Introduction

Breathing systems (support)

In the operating room or intensive care unit the breathing system connects the patient to the ventilator or anaesthesia machine. For resuscitation smaller and simpler portable units are used with a patient mask. For longer periods of time the patient is intubated. A tube is positioned in the patients airways with the help of a laryngoscope, see Fig.8. The patient must be unconscious. The laryngoscope contains battery and light source to ease correct positioning. With the tube correctly positioned in the trachea a cuff is inflated so that a tight coupling with the lungs is obtained.

Figure 8 Laryngoscope and tube insertion.

One-way breathing system

In such systems all the expired gas leaves the system and nothing is recirculated back to the patient. One-way systems are used with ventilators when anaesthetic gases / vapours are not used. Fig.9 shows an example, a simple resuscitation model used for supplying oxygen-rich gas to the patient via a mask, more effectively than by the mouth-to-mouth method. Fresh gas from an oxygen bottle continuously flows into the system; the flow control is outside the illustration as a part of the bottle with manometer, flowmeter and pressure reducing valve. Flow direction valves are an essential part of such a system.

Squeezing the bag starts the inspiration cycle, the gas inlet valve flap A is pushed to the left, and the fresh gas is flowing into the patient. The operator feels the patient lung pressure in her hand and the resistance to gas flow, and perhaps also the rise and fall of the thorax surface. When the operator considers the lungs to be adequately filled the bag squeezing is stopped. The pressure drops and the expiration cycle starts driven by the lung pressure. The valve flap A is pressed to the right and the fresh gas starts to fill the bag. The expiration airway is free out to ambient air. When the operator considers the lung empty, the bag is again squeezed and a new inspiration cycle starts.

The pressure in the bag will not raise proportional to its filling. According to Laplace law the bag pressure P = 2T/r, cf. the subchapter on Gas Physics. However, as r increases also T increases and usually roughly proportional to the circumference of the bag and therefore r. The result is that the pressure in the bag is not very dependent on bag filling. The tubes are often of a spiralled type so that they do not collapse at low pressures (vacuum).

Figure 9 One-way small portable resuscitation system

For short term use a special humidifier is not necessary even if the fresh gas is dry. Usually the parts are single-use low price items, so disinfection or sterilization by the users is not necessary.

Circle system, rebreathing

In these systems a part of the expired gas is returned to the patient. The advantages are: water is returned to the patient airways, heat is returned, use of costly inhalation drugs is reduced. An example of such a system is shown in Fig. 10. The expired gas is partly returned to the circle, partly leaving the system via the pop-off valve. The carbon dioxide is absorbed in the CO₂ canister, this also develops heat. Fresh gas flows continuously into the system from an anaesthesia machine, and the breathing rhythm is determined by the bag squeezing. The patient is intubated with the tube coupled to the Y-piece. The uni-directional valves secures the correct flow direction.

The inspiration starts when the bag is squeezed. The one-way valve 2 is closed, and the content of the bag, humidity included, flows through the absorber and valve 1 and is mixed with the dry fresh gas. The gas mixture enters the lung via the Y-piece. The gas will not continue in the circle because valve 2 is closed. With one hand on the bag the operator palpitates lung filling and lung pressure. When the lungs are adequately filled the operator stops squeezing, valve 2 opens and valve 1 closes. The spring pressure on the pop-off (also called the automatic pressure limiter (ALP) valve plate determines at which pressure the valve opens, and thereby how large part of the gas is expelled. The opening phase of the valve corresponds to the high pressure phase at the end of inspiration.

The surplus gas enters a reservoir open to the ambient air, from the same reservoir a suction system aspirates at a flow rate high enough to secure that no gas escapes into the room. The open reservoir is an important safety measure so that the suction tube can not bring negative pressures to the circle and the patient lungs.

A Y-piece is used as near to the patient as possible in order to separate inspired and expired gas. Dead space is the problem; the first gas inhaled to the lungs is the newly expired gas content of the patient airways. The dead space is the part of a breathing system common to inspiration and expiration. The volume of the trachea is a natural dead space. One-directional valves must be included in such systems to clearly define the inspiration and expiration tubes.

Fig.10 shows the main components of a circle system, however there are many ways of putting the components together. The one-directional valves may for instance be put in the inspiration and expiration tubes, or the bag tube may be connected to the right of the lower one-directional valve. The function in normal mode is perhaps not so different, but if something goes wrong the difference and consequences for the patient may be very dependent on the exact configuration.

Figure 10 Rebreathing circle with uni-directional valves 1 and 2

The bag may be put into a tight bottle, and the outer volume may be cyclically pressurized by a ventilator. Such a bag-in-bottle system is a part of the ventilator.
What happens if the bag is not squeezed? The patient is not ventilated, the fresh gas flows continuously directly to the pop-off valve.

Risk considerations Breathing systems:

The uni-directional valves have important safety functions. If they are not functioning correctly (e.g. open all the time), tube connections swapped, pop-off (scavenging) valve closed, manometer or pressure relief valve at the Y-piece, water condensation in the expiration tubes, too high fresh gas flow. The wheel of a trolley pressing tube to closing.

Humidifiers and nebulizers (therapy)

Passive humidifiers

The humidifier adds water vapour to the breathing system. A simple way is to insert a HME filter (Heat and Moisture Exchanger) at the Y-piece, cf. Fig.12. Expired gas saturated with water is cooled when leaving the patient and the condensed water is absorbed in a sponge with a hygroscopic material. At the same time the latent heat of the condensation process contributes to reduce the patient temperature loss at the next inhalation.

Active humidifiers

Active humidifiers are usually positioned on the inspiratory side of the breathing system. Often the condensation in some part of the system is so strong that water traps must be installed. The problem is that a breathing system is not isothermal. If the gas is water saturated in the warmer parts, the colder parts will cause water condensation. This can be avoided by using an electric heating wire inside the tube.
Hot water humidifier

The inspiration gas is lead over an electrically heated water bath with a sufficiently large contact area between water and gas. Added advantage of heat supply to the patient.

Ultrasonic vibrator humidifier

Ultrasonic vibrating plate near the water surface or water drops falling on the plate creates a water mist.

Gasdriven jet Bernoulli humidifier

is a suction device (see last chapter) aspiring water into the jet stream and thus generating droplets. They may too large to be able to penetrate down to the bronchia.

Nebulizer (aerosols)

The nebulizer is a therapeutic device for the inhalation of pharmaceuticals in aerosol form. Aerosols are particles (powder or droplets) suspended in gases, the therapeutically useful size spectrum is the diameter range 0,5 – 10 μm. The largest particles carry the main amount of substance (volume of a sphere is proportional to r³). For the larger particles sedimentation is an important deposition process. For the smaller particles diffusion is the most important deposition process (collisions with the walls). The smaller the particle, the deeper it penetrates into the lungs towards the alveoli. However, in the upper airways the smaller particles tend to evaporate in the air, the larger to agglomerate.
Two important nebulizer types are based upon jet generation and ultrasonic generation.
Risk considerations Nebulizers:

Condensed water forms traps impeding intended gas flow. Jet humidifier may introduce high pressure in the breathing system. Growth of micro organisms in humid atmospheres.