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.

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’.

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.

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.

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.

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.

6.2.1 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.

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