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2018-08-06T18:24:45Z

Peripheral nerve stimulators

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''Originally from Update in Anaesthesia | www.wfsahq.org/resources/update-in-anaesthesia'' by Graham Bell* and Rachel Homer (*Correspondence Email: gebell@btinternet.com)

Graham Bell*
::Graham Bell
::Consultant in Paediatric
::Anaesthesia

Rachel Homer
::Fellow in Paediatric Anaesthesia
::Royal Hospital for Sick Children,
::Glasgow

=AIRWAY ADJUNCTS=

==Oropharyngeal (“Guedel”) airways==

These are universally popular in children as they
are easy to use and fast to insert. They can be
used to supplement airway manoeuvres such
as jaw-thrust and chin lift during resuscitation,
induction, and emergence from anaesthesia. The
airway should only be inserted when the child is
deeply anaesthetised. If the child is too light the
airway will not be tolerated, potentially leading
to coughing, gagging, expulsion of the airway, or
laryngospasm.

Oropharyngeal airways in children are best
inserted correctly oriented. You should take care
to avoid damage to the delicate soft palate in
young children, and to the posterior pharyngeal
wall if a relatively large airway is used. They are
less likely to cause trauma on insertion than a
nasopharyngeal airway.

==Nasopharyngeal airways==

These are particularly useful in children with a
tendency towards upper airway obstruction or a
reduced level of consciousness. They are better
tolerated than an oropharyngeal airway. To
insert:

::• Lubricate with gel to minimise trauma to the
turbinates
::• Avoid local anaesthetic preparations because
these can alter pharyngeal muscle tone and
impair airway patency
::• Insert gently along the floor of the nose
::• Take care not to exert too much pressure at
the post nasal space where the adenoids may
be damaged.

Nasopharyngeal airways are usually used during
recovery rather than during anaesthesia. A
common indication is for the child at risk of
upper airway obstruction after cleft palate repair.
To prevent damage to the suture line during
insertion, railroad the airway over a smaller
nasogastric tube (confirm that this is in the
stomach first), or allow the surgeon to insert the
nasopharyngeal airway under direct vision.

==Facemasks==

A plastic facemask with an inflatable cuff/
rim gives a better seal on the face, and is less
frightening to the child than the traditional black
rubber facemask. The correct size of facemask
is one that reaches from the cleft of the chin.

==SUMMARY==

Differences in anatomy and
physiology between children
and adults mean that much
of the equipment needed to
anaesthetise children needs to
be specially adapted

Figure 1. Oropharyngeal airways come in a range of sizes suitable for premature neonates up to adults. To
find the correct size, hold with the flange in line with the middle of the incisors. The tip should just reach the
angle of the child’s jaw

to the bridge of the nose, but does not cover the eyes. For
small babies, round masks are available, or turn a shaped mask
‘upside down’ for an acceptable seal.

==Laryngeal mask airways (LMA s)==

The LMA is the most widely available example of a supraglottic
airway device. They are suitable to use during maintenance

Figure 2. The correct diameter of the nasopharyngeal airway is the same as the size of the endotracheal tube for that child (see Table 2).
Estimate the length by the distance from the nostril to the tragus of the ear as show. An uncuffed endotracheal tube makes a satisfactory
nasopharyngeal airway. Either attach a standard connector (to stop the tube from advancing and becoming ‘lost’ in the nose); or split
the tube to facilitate secure taping as shown. Sutures may be tied around the tube to secure the airway as shown.

Figure 3. Clear plastic facemasks

of anaesthesia in cases which you would otherwise manage
by holding a facemask (low aspiration risk), to free the
anaesthetist’s hands for other tasks.

The size 1 LMA is less useful for routine anaesthesia because:
::• It is designed for children <5kg in whom
intubation and ventilation are generally preferred
::• It tends to dislodge, in which case it occludes rather than
maintains the airway
::• Coughing and laryngospasm on insertion are more
common with smaller LMAs.

It may still be useful to rescue a difficult airway, even in a small
baby (see *NEED TO UPDATE LINK* Difficult airway article, page 116).

In adults it is common to mechanically ventilate the lungs
through the LMA. In children there is a higher risk of gastric
insufflation, so this is not widely practiced.

Serious airway damage from the LMA cuff has not been
reported, but be aware that the cuff pressure exceeds that of
a tracheal tube. Capillary perfusion pressure in children is
lower than in adults, thus the potential for injury is probably
greater, even if the cuff is not inflated above the recommended
maximum volume (see Table 1). Be even more cautious in
longer procedures, as lingual oedema has been reported after
LMAs have been in for a long time.

Avoid overinflation of the LMA cuff:
::• Lower cuff pressures tend to give a better seal
::• There is less risk of pressure injury to tissues.

Opinions vary as to when to remove the LMA in children. The
choice is either deeply anaesthetized (“asleep”) or fully awake;
between these extremes, you risk precipitating laryngospasm.
Some suggest that the best time to remove the LMA is when
the pharyngeal reflexes return and the child spits the airway
out.

Use in difficult intubation – see article page 116 *NEED TO UPDATE LINK*

The intubating LMA is not made below a size 3, therefore is
not suitable for young children.

Figure 4. LMAs are available in a range of sizes appropriate for babies up to adults. They may be either single-use or designed for repeat
resterilisation. Rigid versus flexible stems can be easier to insert but more frequently displaced by heavy drapes or when close to the
surgical field

Weight (kg) LMA size Maximum cuff volume
(mls)
<5 1 4
5-10 1 ½ 7
10-20 2 10
20-30 2 ½ 14
30-50 3 20

Table 1. Manufacturers’ guide to LMA sizes

==Laryngoscopes==

The epiglottis is long and U-shaped in infants and small
children. This is advantageous when breast-feeding but
requires a different approach for intubation. A curved blade
placed in the vallecula will not provide a reliable view of the
glottis. Straight blades designed to lift the epiglottis along with
the soft tissues at the base of the tongue give an easier view.
broken during clinically realistic force tests. Manufacturers
label these blades Miller, Robertshaw etc., but they are often
not exact copies of the blades whose name they bear. Both
laryngoscopic view and illumination may vary.

==Endotracheal Tubes (ETT )==
The black marks used for guidance during laryngoscopy
vary widely between manufacturers and between tube types.
Some are meant to indicate the appropriate position at the
cords. Others simply make the tip easier to see. The only
reliable advice that can be given on ETT markings is to ignore
everything except the actual distance markings in centimetres.
A 3.5mm uncuffed ETT is suitable for the normal term
neonate. Use the formulae below to guide ETT selection in
older patients (Table 2). There should be a small leak around
the ETT to avoid excessive pressure on delicate mucosa. If
ventilation is compromised, the leak is too large. The nose
will accommodate the same size ETT as the larynx prior to
puberty.

A neonate’s trachea may be less than 4cm in length. The ETT
should sit at least 1cm above the carina, which is anteriorly
inclined and more symmetrical than in adults. Small flexion/
extension movements may cause accidental endobronchial
intubation or extubation respectively. You must be meticulous
in placing and securing the ETT.

Some children receiving intensive care benefit from a cuffed
tracheal tube. Most paediatric anaesthetists prefer uncuffed
tubes for routine anaesthesia in children up to 8 – 10 years old.

Reasons to choose an uncuffed tube:
::• Larger internal diameter tube can be used
::• Confusing tube markings and long cuff shoulders have led
to frequent endobronchial intubation with traditional
cuffed tubes. The microcuff tube® avoids these problems
::• Overinflation of endotracheal tube cuffs risks the potential
but rare complication of tracheal stenosis

Reasons to choose a cuffed tube:
::• Fewer intubation attempts with different sized tubes
::• Reduced leak – more important in ICU, also in anaesthesia
as better paediatric circle systems are developed and use of
low flow anaesthesia in children increases
There is no good evidence that either cuffed or non-cuffed
tracheal tubes are better.

=BREATHING SYSTEMS=

Semi-open (see article on drawover anaesthesia, p23 NEED TO UPDATE LINK)

Figure 5. The most popular straight blades are [left to right]
the Miller, the Robertshaw and the Oxford [top in (a)].

Many paediatric anaesthetists use a MacIntosh (size3) curved blade in
all children over 1 year old. Others prefer a straight blade until 5
years old. No one design has been shown to be truly superior.
The international standards organization has a size mark for
laryngoscopes from 5 (largest) to 000 (smallest – too small even
for tiny premature babies). A size 1 blade is the correct size for
a normal term baby, and a size 0 for premature neonates who
weigh less than 3kg.

It is possible to intubate with a laryngoscope that is slightly
too big, and rarely possible with one too small. MacIntosh
considered smaller sizes of his original (size 3) blade unnecessary,
as the distal portion of the size 3 blade is straight and therefore
suitable for smaller children. He was correct!

Many disposable laryngoscope blades exist. Some are of high
quality but this cannot be assumed, for example some have

==Semi-closed==
The T-piece is the most popular system for anaesthetising small children in UK with many advantages:
::• Lightweight
::• Compact
::• The bag is easily visible whilst observing the patient
::• Simple to use for spontaneous or controlled ventilation
::• No valves
::• Low internal resistance
::• Low compression volume (i.e. the operator has a good ‘feel’

for the lung compliance).

Jackson-Reece modified the system by adding an open-ended
bag. This bag has several advantages:
::• Respiratory monitor during spontaneous ventilation
::• Easy to convert to manual controlled ventilation
::• Easy to apply PEEP.

In skilled hands the T-piece is the best system for resuscitation.
The application of CPAP / PEEP splints the upper airway
open and so improves the efficacy and ease of ventilation.

However, in less skilled hands, the T-piece may result in
::• Inadequate ventilation (requires practice to use)
::• Gastric insufflation
::• High peak inflation pressures
::• Inadvertent barotrauma.

It is customary to use adult breathing systems in children > 20
– 30kg. To prevent hypercarbia during controlled ventilation,
the fresh gas flow in ml can be predicted by the formula (1000
+ (200 x kg)) (minimum of at least 3 l/minute), which is
approximately 1.5x the minute volume. During spontaneous
breathing in young children, there is no end expiratory pause
and the system becomes less efficient; twice the fresh gas flow
required for controlled ventilation should be used (i.e. about 3
x the minute volume).

Figure 6. T piece attached to oxygen cylinder. The same
connection will fit the outlet of an oxygen concentrator.
Alternatively, the T piece’s green tubing connects directly to an
anaesthetic machine’s common gas outlet. A T piece requires gas
flow.

Figure 7. Humphrey ADE.

Newer versions are, in fact, AE systems;
there is no option for a Mapleson D. The E system (lever down) is
a T-piece; it has no bag and no valve and is used for controlled
ventilation (fresh-gas flow as for the T-piece, above). The A system
(lever up) has the configuration of a parallel Lack circuit. This is
ideal for children breathing spontaneously because of the low
resistance of both the smooth inner bore of the tubing and the
expiratory valve. A flow rate of 3 l/minute is sufficient for most
children before adolescence.

==Closed==
Early circle systems imposed a high work of breathing due to
the resistance of one-way valves. This is much less of a problem
with modern circles but other issues remain:
::• A leak around the tracheal tube means that low flows cannot be used efficiently (solve by using cuffed tracheal tubes)
::• The volume of the breathing system is large compared with minute volume, giving longer equilibration times than in adults
::• The tubing has a significant compression volume, which may alter the characteristics of ventilation during positive pressure ventilation. Avoid by using stiffer tubes and
smaller bellows
::• Paediatric circle systems do not maintain the temperature or humidity of inspired gases adequately.

=IV ACCESS=

==Administering fluids==
Standard blood and fluid administration sets are suitable for
children. A burette should be used for small children (<10kg)
so that the volume of fluid can be measured accurately and to
avoid giving excessive volumes of fluid.

In-line fluid warmers are of great benefit, but they add
considerable deadspace into the system, and require electricity,
and costly single-use disposables. Water baths are not
commonly used to warm IV fluids because contamination
with Pseudomonas species can cause serious infection. Fluids
may be stored in a warming cabinet or other warm place prior
to administration. Some anaesthetists place the sealed bag of
IV fluid on top of the monitor as this becomes warm during
use. This is only partially effective, but makes use of a readily
available source of heat!

==Peripheral cannulae and flushing==
Young children more commonly have a patent foramen ovale.
This means that bubbles administered IV may embolise to
the systemic circulation rather than the lung. Remove all air
bubbles from the fluid administration system. It is difficult to
give guidelines about the amount of air that poses a danger
because speed of injection is as important as volume. Deaths
have occurred after as little as 0.5ml.kg-1 injected rapidly.
The ‘dead-space’ of injection ports and cannulae can be
significant. For example, neonates have been paralysed by
the quantity of suxamethonium present when the hub of
the cannula was flushed post-operatively. Anaesthetists must
flush the cannula carefully with saline after injecting drugs,
particularly if an IV extension has been used. Avoid flushing
with heparinised saline in neonates. Heparinised saline
contains 10 iu.ml-1 of heparin; 10ml given to a 3kg neonate is
33 iu.kg-1 and this could easily affect clotting.

==Central venous catheters==
These are smaller versions of adult catheters, with up to 4
lumens.

===Sites for central venous access===
Femoral venous access is more popular in children than adults.
The tip of the catheter is the most likely place for thrombus
formation, so choose the length of the catheter so that the tip
does not lie at the junction of the renal veins with the inferior
vena cava (IVC). Either short lines (catheter tip in the iliac
vein or lower IVC) or long lines (tip just below the diaphragm)
are appropriate. Central venous pressure monitored from the
latter position is as accurate as from the superior vena cava
(SVC). Confirm the position of longer femoral lines on X-ray
if available, because the line may pass into the contra-lateral
femoral vein, the renal vein or the epidural venous plexus.
In children <2 years old, the J-portion of J wires may exceed
the dimensions of the central vein which will make the wire
difficult to thread.

The umbilical vessels are easily accessible during the first 24
hours of life (and less easily so for a further 3 days). Placing
the tip of an umbilical venous catheter in the correct position
is easier if you adapt the catheter tip to be a unipolar electrode
and monitor the ECG signal until you obtain a characteristic
atrial ECG. Umbilical arterial lines have been linked with the
development of necrotising enterocolitis, although this is not
proven. Umbilical venous catheters are particularly prone to
thrombosis, which may lead to portal hypertension. They
should be replaced as soon as alternative access is practical.
Peripherally inserted central catheters (PICC lines)
These are available as small as 27g and provide access for drugs
or parenteral nutrition. Smaller lines cannot be used for pressure
monitoring or sampling. Hypertonic drugs (e.g. thiopentone)
or drug precipitates (e.g. thiopentone/ suxamethonium) are
likely to block these lines. Flush all medicines in immediately
after administration.

==Intra-osseous needles==
There are discussed in a separate article (see page 242 NEED TO UPDATE LINK)

=DEFIBRILLATORS=

There are no specific defibrillators designed for children. Many
standard defibrillators have paediatric paddles similar to those
shown in Figure 8: the adult paddle slides off the smaller
paediatric paddle. Paddles must be positioned as far apart as
possible to reduce the chances of arcing. Use gel pads (not
shown) to reduce impedance between paddle and patient as
for adults. Self-adhesive pads may be available (see Figure 8).
The appropriate size of paddle is the largest pad that will
fit the child’s chest, as this will reduce both transthoracic
impedence and the potential for chest burns. These pads are
often labelled with a recommended weight range for standard
hospital controlled dose defibrillators, or with an age range for
automatic external defibrillators (AEDs).

The appropriate energy levels for children are different from
those for adults (see Paediatric Resuscitation p264). ‘Advisory’
defibrillators and AEDs prompt the user down adult Advanced
Life Support (ALS) algorithms and so are not recommended in
infants < 1 year old, although they have been used successfully
in young children when there has been no alternative.
Paediatric pads, if used on an AED, will automatically deliver a
lower energy, which varies between defibrillator models. Adult
pads on an AED will deliver up to 360J.

There is evidence that biphasic waveforms are more effective
for the treatment of ventricular fibrillation. Advantages seen in
adult practice include
::• Less time needed to charge
::• Improved first shock success rate (90% compared with 60%)
::• Less myocardial injury
::• Decreased incidence of skin burns
::• Increased battery life of the defibrillator.

Current paediatric recommendations are to select the same
energy (4J.kg-1 in cardiac arrest) for a monophasic or biphasic
defibrillator (see resuscitation articles p267 and 270.)
Figure 8. Paediatric defibrillator pads and paddle positions.

=TEMPERATURE CONTROL=

Good temperature control is vital in paediatric anaesthesia
(see basic sciences article p4). You must monitor temperature
during active warming.

Warming methods include:
::• Forced air heating blankets e.g. the Bair Hugger. These devices deliver warm air up to 43°C for active warming. They can also blow room temperature air to cool patients who are pyrexial.
::• Simple warming blankets reduce conductive losses to the table and are cheap and effective although not as efficient
::• Overhead radiant heaters are also cheap and effective. Pay
strict attention to the safe minimum height of radiant heater above the patient. Patient signs of systemic overheating will be late. Local overheating presents as burns. Radiant heaters increase insensible water loss in neonates
::• Prevent heat loss by insulation when active heating is impractical or unavailable. Almost any insulating material may be used e.g. clean blanket or clothing, cotton wool, tinfoil or bubble wrap
::• Raised operating theatre temperature for babies or small children. You will need to compromise between the thermo neutral temperature for the anaesthetised patient (34°C for
premature neonates, 33°C for term neonates reducing through childhood to 30°C at adolescence) and a comfortable working temperature for staff (up to 25°C).

One solution is to have the temperature relatively high
during induction of anaesthesia, and then reduce it once
the patient is draped and use a forced air warmer if available
to maintain a ‘microclimate’ during surgery
::• In theory circle systems warm and humidify inspired gases.
In practice these effects are small. Some circle systems have
additional heaters to aid gas warming. HME (heat moisture
exchange) filters help
::• Heated humidifiers may be added to any breathing circuit,
and will help warm the patient by warming airway gases.
Monitor gas temperature at the patient end to avoid airway
burns
::• Warm fluids – as above.

=MONITORING=
Standard monitors for a child are the same as for an adult. It
is acceptable to complete the attachment of patient monitors
after induction of anaesthesia to prevent a child becoming
distressed as long as there is no compromise to safety.
Oesophageal or precordial stethoscope
This low-technology device offers continuous, immediate
information on breath and heart sounds without the lag times
associated with oximetry and capnography.

==Pulse oximeter==
Oximetry probes need to be matched to the size of the patient.
Too large a probe will give a poor reading due to light scatter
and easily falls off. When you choose the site carefully, you
can use adult sized probes for younger children e.g. an adult
ear probe can be attached to a digit. These ear probes have
spring closures, which may produce sufficient pressure to cause
ischaemia in a child’s finger. Wrap-around probes are available
and may be used on digits, or in small babies wrapped around
a foot or hand. They have no risk of pressure necrosis but
do produce heat. Burns have occurred in children with low
peripheral perfusion e.g. during cardiopulmonary bypass.
The absorption spectra of foetal and adult haemoglobin are
similar, and at 660nm and 910nm in a basic pulse oximeter
the device is accurate in detecting desaturation of foetal
haemoglobin.

Problems and pitfalls
::• Pulse oximeters are poor indicators of hyperoxia due to the
shape of the oxyhaemoglobin dissociation curve. Whilst the
pathogenesis of retinopathy of prematurity is multifactorial,
it is important to avoid hyperoxia in at risk babies
(particularly premature neonates)
::• Pulse oximeters may be inaccurate in the presence of certain
pigments: methylene blue gives a falsely low reading.
Bilirubin does not affect pulse oximetry
::• Potential inaccuracies in children with congenital heart
disease: the oximeter derives arterial saturation by
subtracting the ‘fixed absorbance’ part of the signal. In
tricuspid regurgitation, the monitor may detect a pulsatile
venous component of the signal, and display the venous
saturation
::• In infants with a physiological shunt through a patent
ductus arteriosus, the pulse saturation will vary between
the right arm (pre-ductal) and other limbs (post-ductal).
The pre-ductal blood supplies the cerebral circulation, so
measure saturation on the right arm as a minimum.

Phone app based pulse oximeters are appearing on the market.
All necessary hardware is incorporated into the finger probe
with a connection to a mobile phone where the phone app
provides the software and display. Some of the probes now
have FDA approval, but experience of their use is limited at
present.

==Apnoea monitors==
These are frequently requested by paediatric anaesthetists to
monitor babies <6 months old after anaesthesia. They are
designed to alarm after 15 or 20 seconds of apnoea. They
detect breathing in one of two ways:
::• By changes in impedance measured through ECG
electrodes on the chest
::• By a change in pressure detected by a sealed mattress or a
small sensor on the abdomen as shown in Figure 8
(abdominal excursions move a fine foil strip in the monitor)
(More common).

Apnoea monitors are cheap and non-invasive but have some
disadvantages:
::• Prone to false positive alarms, and to interference from
mobile phones
::• Will not detect airway obstruction, because movement of
the chest and abdomen still occurs
::• Cardiac pulsation may trigger the monitor, even if the
child is apnoeic.

These monitors should be used as an adjunct to high
dependency nursing care, not a substitute for it.

==Electrocardiography (ECG)==
As children do not usually have ischaemic heart disease, the
ECG is used mainly as a heart rate and rhythm monitor and
rarely to detect ischaemia. At birth, the right ventricle is as
thick as the left. This manifests as right axis deviation and
right ventricular hypertrophy on the ECG (dominant R wave
in V1, deep S wave V5 & V6, inverted T wave V1-4). With
growth, the left ventricle becomes larger than the right. By age
2-5 years, the axis is within the adult range. T wave inversion
reverses by 10 years old when the ECG becomes similar to that
of an adult.

Lead II is usually the best choice in all ages as the prominent P
wave aids rhythm recognition. A common error is that large T
waves may be read as QRS complexes by the monitor causing
double counting of the rate.

==Blood pressure measurement==

Non-invasive blood pressure measurement

All non-invasive blood pressure machines that measure from
a cuff are automated oscillotonometers. The most important
considerations are the width and length of the bladder used in
the cuff (see Table 3).

Figure 10. An apnoea monitor for a baby
BP cuff bladder
width
Should cover
approximately 2/3
of the upper arm
Neonate 3cm
Baby 5cm
Small child 6cm
Large child 9cm
BP cuff bladder
length
Minimum 40% of
the circumference
of the upper limb;
≥ 80% is ideal

Table 3. Determining the appropriate size of a blood pressure
cuff. If the cuff is too small, the reading will be artificially high;
the reverse is true if the cuff is too big. Common Error: generally,
oscillotonometers over-read lower pressures, i.e. they over-read
values within the normal neonatal range.

The machine settings for adult, paediatric or neonatal modes
usually control only the maximum inflation pressure. This is
to avoid excessive pressure and the risk of bruising or trauma
in babies or small children if exposed to the values in a
hypertensive adult.

Direct arterial blood pressure measurement has the same
principles as in adults. If using an umbilical arterial catheter
in neonates, which is relatively long and thin, inspect the trace
for signs of overdamping.

Safety point: take care with manual flushing. Meticulously
avoid bubbles, and note that a brisk 1ml flush in a neonate is
sufficient to push bubbles or thrombus from the radial artery
back as far as the vertebral artery.

==Peripheral nerve stimulators==
Peripheral nerve stimulators may be used in children for
monitoring neuromuscular blockade, or localization of
peripheral nerve. The response of the paediatric neuromuscular
junction to neuromuscular blockade is similar to that of
Update in Anaesthesia | www.wfsahq.org/resources/update-in-anaesthesia page 21
adults. In neonates, the immature junction is sensitive to nondepolarising
muscle relaxants, with reduced prejunctional
reserves of acetylcholine.

==CO2 monitoring/end-tidal agent monitoring/oxygen monitor==
End-tidal gas measurements are unreliable in babies. The tidal
volume is small compared with equipment dead space volume,
fresh gas flow, and leak around the endotracheal tube. Small
tidal volume and rapid respiratory rate confound the monitor’s
real-time analysis.

You should still obtain some CO2 tracing if you are careful to
minimise equipment deadspace, but with a large and variable
A-a gradient.

=CONCLUSION=
Consideration of the anatomical and physiological differences
between paediatric and adult patients makes it easier to select
the correct equipment to anaesthetise all patients safely.
Availability of the appropriate equipment may be the more
difficult problem.
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