Obstetrics

CHAPTER 8 Obstetrics



Anaesthesia for non-obstetric surgery during pregnancy


A total of 1% of patients require GA during pregnancy. In the first trimester, there is a risk of organogenesis; in the third trimester there is a risk of premature labour. Therefore, the second trimester is the safest.


In descending order of preference:


• spinal: method of choice

• epidural: higher doses of LA required and block less effective

• other regional blocks

• combined epidural and GA: lower doses of inhalational agents and narcotics, good postoperative analgesia and can avoid N2O

• GA (NB delayed gastric emptying present from first trimester onwards).




Known teratogenic drugs in humans



• tetracycline: bone defects, dental enamel staining

• warfarin: bone malformations

• alcohol: craniofacial abnormalities

• thalidomide: limb abnormalities

• synthetic progestogens: masculinization of female genitalia

• cocaine

• ACE inhibitors

• possibly diazepam.


Anaesthetic management of LSCS


AAGBI and the Obstetric Anaesthetist’s Association have together recommended a decision-to-delivery interval of ≤ 30 min. Some 78% LSCS in the UK are performed under a regional rather than general anaesthetic.



Regional anaesthesia


Epidurals give excellent/satisfactory analgesia in 91% of mothers. Increased use of regional techniques probably accounts for the continuing reduction in maternal mortality by avoiding risks of failed intubation and aspiration. Comparative Obstetric Mobile Epidural Trial (COMET) 2001 showed that women requesting analgesia for pain relief were more likely to require instrumental delivery if receiving 0.25% bupivacaine boluses rather than low dose bupivacaine infusion or combined spinal/epidural, but no significant difference in LSCS rates between groups (Table 8.1).


Table 8.1 Advantages and disadvantages of epidural anaesthesia






























Advantages Disadvantages
Maternal participation at delivery May take too long to perform if there is fetal distress
Avoids risk of failed intubation Hypotension
Reduced risk of aspiration Risk of patchy, incomplete block
Avoids morbidity from GA drugs Backache
Avoids risk of awareness Urinary retention
Earlier breast-feeding  
Good postoperative analgesia  
Less postnatal depression  


Indications for epidural



• Slow or painful labour

• Pregnancy-induced hypertension

• Cardiac or respiratory disease

• Premature or high risk fetus

• Multiple pregnancy

• Breech delivery

• Trial of labour.


Pain pathways (Fig. 8.1)


First stage


• pain due to cervical dilatation and uterine contractions

• pain transmitted via uterine sympathetic nerves to T10–T12

• some transmission via tubo-ovarian vessels.

image

Figure 8.1 Pain pathways during labour.


Second stage


• pain due to vaginal and perineal stretching and tearing

• pain transmitted via pudendal nerves to S2–S4.

Extrauterine pelvic structures supplied by L5/S1. Particularly stimulated by the fetal head in the occipitoposterior position to cause backache.


LSCS stimulates sensory nerves to T10 in addition to phrenic. Aim to block from T8 to S5.


Aortocaval compression may occur from 20 weeks onwards. In the presence of acute blood loss, cardiovascular compensation may be seriously impaired if the block is extensive. Delivery of the baby relieves the compression.



Epidural test dose


Recommended to avoid complications of inadvertent intravenous injection of bupivacaine. Usually use a dose insufficient to cause total spinal anaesthesia if injected into the intrathecal space.



Catecholamines


Addition of adrenaline (>15 μg) or isoprenaline to the test dose may cause tachycardia within 1 min following intravenous injection. However, this sign may be unreliable because pregnancy may alter physiological changes to catecholamines and the pain of contraction may also cause sudden tachycardia. Catecholamines have also been shown to reduce placental blood flow in animal studies. Therefore, their use is controversial. If used, give between contractions and avoid in hypertensive or pre-eclamptic patients.



Opioids


The addition of 100 μg fentanyl to a test dose has been reported as a reliable indicator of intravascular injection by causing sedation and altered perception.



Air


Air (1 mL) injected intravenously and detected by parasternal Doppler has been proposed as an effective marker of inadvertent intravenous injection. There appear to be no adverse maternal or fetal effects from this small dose of air.



Regional anaesthesia


Platelet count <100 000, abnormal clotting or prolonged bleeding time is a relative contraindication to regional anaesthesia.


Epidurals are best established early and generally unsuitable for emergency procedures. Provide less effective anaesthesia than subarachnoid block and are more likely to require conversion to GA. Epidurals are the technique of choice for labour or LSCS in pre-eclamptic toxaemia (PET), where they provide better haemodynamic stability and improved uteroplacental flow, with avoidance of intubation risks from laryngeal oedema. l-bupivacaine 0.5% is as effective as bupivacaine 0.5% with less risk of CNS or CVS toxicity. Ropivacaine 0.75% is equipotent with bupivacaine 0.5%.


Spinals are the commonest form of anaesthesia for LSCS. Pencil point needles now reduce incidence of post-dural puncture headache requiring blood patch to <0.5%. Colloid fluid preload is more effective than crystalloid in preventing hypotension, but greater risk of anaphylaxis. Avoidance of aortocaval compression and use of vasopressor (e.g. phenylephrine 50–100 μg) significantly reduces hypotension. 2.25 mL 0.5% heavy bupivacaine achieves an adequate block to T4 at term, but larger volumes may be required earlier in pregnancy because of less venous congestion reducing the volume of the epidural space. Addition of opioids (fentanyl/morphine/diamorphine) reduces the incidence of intraoperative visceral pain with little risk of respiratory depression.


Combined spinal-epidural anaesthesia (CSE) may provide better anaesthesia than epidural alone. In a needle-through-needle technique, the spinal needle is advanced <15 mm beyond tip of Tuohy. Alternatively, puncture the subarachnoid space with spinal needle and replace the stylette immediately, then site epidural catheter via Tuohy needle at difference space. Give epidural test dose and then follow with subarachnoid injection. A smaller intrathecal dose (e.g. 1.0 mL 0.5% bupivacaine) followed by epidural increments improves haemodynamic stability; 0.25 mg intrathecal diamorphine ≡ 5 mg epidural diamorphine. There is some concern regarding breaching of the dura increasing infection risk and the complexity of the technique.


Epidural volume extension (EVE) A low subarachnoid block can be extended in a cephalad direction by an epidural injection of 10 mL of normal saline given within 5 min of the initial subarachnoid block. Probably related to compression of the subarachnoid space by the epidural saline, resulting in cephalad spread of local anaesthetic within the subarachnoid space. EVE allows CSE to be performed with small initial intrathecal doses of local anaesthetic and, as saline is used for the epidural ‘top-ups’, the total dose of local anaesthetic used is reduced.


Continuous spinal anaesthesia using an ultra fine bore catheter threaded through a spinal needle has been associated with cauda equine syndrome and is little used in the UK.



General anaesthesia


Technique of choice for emergency LSCS. Probably safer than an epidural if there is severe hypertension, uncorrected hypovolaemia, fetal distress, coagulopathy or risk of convulsions. GA morbidity most commonly due to anaphylaxis or airway crises. Short-acting antihypertensive drugs may be necessary to prevent intubation-induced hypertension, which causes CVA, pulmonary oedema and reduced placental blood flow. (Unlike thiopentone induction or lidocaine, remifentanil 1 μg.kg−1 prevents the haemodynamic changes at intubation). Beware of laryngeal oedema. Magnesium increases sensitivity to non-depolarizing neuromuscular blockers by inhibiting presynaptic calcium-facilitated neurotransmitter release.


Postoperative. Pre-eclampsia may not begin to resolve until 3–4 days post-delivery. Therefore, monitor BP and urine output carefully; 60% of patients who develop pulmonary oedema do so >48 h after delivery.



Failed intubation


Difficult intubation occurs in 1:300 (1:2000 in normal population). More common in Africans and Afro-Caribbeans.


It is the commonest cause of anaesthetic-related maternal deaths (it is not failure of intubation but subsequent failure of oxygenation that kills). Difficult intubation is due to left lateral tilt, increased weight, increased breast size, full dentition, laryngeal oedema, incorrect application of cricoid pressure, minimum dose of induction agent and attempted intubation before onset of neuromuscular blockade.


Perform no more than three attempts at intubation. Know the failed intubation drill which is being modified in many hospitals to use the laryngeal mask. Maintain cricoid pressure (if it does not prevent LMA insertion) which reduces aspiration risk and stops gastric distension if the patient is being bagged).

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Aug 28, 2016 | Posted by in ANESTHESIA | Comments Off on Obstetrics

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