Introduction and Definition

High Flow Nasal Oxygen (HFNO) therapy, also known as Heated Humidified High Flow Nasal Cannula (HFNC), is a non-invasive respiratory support modality.
It delivers heated and fully humidified oxygen at flow rates that meet or exceed the patient's peak inspiratory flow demand.
High flow is generally defined by age and size parameters.
It is considered > 1 L/min for neonates, > 3 L/min for toddlers, and > 5 L/min for older children.

Physiological Mechanisms of Action

HFNO provides a continuous set flow that effectively reduces inspiratory airway resistance.
It acts through several physiological benefits to enhance oxygenation and ventilation.

Mechanism	Clinical Effect
Dead Space Washout	Continuous high flow flushes the nasopharyngeal dead space. This expels retained $CO_2$, prevents rebreathing, and raises the effective fraction of inspired oxygen ($FiO_2$).
Positive End-Expiratory Pressure (PEEP)	Generates an undetermined but significant pharyngeal distending pressure. This mild PEEP splints the lower airways, overcomes resistance, and opens atelectatic alveoli.
Increased Lung Volume	By recruiting collapsed alveoli, it significantly increases the end-expiratory lung volume (EELV) and functional residual capacity. This improves ventilation-perfusion matching.
Active Humidification	Delivery of gas heated to $34-37^{\circ}C$ with nearly 100% relative humidity preserves mucosal integrity. It optimizes ciliary function, promotes muco-ciliary clearance, and prevents secretion thickening.
Reduced Work of Breathing	The heated and humidified gas reduces the metabolic energy expended on conditioning inspired air. This ultimately unloads the respiratory muscles.

Category	Specific Clinical Scenarios
Primary Respiratory Failure	Acute hypoxemic respiratory failure (Type I) with $SpO_2$ < 90%. Severe respiratory distress due to acute bronchiolitis, asthma, or community-acquired pneumonia.
Neonatal Conditions	Apnea of prematurity, respiratory distress syndrome (RDS), and transient tachypnea of the newborn. Used as an alternative or weaning step from Continuous Positive Airway Pressure (CPAP).
Post-Operative Support	Respiratory support following major surgeries, such as tonsillectomy and adenoidectomy. Weaning therapy from invasive mechanical ventilation or CPAP/BiPAP.
Procedural Support	Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE) extends the safe apnea time during difficult intubation, general anesthesia induction, and painless endoscopic procedures.

Category	Specific Clinical Scenarios
Absolute Contraindications	Complete upper airway obstruction, choanal atresia, basilar skull fractures, facial trauma, and un-drained pneumothorax.
Relative Contraindications	Impending respiratory failure requiring immediate intubation. Decreased level of consciousness or inability to protect the airway. Severe hypercapnic respiratory failure ($pH < 7.25$) where non-invasive ventilation is preferred.

The HFNO system requires specific integrated components to function optimally.
Air-Oxygen Blender: Allows precise titration of $FiO_2$ from 21% to 100% independent of the flow rate.
Heated Humidifier: Actively warms the gas mixture to $34-37^{\circ}C$.
Heated Delivery Circuit: Insulated tubing prevents temperature drops and circuit condensation (rainout).
Nasal Interface: Soft, beveled nasal prongs are used. The prongs must be non-occlusive and should occupy less than 50% of the nostril aperture to allow egress of expired gas.

Close continuous monitoring is mandatory, especially during the first 1 to 2 hours of initiation.
Key clinical parameters include respiratory rate, heart rate, work of breathing, and continuous pulse oximetry ($SpO_2$).
Target $SpO_2$ is generally maintained between 92% and 97%.

Identifying early failure is critical to prevent delayed intubation and clinical deterioration.
Clinical Signs of Failure: An increasing $FiO_2$ requirement > 50%, worsening respiratory acidosis ($PCO_2 > 60$ mm Hg, $pH < 7.20$), or recurrent apnea.
ROX Index: A validated objective tool used to predict HFNO failure.
- Calculation: $(SpO_2 / FiO_2) / Respiratory Rate$.
- Interpretation: A lower score indicates a higher risk of failure and need for mechanical ventilation.
- Predictors for Failure: ROX index < 2.85 at 2 hours, < 3.47 at 6 hours, or < 3.85 at 12 hours strongly predict the need for intubation.

Weaning should be considered only when the patient shows sustained clinical stabilization for 12 to 24 hours.
Step 1: The $FiO_2$ is systematically reduced first. It should be weaned to a target of 30% to 40%.
Step 2: Once $FiO_2$ is stabilized at lower limits, the flow rate is gradually decreased.
Flow is reduced by 0.5 to 1.0 L/kg/min every 4 to 12 hours under close clinical monitoring.
Step 3: Discontinuation. HFNO therapy is typically ceased when the flow rate drops below 0.5 L/kg/min (or 2 L/min) and $SpO_2$ is maintained > 92% on minimal $FiO_2$.
The patient may then be transitioned to simple low-flow nasal prongs or room air.

Though non-invasive, HFNO is associated with specific complications requiring proactive nursing care.
Gastric Distension: The continuous positive pressure can cause air swallowing and abdominal distension. An orogastric or nasogastric tube is often recommended for air decompression.
Nasal Trauma: Although less common than with CPAP, pressure areas on the nasal septum or skin irritation can occur. Meticulous attention to prong sizing and securement is required.
Air Leaks: Over-distension can rarely cause pneumothorax or pneumomediastinum, especially if the prongs occlude the nares entirely.
Hemodynamic Alterations: High intra-thoracic pressures may occasionally impede venous return, causing mild hypotension.