than 500 ml from the genital tract after the birth of the child. It may be immediate or if it occurs between 24 hours and 6 weeks is classified as primary and beyond this period is termed second- ary haemorrhage. This can be following placenta previa and abruption or when products are retained in the uterus preventing sufficient retrac- tion to stem bleeding or ineffective uterine contraction. Further conditions which fit into the description of foetal distress and determine C/S are described below. Prolapsed (umbilical) cord involves the downward displacement of the cord before the foetus presents. With vasa previa a foetal blood vessel lies over the os and is in danger of rupture and shoulder dystocia is failure of the foetal shoulders to traverse the pelvis after delivery of the head. This is more likely to progress to episiotomy and application of external pelvic pressure with the mother in the lithotomy or left lateral position than open surgery. All of the above conditions can threaten the viability of the foetus and lead to C/S. In fact anything that interferes with foetal oxygenation will cause foetal distress (Chamberlain, 1995). Approximately 30% of breech births also result in emergency C/S (Dobson, 2004). Even though obstetric anaesthesia is specialised and to some extent standardised in procedure, it does not conform or adhere to a universal model or algorithm but in general will involve the avoidance of drugs and agents that cross the placental barrier, depress foetal vital signs, cause myocardial or respiratory depression and initiate untimely uterine contractions. Preoperative preparation, whether elective or emergency would have involved establishing an IV line and measures to control and neutralise gastric acid with oral antacids given as close to theatre time as possible. Fasting would only be an issue in the case of an elective procedure. Premedication is not standard or indeed desirable, especially narcotics and drugs used for sedation due to the depressant effect on the foetus (Carrie et al., 2000). Nevertheless, pethidine remains standard with midwives and delivery room staff during expected normal birth and this must be kept in mind if such a scenario converts to one requiring anaesthesia and surgery. The potential for aortocaval compression in patients at this stage determines that they should never be allowed to lay flat. During transfer to theatre this may require the mother to assume the left lateral position and once on the operating table should be positioned supine with a 15-degree left- sided tilt (Nelson, 1999). This may involve the use of a wedge or actual lateral rotation of the table itself (Harvey, 2004). In order to maintain adequate oxygen saturation levels, pre-oxygenation is mandatory. Besides the immediate benefits to mother and baby it provides an oxygen reserve which may be required during intubation. There is no definitive duration for pre-oxygenation but to be fully effective it should be a minimum of 3 minutes. This is thought to be sufficient time to not only saturate the red cells but provide extra reserves by displacing a degree of pulmonary nitrogen and being taken up by the plasma. The potential for vomiting and regurgitation has already been referred to and therefore employment of a rapid sequence induction is mandatory. Following pre-oxygenation and ongoing explana- tion to the patient, the anaesthetist will begin induction. Carrie et al.(2000) state that with the exception of the frequently used muscle relaxants, most drugs used in anaesthesia readily cross the placental barrier in significant quantities. Ryan (2000) states that as regurgitation can start once induction and neuromuscular blocking agents have been given, cricoid pressure (Sellick’s manoeuvre), must be applied by the AP who should be suitably trained and familiar with this crucial procedure. If applied correctly, it prevents stomach contents reaching the patient’s airway and entails applying pressure with the thumb and first two fingers downward upon the cricoid cartilage (Ryan, 2000). This acts to compress the oesophagus between the trachea and cervical spine, closing off the oesophagus. Classical Sellick’s manoeuvre actually involves counter pressure with the assistant’s other hand cupped behind the patient’s neck. Obstetric anaesthesia 123 A number of variations are in common practice, namely using one hand while the other is free to pass intubation equipment which should be prepared and to hand. The pressure should be maintained until the endotracheal tube (ETT) is in place and the cuff inflated and only released upon the instruction of the anaesthetist. Immediate fixation of the ETT is essential in order to prevent inadvertent displacement. Besides the aspiration hazards associated with incorrectly applied cricoid pressure, there is the possibility that it could also hinder visualisation of intubation landmarks. Anaesthesia is kept deliberately light due to the depressant effects on the foetus but a consequence of this is maternal awareness. Therefore any narcotic agents and inhalational supplementation are held back until the foetus is delivered and only then is anaesthesia deepened. It is at this point that the tilted table will need levelling out. Suxamethonium remains the drug of choice for intubation followed by a non-depolarising muscle relaxant as part of the maintenance regime. Nevertheless, suxamethonium is not without unde- sirable properties. Even though it allows intubation within approximately 30 seconds, there is a period when no spontaneous breathing can take place and any attempt to apply positive ventilation via the facemask could force gases into the stomach and so exacerbate the tendency to regurgitation. It is here that the value of pre-oxygenation may be realised. The longer acting non-depolarising muscle relaxant is given when the effects of the depolarising agent have abated. The muscle fasi- culation produced increases intragastric pressure and the paralysis produced increases the potential for regurgitation. In the event of aspiration the AP needs to be familiar with treatment protocols which could involve head-down tilt of the operat- ing table, lateral positioning, suction, ventilation with 100% oxygen followed by drug therapy including bronchodilators, steroids, antibiotics and depending on severity, pulmonary lavage, chest physiotherapy and the possibility of mechan- ical ventilation combined with positive end expira- tory pressure (PEEP) in the most severely affected. The signs of aspiration may include laryngospasm, bronchospasm, airway obstruction, tachypnoea, tachycardia and a fall in oxygen saturation with the possibility of hypotension and cyanosis (Wenstone, 2000). Signs may be immediate or manifest at a later stage leading to misdiagnosis. If the situation happened during induction for an emergency procedure, the anaesthetist would be responsible for prioritising and synchronising actions between treatment of the aspiration and continuing surgery. All of the commonly used inhalational agents readily cross the placental barrier and the concen- tration in the foetal blood quickly approaches the levels in the mother. An additional contraindica- tion is that in general they hinder uterine contrac- tion and so increase the potential for post-partum haemorrhage. Nevertheless, many have been reported to have benefits in labour using sub- anaesthetic concentrations in conjunction with oxygen þ nitrous oxide, when they have minimal effect on the foetus and uterine contraction (Rudra, 2004). Respiratory changes during pregnancy enhance anaesthetic uptake as the increase in resting ventilation delivers more agent into the alveoli (Ciliberto, 1998). Alongside analgesics, anti-emetics and anti- biotics, oxytocics are the only other drugs com- monly used. Even though they are not anaesthetic related, they are standard in the obstetric anaes- thetist’s pharmacology armamentarium. They are administered via a single shot, intended to bring about uterine contraction as the foetus is being delivered, or as an infusion if the surgeon indicates that the uterus is flaccid and lacking tone. Oxytocics are also referred to as uterotonics. There are three in common use: syntocinon, ergo- metrine and syntometrine, which is a combined preparation of the other two. They have the action of contracting the myometrium, although in differing manners and for varying durations. This action can actually compromise placental blood flow and lead to foetal hypoxia. Ergometrine especially has the additional unwanted side effects 124 T. Williams of causing nausea and vomiting and can induce a general vasoconstriction leading to a rise in blood pressure, an effect unwanted if the mother is already hypertensive. Interestingly, according to Ciliberto and Marx (1998) the auto transfusion of blood from the contracting uterus reduces the impact of maternal blood loss at birth. Besides the high profile C/S and emergencies involving severe blood loss, there are a number of procedures common to obstetric anaesthesia which are viewed as less serious, however many of the inherent risks are still present. Forceps delivery and vacuum extraction,or vontouse, usually take the form of a trial and if unsuccessful, progresses to C/S and so accord- ingly involve an anaesthetic pre-planned with this in mind. Nevertheless, as some form of analgesia is usual for these procedures, pudendal block, caudal or epidural should be anticipated. Although ectopic pregnancy is increasingly preceded by laparo- scopic investigation, the anaesthetic approach will be as for an emergency utilising rapid sequence induction and being prepared for major haemor- rhage and shock. ERPC involves post-partum bleeding because of debris, which prevents effec- tive retraction of the uterus. General anaesthesia is the norm for these patients, with time since delivery and eating determining technique. The potential for embolism, particularly throm- boembolism, is ever present with any speciality, indeed with any patient undergoing a lengthy hospital stay but there are increased factors with obstetric patients, especially those requiring surgery. Amniotic embolism is unique to the obstetric situation and occurs usually during or just after delivery when amniotic fluid gains access to the circulation, possibly due to placental abrup- tion, leading to shock and obstruction of pulmo- nary blood flow and triggering an anaphylactoid response. Effects are devastating, immediate and usually fatal, not least because of the unfamiliar and uncommon nature of the condition in delivery suites. Immediate signs would include hypocarbia, hypoxia and hypotension. Thrombo prophylactic measures to prevent deep venous thrombosis (DVT), which is a precursor to pulmonary embo- lism, tend to centre on physical measures such as thrombo-embolism deterrent (TED) stockings or flowtron devices intended to maintain venous blood flow by external massaging. Monitoring for obstetric anaesthesia differs little if at all from standard anaesthetic monitoring and adheres to the recommendations of the Association of Anaesthetists of Great Britain and Ireland (AAGBI, 2000) and OAA. The AAGBI regard it as essential that core standards of monitoring apply whenever a patient is anaesthetised, irrespective of duration or location. Whether involving general anaesthetic or regional analgesia, minimum moni- toring will include, pulse oximetry, non-invasive blood pressure and electrocardiography, with the addition of inspired oxygen and end-tidal carbon dioxide monitoring in the case of general anaes- thesia. Despite the view that nothing replaces personal vigilance, there is substantial evidence that monitoring reduces risks of incidents. The Australian Incident Monitoring Study (1993) reported that 52% of incidents were detected first by a monitor with the pulse oximeter and capnograph being predominant in this detection. Even though they are not standard, methods of monitoring potential awareness, as with all branches of anaesthesia, are finding their way into the speciality. Eclampsia is an associated condition, although not necessarily anaesthesia related and can involve the anaesthetic team antenatally. The condition may occur before, during or shortly after delivery (Chamberlain, 1995) and is characterised by convulsions which may develop if pre-eclampsia is left untreated. Actual causation is unknown but insufficient blood flow to the uterus is suspected. Placental abruption often accompanies the condi- tion. It is during the pre-eclamptic stage at which the anaesthetist and AP might become involved when the patient will require intensive care management prior to possible delivery of the foetus by C/S. If pre-eclampsia progresses it may become necessary to sedate the patient and Obstetric anaesthesia 125 introduce positive pressure ventilation along with invasive blood pressure and central venous pres- sure monitoring. Pre-eclampsia usually occurs after the 20th week of gestation and involves hypertension, proteinuria, oedema and oliguria and is classified as mild, moderate or severe (Torr & James, 1998). Depending on the degree of effect, the patient may also suffer cerebral irrita- bility, visual disturbance, pulmonary oedema and hypoxia. Management will involve reducing the blood pressure, controlling the convulsions, correcting the fluid balance and any coagulation abnormalities. Hydralazine is commonly used to treat the hypotension and magnesium sulphate is regularly the drug of choice in the treatment of convulsions and works by producing cerebral vasodilatation. Sedation is essential and benzo- diazepines are often considered but due to the possible detrimental effects on the maternal airway and foetus, must be used with care and in conjunction with suitable monitoring. Pethidine is also contraindicated as the metabolites produced during its breakdown can actually cause convul- sions (Rudra, 2004). Fruesemide remains the standard diuretic in the treatment of the oedema. Convulsions can be triggered by noise, bright lights and activity that can invoke anxiety so if the patient does have to be taken to theatre the AP will be a prime mover in controlling anything that may have a detrimental effect, i.e. bright theatre lighting, increased staff activity and any accompanying noise normally generated when setting up theatre. The value of regional analgesia is well estab- lished (within obstetric anaesthesia), especially epidural and spinal techniques. The regular use of the former became popular as an epidural service on delivery suites providing a pain-free, awake birth. If vaginal birth became difficult and proceeded to forceps or C/S, the facility for analgesia was already in place, avoiding the need for general anaesthesia with all its inherent problems of airway and aspiration management. The indwelling epidural catheter could be used to supplement necessary analgesia for surgery and post-operative pain management. The realisation of the benefits of regional analgesia then led to the spinal approach becoming popular for both elective and emergency C/S. The single-shot technique however can be unsuitable should surgery duration outlast analgesic effect, while the need for post-operative pain control has to be provided by additional means. Almost as a natural next step, the combined spinal/epidural (CSE) or combined spinal/epidural analgesia (CSEA) has gained popularity as it provides the rapid on-set of spinal combined with the longer-term facility of epidural while being somewhat more selective with sensory and motor blockade. The technique can be performed through one lumbar interspace by firstly inserting a Tuohy needle into the extradural space then using it as a guide for introducing the smaller gauge spinal needle into the subarachnoid space, referred to as needle-through-needle technique (Carrie et al., 2000). Following injection of analgesic solution and needle withdrawal, a catheter is introduced into the epidural space. The alternative technique involves inserting the needles through separate lumbar spaces. Epidural needles are usually in the range of 16À18 G and spinal needles are much finer, e.g. 26À27 G. This finer gauge and specialised low trauma tips reduce leakage of cerebrospinal fluid (CSF) and in turn post-dural puncture headache (PDPH). Whitacre and Sprotte are the two main needle designs at present. Both techniques have many plus factors for mother, baby and anaesthetist although in spite of the benefits, there are potential drawbacks. Local analgesic solution toxicity is a continuing danger with epidural as repeated doses via the catheter can lead to accumulation, especially when being used for surgery and continuing post-operative pain relief. Hypotension due to sympathetic blockade is common to both techniques, although there is a much more rapid onset with the spinal route which is a particular danger in obstetrics as beside a primary hazard to the mother, placental perfusion is compromised and can lead to foetal distress (Chamberlain, 1995). Measures to offset this possibility include pre-loading with IV fluids 126 T. Williams and/or vasopressor drugs such as ephedrine, which can be prepared as an IV infusion, stand-by syringe containing 50 mg in 10 ml, or is sometimes given prophylactically preoperatively via intramuscular injection (Oyston, 2000). As the subarachnoid space contains CSF and the extradural space is a fluid-free potential space, the properties of the drugs for each differ. To prevent spinal drugs from the natural tendency of rising within the CSF, they have a higher specific gravity. This is created by presenting the drug in dextrose, making it ‘heavy’, as with heavy marcaine which is 0.5% bupivacaine in 8% dextrose. As the epidural space is fluid-free, ‘normal’ drug solutions are used. Lignocaine and marcaine of varying strengths and percentages have been popular as well as those containing a vasoconstrictor such as adrenaline. The intention is to obtain adequate sensory nerve analgesia combined with sufficient motor block- ade. More recently ropivacaine and levobupiva- caine have gained popularity. Both are said to be longer acting and particularly with the latter, have reduced motor blockade and toxicity effects (Arias, 2002). The actual drug volume requirement is less with spinal than epidural thus reducing the potential for toxic overdose. Local analgesic can be administered in lower concentrations when used in combination with preservative-free opioids such as fentanyl, sufentanil, morphine as well as pethidine and diamorphine to provide effective, synergistic analgesia while also reducing motor blockade. Nevertheless, they still carry the danger of respiratory depression, nausea and vomiting and urine retention. Both techniques create the desired density of block but attention to spread is also important and an area from nipple to perineum is desirable especially to block peritoneal pain during surgery. The block produced is adequate for surgery and any required sedation is commonly provided by an oxygen 50% and nitrous oxide 50% mix. Continuous spinal using an indwelling catheter has not proven popular, mainly due to the difficulties surrounding threading of a 30 G catheter through a 26 G needle and consequent resistance to injection and flow. Pudendal block is the only other commonly found regional technique and is used for episiotomy but may not provide adequate analgesia for forceps delivery or procedures that involve extensive manipulation (Rudra, 2004). Opinion and debate continue over patient positioning when performing epidural and spinal, especially for C/S, either lateral or sitting position with legs over the edge of trolley, bed or operating table. Additionally, left or right lateral also insti- gates discussion, initially with regard to unilateral block, however, there are proponents of both left and right lateral. The thinking of the former relates to vena-caval occlusion and the fact that the patient will be in left tilt during surgery is used to support the latter view. Two uncommon problems associated with regional techniques that the AP should be familiar with are total spinal and blood patch for dural puncture. Total spinal happens when local analge- sic solution spread is too advanced and affects cranial nerves, leading to paralysis of respiratory muscles, loss of consciousness, hypotension and bradycardia. It is more likely to happen during epidural when the needle may inadvertently pierce the dura mater and the large volume of analgesic solution is injected into the subarachnoid space (Carrie et al., 2000). Blood patch is carried out for the relief of post-spinal headache and is an attempt to plug the dural leak with 10À20 ml of the patient’s venous blood injected into the extradural space (Smith & Williams, 2004). There is no one dominant or recognised pain-care regime common to obstetric anaesthesia. Regional analgesia by nature can provide its own pain relief and there is a growing use of PCEA (patient controlled epidural analgesia). The obvious discom- fort and distress of pain to the mother can cause hyperventilation which may lead to maternal hypercarbia, respiratory alkalosis and metabolic acidosis. Consider the already compromised oxygen consumption associated with labour and the need for effective analgesia becomes apparent. Therefore pain control can involve a pre-, inter- and post-operative role for the anaesthetist. Obstetric anaesthesia 127 Even though pethidine is not popular with anaes- thetists, it persists in the normal delivery setting whereas fentanyl is commonly the drug of choice during surgery although many alternatives, includ- ing nubaine and tramadol are not uncommon. Post- operatively, morphine maintains a place whether administered in the traditional intravenous and intramuscular routes or via a titrated PCA system. It is clear that the obstetric AP has a role within both outlying areas as well as theatres, however, there also exists a diverse level of input by APs throughout different centres. In some it is simply the on-call or stand-by member who attends in the event of an obstetric anaesthetic, whereas in others they have a permanent involvement and profile within the obstetric unit. The author has worked in many centres in a number of national and international locations and is aware of differing practices so has therefore attempted to limit naming specific drugs, equip- ment and making reference to particular routines as this can be misleading. The intention has been to present the information in this chapter from the viewpoint, level and need of the post-registration AP. While having to interpret and incorporate this knowledge into their own clinical role, APs must also maintain awareness of personal limitations and be continually mindful of their professional codes, standards and scope of practice (Health Professions Council, 2003). REFERENCES Arias, M. G. (2002). Levobupivacaine. A long acting local anaesthesia, with less cardiac and neurotoxicity. Update in Anaesthesia (Online) 14(17). Available at http://www.nda.ox.ac.uk/wfa/html/u14/u1407.0.1htm (Accessed 3 May 2005). Association of Anaesthetists of Great Britain and Ireland. (2000). Recommendations for Standards of Monitoring During Anaesthesia and Recovery (Online). Available at: http://www.aagbi.org/pdf (Accessed 14 April 2005). Brighouse, D. (2002). Obstetric emergencies. Anaesthesia and Intensive Care Medicine, 3(2), 48À54. Carrie, L. E. S., Simpson, P. J. & Popat, M. T. (2000). Understanding Anaesthesia, 3rd edn. Oxford: Butterworth Heinemann. Chamberlain, G.V. P. (1995). Obstetrics by Ten Teachers, 16th edn. London: Arnold. Ciliberto, C. F. & Marx, G. F. (1998). Physiological Changes Associated with Pregnancy. Available at: www.nda.ox.ac.uk/wfsa/html/uO9003.htm (Accessed 9 March 2005). Dobson, A. (2004). A critical analysis of caesarean section procedure. Journal of Operating Department Practice, 1(8), 16. Ducloy, A. & de Flandre, M. J. (2002). Obstetric Anaes- thesia À Placental Abruption. Update in Anaesthesia (Online), 14(17). Available at: http://www.nda.ox.ac.uk/ wfsa/html (Accessed 5 April 2005). Faura, E. A. M. (2004). Anaesthesia for the Pregnant Patient. Available at: www.daccx.bsd.uchicago.edu/ manuals/obstetric/obstetricanaesthesia.html (Accessed 30 March 2005). Harvey, P. (2005). The role of the ODP in obstetric haemorrhage. Journal of Operating Department Practice, 1(11), 18. Health Professions Council. (2003). Standards of Conduct, Performance and Ethics (Online). Available at: www.hpc.uk.org (Accessed 16 May 2005). Morgan, B. M. (1987). Foundations of Obstetric Anaesthe- sia & Analgesia. London: Baillie ´ re Tindall. Nelson, G. L. (1999). Fundamentals of pain relief. In A. Davey & C. S. Ince, eds., Fundamentals of Operating Department Practice. London: Greenwich Medical Med- ia Ltd, pp. 245À57. Oyston, J. (2000). A Guide to Spinal Anaesthesia for Caesarean Section. Virtual Anaesthesia Textbook (Online). Available at: http://www.virtual-anaesthesia- textbook.com (Accessed 29 April 2005). Owen, P. (2002). Pelvic Arthropathy During Pregnancy. Available at: www.Netdoctor.co.uk/diseases/facts/ pelvicarthropathy.htm (Accessed 26 March 2005). Rudra, A. (2004). Pain Relief in Labour. Update in Anaesthesia (Online), 18(3). Available at: http:// www.nda.ox.ac.uk/wfsa/html (Accessed 9 March 2005). Ryan, T. (2000). Fundamentals of obstetric and emergency anaesthesia. In Fundamentals of Operating Department Practice. London: Greenwich Medical Media. Simpson, P. & Popat, M. (2001). Understanding Anaes- thesia, 4th edn. Oxford: Butterworth Heinemann 128 T. Williams Smith, B. & Williams, T. (2004). Operating Department Practice AÀZ. London: Greenwich Medical Media. Torr, G. J. & James, M. F. M. (1998). The role of the anaes- thetist in the management of pre eclampsia. Update in Anaesthesia (Online). Issue 9 (4). Available at: http:// www.nda.ox.ac.uk/wfsa/html/uO9/uO9/_012.htm (Accessed 19 April 2005). Tortora, G. J. & Grabowski, S. R. (2003). Principles of Anatomy & Physiology. New York: John Wiley & Sons. Yuill, G. & Gwinnutt, C. (2003). Postoperative Nausea and Vomiting. (Online) Issue 17, article 2. Available at: www.nda.ox.ac.uk/wfsa/html (Accessed 21 March 2005). Wenstone, R. (2000). Identification and Management of Anaesthetic Emergencies. Fundamentals of Operating Department Practice. London: Greenwich Medical Media. USEFUL RESOURCES Association of Operating Department Practitioners www.aodp.org GASNET: http://gasnet.med.yale.edu/gta/ Health Professions Council www.hpc-uk.org Medline/Patient UK www.patient.co.uk Association for Perioperative Practice www.afpp.org.uk National Electronic Library for Health www.nelh.uk Nursing & Midwifery Council www.nmc-uk.org Obstetric Anaesthesia & Analgesia. Available at: www.the mediweb.net/obstetrics/Anaes%20Considerations.htm Obstetric Anaesthetic Association www.oaa-anaes.ac.uk Royal College of Anaesthetists www.rcoa.ac.uk Royal College of Nurses www.rcn.org.uk Obstetric anaesthesia 129 13 Understanding blood gases Helen McNeill Key Learning Points • Understand the sampling methods for arterial blood gases • Understand and interpret arterial blood gas results. This will include: • Oxygen transport in the body • Mechanisms of normal acid-base balance • Disturbances of acid-base balance • Step-by-step guide to arterial blood gas analysis • Clinical scenarios Introduction Arterial blood gas (ABG) analysis is now common- place in perioperative and acute-care settings and is used to aid diagnosis and to monitor the progress of the patient and the response to any interven- tions. It is essential that staff working in the perioperative environment understand the key principles of ABG analysis so that results can be dealt with quickly and appropriately, thereby improving the safe management of the patient. Arterial blood gas analysis is often central to the management of the patient who is either already critically ill or is at risk of deterioration in their condition (Simpson, 2004). Many patients cared for within the perioperative environment will fall into one of these two categories and this makes ABGs one of the most common tests performed in theatres. Jevon and Ewens (2002) also suggest indications for ABG analysis may include respira- tory compromise, evaluation of interventions such as oxygen therapy and respiratory support, as a preoperative baseline and following a cardio- respiratory arrest. It is imperative to note at the start of this chapter that, just as with any investigation, ABGs must always be interpreted in conjunction with other clinical information about the patient (Adam & Osborne, 1997). A thorough clinical examination and assessment of a patient will present many clues about the physiological status of that individual À ABG analysis just adds another piece to that jigsaw. Additionally, what may be an ade- quate set of results for one person may be entirely unacceptable for another, depending on their current diagnosis and any pre-existing illnesses. Table 13.1 shows the basic parameters measured by blood gas analysers and their normal values. To help you understand and interpret these values, this chapter will cover some of the fundamentals of the physiology of acid-base balance, alveolar ventilation and oxygenation. Once you are aware of the related physiology, the interpretation of ABG results will be much easier as you will be able to think more clearly about what could be hap- pening to your patient. This chapter also contains a section on the collection and handling of ABG samples. It is worth remembering that, as with any skill, to become really good at ABG analysis you must Core Topics in Operating Department Practice: Anaesthesia and Critical Care, eds. Brian Smith, Paul Rawling, Paul Wicker and Chris Jones. Published by Cambridge University Press. ß Cambridge University Press 2007. 130 practise! There are some examples at the end of the chapter to get you started, but there is nothing like learning in the real world À so, look at real patients with real ABG results and apply what you learn in this chapter to genuine clinical situations. Only then will your learning become embedded and ABG analysis become second nature. Sampling arterial blood gases It is important to collect and handle the ABG sample carefully in order to reduce the possibility of inaccurate readings. The relevant local policies and health and safety precautions relating to blood sampling must, of course, be adhered to reduce the risk of needle stick injuries. There are three possible ways of obtaining an ABG sample: 1. From an indwelling arterial catheter. 2. From an arterial puncture (stab), usually from the radial or femoral artery. 3. Capillary sample from the earlobe. In the perioperative setting it is likely that most samples will be taken from an indwelling arterial catheter. Such arterial lines are not without risk to the patient and are only appropriate for use in areas where the patients are closely monitored and observed (Woodrow, 2004). It is good practice to ensure that arterial catheters are clearly identified and labelled so they are not mistaken for a venous cannula. Garretson (2005) suggests that there are three major causes of complications in indwelling arterial catheters: haemorrhage, thrombosis and infection. Accidental removal or disconnection of the catheter are the most common causes of haemorrhage and could lead to significant blood loss if they were to go unnoticed. Vigilance is paramount in prevention of this problem and the catheter should be well secured and the insertion site and transducer line kept visible and directly observed whenever possible. Thrombosis is rare, but if a clot were to form in the lumen of the catheter this could be flushed into the arterial circulation and compromise the blood flow distal to the catheter site (Garretson, 2005). Correct maintenance of the pressure trans- ducer system will ensure a continuous flush of saline (usually around 3 ml/hour) to help maintain patency of the catheter. The colour, temperature and sensation of the limb should be observed to assess for any signs of compromised circulation. Blanching or discolouration should be reported immediately to medical staff (Moore, 2000). As with any invasive line, there is a risk of infection with an indwelling arterial catheter if strict hand washing and asepsis are not observed during both insertion and sampling (Moore, 2000). It is also important that the lines and sample ports are kept free from blood and other debris. Signs of infection include localised redness, warmth and discharge at the insertion site and the patient may develop a pyrexia (Garretson, 2005). Arterial puncture or ‘stab’ from the radial or femoral arteries is usually the method of choice for sampling if there is no indwelling arterial cannula. Complications can include spasm of the artery, clot formation within the lumen of the blood vessel, haematoma formation and bruising (Williams, 1998). These can potentially compromise blood flow distal to the puncture site; consequently the radial artery is the optimal site as patients usually have a good collateral blood supply via the ulnar artery. Prior to a radial arterial stab or cannulation, a simple Allen’s test can be performed to deter- mine the adequacy of collateral circulation to the hand via the ulnar artery (Moore, 2000). Table 13.1 Normal values for arterial blood gases Parameter Normal values pH 7.35À7.45 PaO 2 11À13 kPa (80À100 mmHg) PaCO 2 4.5À6 kPa (35À45 mmHg) Standard bicarbonate 22À26 mmol/l Base excess/base deficit À2toþ2 mmol/l SaO 2 93À98% Understanding blood gases 131 The Allen’s test can be performed by following these steps: 1. Occlude both the ulnar and radial arteries to the hand. 2. Ask the patient to clench their fist several times until their hand goes pale. 3. Release the pressure on the ulnar artery and observe colour of the hand. If the ulnar artery has a good blood flow the hand should return to the normal colour within 5À7 seconds. Any delay indicates poor ulnar circulation and an alternative site should be used (Moore, 2000). Arterial puncture can be painful for the patient. Crawford (2004) found that 49% of patients reported a pain score of 5 or more on a visual analogue pain scale of 0À10. Williams (1998) recommends the use of local anaesthesia prior to the puncture. Arterial puncture sites take longer to stop bleeding than venous ones so it is recom- mended that pressure is applied for at least 5 minutes to reduce the risk of bruising and haematoma formation (Williams, 1998; Crawford, 2004; Woodrow, 2004). Patients with prolonged clotting may need longer than 5 minutes and must be assessed individually. Capillary samples from the earlobe may be used occasionally, particularly in patients requiring multiple samples who do not have an indwelling arterial cannula. Woodrow (2004) suggests that the difference between the arterial and ear lobe capillary sample is not clinically significant, however, Williams (1998) argues that whilst the PaCO 2 does not vary significantly the accuracy of the PaO 2 reading is dependent on good sampling technique from a warmed, vasodilated earlobe. A heparinised syringe must be used so that the blood does not clot in the tubing of the blood gas analyser and there are many commercially prepared blood gas syringes available that are pre- heparinised. To prevent the exchange of carbon dioxide and oxygen between air and the blood sample all bubbles must be expelled and the syringe sealed with an airtight stopper. As the constituents of blood continue to remain metabolically active for some time after the sample is drawn it is advisable to analyse the blood as soon as possible to ensure accuracy. Many operating departments will have rapid access to an analyser but if the sample needs transporting to a laboratory it should be cooled quickly (Williams, 1998). Cooling the sample has the effect of slowing the metabolism of the blood cells and will prolong the time available for analysis to about 1 hour (Woodrow, 2004). It is common practice to place the syringe into ice to cool the sample but Woodrow (2004) suggests there is anecdotal evidence that this may cause haemolysis and so recommends that iced water is used as long as it does not cause undue delays in transporting the sample. As the course of treatment a patient receives is often based on the ABG results, it is imperative that all possible measures are taken to optimise the accuracy of the readings. Many modern blood gas analysers automatically calibrate themselves at predetermined intervals and require little in the way of maintenance (Williams, 1998). It is, however, essential that practitioners liaise closely with their hospital laboratory services to ensure that the manufacturer’s guidelines and local Trust policies for quality control and health and safety are adhered to. What can ABGs tell you? Arterial blood gases will provide a set of values that can be used to determine key aspects of the patient’s condition. These values can be broadly categorised into: 1. oxygenation status 2. adequacy of alveolar ventilation 3. acid-base balance. The oxygenation status of the patient can be determined by looking at the partial pressure of oxygen in arterial blood (PaO 2 ). Additionally, many machines will also provide a reading of the arterial oxygen saturation (SaO 2 ) and haemoglobin (Hb) if they have the addition of a co-oximeter. 132 H. McNeill [...]... technique involved the use of a sheep’s bladder and a sharpened quill (Major, 16 67) Intravenous solutions of wine, ale or opium were injected into the veins of dogs (the technique was used as part of an overall study of the human circulatory system) As often happens during any experimentation, an incidental observation was made The observation in this case was Core Topics in Operating Department Practice: Anaesthesia. .. pH above 7 is alkaline and indicates a decreasing level of hydrogen ions The body will always strive to maintain the pH within the very narrow range of 7. 35 7. 45, which, as you can see, is actually very slightly alkaline as it is above 7 In terms of acid-base balance any reading below 7. 35 is seen as acidic and any reading above 7. 45 is alkalotic, however, for the purposes of ABG analysis 7. 4 is often... called bases) combine with free Hþ and help to prevent increases in Hþ levels A strong alkali will more readily bind to Hþ than a weak alkali The pH scale is a measure of the concentration of hydrogen ions (Hþ) and ranges between 1 (very strong acid) and 14 (very strong alkali), with 7 being the neutral point in the middle of the scale • A pH below 7 is acidic and indicates an increasing level of hydrogen... ABGs Buffers react within seconds to changes in pH and work by removing or replacing hydrogen ions (Martini, 2001) They are the first line of defence against disturbances of acid-base balance; however, buffers do have a limited capacity and are a short-term measure only There are numerous systems in the body that can buffer acids including haemoglobin, phosphates, plasma proteins and the carbonic acid-bicarbonate... the information above, what do you think could have caused this disorder? Scenario 2 Mike is a 28-year-old man who is anxiously awaiting theatre following an open femoral fracture sustained during a climbing accident He has had opiates for pain and 3 l of crystalloid to manage his hypovolaemia He starts complaining of feeling light-headed and appears dyspnoeic, despite being on oxygen at 3 L/min pH... (2004) Interpretation of arterial blood gases: a clinical guide for nurses British Journal of Nursing, 13(9), 522À8 Treacher, D F & Leach, R M (1998) Oxygen transport À basic principles British Medical Journal, 3 17, 1302À6 Williams, A (1998) Assessing and interpreting arterial blood gases and acid-base balance British Medical Journal, 3 17, 71 67 Woodrow, P (2004) Arterial blood gas analysis Nursing Standard,... pH and the PaCO2 (Moore, 2000) Nevertheless, if these conditions alter, as they frequently do in acute illness, the curve will shift resulting in a change to the affinity of haemoglobin for oxygen • Left shifts of the curve À alkalosis, a low PaCO2, and hypothermia cause a left shift of the curve resulting in an increased affinity of haemoglobin for oxygen Oxygen will, therefore, easily bind Understanding... levels His pH is high indicating an alkalosis The PaCO2 is low and the SBC and BE are normal so this indicates a respiratory alkalosis with no compensatory changes in the metabolic parameters It would be easy to write this off as an anxiety or pain Adam, S K & Osborne, S (19 97) Critical Care Nursing À Science and Practice Oxford: Oxford University Press Cooper, N (2004) Acute care: arterial blood gases... contamination of the peritoneum and he had episodes of hypotension intra-operatively, despite being given large volumes of intravenous fluids He has now been in recovery for 2 hours and is on 40% oxygen His blood pressure is 118/58 and he is passing adequate urine volumes pH PaO2 PaCO2 SBC BE 7. 18 20.9 3.41 9.2 À16.4 What do these gases show? Use the step-by-step guide and the flow to help you Using... 45À52 14 Total intravenous anaesthesia Kevin Henshaw Key Learning Points • Understand the flexibility of TIVA to offer more independent control over each component of anaesthesia • Describe the pharmacokinetic interaction of the drug on the human body • Understand the pharmacodynamics of the drug on the human body • Appreciate the movement and elimination of any drug from the body and the dependency . contains a section on the collection and handling of ABG samples. It is worth remembering that, as with any skill, to become really good at ABG analysis you must Core Topics in Operating Department. Nevertheless, pethidine remains standard with midwives and delivery room staff during expected normal birth and this must be kept in mind if such a scenario converts to one requiring anaesthesia and surgery. The. (H þ ) and ranges between 1 (very strong acid) and 14 (very strong alkali), with 7 being the neutral point in the middle of the scale. • A pH below 7 is acidic and indicates an increasing level