Issue: June 2010
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for today's Anaesthesia Professional|
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Issue: June 2010 |
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| for today's Anaesthesia Professional |
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Pre-operative assessment
Early identification and planning of obstetric care by multidisciplinary teams was emphasised in the last CEMACH report 1. To this end all maternity units should now have obstetric anaesthetic antenatal clinics dealing with relevant anaesthetic problems, and obstetric or medical illnesses that need anaesthetic input. The problems of the obese patient from both a regional and a general anaesthetic point of view need special consideration. Anaesthetists are suggested to review patients who are morbidly obese (BMI > 35). Apart from predicting problems these clinics allow planning for more complicated parturient management. All elective caesarean section patients should now be seen in pre-assessment clinics by anaesthetists.Classification of caesarean section
The degree of urgency of caesarean section has long been a source of tribulation between obstetricians and anaesthetists. Communication about the need of immediacy of caesarean section by obstetricians has often been subjective with a variety of results. The terms ‘crash section,’ emergency, semi-emergency, semi-elective and elective were used but meant different things to different members of the team. A ‘30 minute’ rule was used for emergency surgery but this is also subjective in terms of the potential emergency nature of the surgery. NICE guidelines and the Royal Colleges of both Obstetricians and Anaesthetists now recommend that classification 2 be carried out on every case according to the following schedule: Category 1: immediate threat to the life of the mother or fetus Category 2: maternal or fetal compromise which is not immediately life threatening Category 3: needing early delivery but no maternal or fetal compromise Category 4: at a time to suit the woman and maternity team.Equipment and staffing
Epidural equipment is standardised in the main with 16 or 18 gauge needles being in common practice. Newer refinements may occur soon with smaller and stronger epidural needles being manufactured. Newer advances in catheter material may allow for smaller atraumatic catheters to be manufactured. Safer loss of resistance devices which allow for two handed advancement of the needle and thus a potentially lower dural puncture rate have also been developed and the Epidrum® is one such example. This device has a small one ml priming pump attached to it which collapses once the epidural space is located. Newer equipment that is often deemed necessary in the delivery suite include a thromboelastograph for instantaneous clotting profiles, haemoglobin measurement from oximetry, oesophageal Doppler cardiac output monitoring, a Level 1 tranfusor device to assist in major haemorrhage and major patient transfer equipment for the transfer of the critically ill patient to the Intensive Care Unit. Maternity High Dependency Units of a Level 2 standard are now becoming an integral part of maternity units. Staffing of these areas is traditionally done by midwives. There are now several units with trained High Dependency Unit nurses assisting in the care of the ill parturient. Medical care can be delivered by these experienced staff and maternity care by midwives. More midwives are now trained in managing and understanding the sick mother.Nerve injuries after neuraxial anaesthesia
Anaesthetists are often aware of obstetric palsies as regional anaesthesia is often blamed for them. However, the most common palsies are intrinsic and have a reported incidence of 0.6 to 92 per 10,000 deliveries. Stretch or compression injury to the lumbosacral plexus or lower extremity peripheral nerves in childhood is blamed for them but compression of the nerve vascular supply mechanism is another possibility. Nullparity, a prolonged second stage of labour, cephalo-pelvic disproportion, non-vertex presentation and forceps deliveries are associated with palsies. The most common is a lateral femoral neuropathy known as meralgia paresthetica. The nerve arises from the lateral border of the psoas muscle medial to the anterior superior iliac spine and then passes under or through the inguinal ligament and is vulnerable to compression. It is sensory to the lateral thigh and is also easily injured in caesarean section. Other nerves likely to be injured in childbirth include the femoral, obturator, sciatic and the common peroneal nerves 3. Anaesthetic related nerve damage does exist but is rare. Estimates in obstetric anaesthesia range from 1.2 – 0.3 per 100,000 procedures. Damage caused can include spinal haematoma, epidural abscess, traumatic spinal cord injury (from the needle), intracranial subdural haematoma, abducens nerve palsy and the development of Horner’s syndrome. Direct spinal cord trauma is most worrying as it can lead to permanent spinal cord injury and is most likely to arise from spinal or combined spinal epidural anaesthesia. Recommendations for minimising this complication include choosing a lumbar puncture site below L3, halting needle advancement if the patient perceives pain or paraesthesia and injecting anaesthetic solutions only if all pain and paraesthesia has disappeared 4.Magnesium
Magnesium is a critical physiological cation and its deficiency in pregnant women may contribute to the development of pre-eclampsia, impaired neonatal development and to metabolic problems extending into adult life. Neurologically the inhibition of calcium channels and antagonism of the NMDA receptor raises the potential of neuronal protection and magnesium administration to prematurely labouring mothers may reduce the incidence of neonatal cerebral palsy. In the management of eclampsia magnesium has been shown to terminate convulsions and prevent further convulsions. It is also the agent of choice for the prevention of convulsions but the argument as to which pre-eclamptic mother should receive magnesium is unresolved. Magnesium therapy has potential complications in its own right and it is usually received for patients with severe disease 5.Effects of labour analgesia on the foetus
Labour pain and stress are associated with a progressive foetal acidosis. In terms of analgesia nitrous oxide relieves labour pain more effectively than pethidine and it is worth noting that if Entonox is found to be ineffective it is useless to offer pethidine. Pethidine is not recommended but is available for labour analgesia. It causes neonatal respiratory depression which is most severe if pethidine is given 3-5 hours prior to delivery but the depression is only slight if pethidine is given within an hour of delivery. Epidural analgesia in labour, whilst it has some short-term maternal side effects, when compared to systemic analgesia is shown to be consistently beneficial in terms of not only Apgar score but also of acid-base status and is less likely to impair breast feeding. Epidural analgesia is associated with maternal pyrexia but this does not appear to be of any consequence to the neonate because if it were there would be an increased incidence of fetal acidosis after epidural analgesia in labour whereas in reality the reverse is true 6.Coagulation in pregnancy
Pregnancy is associated with physiological changes that include an increase in the majority of clotting factors and a decrease in natural anticoagulants and a reduction in fibrinolytic activity. The mother becomes hypercoagulable and risks thromboembolism. The changes reverse to normal about 4 weeks post-partum. Pregnancy can induce changes in platelet numbers and function which can complicate the provision of regional analgesia and anaesthesia. Most practitioners will perform regional anaesthesia if the platelet count is above 80 x 109/l if the trend of platelet function is stable 7. The use of low dose heparin is becoming more common and often decisions about the provision of regional anaesthesia are difficult. Normally the following rules are utilised:Pharmacogenetics
Pharmacogenetics is the study of the variability in drug response due to genetic variability. The first observation of this influence was made in the 1950s when the genetic influence of the metabolism of suxamethonium was noted. It has been highlighted recently because of the variability of vasopressor requirement during spinal anaesthesia for caesarean section seems to have a genetically affected response distribution. This response is clearly affected by the ß2AR genotype and it has been noted that women who are Gly16 homozygous and those who are heterozygous or homozygous for the Glu27 variant require significantly less vasopressors (ephedrine) for the treatment of hypotension during spinal anaesthesia. These 2 haploids appear to ‘protect’ women from requiring higher doses of vasopressors and these haploids are more likely to occur in Caucasian patients. These differences in the population may explain why a simple recipe approach to treating hypotension after regional anaesthesia is unsuccessful. Genetic differences may also explain why some ethnic groups respond better or are more resistant to blood pressure control in pre-eclampsia 8. Analgesic drugs, especially, codeine are also of significance in this sphere. Codeine is metabolised the cytochrome P450 enzymes. The gene coding for this enzyme is polymorphic and there are more than 75 different CYP2D6 alleles which results in a hugely variable enzyme activity which can range from 1 to 200%. The action of codeine varies from each individual. Poor metabolisers do not achieve analgesia whilst they encounter the side effects such as nausea and vomiting. Conversely codeine intoxication can be anticipated with ultra-rapid CYP2D6 metabolism. There is anxiety about codeine being used in breast feeding mothers. A recent FDA warning followed the death of a breast fed 13 day old neonate was issued because the neonate was thought to have died from a morphine overdose as the mother was taking codeine and was a CYP2D6 metaboliser 9.Haemorrhage
World wide haemorrhage is the biggest cause of maternal death. It is variably defined and loss is always difficult to assess. In England and Wales the Health Care Commission defined ‘significant’ haemorrhage as > 1000 ml and ‘major’ loss as > 2500 ml. The Scottish Confidential Audit of Severe Maternal Morbidity (SCASMM) considered major haemorrhage as a loss of > 2500 ml or a transfusion of 5 or more units or treatment for coagulopathy. It is stating the obvious but obstetric haemorrhage is notoriously difficult to estimate and is normally under-estimated. Recent thinking in the treatment of major haemorrhage of relevance to anaesthetists is to increase the ratio of clotting factors to packed cells to a ratio of 1:2 or even 1:1 to prevent the onset of coagulopathy. Attempts should be made to avoid further precipitant factors which increase disseminated intravascular coagulation such as shock, acidosis and hypothermia. Adjuncts to treatment with surgery and drugs include the use of cell salvage. A cell saver cuts down on donor blood usage and is cost effective but it must be emphasised that salvaged blood has no coagulation factors. Concerns about amniotic fluid embolism and rhesus immunisation problems have been unfounded. Interventional radiology services are now nationally recommended to be available for the use in the management of placenta accrete and praevia and the use of such facilities has proven to be life saving 10.References
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If you are interested in responding to one of the articles featured inUltrasound
Ultrasound (US) is simply sound waves, like audible sounds. Although some physical properties are dependent on the frequency, the basic principles are the same. Sound consists of waves of compression and decompression of the transmitting medium (e.g. air or water), travelling at a fixed velocity. Sound is an example of a longitudinal wave oscillating back and forth in the direction the sound wave travels, thus consisting of successive zones of compression and rarefaction. Transverse waves are oscillations in the transverse direction of the propagation.An Ultrasound Machine
A basic ultrasound machine among various parts for functioning has the following key components.History
The first people to report on the use of US were Bogin & Stulin, for lumbar puncture in the Russian literature in 19711. Then a group of Anaesthesiologists from Arizona, Cork and co-workers used the US to locate the neuraxial anatomy2. Between 2001 and 2004 Grau and co-workers from Germany have done enormous work to evaluate the use of US for epidural punctures3,4.Imaging of spine: Basic considerations
The various structures of the spine are located at a depth that requires low-frequency US (5-2 MHz). The neuraxial structures also need curved array transducers of US imaging. Low frequency probes provide good penetration but at the expense of resolution, however high frequency probes can be used, but lacks penetration. The low frequency probe would be more appropriate for neuraxial structures as it penetrates deeper.Spinal sono-anatomy
The key to understand and comprehend the sono-anatomy information being produced by the US is a thorough knowledge of the spine anatomy. The US of the spine can be performed using two acoustic windows. One is the midline transverse (axial scan)5,6, or the longitudinal (sagittal) axis7. The longitudinal scan can be performed either midline or para-median. Both the acoustic windows complement each other and are very useful for central neuraxial blocks. The longitudinal scan helps is the identification of the sacrum, the articular processes, ligamentum flavum and dura mater(Figure 1). |
| Figure 1. Transverse and Longitudinal views of the lumbar spine |
Lumbar spine imaging
The lumbar spine can be imaged either by the transverse or longitudinal scans as mentioned above. In the transverse scan the US transducer is placed transverse to the spine with patient in lying or sitting position. The spinous process is then visualised as a hyperechoeic reflection. This scan is useful to identify the midline. However because it blocks the structure below it may not be useful to see the neuraxial structures. In which case if the transducer is moved cranially or caudally, it is possible to see the deeper structures like the ligamentum flavum, the posterior dura, thecal sac and anterior dura8. For the longitudinal scan, the transducer is placed vertically either in the midline or para-median, perpendicular to the long axis of the spine. The images seen are hyperechoeic saw like pattern which represents the articular processes and the interspaces. These spaces consist of the ligamentum flavum, posterior dura and deeper, the anterior dura. Thus the two scans help in complementing each other to define the level, midline, depth and ideal insertion point.Conclusion
The advent of USG has added to our armoury of equipment, a tool to safely visualise the neuraxial structures and aid in the placement of central neuraxial blocks. It has also been demonstrated to increase accuracy, forecasting difficult epidurals and reducing patient discomfort. It is a safe, non-invasive, validated, radiation free tool with a sharp learning curve and very effective in obstetrics, where the traditional method of assessing the intervertebral spaces by palpation has not proved to be reliable.References
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