Core Topics in Operating Department Practice Anaesthesia and Critical Care – Part 7 pot

The value of regional analgesia is well estab-lished within obstetric anaesthesia, especially epidural and spinal techniques.. The indwelling epidural catheter could be used to supplemen

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 scenarioconverts to one requiring anaesthesia and surgery.The potential for aortocaval compression inpatients at this stage determines that they shouldnever be allowed to lay flat During transfer totheatre this may require the mother to assume theleft lateral position and once on the operating tableshould 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 tableitself (Harvey, 2004)

In order to maintain adequate oxygen saturationlevels, pre-oxygenation is mandatory Besides theimmediate benefits to mother and baby it provides

an oxygen reserve which may be required duringintubation There is no definitive duration forpre-oxygenation but to be fully effective it should

be a minimum of 3 minutes This is thought to besufficient time to not only saturate the red cellsbut provide extra reserves by displacing a degree ofpulmonary nitrogen and being taken up by theplasma

The potential for vomiting and regurgitation hasalready 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 begininduction Carrie et al (2000) state that with theexception of the frequently used muscle relaxants,most drugs used in anaesthesia readily cross theplacental barrier in significant quantities Ryan(2000) states that as regurgitation can start onceinduction and neuromuscular blocking agents havebeen given, cricoid pressure (Sellick’s manoeuvre),must be applied by the AP who should be suitablytrained and familiar with this crucial procedure

If applied correctly, it prevents stomach contentsreaching the patient’s airway and entails applyingpressure with the thumb and first two fingersdownward upon the cricoid cartilage (Ryan,2000).This acts to compress the oesophagus betweenthe trachea and cervical spine, closing off theoesophagus Classical Sellick’s manoeuvre actuallyinvolves counter pressure with the assistant’sother 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 positionoperat-ing, 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 saturationwith the possibility of hypotension and cyanosis(Wenstone, 2000) Signs may be immediate ormanifest at a later stage leading to misdiagnosis

If the situation happened during induction for anemergency procedure, the anaesthetist would beresponsible for prioritising and synchronisingactions between treatment of the aspiration andcontinuing surgery

All of the commonly used inhalational agentsreadily cross the placental barrier and the concen-tration in the foetal blood quickly approaches thelevels in the mother An additional contraindica-tion is that in general they hinder uterine contrac-tion and so increase the potential for post-partumhaemorrhage Nevertheless, many have beenreported to have benefits in labour using sub-anaesthetic concentrations in conjunction withoxygen þ nitrous oxide, when they have minimaleffect on the foetus and uterine contraction (Rudra,

2004) Respiratory changes during pregnancyenhance anaesthetic uptake as the increase inresting ventilation delivers more agent into thealveoli (Ciliberto, 1998)

Alongside analgesics, emetics and biotics, oxytocics are the only other drugs com-monly used Even though they are not anaestheticrelated, they are standard in the obstetric anaes-thetist’s pharmacology armamentarium They areadministered via a single shot, intended tobring about uterine contraction as the foetus

anti-is being delivered, or as an infusion if thesurgeon indicates that the uterus is flaccid andlacking tone

Oxytocics are also referred to as uterotonics.There are three in common use: syntocinon, ergo-metrine and syntometrine, which is a combinedpreparation of the other two They have the action

of contracting the myometrium, although indiffering manners and for varying durations Thisaction can actually compromise placental bloodflow and lead to foetal hypoxia Ergometrineespecially 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 asthrombo-embolism deterrent (TED) stockings orflowtron devices intended to maintain venousblood flow by external massaging

Monitoring for obstetric anaesthesia differs little

if at all from standard anaesthetic monitoring andadheres to the recommendations of the Association

of Anaesthetists of Great Britain and Ireland(AAGBI, 2000) and OAA The AAGBI regard it asessential that core standards of monitoring applywhenever a patient is anaesthetised, irrespective ofduration or location Whether involving generalanaesthetic or regional analgesia, minimum moni-toring will include, pulse oximetry, non-invasiveblood pressure and electrocardiography, with theaddition of inspired oxygen and end-tidal carbondioxide monitoring in the case of general anaes-thesia Despite the view that nothing replacespersonal vigilance, there is substantial evidencethat monitoring reduces risks of incidents TheAustralian Incident Monitoring Study (1993)reported that 52% of incidents were detectedfirst by a monitor with the pulse oximeter andcapnograph being predominant in this detection.Even though they are not standard, methods ofmonitoring potential awareness, as with allbranches of anaesthesia, are finding their wayinto the speciality

Eclampsia is an associated condition, althoughnot necessarily anaesthesia related and can involvethe anaesthetic team antenatally The conditionmay occur before, during or shortly after delivery(Chamberlain, 1995) and is characterised byconvulsions which may develop if pre-eclampsia

is left untreated Actual causation is unknown butinsufficient blood flow to the uterus is suspected.Placental abruption often accompanies the condi-tion It is during the pre-eclamptic stage atwhich the anaesthetist and AP might becomeinvolved when the patient will require intensivecare management prior to possible delivery ofthe foetus by C/S If pre-eclampsia progresses itmay 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 tothe spinal approach becoming popular for bothelective and emergency C/S The single-shottechnique however can be unsuitable shouldsurgery duration outlast analgesic effect, while theneed for post-operative pain control has to beprovided by additional means Almost as a naturalnext step, the combined spinal/epidural (CSE) orcombined spinal/epidural analgesia (CSEA)has gained popularity as it provides the rapidon-set of spinal combined with the longer-termfacility of epidural while being somewhat moreselective with sensory and motor blockade Thetechnique can be performed through one lumbarinterspace by firstly inserting a Tuohy needleinto the extradural space then using it as aguide for introducing the smaller gauge spinalneedle into the subarachnoid space, referred to asneedle-through-needle technique (Carrie et al.,

2000) Following injection of analgesic solutionand needle withdrawal, a catheter is introducedinto the epidural space The alternative techniqueinvolves inserting the needles through separatelumbar spaces Epidural needles are usually in therange of 16
18 G and spinal needles are muchfiner, e.g 26
27 G This finer gauge and specialisedlow trauma tips reduce leakage of cerebrospinalfluid (CSF) and in turn post-dural punctureheadache (PDPH) Whitacre and Sprotte are thetwo main needle designs at present

Both techniques have many plus factors formother, baby and anaesthetist although in spite

of the benefits, there are potential drawbacks Localanalgesic solution toxicity is a continuing dangerwith epidural as repeated doses via the catheter canlead to accumulation, especially when being usedfor surgery and continuing post-operative painrelief Hypotension due to sympathetic blockade

is common to both techniques, although there is amuch more rapid onset with the spinal routewhich is a particular danger in obstetrics asbeside a primary hazard to the mother, placentalperfusion is compromised and can lead to foetaldistress (Chamberlain, 1995) Measures to offsetthis 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 techniqueand is used for episiotomy but may not provideadequate analgesia for forceps delivery orprocedures that involve extensive manipulation(Rudra,2004)

Opinion and debate continue over patientpositioning when performing epidural and spinal,especially for C/S, either lateral or sitting positionwith legs over the edge of trolley, bed or operatingtable Additionally, left or right lateral also insti-gates discussion, initially with regard to unilateralblock, however, there are proponents of both leftand right lateral The thinking of the former relates

to vena-caval occlusion and the fact that thepatient will be in left tilt during surgery is used

to support the latter view

Two uncommon problems associated withregional techniques that the AP should be familiarwith are total spinal and blood patch for duralpuncture Total spinal happens when local analge-sic solution spread is too advanced and affectscranial nerves, leading to paralysis of respiratorymuscles, loss of consciousness, hypotension andbradycardia It is more likely to happen duringepidural when the needle may inadvertently piercethe dura mater and the large volume of analgesicsolution is injected into the subarachnoid space(Carrie et al.,2000) Blood patch is carried out forthe relief of post-spinal headache and is an attempt

to plug the dural leak with 10
20 ml of the patient’svenous blood injected into the extradural space(Smith & Williams,2004)

There is no one dominant or recognised pain-careregime common to obstetric anaesthesia Regionalanalgesia by nature can provide its own pain reliefand there is a growing use of PCEA (patientcontrolled epidural analgesia) The obvious discom-fort and distress of pain to the mother can causehyperventilation which may lead to maternalhypercarbia, respiratory alkalosis and metabolicacidosis Consider the already compromisedoxygen consumption associated with labour andthe 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), 48
54

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) PhysiologicalChanges Associated with Pregnancy Available at:www.nda.ox.ac.uk/wfsa/html/uO9003.htm (Accessed 9March 2005)

Dobson, A (2004) A critical analysis of caesarean sectionprocedure Journal of Operating Department Practice,1(8), 16

Ducloy, A & de Flandre, M J (2002) Obstetric thesia
Placental Abruption Update in Anaesthesia(Online), 14(17) Available at:http://www.nda.ox.ac.uk/wfsa/html(Accessed 5 April 2005)

Anaes-Faura, E A M (2004) Anaesthesia for the PregnantPatient Available at: www.daccx.bsd.uchicago.edu/manuals/obstetric/obstetricanaesthesia.html(Accessed

30 March 2005)

Harvey, P (2005) The role of the ODP in obstetrichaemorrhage Journal of Operating DepartmentPractice, 1(11), 18

Health Professions Council (2003) Standards ofConduct, Performance and Ethics (Online) Availableat:www.hpc.uk.org(Accessed 16 May 2005)

Morgan, B M (1987) Foundations of Obstetric sia & Analgesia London: Baillie´re Tindall

Anaesthe-Nelson, G L (1999) Fundamentals of pain relief In

A Davey & C S Ince, eds., Fundamentals of OperatingDepartment Practice London: Greenwich Medical Med-

ia Ltd, pp 245
57

Oyston, J (2000) A Guide to Spinal Anaesthesia forCaesarean 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 inAnaesthesia (Online), 18(3) Available at: http://www.nda.ox.ac.uk/wfsa/html (Accessed 9 March2005)

Ryan, T (2000) Fundamentals of obstetric and emergencyanaesthesia In Fundamentals of Operating DepartmentPractice London: Greenwich Medical Media

Simpson, P & Popat, M (2001) Understanding thesia, 4th edn Oxford: Butterworth Heinemann

Anaes-128 T Williams

Smith, B & Williams, T (2004) Operating Department

Practice A
Z 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

National Electronic Library for Healthwww.nelh.uk

Nursing & Midwifery Councilwww.nmc-uk.org

Obstetric Anaesthesia & Analgesia Available at:www.themediweb.net/obstetrics/Anaes%20Considerations.htmObstetric Anaesthetic Association

www.oaa-anaes.ac.ukRoyal College of Anaesthetistswww.rcoa.ac.uk

Royal College of Nurseswww.rcn.org.uk

Obstetric anaesthesia 129

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 tory compromise, evaluation of interventionssuch as oxygen therapy and respiratory support,

respira-as a preoperative brespira-aseline and following a respiratory arrest

cardio-It is imperative to note at the start of this chapterthat, just as with any investigation, ABGs mustalways be interpreted in conjunction with otherclinical information about the patient (Adam &Osborne, 1997) A thorough clinical examinationand assessment of a patient will present manyclues about the physiological status of thatindividual
ABG analysis just adds another piece

to that jigsaw Additionally, what may be an quate set of results for one person may be entirelyunacceptable for another, depending on theircurrent diagnosis and any pre-existing illnesses.Table13.1shows the basic parameters measured

ade-by blood gas analysers and their normal values Tohelp you understand and interpret these values,this chapter will cover some of the fundamentals

of the physiology of acid-base balance, alveolarventilation and oxygenation Once you are aware ofthe related physiology, the interpretation of ABGresults will be much easier as you will be able tothink more clearly about what could be hap-pening to your patient This chapter also contains

a section on the collection and handling of ABGsamples

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
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 andinfection Accidental removal or disconnection ofthe catheter are the most common causes ofhaemorrhage and could lead to significant bloodloss if they were to go unnoticed Vigilance isparamount in prevention of this problem and thecatheter should be well secured and the insertionsite and transducer line kept visible and directlyobserved whenever possible

Thrombosis is rare, but if a clot were to form

in the lumen of the catheter this could be flushedinto the arterial circulation and compromise theblood flow distal to the catheter site (Garretson,

2005) Correct maintenance of the pressure ducer system will ensure a continuous flush ofsaline (usually around 3 ml/hour) to help maintainpatency of the catheter The colour, temperatureand sensation of the limb should be observed toassess for any signs of compromised circulation.Blanching or discolouration should be reportedimmediately to medical staff (Moore,2000)

trans-As with any invasive line, there is a risk ofinfection with an indwelling arterial catheter ifstrict hand washing and asepsis are not observedduring both insertion and sampling (Moore,2000)

It is also important that the lines and sample portsare kept free from blood and other debris Signs ofinfection include localised redness, warmth anddischarge at the insertion site and the patient maydevelop a pyrexia (Garretson,2005)

Arterial puncture or ‘stab’ from the radial orfemoral arteries is usually the method of choice forsampling if there is no indwelling arterial cannula.Complications can include spasm of the artery, clotformation within the lumen of the blood vessel,haematoma formation and bruising (Williams,

1998) These can potentially compromise bloodflow distal to the puncture site; consequently theradial artery is the optimal site as patients usuallyhave a good collateral blood supply via the ulnarartery

Prior to a radial arterial stab or cannulation, asimple Allen’s test can be performed to deter-mine the adequacy of collateral circulation tothe hand via the ulnar artery (Moore, 2000)

Table 13.1 Normal values for arterial blood gases

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

5
7 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 0
10 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

PaCO2does not vary significantly the accuracy of

the PaO2reading 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 thesample is drawn it is advisable to analyse theblood 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 theeffect of slowing the metabolism of the bloodcells and will prolong the time available for analysis

to about 1 hour (Woodrow, 2004) It is commonpractice to place the syringe into ice to cool thesample but Woodrow (2004) suggests there isanecdotal evidence that this may cause haemolysisand so recommends that iced water is used as long

as it does not cause undue delays in transportingthe sample

As the course of treatment a patient receives isoften based on the ABG results, it is imperative thatall possible measures are taken to optimise theaccuracy of the readings Many modern bloodgas analysers automatically calibrate themselves

at predetermined intervals and require little inthe way of maintenance (Williams, 1998) It is,however, essential that practitioners liaise closelywith their hospital laboratory services to ensurethat the manufacturer’s guidelines and local Trustpolicies for quality control and health and safetyare adhered to

What can ABGs tell you?

Arterial blood gases will provide a set of valuesthat can be used to determine key aspects of thepatient’s condition These values can be broadlycategorised into:

132 H McNeill

The adequacy of alveolar ventilation is reflected by

the partial pressure of the arterial carbon dioxide

level (PaCO2) and provides invaluable

informa-tion about respiratory funcinforma-tion Examinainforma-tion of the

acid-base status will provide useful information

about both the respiratory and metabolic

compo-nents of acid-base balance Each of these three

categories will now be discussed in more detail

Oxygenation

Often, one of the primary reasons for ABG analysis

is to ascertain the oxygenation status of the patient

The body has no means of storing oxygen and so

cells are dependent on a continuous supply that

meets their metabolic needs If the supply of

oxygen does not meet demand then tissue hypoxia

will develop and the vastly inefficient process of

anaerobic metabolism will commence, with the

resultant production of lactic acid (Fitz-Henry &

Lewis,2001)

There are two sites within the body where oxygen

transfer occurs: at the lungs from the alveoli to the

haemoglobin in red blood cells, and at tissue level

from the haemoglobin to the mitochondria in the

tissue cells (Moore, 2000) A significant factor in

oxygen transfer is the natural affinity of

haemoglo-bin for oxygen This relates to how easily oxygen

will bind to haemoglobin in the lungs and how

readily it will be released to the tissues Successful

oxygen transfer is, of course, also entirely

depen-dent on adequate respiratory and cardiovascular

function

The diffusion of oxygen relies upon the passive

movement of oxygen down a partial pressure

gradient (i.e concentration gradient), which allows

oxygen molecules to cross the tissue barriers

(Leach & Treacher,1998) As the concentration of

oxygen found in the alveoli (PAO2) is higher than

that of the deoxygenated blood in the pulmonary

capillaries, oxygen will diffuse across the

alveolar-capillary membrane This is known as the A-a

gradient Similarly, at tissue level, the concentration

of oxygen in arterial blood (PaO2) is greater than that

of the tissue cells, therefore, oxygen will diffuse

from the capillaries into the tissues The A-agradient can be manipulated by the administration

of oxygen therapy as increasing the PAO2 canimprove the PaO2and SaO2and oxygen delivery tothe tissues (Treacher & Leach,1998)

The alveolar-capillary oxygen gradient cates that the inspired oxygen level must always

indi-be higher than the arterial oxygen level In ahealthy subject, the difference between the two isabout 10 kPa This is because at sea level, 1% O2

is approximately the same as 1 kPa, therefore, apatient breathing room air (21% O2) should have

a PaO2 of greater than 11 kPa (ResuscitationCouncil,2004) This ‘rule of 10’ provides a usefulestimate of what you could expect your patient’sPaO2 to be (Simpson,2004) Consequently, if youhave a patient who is on 40% oxygen you wouldexpect their PaO2 to be approximately 30 kPa Ifthe difference between the inspired oxygen andthe PaO2 is greater than 10 this is suggestive ofpulmonary disease causing a mismatch betweenthe ventilation of the alveoli and the perfusion ofthe pulmonary capillaries

As mentioned at the beginning of this chapter,many blood gas analysers will provide the SaO2

expressed as a percentage In terms of oxygenation,

it is crucial to remember that saturation ments do not take into account the haemoglobinlevel of the patient (Woodrow,2004) For example,one of your patients could have an Hb of 6 g/dl andanother an Hb of 12 g/dl It is clear that the patientwith an Hb of 12 g/dl will have a much greater totaloxygen content of the blood than the patient with

measure-an Hb of 6 g/dl, even if their SaO2 was exactlythe same

The oxygen-haemoglobin dissociation curveThe relationship between PaO2 and SaO2 iscomplex and is reflected by the S-shaped oxygen-haemoglobin dissociation curve, illustrated inFigure13.1

If you observe the curve in Figure13.1it can beseen that one of the most distinctive features is theS-shape, with a steep slope followed by a flattened

Understanding blood gases 133