisoquinolinium compound. Mivacurium is rapidly hydrolysed by plasma cholinesterase, has an onset time similar to that of atracurium and likewise may cause a release of histamine accompanied by a flush and cardiovascular alterations. There are no data yet available comparing mivacurium with suxamethonium in patients presenting for caesarean section. Rocuronium is a new steroidal agent of intermediate duration which will soon be introduced into clinical practice. This compound may prove to be the first real alternative to suxamethonium for rapid sequence induction as good tracheal intubation conditions can be achieved within 60 seconds (Puhringer et al, 1992). When used for caesarean section, 0.6mg/kg produced satisfactory intubating conditions in 90% of patients within 80 seconds (Abouleish et al, 1994). Maintenance of anaesthesia

Inhalational anaesthetics Nitrous oxide (N20). Nitrous oxide is an agent with limited lipid solubility (oil/gas partition coefficient 1.4) that crosses the placenta readily because of its low molecular weight (44 daltons) and reaches maternal and fetal brain rapidly because of its low blood solubility (blood/gas partition coefficient 0.47). Within 3 to 4 minutes of N20 anaesthesia, there is an almost complete equilibration between maternal and fetal blood concentrations as indicated by a UV/MV ratio of 0.83 (Marx et al, 1970). As fetal uptake increases with the duration of N~O exposure, tissue saturation reaches 87% after 15-19 minutes of maternal administration (Marx et al, 1970). Hence, there is a risk of neonatal depression attributable to central anaesthetic effects of N20. By increasing the inspired concentration of N20 to more than 66% and extending the time until delivery, the rate of tissue uptake may result in reduced neonatal Apgar scores (Stenger et al, 1969). Nitrous oxide does not cause relaxation of uterine muscle nor does it interfere with the uterine response to oxytocin (Marx and Bassell, 1985); therefore, it can be administered throughout the caesarean section without any reservation.

Halothane, enflurane, isoflurane. These potent volatile anaesthetics have similar molecular weights (halothane 197, enflurane and isoflurane 184 daltons) but differ in degree of lipophilicity (oil/gas partition coefficients

664

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SCHNEIDER

for halothane 224, for enflurane and isoflurane 98.5 and 97.8, respectively) (Wong and Sundin, 1995). All of them rapidly pass from the maternal into the fetal circulation; thus isoflurane as well as halothane was detected within 2 minutes of administration in the fetal lamb (Biehl et al, 1983a,b). These newer agents have largely replaced diethyl ether, which had been used successfully for more than a century since its introduction into obstetric practice by James Young Simpson in 1847. These anaesthetic agents are usually administered in 0.5 MAC concentrations to provide a satisfactory level of anaesthesia in combination with N20 until delivery of the baby. In the pregnant ewe, administration of halothane or isoflurane in concentrations of less than 1 to 1.5 MAC did not compromise uteroplacental blood flow, fetal oxygenation or acid-base balance, whereas administration of 2 MAC for more than one hour was associated with fetal acidosis (Palahniuk and Shnider, 1974). Neonatal depression is avoidable, even in the presence of 50% NiO, by limiting the concentration of these anaesthetics to 1 MAC equivalents and by keeping the incision-delivery intervals shorter than 11 minutes (Warren et al, 1983). This does not preclude the implementation of the so-called 'overpressure' technique (McCrirrick et al, 1994), which consists of starting an obstetric anaesthetic with isoflurane 2% for 5 minutes, followed by isoflurane 1.5% for the next 5 minutes, and 0.8% thereafter. The impact of induction-to-delivery and uterine incision-to-delivery intervals on neonatal outcome has been studied under the conditions of general and spinal anaesthesia; under general anaesthesia (66% N20 in oxygen), the incidence of neonatal acidosis was significantly higher with induction-to-delivery intervals of more than 8 minutes and uterine incisionto-delivery intervals of more than 3 minutes, respectively (Datta et al, 1981). Equipotent doses of these anaesthetics produced similar dosedependent uterine relaxation (Munson and Embro, 1977) and there were no clinically relevant differences in postpartum blood loss or responsiveness to oxytocin when halothane 0.5%, enflurane 1.0% and isoflurane 0.75% in 50% N20 were evaluated (Warren et al, 1983).

Desflurane and sevoflurane. These new volatile anaesthetics have similar molecular weights (desflurane 168 and sevoflurane 182 daltons), extremely low blood solubilities (blood/gas partition coefficient for desflurane 0.42, for sevofiurane 0.68) and comparable lipophilicities (oil/gas partition coefficients for desflurane 18.7, for sevoflurane 53.4) (Wong and Sundin, 1995). Because of these unique physical properties, both anaesthetics can induce surgical anaesthesia very rapidly. Desflurane is about 2 to 3 times less potent than sevoflurane (MAC desflurane 5.7%, MAC sevofiurane 1.7%) and is distinguished from sevoflurane by its chemical stability and minimal metabolism; in this regard desfiurane is comparable to isoflurane (Wong and Sundin, 1995). The experience with these new agents in obstetrics is very limited and data on neonatal outcome are sparse. According to a preliminary study on desflurane during caesarean delivery, 6% desflurane in oxygen provided safe and effective surgical anaesthesia without evidence of neonatal depression as assessed by Apgar scores,

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ANAESTHESIA

FOR CAESAREAN

SECTION REVISITED

665

umbilical blood gases and neurobehavioural scores; nonetheless, the time to eye opening was not shortened (Wallace et al, 1993). Similarly, no maternal or neonatal problems nor complications were reported when sevoflurane was administered for obstetric anaesthesia (Tatekawa et al, 1993). However, there was a significant increase in maternal and fetal fluoride levels (14.7 and 6.2 ~tmol/1, respectively) which were still well below what is considered to be toxic. Thus, no alterations in kidney or liver function were observed. These data were corroborated by another study in which sevoflurane 1% was utilized and fluoride levels were still elevated 24 hours after delivery (Gambling et al, 1994). When comparing neonatal outcome, there were no differences in neurobehavioural adaptation of neonates during the first 24 hours of life following caesarean section under general anaesthesia (sevoflurane 1% or isoflurane 0.5%) or spinal anaesthesia (Sharma et al, 1994). Despite these preliminary findings and favourable results, we are far from being in a position to define the role of desfturane and sevoflurane in the practice of modern day obstetric anaesthesia. The impact of additional costs versus potential benefits clearly necessitates further studies (Saidman, 1991).

Opioids in balanced anaesthesia Opioids are sometimes used to ablate the hypertensive response to laryngoscopy and endotracheal intubation. Although fentanyl in doses of 1 ~tg/kg has been demonstrated to be safe with respect to neonatal outcome (Eisele et al, 1982), higher doses may cause neonatal respiratory depression as they do in adults. Placental transfer of fentanyl is rapid and peak fetal concentrations occur within 5 minutes after administration (Capogna and Celleno, 1993). Therefore, it is our practice to give opioids along with other drugs used to produce balanced anaesthesia as soon as the administration of volatile anaesthetics (with the exception of N20) is discontinued following delivery of the baby. We administer midazolam at the same time as the opioid in order to minimize the risk of maternal awareness. S O M E C O M P L I C A T I O N S OF G E N E R A L ANAESTHESIA

Management of an unexpected difficult airway and failure at intubation A difficult airway and unsuspected failure at intubation are not fiction in obstetric practice but dreadful facts which continue to contribute to maternal morbidity and mortality as detailed above. Proper management of these situations is of paramount importance to reduce the incidence of associated complications. This means that every anaesthetist should have a protocol to follow in case of unsuspected problems in airway control. Such a protocol should be simple, straightforward and accepted by everyone involved in the care of such an endangered patient. There is no doubt that the prime

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objective must be to ensure maternal oxygenation and maternal survival. Because 'mothers do not die from failure to intubate, they die either from failure to stop trying to intubate or from undiagnosed esophageal intubation' (Scott, 1986), several algorithms have been proposed for coping with the variety of conditions encountered in the course of such a situation (King and Adams, 1990; Benumof, 1991; Lussos and Datta, 1993; Brimacombe and Berry, 1994). These protocols are based on a sequence of decisions and actions which should be strictly complied with. Early communication with the obstetrician is necessary in order to inform him of any problem related to airway management and to determine the appropriate actions. If someone is available who has more experience in the field of obstetric practice or difficult airway management, this person should be sent for to perform the second attempt at endotracheal intubation. In the meantime, the position of the patient should be optimized and mask ventilation started while cricoid pressure is applied by an assistant. Alternative material for difficult intubation (different laryngoscope, laryngeal mask, transtracheal cannula, suction catheter, gum-elastic bougie) may be useful in such a situation. Attempts at intubation should be limited to a maximum of three, as multiple attempts may cause soft tissue trauma thus impairing the chances of a proper mask ventilation (Cormack and Lehane, 1984). If unsuccessful, further anaesthetic care and management should be decided on in view of the following considerations: --Ventilation and oxygenation are possible versus impossible. - - A delay of caesarean delivery would be acceptable versus unacceptable. The responses to these points lead to a series of potential scenarios: 1.

2.

Mask ventilation and effective oxygenation are feasible, but there is: - - N o absolute indication for continuing the caesarean section such as severe fetal compromise. In this case, the patient should be allowed to awaken (Lussos and Datta, 1993). Then, an alternative method of anaesthesia (regional anaesthetic) or a different approach to the airway (awake fibreoptic intubation) should be chosen. - - A n absolute indication for continuing the caesarean section because of severe fetal distress as judged by the obstetrician. In this case, mask ventilation should be continued using a potent volatile anaesthetic in oxygen until spontaneous respiration returns. Cricoid pressure should not be released. Mask ventilation and effective oxygenation are impossible thus bringing the anaesthetist face to face with one of the most challenging emergencies in obstetric anaesthesia, which is burdened with many pitfalls: --Irrespective of the indication to perform a caesarean delivery and the degree of emergency, the patient should be managed according to the guidelines detailed in the algorithm for failed intubation (Figure 1). There is no time to waste before starting to use ancillary techniques of airway management. If failure of ventilation by face mask cannot be overcome by insertion of an oropharyngeal airway, the laryngeal mask airway is the next step to be taken without releasing cricoid

GENERAL ANAESTHESIA FOR CAESAREAN SECTION REVISITED

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pressure during insertion (Brimacombe and Berry, 1994). Although a laryngeal mask airway cannot prevent pulmonary aspiration of gastric contents, there are a number of case reports describing its successful use in failed obstetric intubation (Brimacombe and Berry, 1994). If the insertion of the laryngeal mask airway is difficult or impossible, cricoid pressure may be temporarily interrupted or reduced in order to ease the insertion of the airway; thereafter, cricoid pressure should be resumed. If this attempt at ventilation by laryngeal mask airway is unsuccessful, ventilation by facemask should again be attempted, if possible with the assistance of one person lifting the jaw while the other performs bag ventilation. If this approach fails, puncturing of the cricothyroid membrane using a 12 or 14-gauge cannula or a commercial system for emergency cricothyroidotomy has to be considered as the last resort (King and Adams, 1990). When a transtracheal catheter is used, ventilation can be achieved by intermittent jets of high pressure oxygen provided by a manual jet ventilator, otherwise, a large bore device used for cricothyroidotomy may be attached to a resuscitation bag or a standard breathing system. Emergency tracheostomy is not recommended as this surgical procedure may exacerbate rather than resolve acute airway problems (Brimacombe and Berry, 1994). If all these manoeuvres fail to prevent severe hypoxia and spontaneous respiration does not re-occur, cardiorespiratory resuscitation is necessary. At this point, the obstetrician will be asked to perform caesarean delivery in order to obviate caval obstruction by the gravid uterus and to improve the effectiveness of maternal resuscitation.

Aspiration of gastric contents According to the last Report on Confidential Enquiries into Maternal Deaths in the United Kingdom 1988-1990, two maternal deaths among those believed to be directly due to anaesthesia were attributed to aspiration of gastric contents and in an additional four out of ten deaths, aspiration was considered as a contributory factor (Department of Health and Social Security, 1994). These results indicate that this problem is present and the onus of prevention rests with the anaesthetist. Should aspiration occur, treatment is directed at removing as much of the aspirated material as possible by suction and at minimizing the risk of secondary pulmonary changes by positive pressure ventilation with positive end-expiratory pressure similar to the settings used for the adult respiratory distress syndrome (ARDS). Prophylactic administration of antibiotics or steroids has not been shown to be of any value.

THE POSTDELIVERY PHASE Post-operative care of patients focuses on two main goals: patient safety and pain relief. Patient safety has markedly improved in response to the

668

M. C. SCHNEIDER

I

Failedintubationl

FM ventilation with CP ~

FM ventilation with CP O

¢

¢

UGnt caesarean?

-

LMA inssertion

~

with/withoutCP_

¢

¢ LMA ventilation with CP ~

,~ Allow patient to awaken

2-3 more attempts at intubation (ancillary techniques) maximum < 2-3 minutes

LMA ventilation with~P

Face mask (FM)

Alternative anaesthetic FM ventilation ® techniques: --regional, --fibreoptic Cricothyroid ] --awake intubation puncture

Successful

Unsuccessful

Jet ventilation

General anaesthesia with secure airway

General anaesthesia with FM and CP

No success CPR!

¢

¢

¢

Figure 1. Protocol for fmled lntubatlon J , possible, O, impossible, FM, face mask, LMA, laryngeal mask airway; CP, CrlCOldpressure; CPR, car&opulmonary resuscitation Adapted from Lussos and Datta (1993) and Brimacombe and Berry (1994) with permission

GENERAL ANAESTHESIA FOR CAESAREAN SECTION REVISITED

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favourable impact of modern anaesthetic practice, better monitoring, stricter application of standards for care and emphasis on the presence of trained and competent staff. In the United Kingdom, substandard care related to caesarean section and/or post-operative management was implicated in 21 maternal deaths between 1988-1990 (Department of Health and Social Security, 1994). This indicates the importance of extending the high anaesthetic standards implemented during surgery to postoperative care units. This is a broad area where considerable quality improvement can and should be achieved in the future. Provision of adequate analgesia is the other domain where important developments have taken place during the past decade. Patient-controlled analgesia is now available in an increasing number of obstetric units and offered as a complement to the well established methods of neuraxial pain control. The place of pre-emptive analgesia in obstetrics has not yet been clearly defined, but some studies indicate that the amount of opioid analgesic needed to treat post-operative pain can be reduced by administration of diclofenac (Bush et al, 1992; Rorarius et al, 1993).

SUMMARY Despite enormous advances in the safety of obstetric anaesthetic practice, general anaesthesia has fallen into disuse. Alterations in physiological functions and anatomy contribute to the increased incidence of hazards and mishaps related to general anaesthesia in pregnant patients. Careful preoperative evaluation and premedication using drugs which reduce the risk of aspiration may be beneficial in reducing the frequency of complications and, therefore, should be part of routine practice. The importance of monitoring, correct patient positioning and pre-oxygenation is emphasized, and an algorithm for the management of a difficult airway is presented. The most popular drugs used for general anaesthesia during caesarean section are discussed with special reference to untoward fetal side effects. Finally, the importance of the post-delivery care is discussed.

Acknowledgement l wish to thank Joan Ethnger for her excellent secretarial work

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B enumof JL (1991) Management of the &fficult adult airway. Anesthesiology 75:1087-1110 Biehl DR, C6t6 J, Wade JG et al (1983a) Uptake of halothane by the foetal lamb m utero Canadian Anaesthettsts' Soctety Journal 3 0 : 2 4 - 2 7 B lehl DR, Yarnell R, Wade JG & Sitar D (1983b) The uptake of isoflurane by the foetal lamb m utero effect on regional blood flow Canachan Anaesthetists' Soctety Journal 30:581-586 Bland BAR, Lawes EG, Duncan PW et al (1987) Comparison of mldazolam and thlopental for rapid sequence anesthetic induction for elective cesarean section. Anesthesta and Analgesta 66: 1165-1168 Brimacombe J & Berry A (1994) The laryngeal mask airway for obstetric anaesthesia and neonatal resuscitation lnternattonal Journal of Obstemc Anesthesta 3:211-218 Brock-Utne JG, Downing JW & Seedat F (1977) Laryngeal oedema associated with pre-eclamptic toxaemla Anaesthesta 32: 556-558. Brock-Utne JG, Rubm J, Welman S e t al (1978) The effect of glycopyrrolate (Robmul) on the lower oesophageal sphincter Canadtan Anaesthetists' Society Journal 25: 144-146. Brock-Utne JG, Dow TGB, Dimopoulos GE et al (1981) Gastric and lower oesophageal sphincter (LOS) pressures in early pregnancy Brttish Journal of Anaesthesta 53: 381-384. Bush DJ, Lyons G & Macdonald R (1992) Diclofenac for analgesia after Caesarean section Anaesthesza 47:1075-1077 Camann WR & Ostheimer GW (1990) Physiological adaptations dunng pregnancy International Anestheslology Chmcs 2 8 : 2 - 1 0 Capogna G & Celleno D (1993) The effects of anaesthetic agents on the newborn In Reynolds F (ed) The Effects on the Baby of Maternal Analgesta and Anaesthesta, pp 221-236 London WB Saunders Company Ltd. Carp H, Jayaram A & Stoll M (1992) Ultrasound examination of the stomach contents of parturlents Anesthesta and Analgesta 74:677-682 Chadwick HS, Posner K, Caplan RA et al (1991) A comparison of obstetnc and nonobstetnc anesthesia malpractice clmms. Anesthestology 74:242-249 Conklm KA (1984) Should precurarization be used an obstetrics? Anesthestology 6 0 : 3 8 4 (letter) Cormack RS & Lehane J (1984) Difficult tracheal intubation m obstetrics Anaesthesta 39: 1105-1111 Cotton BR & Smith G (1984) The lower oesophageal sphincter and anaesthesia Brtttsh Journal oJ Anaesthesta 5 6 : 3 7 - 4 6 Crawford JS (1971) Suxamethonmm muscle pains and pregnancy British Journal of Anaesthesla 43" 677-680 Crawford JS, Burton M & Davies P (1972) Time and lateral tilt at Caesarean section. Brlttsh Journal of Anaesthesia 44:477-484 Dailey PA, Fisher DM, Shnider SM et al (1984) Pharmacoklnetics, placental transfer, and neonatal effects of vecuronlum and pancuronium administered during cesarean section. Anesthestology 60:569-574 Dailland P, Cockshott ID, Llrzm JD et al (1989) Intravenous propofol during Cesarean section placental transfer, concentrations in breast milk, and neonatal effects A preliminary study Anesthestology 71" 827-834 Datta S & Bnwa J (1981) Modified laryngoscope for endotracheal lntubation of obese patients (letter) Anesthesta and Analgesta 60:120-121 Datta S, Ostheimer GW, Weiss JB et al (1981 ) Neonatal effect of prolonged anesthetic induction for cesarean section Obstetrtcs and Gynecology 58:331-335 Datta S, Hurley R J, Naulty JS et al (1986) Plasma and cerebrosplnal fluid progesterone concentratmns m pregnant and nonpregnant women Anesthesta andAnalges~a 65:950-954 Datta S, M1gliozzi RP, Flanagan HL & Kneger NR (1989) Chronically administered progesterone decreases halothane requirements in rabbits Anesthesta andAnalgesta 68" 46-50 Department of Health and Social Security (1991) Report on Confidenttal Enqutrtes tnto Maternal Deaths m the Umted Kmgdom 1985-1987, pp 73-87 London. HMSO Department of Health and Social Security (1994) Report on Confidennal Enquiries into Maternal Deaths m the Untted Kingdom 1988-1990, pp 80-99 London' HMSO. Dewan DM, Floyd HM, Thlstlewood JM et al (1985) Sodium otrate pretreatment in elective cesarean sectmn patients Anesthesta and Analgesia 64: 34-37. Dich-Nlelsen J & Holasek J (1982) Ketamine as induction agent for Caesarean section. Acta Anaesthesiologtca Scandmavtca 26:139-142 Downing JW, Buley RJR, Brock-Utne JG & Houlton PC (1979) Etomidate for reduction of

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anaesthesia at Caesarean section, companson with thlopentone Brtnsh Journal of Anaesthesta 51: 135-140. Eckstem K-L & Marx GF (1974) Aortocaval compression and uterine displacement. Anesthesiology 40:92-96 Eger El II (1974) Anesthetic Uptake andActton, pp 122-145. Baltimore: Williams & Wllkms. Elsele JH, Wright R & Rogge P (1982) Newborn and maternal fentanyl levels at cesarean section Anesthesta and Analgesta 61:179-180 (abstract) Elkington KW (1991) At the water's edge. where obstetrics and anesthesia meet (clinical commentary). Obstetrws and Gynecology 77:304-308 Endler GC, Marlona FG, Sokol RJ & Stevenson LB (1988) Anesthesia-related maternal mortality in Michigan, 1972 to 1984 Amertcan Journal of Obstetrics and Gynecology 159:187-193 Flynn PJ, Frank M & Hughes R (1984) Use of atracurium in Caesarean section Brtttsh Journal of Anaesthesta 56: 599~i05. Gambling DR, Sharma SK, White PF et al (1994) A comparison of sevoflurane/N20 and lsoflurane/N20 for cesarean birth: maternal effects and fluoride levels Anesthesiology 81: All30 Gibbs CP, Schwartz DJ, Wynne JW et al (1979) Antacid pulmonary asptration m dog Anesthesiology 51:380-385 Gibbs CP, Knscher J, Peckham BM et al (1986) Obstetric anesthesia' a national survey Anesthesiology 65: 298-306. Gin T & Chart MTV (1994) Decreased minimum alveolar concentration of isoflurane an pregnant humans. Anesthestology 81:829-832 Gin T, Yau G, Jong W et al (1991) Dlsposmon of propofol at ceasarean section and in the postpartum period British Journal of Anaesthesia 67:49-53 Gmtzler AR (1980) Endorphm-medlated increases in pmn threshold during pregnancy. Science 210: 193-195 Glosten B (1994) Obstetric anesthesia risk a review of recent literature. Internattonal Journal of Obstetric Anesthesta 3: 7-12. Gregory MA, Gm T, Yan G e t al (1990) Propofol infusion anaesthesia for Caesarean section Canadian Journal of Anaesthesla 37: 514-520. Grimes DA (1994) The morblthty and mortality of pregnancy' still risky business American Journal of Obstetrtcs and Gynecology 170:1489-1494 Hawkins JL, Gibbs CP & Orleans M (1994) Obstetnc anesthesm workforce survey--1992 versus 1981 Anesthestology 81:Al128 Hey VMF & Ostlck DG (1978) Metoclopramide and the gastro-oesophageal sphincter, a study in pregnant women with heartburn. Anaesthesta 33: 462-465. Hey VMF, Cowley DJ, Ganguh PC et al (1977) Gastro-oesophageal reflux in late pregnancy. Anaesthesza 32:372-377 Hodgkmson R, Bhatt M, Kim SS et al (1978) Neonatal neurobehavioral tests following cesarean section under general and spinal anesthesia Amerwan Journal of Obstetrtcs and Gynecology 132:670~574 Howe JP, McGowan WAW, Moore Jet al (1981) The placental transfer of clmettdine Anaesthesia 36:371-375 Iwasaki H, Collins JG, Salto Y & Kerman-Hmds A (1991) Naloxone-sensmve, pregnancy-induced changes in behavioral responses to colorectal distention: pregnancy-induced analgesia to v~sceral stimulation Anesthestology 74: 927-933. Jones MM, Joyce TH, Adenwala J & Mawji F (1985) Comparison of thlopental-nitrous oxlde-halothane with ketamine-oxygen-halothane as anesthetic agents for cesarean section Anesthesia and Analgesm 64:233 (abstract) King TA & Adams AP (1990) Failed tracheal mtubatlon Brlttsh Journal of Anaesthesta 65: 400-414. Knuttgen HG & Emerson K (1975) Physiologic response to pregnancy at rest and dunng exercise Journal of Applied Phystology 36: 549-553. Kosaka Y, Takahashl T & Mark LC (1969) Intravenous thlobarbiturate anesthesia for cesarean section Anesthesiology 31: 489-506. Lewis M & Crawford JS (1987) Can one nsk fasting the obstetric patient for less than 4 hours? British Journal of Anaesthesta 59:312-314 Lewis M, Keramatl S, Benumof JL & Berry CC (1994) What is the best way to determine oropharyngeal classification and mandibular space length to predict difficult laryngoscopy? Anesthestology 81:69-75

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