High Flow Nasal Oxygen Therapy

Mechanism of Action and Physiological Benefits

• High-flow nasal cannula (HFNC) oxygen therapy is a modality of non-invasive ventilation (NIV) that delivers heated and humidified, very high flow oxygen through the nose.
• It acts by providing a set flow that reduces airway resistance and generates an undetermined, yet significant, pharyngeal distending pressure.
• This distending pressure helps to stent the lower airways, overcome airway resistance, and open atelectatic alveoli, thereby reducing the patient’s overall work of breathing.
• The continuous high flow washes out nasopharyngeal dead space, which improves oxygenation and facilitates carbon dioxide ($C{O}_2$) clearance.
• HFNC therapy also assists in muco-ciliary clearance, inhibits the broncho-constrictor reflex, and increases the end-expiratory lung volume.

Clinical Indications

• HFNC is primarily utilized for children presenting with acute hypoxemic respiratory failure or severe respiratory distress.
• It is heavily utilized and predominantly studied in infants suffering from acute bronchiolitis.
• It serves as a critical respiratory support option in severe community-acquired pneumonia to prevent further clinical deterioration.
• In infants experiencing severe respiratory distress or respiratory failure, the timely application of HFNC may successfully avert the need for mechanical ventilation and endotracheal intubation.

Advantages Over Traditional Non-Invasive Ventilation

• Contrary to standard NIV modalities like Continuous Positive Airway Pressure (CPAP) or Bi-level Positive Airway Pressure (BiPAP), HFNC does not require a tight-fitting nasal or facial mask interface.
• The absence of a restrictive mask significantly improves patient tolerance, minimizes claustrophobia, and facilitates easier application in the Pediatric Intensive Care Unit (PICU).
• In resource-limited settings, an indigenous HFNC circuit can be effectively assembled using an oxygen-air blender to regulate the Fraction of Inspired Oxygen ($Fi{O}_2$), a servo-controlled heated wire humidifier, corrugated tubing, and appropriately sized nasal prongs.

HFNC Settings and Flow Rate Guidelines

• The definition of “high flow” varies according to the age and size of the pediatric patient: a flow rate $>1\text{ L/min}$ is considered high flow for neonates, $>3\text{ L/min}$ is high flow for toddlers, and $>5\text{ L/min}$ is classified as high flow for older children.
• To define “At-risk” Pediatric Acute Respiratory Distress Syndrome (PARDS) in non-intubated children, the minimum high flow oxygen requirements are $<1\text{ year}:2\text{ L/min}$, $1-5\text{ years}:4\text{ L/min}$, $5-10\text{ years}:6\text{ L/min}$, and $>10\text{ years}:8\text{ L/min}$.
• The humidifier temperature must be maintained optimally around $32-36^{\circ }\text{C}$ to ensure the gas is adequately warmed and humidified.
• The $Fi{O}_2$ should be carefully titrated to achieve and maintain a target oxygen saturation ($Sp{O}_2$) of $92$.

Patient Weight	Recommended HFNC Flow Rate	Circuit Type Required
$<12\text{ kg}$	$2\text{ L/kg/min}$	Pediatric circuit
$12-15\text{ kg}$	$2\text{ L/kg/min}$	Adult circuit
$15-30\text{ kg}$	$35\text{ L/min}$	Adult circuit
$30-45\text{ kg}$	$45\text{ L/min}$	Adult circuit

• Alternatively, weight-based flow rates can be set at $1-2\text{ L/kg/min}$ for the first $10\text{ kg}$ of body weight, and $0.5\text{ L/kg/min}$ for the next $10\text{ kg}$, up to an absolute maximum of $20-25\text{ L/kg/min}$.

Monitoring, Weaning, and Escalation

• The effectiveness of HFNC must be continuously assessed within the first 1 to 2 hours by monitoring the respiratory rate, heart rate, work of breathing, and $Fi{O}_2$ requirements.
• Once the child’s clinical trajectory shows sustained improvement, weaning is initiated by first reducing the $Fi{O}_2$ down to $30$.
• Following the $Fi{O}_2$ reduction, the flow rate is systematically decreased to $0.5-1\text{ L/kg/min}$ before transitioning the child to simple nasal prongs or a face mask.
• Escalation to endotracheal intubation or more advanced mechanical ventilation is strictly indicated if the child exhibits worsening severe respiratory distress, cyanosis, central nervous system signs of hypoxia (like lethargy or coma), or an inability to maintain a target $Sp{O}_2\ge 92$ despite optimal HFNC support.

Limitations and Controversies

• Whether HFNC is equivalent or inferior in efficacy compared to traditional NIV remains debatable due to a lack of sufficient, high-quality pediatric data.
• A large systematic review encompassing over 2100 children concluded that HFNC, although safe, does not offer superior clinical benefits for children with bronchiolitis when compared to other oxygen delivery modes; consequently, NICE guidelines continue to recommend CPAP for impending respiratory failure in these patients.
• Furthermore, in the specific management of COVID-19 pneumonia for children at risk of PARDS, CPAP or BILEVEL devices are explicitly preferred over HFNC, particularly if the $Sp{O}_2/Fi{O}_2$ (S/F) ratio falls between 221 and 264.