6. Equipment and Monitoring for Pediatric Anesthesia
KeywordsAnesthetic 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. 4.5–4.7.
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.
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.
An infant breathing spontaneously is most prone to the problem of deadspace as the tidal volume may be close to the equipment deadspace volume.
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 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.
188.8.131.52 Rebreathing and Fresh Gas Flow
Advantages and disadvantages of the T-piece circuit
Advantages of T-piece circuit
Disadvantages of T-piece circuit
Complex to assemble if not familiar
Low resistance, no valves
Inefficient, with high FGF required for large child, particularly during spontaneous ventilation
Fast wash in
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.
Suggested initial fresh gas rates for T-piece circuit in different age groups
Initial fresh gas rate (L/min)
Neonate and infant
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.
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.
The advantages of the circle for pediatric anesthesia are familiarity, economy and efficiency, built-in scavenging, airway humidification and the ability to mechanically ventilate.
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.
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.
Small filters are not suitable for large patients—the filter does not block pathogens and its resistance is too high.