It is without question that adequate oxygen supply to all tissues and cells is essential for life. For oxygen to be delivered to the cells, there must be adequate oxygen exchange in the lungs to the blood, and therefore adequate pulmonary function is critical.

The Respiratory System

The respiratory system works in conjunction with the blood and cardiovascular system to provide the body with the required exchange of oxygen and carbon dioxide.

External respiration is the gaseous exchange of O₂ and CO₂ between the environment (air in the lungs) and the blood. Pulmonary ventilation and diffusion of gases through the alveolar membrane of the lungs is required in order for external respiration to take place.

Internal or tissue respiration is the exchange of O₂ and CO₂ between the blood in the capillaries and the tissue cells.

The respiratory system is referred to in two parts:

• Upper respiratory system: structures that are outside of the lungs—nose, pharynx, larynx, trachea
• Lower respiratory system: structures within the lungs—bronchi, bronchioles, alveolar ducts and alveoli

Bronchi and Bronchioles

The primary bronchi are the first branches of the trachea. They are quite large with C-shaped chondral rings like the trachea. They continue to branch, becoming smaller as they progress, resulting in secondary and tertiary bronchi. They are kept tubular by flattened, overlapping, curved cartilages. As branching continues into the smaller bronchioles, the cartilage component to their walls disappears when the diameter has reduced to 1 mm or less. The respiratory bronchioles then give rise to the alveolar ducts and alveoli.

After the bronchi enter the lung, each main bronchus divides into even smaller bronchi and then finally into even smaller bronchioles. The bronchioles continue to subdivide down to the smallest air passages—the microscopic alveolar ducts. The alveolar ducts then end in groups of alveoli—a bit like a bunch of grapes—called alveolar sacs.

The bronchial tree is not a rigid tube; the diameter of each one can be adjusted by the smooth muscle fibres in its wall. The autonomic (unconscious) portion of the nervous system controls this smooth muscle.

Bronchodilation—This is when the smooth muscle relaxes (e.g., during times of intense physical activity), allowing the air passageways to dilate to their maximum capacity which helps the respiratory effort move the greatest amount of air back and forth into the alveoli with each breath.

Bronchoconstriction—During times of rest, the smooth muscle partially contracts, reducing the size of the air passageways which in turn lessens the workload for the respiratory tract. However, some respiratory irritants and disease can cause severe bronchoconstriction, making it difficult for the animal to breathe.

Alveolar Ducts and Alveoli

External respiration takes place in the alveoli. This is when oxygen and carbon dioxide are exchanged between blood and the air in the alveoli. The rest of the respiratory structures just move air in and out of the alveoli.

The alveoli are tiny, thin-walled sacs that are surrounded by a network of capillaries. The wall of each alveolus is made up of the thinnest epithelium in the body—simple squamous epithelium. The capillaries that surround the alveoli are also made up of simple squamous epithelium.

As these two layers are so thin, they freely allow oxygen and carbon dioxide to diffuse between the air and blood.

Each alveolus is lined with a thin layer of fluid that contains a substance called surfactant. This surfactant helps reduce the surface tension (the attraction of water molecules to each other) of the fluid. This prevents the alveoli from collapsing as air moves in and out during each breath.

Respiratory Membrane

This is the alveolar and capillary wall through which gas exchange takes place. The alveoli make up the bulk of pulmonary tissue.

Dyspnoea can be defined as the sensation of difficulty in breathing, which is experienced by patients suffering compromised respiratory function.

The sensation is initiated by either hypoxaemia (low oxygen levels) or hypercapnia (high carbon dioxide levels).

Signs of Dyspnoea/Respiratory Distress

• Open-mouth breathing
• Panting
• Tachypnoea
• Extension of head and neck
• Abducted elbows
• Exaggerated abdominal component

Airway Obstruction and Injuries to the Respiratory Tract

Causes of airway obstruction and respiratory dysfunction may include:

• Trauma—hit by car, dog attack
• Foreign bodies
• Degenerative physical conditions—such as laryngeal paralysis, tumours, collapsing trachea
• Haemothorax (blood in the chest cavity)
• Pneumothorax (air in the chest cavity)
• Chest wall trauma (flail chest)
• Lung disorders—primary (e.g., chronic bronchitis) or secondary (e.g., due to cardiac failure)
• Respiratory paralysis
• Physical deformity (e.g., brachycephalic—bulldogs/pugs, etc.)

Pneumothorax

A pneumothorax is caused by leakage of air from the airways or parenchyma into the pleural space. It is often caused by blunt trauma to the chest (e.g., hit by car).

Many patients that have sustained trauma to the chest may have a small leak that seals over quickly and may not produce clinical signs.

• Open pneumothorax. A pneumothorax is termed ‘open’ when there is a penetrating wound to the thoracic cavity and air is being ‘sucked in’ through the wound. First-aid action is to cover the open wound immediately to reduce air being introduced.
• Closed pneumothorax. The most common type seen, especially following blunt trauma. There is no penetrating wound. The air is leaking within the pleural space.
• Tension pneumothorax. As the air leaks into the pleural space, it is like a one-way valve; the air leaks in but is not removed. When the pressure from the leaking air becomes higher than atmospheric pressure, this is termed a tension pneumothorax—a life-threatening condition, and thoracocentesis must be performed immediately.

Haemothorax

A haemothorax is defined as blood within the pleural cavity. This can result from trauma or accumulation of blood due to, for example, a clotting disorder, and an active bleed has occurred. A haemothorax is initially stabilized (depending on cause) by performing thoracocentesis to remove blood.

Pyothorax

A pyothorax is caused by bacterial contamination of the pleural space; both cats and dogs can present with this condition. However, the causes can vary greatly. In cats, the most common cause of bacterial contamination is from the oral cavity of other cats—bite wounds to the chest.

Other causes of pyothorax include:

• Bacterial contamination from penetrating chest injuries
• Migrating foreign bodies (grass seeds, etc.)
• Inhalation and migration of foreign body
• Rupture or perforation of the oesophagus
• Rupture or perforation of the trachea
• Bacterial pneumonia leading to lung abscessation
• Bacterial spread from other site (systemic sepsis)

Flail Chest

When a rib segment is broken and becomes free-floating, it is termed a flail chest. Pain and dyspnoea are observed, and often the flail segment can be located by visualization of the chest wall.

Pleural Effusion

The pleural lining is the lining of the thoracic cavity and outer surface of the lung; the lining covering the outer portion of the lung is the visceral pleura, and the lining covering the mediastinum and diaphragm is the parietal lining. These two linings are separated by a thin layer of fluid to reduce friction between the two surfaces. A pleural effusion is defined as an excessive collection of fluid between the visceral and parietal pleura.

Causes of pleural effusion include:

• Neoplasia
• Heartworm
• Congestive heart failure
• Trauma
• Hypoalbuminaemia

Delivering Oxygen

Provide supplemental oxygen. It is essential that the patient receives supplemental oxygen in the least stressful manner. The patient should be placed in sternal recumbency as this facilitates easier breathing. The method chosen is dependent upon the patient presentation, disease and availability within the clinic.

Methods include:

• ‘Flow-by’/’Fly-by’
• Face mask
• Head tent
• Oxygen box
• Nasal oxygen—not recommended for immediate stabilisation, as too stressful to patient and time consuming

Fly-by/Flow-by Oxygen

This is an excellent way to provide oxygen for a distressed patient on presentation.

Equipment required:

• Oxygen supply
• If this is not available, the anaesthetic machine can be used; however, ensure that the gaseous anaesthetic is turned off and the system has been flushed, so no anaesthetic residue is in the line.
• A humidifier is preferred for any prolonged administration time.
• Specific oxygen tubing is available that can be attached to the regulator/humidifier. This tubing can also be attached to the oxygen port on the anaesthetic machine with a small adaptor.

Procedure:

• Hold tubing as close to the patient’s airway as possible.
• If the patient has reduced consciousness, you may be able to place the tubing just inside the mouth without distressing it.
• Have the oxygen on the highest flow rate.

Face Mask

Although a face mask will provide a greater amount of inspired oxygen than ‘fly by’, it can be very stressful for the patient and often make the situation worse. Imagine if you could not breathe and someone tried to place something over your airway!

The mask, if used, does not have to be an airtight fit, as expired carbon dioxide needs to be released.

High oxygen flow rates should be used and if full coverage of the mouth/nose is not possible, hold as close to airway as tolerated.

Equipment:

• Mask of appropriate size
• Oxygen supply
• Tubing
• The anaesthetic machine is often used when providing oxygen via a mask, as the mask fits snugly on the end of the appropriate anaesthetic circuit.
• Again, remember to ensure that the gaseous agent is turned off and the circuit has been thoroughly flushed prior to use.
• Masks can also be used with a humidified oxygen source with the correct adaptor.

Head Tent/Box Oxygen

This is a simple and effective technique for providing supplemental oxygen, although may not be tolerated by some patients.

Equipment required:

• Elizabethan collar (or similar)
• Glad® wrap
• Oxygen tubing
• Oxygen supply
• Tie for collar

Procedure:

• Collect appropriately sized Elizabethan collar and assemble.
• Cover the collar front with Glad® wrap/cling film, leaving a small gap at the top for expired carbon dioxide to be excreted.
• Secure Glad® wrap/cling film with tape.
• Position end of oxygen tubing on the bottom of the e-collar (away from gap) and secure in place with adhesive tape.
• Place e-collar on patient and secure with tie.
• Provide oxygen at a flow rate of 8–10 L/min.

Considerations to be aware of include:

• Facial trauma. Although this is an excellent way to provide oxygen support when nasal oxygen cannot be placed due to facial trauma, the patient must be closely monitored for pooling of fluids (e.g., blood, saliva) within the head tent.
• Heat. The head tent can become quite hot if a suitable area has not been left unwrapped. Careful monitoring should be provided to ensure panting from excessive heat or discomfort from heat within the box does not occur.
• Patient observation. As the head of the patient is enclosed within the oxygen tent, monitoring of mucous membranes, capillary refill time and pupil size and reaction are impeded. Therefore, other monitoring techniques should be implemented.

Oxygen Cage/Box

Oxygen cages are commercially available or can be homemade. Using a cage is a noninvasive and less-stressful way to provide supplemental oxygen. It is necessary to have a ‘clear’ box so the patient can be visually assessed at all times.

• Commercial oxygen cage
  The commercial oxygen cage enables ‘pure’ oxygen to be delivered to the cage, eliminating the use of an anaesthetic circuit.
• Homemade oxygen cage
  This oxygen cage has been made from a plastic storage box. Several sizes can be made.
• A hole is drilled at one end of the box to facilitate the placement of the oxygen source via the anaesthetic circuit.
• Two holes are drilled at the other end of the box to facilitate removal of expired carbon dioxide.
• Although not as clear as the commercial anaesthetic cage, the patient can still be continuously observed.
• Disadvantages
• These boxes can become quite hot, and therefore careful monitoring should be implemented to prevent this.
• When the lid is removed from the box, the oxygen content will drop dramatically and need to be replenished when re-sealed.
• High flow rates of oxygen are required.