Breathing systems

1. Three ports: the inlet, the patient and the exhaust ports. The latter can be open to the atmosphere or connected to the scavenging system using a shroud.

2. A lightweight disc rests on a knife-edge seating. The disc is held onto its seating by a spring. The tension in the spring, and therefore the valve’s opening pressure, are controlled by the valve dial.

1. This is a one-way, adjustable, spring-loaded valve. The spring is used to adjust the pressure required to open the valve. The disc rests on a knife-edge seating in order to minimize its area of contact.

2. The valve allows gases to escape when the pressure in the breathing system exceeds the valve’s opening pressure.

3. During spontaneous ventilation, the patient generates a positive pressure in the system during expiration, causing the valve to open. A pressure of less than 1 cm H₂O (0.1 kPa) is needed to actuate the valve when it is in the open position.

4. During positive pressure ventilation, a controlled leak is produced by adjusting the valve dial during inspiration. This allows control of the patient’s airway pressure.

1. Malfunction of the scavenging system may cause excessive negative pressure. This can lead to the APL valve remaining open throughout respiration. This leads to an unwanted enormous increase in the breathing system’s dead space.

2. The patient may be exposed to excessive positive pressure if the valve is closed during assisted ventilation. A pressure relief safety mechanism actuated at a pressure of about 60 cm H₂O is present in some designs (Fig. 4.3).

3. Water vapour in exhaled gas may condense on the valve. The surface tension of the condensed water may cause the valve to stick. The disc is usually made of a hydrophobic (water repelling) material, which prevents water condensing on the disc.

1. It is made of anti-static rubber or plastic. Latex-free versions also exist. Designs tend to be ellipsoidal in shape.

2. The standard adult size is 2 L. The smallest size for paediatric use is 0.5 L. Volumes from 0.5 to 6 L exist. Bigger size reservoir bags are useful during inhalational induction, e.g. adult induction with sevoflurane.

1. Accommodates the FGF during expiration acting as a reservoir available for the following inspiration. Otherwise, the FGF must be at least the patient’s peak inspiratory flow to prevent rebreathing. As this peak inspiratory flow may exceed 30 L/min in adults, breathing directly from the FGF will be insufficient.

2. It acts as a monitor of the patient’s ventilatory pattern during spontaneous breathing. It serves as a very inaccurate guide to the patient’s tidal volume.

3. It can be used to assist or control ventilation.

4. When employed in conjunction with the T-piece (Mapleson F), a 0.5 L double-ended bag is used. The distal hole acts as an expiratory port (Fig. 4.4).

1. Because of its compliance, the reservoir bag can accommodate rises in pressure in the breathing system better than other parts. When grossly overinflated, the rubber reservoir bag can limit the pressure in the breathing system to about 40 cm H₂O. This is due to the law of Laplace dictating that the pressure (P) will fall as the bag’s radius (r) increases: P = 2(tension)/r.

2. The size of the bag depends on the breathing system and the patient. A small bag may not be large enough to provide a sufficient reservoir for a large tidal volume.

3. Too large a reservoir bag makes it difficult for it to act as a respiratory monitor.

1. Corrugated rubber or plastic tubing (usually 110–180 cm in length) and an internal volume of at least 550 mL.

2. A reservoir bag, mounted at the machine end.

3. APL valve situated at the patient end.

1. During the first inspiration, all the gases are fresh and consist of oxygen and anaesthetic gases from the anaesthetic machine.

2. As the patient exhales (Fig. 4.5C), the gases coming from the anatomical dead space (i.e. they have not undergone gas exchange so contain no CO₂) are exhaled first and enter the tubing and are channelled back towards the reservoir bag which is being filled continuously with FGF.

3. During the expiratory pause, pressure built up within the system allows the FGF to expell the alveolar gases first out through the APL valve (Fig. 4.5D).

4. By that time the patient inspires again (Fig. 4.5B), getting a mixture of FGF and the rebreathed anatomical dead space gases.

5. It is a very efficient system for spontaneous breathing. Because there is no gas exchange in the anatomical dead space, the FGF requirements to prevent rebreathing of alveolar gases are theoretically equal to the patient’s alveolar minute volume (about 70 mL/kg/min).

6. The Magill system is not an efficient system for controlled ventilation. A FGF rate of three times the alveolar minute volume is required to prevent rebreathing.

1. 1.8 m length coaxial tubing (tube inside a tube). The FGF is through the outside tube, and the exhaled gases flow through the inside tube (Fig. 4.6A).

2. The inside tube is wide in diameter (14 mm) to reduce resistance to expiration. The outer tube’s diameter is 30 mm.

3. The reservoir bag is mounted at the machine end.

4. The APL valve is mounted at the machine end eliminating the drag on the connections at the patient end, which is a problem with the Magill system.

1. A similar mechanism to the Magill system except the Lack system is a coaxial version. The fresh gas flows through the outside tube whereas the exhaled gases flow through the inside tube.

2. A FGF rate of about 70 mL/kg/min is required in order to prevent rebreathing. This makes it an efficient breathing system for spontaneous ventilation.

3. Since it is based on the Magill system, it is not suitable for controlled ventilation.

1. A reservoir bag. In the B system, corrugated tubing is attached to the bag and both act as a reservoir.

2. An APL valve at the patient’s end.

3. FGF is added just proximal to the APL.

1. A length of coaxial tubing (tube inside a tube). The usual length is 180 cm, but it can be supplied at 270 cm (for dental or ophthalmic surgery) and 540 cm (for magnetic resonance imaging (MRI) scans where the anaesthetic machine needs to be kept outside the scanner’s magnetic field). Increasing the length of the tubing does not affect the physical properties of the breathing system.

2. The fresh gas flows through the inner tube while the exhaled gases flow through the outside tube (Fig. 4.8). The internal lumen has a swivel mount at the patient end. This ensures that the internal tube cannot kink, so ensuring delivery of fresh gas to the patient.

3. The reservoir bag is mounted at the machine end.

4. The APL valve is mounted at the machine end.

1. During spontaneous ventilation, the patient’s exhaled gases are channelled back to the reservoir bag and become mixed with fresh gas (Fig. 4.9B). Pressure build-up within the system will open the APL valve allowing the venting of the mixture of the exhaled gases and fresh gas (Fig. 4.9C).

2. The FGF required to prevent rebreathing (as seen in Fig. 4.9D) during spontaneous ventilation is about 1.5–2 times the alveolar minute volume. A flow rate of 150–200 mL/kg/min is required. This makes it an inefficient and uneconomical system for use during spontaneous ventilation.

3. It is a more efficient system for controlled ventilation. A flow of 70–100 mL/kg/min will maintain normocapnia. A flow of 100 mL/kg/min will cause moderate hypocapnia during controlled ventilation.

4. Connection to a ventilator is possible (Fig. 4.10). By removing the reservoir bag, a ventilator such as the Penlon Nuffield 200 can be connected to the bag mount using a 1-m length of corrugated tubing (the volume of tubing must exceed 500 mL if the driving gas from the ventilator is not to enter the breathing system). The APL valve must be fully closed.

5. A parallel version of the D system is available.

1. The internal tube can kink, preventing fresh gas being delivered to the patient.

2. The internal tube can become disconnected at the machine end causing a large increase in the dead space, resulting in hypoxaemia and hypercapnia. Movement of the reservoir bag during spontaneous ventilation is not therefore an indication that the fresh gas is being delivered to the patient.