and Monitoring for Pediatric Anesthesia

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© Springer Nature Switzerland AG 2020
Craig Sims, Dana Weber and Chris Johnson (eds.) A Guide to Pediatric Anesthesiadoi.org/10.1007/978-3-030-19246-4_6



6. Equipment and Monitoring for Pediatric Anesthesia



Craig Sims1   and Tom Flett1  


(1)
Department of Anaesthesia and Pain Management, Perth Children’s Hospital, Nedlands, WA, Australia

 



 

Craig Sims (Corresponding author)



 

Tom Flett



Keywords

Anesthetic breathing circuits for childrenT-piece circuitBreathing system deadspaceNeonatal ventilatorPressure controlled ventilation in children


Children can be as small as several hundred grams or as large as adults, and so a range of equipment sizes and types is required. This chapter focuses on aspects of equipment and monitoring specifically for children, and factors to consider when using adult equipment for children. Equipment for the airway is discussed in Chap. 4, Sects. 4.​54.​7.


6.1 Breathing Circuits


Although the T-piece is the classic circuit for children, many circuits can be used safely for pediatric anesthesia. For a circuit to be suitable for children it must have low deadspace and low resistance. Preferably, the circuit should have a small compressible volume, be lightweight, compact, efficient and easy to use.


6.1.1 Deadspace


The deadspace of a circuit is the portion of the circuit between the patient and the point that fresh gas enters. For a circle this is at the Y-piece where inspiratory and expiratory limbs meet. For a T-piece this is at the side arm of the ‘T’ where the fresh gas enters. For a Bain circuit it is at the end of the circuit where the inner fresh gas line joins the expiratory limb. Deadspace is increased by angle connectors, filters, Cobbs connectors, respiratory monitors and facemasks. It causes rebreathing and requires the patient to increase minute ventilation to maintain normocarbia. Children have small tidal volumes and increased deadspace may form a significant proportion of tidal volume. For this reason, deadspace can be a problem particularly in small children who are breathing spontaneously, and is one of the reasons why neonates and infants tend to be ventilated during anesthesia.



Note


An infant breathing spontaneously is most prone to the problem of deadspace as the tidal volume may be close to the equipment deadspace volume.


6.1.2 Resistance


Resistance of breathing circuits adds to the work of breathing. Neonates and infants have difficulty increasing their respiratory effort for more than a short period of time and are particularly at risk of problems from circuit resistance. Resistance in a circuit arises from the hoses, valves and attached filters. In practice however, the greatest source of resistance in anesthesia is the shaft of the ETT or LMA.


6.1.3 The T-Piece Circuit


The original T-piece was a simple metal ‘T’ designed by Dr. Phillip Ayre to use in babies undergoing cleft palate repair (Fig. 6.1). This simple device offers no resistance and minimal deadspace, but there is entrainment of room air and dilution of the anesthetic gases. Dr. Jackson Rees from the Liverpool Children’s Hospital added an open-tailed bag. The bag allows breathing to be monitored or assisted. This is the circuit now referred to as the ‘T-piece circuit’.

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Fig. 6.1

Evolution of the original T-piece to become the T-piece circuit in contemporary practice


The total volume of the expiratory limb and bag of the T-piece must be greater than the tidal volume. It does not matter if the expiratory limb is very long or short. Long expiratory limbs can be used when the patient is remote from the anesthetist, such as in MRI. Different sized bags can be used on the expiratory limb—commonly a 500 mL bag for neonates and infants, and a 1 L bag for children. Two liter bags are also available but difficult to hold and use properly. During mechanical ventilation, the bag is replaced by a hose between the expiratory limb and the ventilator.


6.1.3.1 Rebreathing and Fresh Gas Flow


During expiration, exhaled gas mixes with fresh gas flow in the expiratory limb. During the expiratory pause (the time between end expiration and beginning of inspiration), more fresh gas accumulates in the expiratory limb, pushing exhaled gas further down the limb and away from the patient. During inspiration, fresh gas enters the patient along with fresh and exhaled gas from the expiratory limb (Fig. 6.2). The proportions of fresh gas and exhaled gas breathed by the patient depend on several factors. These are the minute volume, including the rate and respiratory pattern, the CO2 production, and the fresh gas flow.

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Fig. 6.2

The respiratory cycle and gas within the T-piece circuit


The fresh gas flow needs to be greater than the peak inspiratory flow rate, or five times the minute ventilation, to eliminate all rebreathing. However, a small amount of rebreathing is acceptable and provides humidification and reduced volatile agent use. Various formulae for the acceptable fresh gas flow have been suggested, but 2.5–3 times the minute ventilation is commonly used (although rates as low as 1.5 times minute ventilation are possible in adults with their slower respiratory patterns). These formulae pre-date ETCO2 monitoring and nowadays fresh gas flow can set to an initial level (Table 6.1) and then adjusted individually according to an acceptable FiCO2.


Table 6.1

Advantages and disadvantages of the T-piece circuit



































Advantages of T-piece circuit


Disadvantages of T-piece circuit


Light weight


Complex to assemble if not familiar


Low resistance, no valves


Variable rebreathing


Low deadspace


Inefficient, with high FGF required for large child, particularly during spontaneous ventilation


Fast wash in


Low humidity


Requires learnt technique to hold rebreathing bag correctly


Low compliance (1 mL/cmH2O) and ability to ‘feel’ compliance of chest or detect leak in system


Can be difficult to scavenge, and the bag may twist and obstruct expiration and the outflow of gas


Portable for recovery or outside anesthetic locations


Not able to mechanically ventilate with modern anesthetic workstations


Compact with whole circuit in field of view

 

Whole circuit can be sterilized and no filter required

 

The respiratory pattern also affects the circuit’s efficiency. During spontaneous ventilation, there is only a short pause between the end of expiration and the beginning of inspiration, so relatively high fresh gas flow is required. During IPPV, the expiratory pause is longer and a lower fresh gas flow is possible.


6.1.3.2 Advantages


There are several advantages of the T-piece, as outlined in Table 6.2. The compact nature of the T-piece allows the whole circuit to be in the field of view with no need to reach out to adjust spill valves. The other major advantage is the small compression volume which allows lung compliance to be assessed, and easier manual ventilation of poorly compliant lungs.


Table 6.2

Suggested initial fresh gas rates for T-piece circuit in different age groups




















Patient size


Initial fresh gas rate (L/min)


Neonate and infant


3


Child


6


Adolescent


9



Fresh gas rates can be adjusted after monitoring ETCO2 and rebreathing (FiCO2) during use


The feedback, or feel of the lung compliance with the circuit has led to the term ‘educated hand’ for ventilation with the T-piece. There has been criticism that the educated hand does not exist and the anesthetist cannot feel or assess the child’s compliance any better than with mechanical ventilation. However, there are two points of detail to ensure that the hand is ‘educated’. The first is to keep the volume of the bag small. A suitable size bag is selected for the patient size and it is kept only partly filled. A large bag bulges out around the hand and increases compression volume. Small infants benefit from a two-handed bag squeeze technique, where one hand encircles a partly inflated bag and the second hand controls the occlusion of the tail (Fig. 6.3). The second is to keep the fresh gas flow rate as low as possible while allowing for the size of the patient. A high fresh gas flow rate makes the bag feel tight and it becomes harder to assess compliance.

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Fig. 6.3

Two-handed technique to ventilate small child with poorly compliant lungs. Keeping the T-piece bag small and avoiding an excessively high fresh gas flow rate allows the assessment of compliance and more effective ventilation


6.1.3.3 Disadvantages


Perhaps the greatest disadvantage of the T-piece is the time it takes to become skilled in its use. It is held differently to all other circuits and skill is required to occlude the tail correctly to deliver continuous positive airway pressure (CPAP) and ventilation. The skill to perform this takes time to learn and discourages many from the circuit and its advantages.


Although it looks simple, the T-piece is complex in form and function. It is made up of several components that can be incorrectly assembled. Its function is complex because of the interaction between the factors that affect rebreathing and thus ETCO2. Squeezing the bag faster doesn’t necessarily reduce the ETCO2 as it would with a circle system (Fig. 6.4). Faster respiratory rates shorten the expiratory pause, which then shortens the time for fresh gas to accumulate in the expiratory limb. The shorter expiratory pause increases rebreathing unless the fresh gas rate is also increased or is already high relative to the minute ventilation.

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Fig. 6.4

Squeezing the bag faster on a T-piece circuit does not necessarily reduce the ETCO2. Increasing the respiratory rate and increase minute ventilation, but it also shortens the expiratory time and there is more rebreathing unless the fresh gas flow rate is increased


Another disadvantage is that the T-piece cannot be attached to modern ventilators that are integrated within the anesthetic machine and cannot be separated from the circle, thus preventing mechanical ventilation.


Finally, the circuit can be difficult to scavenge, and is used in some countries without scavenging. Scavengers may be difficult to attach and remove from the tail of the bag, and may kink the tail and obstruct outflow from the circuit and expose the child to barotrauma. Some variants of the T-piece include a valve with a scavenging port between the expiratory limb and bag, but this makes the circuit more cumbersome and introduces the risk of barotrauma if the valve is left closed. A convenient and safe scavenging system described by Keneally and Overton is used in many Australian and New Zealand centers.


6.1.4 The Circle Circuit


In the past, it was thought that the circle could only be used for larger children because of the resistance from the inspiratory and expiratory valves. This is now known to be incorrect, and the circle circuit is the commonest circuit in pediatric anesthesia.


Children of any age can be managed using an adult circle circuit provided ventilation is controlled or assisted in neonates and infants. When using a circle system, the standard 22 mm diameter hoses are usually replaced with 15 mm diameter hoses, and the 2 L bag replaced with a 500 or 1000 mL bag. These changes are not essential but reduce the bulk and weight of the circuit, reduce circuit volume and compression volume, and reduce wash-in time. The volume of the soda lime absorber also affects compression volume.


6.1.4.1 Advantages


The advantages of the circle for pediatric anesthesia are familiarity, economy and efficiency, built-in scavenging, airway humidification and the ability to mechanically ventilate.


6.1.4.2 Disadvantages


The circle circuit has a larger compression volume than the T-piece. As the rebreathing bag is squeezed, part of the volume enters the patient but a proportion goes into compressing the gas within the hoses and absorber. The compression volume can make it more difficult to assess lung compliance in neonates, and is one of the reasons why the T-piece remains popular in this patient group. Other minor disadvantages are the circle system’s bulk and weight, slower washin and washout rates, and need to use a filter to protect the absorber and hoses from contamination.



Keypoint


The circle circuit is being used more commonly for small infants and children. The biggest advantage of the T-piece is its low compression volume, which allows successful manual ventilation of the smallest patient and in the most difficult-to-ventilate situations— the circle circuit is fine when ventilation is going well, but its large compression volume makes assessment of ventilation difficult when things aren’t going well.


6.2 Breathing Filters


As in adults, filters provide humidification and prevent microbial contamination of the anesthetic circuit. Although the same general considerations apply in children and adults, three areas are of importance when using filters for pediatric anesthesia.


6.2.1 Filter Deadspace and Resistance


Filters are usually placed between the patient and the T-piece or the Y-piece of the circle and add to the deadspace of the breathing system. During spontaneous breathing, the tidal volume may be only a few mL/kg and deadspace needs to be minimized to stop rebreathing. The deadspace of filters for infants and babies is usually 8–10 mL, and 20–25 mL for larger children.


Resistance from the filter increases work of breathing. It becomes important when a very small baby is breathing spontaneously through a filter, or when a filter that is too small is being used for a larger child. The resistance of the filter may reduce the amount of gas leaving the circuit during inhalational induction when there is no mask seal and there is neither a negative inspiratory pressure from the child nor a positive pressure on the rebreathing bag forcing gas out through the filter.


6.2.2 Anti-microbial Efficiency


Pleated, hydrophobic membrane filters are considered best for pediatric use, but there is wide variation in the performance of filters from different manufacturers. Their smaller size makes them inefficient and ineffective when tested under adult-sized conditions. However, when tested at conditions closer to the inspiratory flow rates that a small child would generate, the filters perform almost as well as adult sizes. However, some professional societies have guidelines that recommend a new sterile circuit be considered for each case.



Note


Small filters are not suitable for large patients—the filter does not block pathogens and its resistance is too high.

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Nov 27, 2021 | Posted by in ANESTHESIA | Comments Off on and Monitoring for Pediatric Anesthesia
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