The Use of Nasal High Flow Oxygen Therapy in Critical Neonates

Background

Noninvasive ventilation (NIV) is commonly used in human intensive care to provide ventilatory support without the need for intubation. Noninvasive ventilation is generally preferred to mechanical ventilation (MV) due to the large number of possible adverse consequences of MV (such as airway trauma and pneumonia). Management of patients requiring MV is also expensive and labour intensive. In preterm neonates, long-term adverse developmental effects such as bronchopulmonary dysplasia can also occur. Multiple techniques exist to provide NIV in humans including nasal high flow oxygen therapy (HFOT), continuous positive airway pressure (CPAP) and nasal intermittent positive pressure ventilation.

Respiratory support in neonatal foals has traditionally relied on low flow oxygen therapy and limited options have been available for foals requiring a greater degree of respiratory support. The availability of MV for equine neonates is limited and requires specialist expertise and intensive nursing support. The use of nasal CPAP and HFOT have recently been described in foals [1,2]. These techniques offer promise in helping to bridge the gap between low flow oxygen insufflation and MV. This abstract describes the technique used to provide HFOT in neonatal foals.

How HFOT works

Air is blended with oxygen to give a set fraction of inspired oxygen (FiO2). This is passed through a humidifier and is then delivered to the patient at a high flow rate (up to 60 L/min) via a wire-heated circuit and nasal prongs.

There are four main physiological benefits of HFOT.

• Reduced inspiratory resistance: the high flow rate of oxygen/ air mix lessens the work to inhale and results in improvements in respiratory effort.
• Washout of nasopharyngeal dead space: the high flow rate washes out the nasopharyngeal dead space which eliminates rebreathing and ensures constant provision of the desired fraction of inspired oxygen.
• Provision of CPAP: the high flow rate provides a degree of positive airway pressure which helps recruit alveoli and maintain alveolar inflation. The exact effect on airway pressure in foals is not known. In humans a flow rate of 0.7 L/kg/min has been shown to create a positive mean airway pressure of 7 cmH2O [3].
• Heating and humidification: the high flow rate of oxygen/ air mix is heated (usually to 37°C) and humidified to 100%. This prevents airway desiccation and irritation, stops bronchospasm and obstruction of airways by mucus.

Equipment required to deliver HFOT

• High flow unit (AIRVO 2 Optiflow system; Fisher and Paykel Healthcare)
• Nasal catheters (24 Fr thoracic chest drains)
• Y connector (8 mm to 10 mm)
• Adaptor from human nasal interface
• Oxygen supply

What are the indications for HFOT?

HFOT can be used in any foal that requires a greater deal of respiratory support than can be provided by low flow oxygen insufflation. We have used HFOT in foals with a variety of diagnoses including neonatal maladjustment syndrome, sepsis, meconium aspiration and prematurity. The technique is unlikely to be able to provide adequate respiratory support in foals with respiratory failure and is not suitable for foals that have inadequate spontaneous respiration. The machine is static and the foal is fixed to the machine via the heated circuit. The technique is consequently best suited to recumbent or minimally ambulatory foals. The maximum flow rate is 60 L/min; this limits the use of the machine to foals less than 80–100 kg in weight.

How to set up HFOT and initial settings

24 French chest drains (or similar nasal cannulae) should be placed up both nostrils (sutured in position), ending at the level of the mid pharynx (measured to medial canthus of eye). It is essential that these cannulae do not create a fixed seal in the nasal passages. Air leak is essential and the cannulae should not obstruct more than 50% of the nasal diameter. These cannulae are connected via an adaptor to the heated circuit and then to the HFOT machine.

Initial flow rate should be set at between 0.5 and 1 L/kg/min. For a 50 kg foal, a starting total flow rate of 40 L/min is often used. Temperature should be set at 37°C. Oxygen flow rate should be titrated to maintain normoxaemia. Oxygen can be piped into the machine and the rate adjusted to obtain the desired fraction of inspired oxygen.

Monitoring HFOT

The efficacy of HFOT should be measured via clinical effect and if possible arterial blood gas analysis. Maximum clinical effect should be visible in 1–2 hours, and in most foals a visible improvement in respiratory effort is observed after the initiation of therapy. The technique may not provide adequate support in foals with severe clinical disease and MV may still be needed in some cases.

Adverse effects

The most commonly reported side effects in humans include gastric distension and pneumothorax caused by ‘air leak syndrome’. No significant adverse effects have been noted in neonatal foals. Mild abdominal distension has been noted in foals which is assumed to be related to aerophagia caused by pharyngeal dysfunction.

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