Diagram 1

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The body employs many defences to protect the lungs, including small hairs (cilia) lining the trachea and bronchi supporting a constant stream of mucus out of the lungs, and reflex coughing and sneezing to dislodge mucus contaminated with dust particles or micro-organisms.

Viral infections in younger children are the most common cause/trigger of symptoms such as wheeze, cough and rhinorrhoea to name a few. It is not unusual for children to have six to eight ‘flu’s’ per year, mostly during the colder months.

Acute hypoxaemic respiratory diseases such as pre-school wheeze (reactive airways disease), pneumonia, bronchitis and pneumonitis to name a few, are acute inflammatory diseases (infectious and non-infectious) of the lower respiratory tract, resulting in obstruction of the small airways/alveoli – Diagram 2 and Diagram 3.  Hence they have signs of reduced alveolar surface (atelectasis) resulting in an Oxygen requirement and airway obstruction (wheezing).

Diagram 2

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Diagram 3

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When is additional increased Oxygen delivery required and what does the clinician need to know?

Reduced alveolar surface – If the alveoli surface is reduced, then an Oxygen requirement for the patient can occur.  To fix this problem, we need to reverse the atelectasis and consolidation in the lung.  Continuous Positive Airway Pressure or CPAP as it is commonly referred to can perform this function and fix this problem (blowing up the lung).

Improving Shunt Fraction – Shunt Fraction is the percentage of blood circulated out by the heart that is bypassed functioning alveoli and not completely Oxygenated and can arise when there is atelectasis occurring.  CPAP can fix this problem.

Poor Ventilation Inhomogeneity – Ventilation Inhomogeneity is when part of the lungs are less well ventilated than others or even blocked.  CPAP can fix this problem.

Swelling Alveolar-Capillary Membrane – If the alveolar-capillary membrane is thickened due to inflammation and secretion, Oxygen diffuses poorly/inadequately and less Oxygen is taken up into the blood. This problem can be fixed by giving more Oxygen into the inspired air (increase FiO2 – fractional concentration of inspired Oxygen).

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What is Oxygen?

Oxygen is a commonly used therapy for hypoxaemia (low blood oxygen levels) in the management of respiratory illness/disease in the paediatric population.

Oxygen is a drug, which should be administered with care at all times. If Oxygen is given in a too high quantity, then Oxygen toxicity can occur, which can harm the lungs, the brain, and the heart. An example of how this knowledge has changed practice, is with newborns who are currently being resuscitated in room air due to the risks associated the adverse effects and toxicity of Oxygen.

The purpose of supplemental Oxygen is indicated to relieve the following:

• Hypoxaemia – inadequate supply of Oxygen in blood. (An inadequate supply of Oxygen in tissues is called Hypoxia).
• Reduce myocardial stress due to hypoxaemia that is the stress to the muscle tissue of the heart.

Signs and Symptoms of Hypoxaemia

1. Confused, agitation
2. Hypertension but at a later stage hypotension
3. Tachypnoea
4. Cool, clammy skin
5. Tachycardia
6. Dyspnoea
7. Shallow and/or laboured breathing

Dangers of Oxygen include:

• Oxygen toxicity if exposure is high and prolonged
• Atelectasis (blockage of the bronchioles by mucus or by pressure on the outside of the lung, preventing normal Oxygen absorption to healthy tissues).
• Carbon dioxide retention causes respiratory drive depression
• Oxygen-derived free radicals which are the end product of too much Oxygen cause cellular damage to the lungs.

Infants have anatomical and physiological features that differ from older children, teenagers and adults.

These include:

• Obligatory nose breathers easily obstructed with secretions
• High larynx which can cause increased resistance when breathing
• Ineffective use of accessory muscles (eg. Sternomastoid results in head bobbing rather than the increased chest expansion).
• Infants have a relative smaller lung volume to start with and have therefore less reserve and can develop hypoxemia much quicker than adults or older children

Infants are born with small airways, which can develop oedema and narrow airways when they have a viral infection, such as bronchiolitis, with coryzal symptoms. Children are exposed to an average of flu-like symptoms 6-8 times a year and some of these children develop reactive airway disease triggered commonly by viral infections and can sometimes turn into an allergic trigger such as asthma.

Why do infants rapidly get sick with a respiratory virus or bacterial infection?

The chest wall includes the ribs and the intercostal muscles. The ribs initially develop as cartilage. The chest wall functions as a pump, which performs the respiratory movements driving respiration itself. Infant’s ribs lie horizontally, and chest movements are therefore less efficient in early life than later life when the child adopts a more upright posture. Physiologically, infants have very few type 1 muscles fibres (slow contraction muscle fibres) and hence fatigue more easily.

Infant’s bronchioles develop oedema, which causes them to narrow and close (Diagram 2 and Diagram 3), and thus the infant finds it more difficult to breath. Again, the relative small size of the bronchi in infants makes them more likely to be obstructed by secretions or due to airway wall swelling (Diagram 2 and Diagram 3.)

The diaphragm is the principal muscle of respiration in childhood. It consists of a fibro-muscular sheet of tissue that separates the thorax from the abdomen. It is comprised of a central membranous tendon to which the muscles of the diaphragm are attached. Diaphragmatic muscles are more easily fatigued in infancy because they contain a smaller proportion of fatigue-resistant muscle fibres than in later life.

The main trigger for infant’s respiratory illness and obstructive airway disease is due to a viral infection with little reaction of the smooth muscle of small airways. Therefore, it’s unlikely that small infants react to bronchodilators. However, when infants grow older, and especially children aged 1-4 year develop reactive airways and are more likely to have reactive airways disease (RAD), also known as pre-school wheeze, and hence may respond to bronchodilators.

Children greater than 4 years of age have a more distinct airway reactive pattern of allergies such as allergy pollens (or exercise, cold air). Therefore, a definition of asthma or reactive airway in children aged 1-4 year group is hard to diagnose. In general, it can be said, that in the under 1 year of age group, reactive airways are less likely, and in the 1-4 years we often don’t know and need to test if they respond to bronchodilators. Asthma should be diagnosed preferentially after the age of 4.

Respiratory diseases, particularly the 1–4 year age group have differing diagnosis including bronchiolitis in this age group, which is triggered by a viral illness. The most common disease is Respiratory Syncytial Virus (RSV). Some children can develop less airway disease but more parenchymal disease with infections being either bacterial or viral. Pulmonary parenchyma is a term that refers to the parts of the lungs involved in gas transfer at the alveolar level. These include the alveoli, interstitial tissue, blood vessels, bronchi and bronchioles. These children can present with a classic pneumonia picture, which means certain sections or lobes of lungs, are opacified on a chest x-ray, or they present with a more diffuse pattern of the infection, which is called pneumonitis. Some children can very rapidly develop a bilateral chest infiltrate with a high oxygen requirement and potentially require respiratory support, which is called Acute Respiratory Distress Syndrome (ARDS). Children with ARDS are characterized by widespread inflammation in the lungs. ARDS is not a particular disease, but rather a clinical phenotype (set of observable characteristics of an individual resulting from the interaction of an individual organism within the environment), which may be triggered by various pathologies as outlined, such as pneumonia and sepsis.

A proportion of children present to hospital with an oxygen requirement and airway obstruction/wheeze and can respond quite well to bronchodilators (eg. Salbutamol) and steroids. Some of these cases are triggered by a viral infection, however, can also be triggered by inhaled allergens or other allergies. These children can improve within the first few hours of treatment and are then called RAD.

The overarching description of all these children is acute hypoxaemic respiratory failure (AHRF), which in principle describes these children as having respiratory distress, with increased work of breathing, and an oxygen requirement. For this study, we enrol a large variety of specific individual diseases, however we always target this overarching diagnostic group (AHRF) with High Flow Nasal Cannula (HFNC) therapy treatment, as we are interested in investigating the physiological affect of HFNC therapy and not disease specific response.

To support the breathing with HFNC therapy we must understand that the flow requirement does not increase linearly with weight or growth. The respiratory flow and volume changes during the respiratory cycle follow the growth of the child.  After the age of 2-4 years the relative size of the lung and the requirement of respiratory support starts to be more equal to adults, hence the initial 2L/kg/min of support needs to be adapted. Therefore a 20 kg child needs relatively less HFNC rate compared to a 5 kg infant.

The demand for flows when using HFNC follows the length and growth of a normal child. (Note the growth chart in Diagram 4).