Visual Guide to High Flow Nasal Cannula in Pediatrics

The High Flow Nasal Cannula (HFNC) is a system of non-invasive respiratory support which administers a gas flow (air and/or oxygen) much higher than conventional. Its effectiveness is based on the synergy of three key components that are delivered conditioned for maximum comfort and effectiveness.

Heated Gas (34-37°C)

Prevents dryness, improves comfort and optimizes the function of cilia to clean the airway.

Humidified Gas (100%)

It fluidizes secretions, facilitates their expulsion and prevents the formation of dangerous mucous plugs.

High Flow (1-2 L/kg/min)

It is the main therapeutic component that reduces the work of breathing through several mechanisms.

Key Concept Learned: CNAF is not just "oxygen." It is a high flow gas conditioning therapy which combines heat, humidity and flow to treat respiratory failure.

CNAF acts through four main physiological effects that combine to provide relief to the patient.

Dead Space Wash

The constant flow "sweeps" the CO₂ from the pharynx. This enriches each inhalation with oxygen, making ventilation more efficient and reducing effort.

Low level PEEP effect

It generates a gentle positive pressure (1-5 cmH₂O) that keeps the alveoli open (alveolar recruitment), improving oxygenation and lung mechanics.

Mucociliary Clearance

Conditioned gas optimizes ciliary function. Secretions move more easily, preventing obstruction and atelectasis.

Satisfies Inspiratory Demand

By providing a flow equal to or greater than what the patient needs, the work of the respiratory muscles is drastically reduced. The child "breathes without effort."

Key Concept Learned: The main objective and effect of the CNAF is the reduction in work of breathing through a set of physiological mechanisms.

Indications

Acute Bronchiolitis: The star indication and with the greatest evidence.
Asthmatic Crisis: As an adjuvant in mild-moderate cases.
Pneumonia: For the management of hypoxemic respiratory failure.
Post-extubation support: To prevent failure and facilitate the transition.
Heart Failure: Reduces preload and work of breathing.
Support in procedures under sedation.

Contraindications

Obstruction of the upper airway (choanal atresia).
Severe facial or head trauma.
Undrained pneumothorax.
Sensory depression or apnea (risk of aspiration).
Imminent respiratory failure (needs intubation).

Key Concept Learned: HFNC is for patients with respiratory distress that still has its own impulse. It is not for those who have stopped breathing or have a physical obstruction.

Initial Therapeutic Programming

FLOW

1-2L/kg/min

It is the KEY parameter to reduce the work of breathing. Start with 2 L/kg/min if the difficulty is moderate/severe.

FiO₂

> SpO₂ 92%

Adjust for saturation target. Start with 50-60% if hypoxemia is present and quickly titrate downward.

TEMPERATURE

34-37°C

Start at 37°C for maximum comfort and mucociliary effectiveness. It is essential for tolerance to high flows.

Monitoring: Does it work?

The response is evaluated in the first 30-60 minutes. We look for improvement in:

Respiratory Work: Less pulling, fluttering, and wobble.
Respiratory and Heart Rate: They should decrease towards normal.
Oxygenation: SpO₂ improves and allows FiO₂ to be lowered.
Overall comfort: The patient seems calmer, he can even sleep.

Key Concept Learned: He Flow treats the work of breathing. The FiO₂ treats hypoxemia. Do not confuse their functions when adjusting parameters.

When to Start?

When there is sustained clinical improvement (>12-24h), low respiratory work and stable and decreasing FiO₂ (ideally < 40%).

Graphic Weaning Process

1

Lower FiO₂

It is the first passed. Reduce progressively until reaching 21% (room air), maintaining SpO₂ > 92%.

2

Lower Flow

Once with FiO₂ 21%, decrease the flow in increments of ~25% every 4-12h as tolerated.

3

Remove or Change

With low flow (e.g. < 0.5 L/kg/min or < 5 L/min), you can switch to a low-flow cannula or remove the support completely.

Key Concept Learned: Weaning is an orderly process: first the O₂ (FiO₂) is removed, then the flow support (Flow). Patience is key to avoiding failure.

Case 1: Infant with Bronchiolitis

Clinic: 3 months, 5 kg, RR 70, severe indrawing, SpO₂ 88%.
Action: Start CNAF a 10L/min (2L/kg/min) and FiO₂ 60%.
Result: At one hour, RR 55, less draft, SpO₂ 95%. FiO₂ is lowered to 40%.
Lesson: Starting with therapeutic flow is crucial.

Case 2: Post-Extubation

Clinic: 2 years, 12 kg, post-extubated with mild stridor.
Action: Start CNAF a 18L/min (1.5L/kg/min) and FiO₂ 40%.
Result: Patient comfortable, without stridor. Reintubation is prevented.
Lesson: The PEEP effect is protective in this scenario.

Case 3: Asthmatic Crisis

Clinic: 5 years old, 20 kg, with respiratory difficulty despite bronchodilators.
Action: Start CNAF a 20L/min (1L/kg/min) and FiO₂ 21% (no hypoxemia).
Result: Improves WOB, allowing better delivery of the drug.
Lesson: HFNC as a support to reduce the work of breathing.

Case 4: Choice of Cannula

Clinic: 3 kg neonate.
Mistake: A cannula is used that occupies 90% of the nostril.
Correct Action: Switch to a neonatal cannula that does not occupy >50%.
Lesson: A cannula that is too large is dangerous. Air leakage is necessary and safe.

1. What is the main mechanism of HFNC to reduce the work of breathing?

A) Contribution of FiO₂ 100% | B) Washing of the nasopharyngeal dead space | C) Cooling of the airway | D) Generation of PEEP of 20 cmH₂O

2. Baseline parameters for an 8 kg infant with moderate bronchiolitis:

A) Flow 8 L/min, FiO₂ 21% | B) Flow 16 L/min, Temp 30°C | C) Flow 16 L/min, FiO₂ 50%, Temp 37°C | D) Flow 2 L/min, FiO₂ 100%

3. What is the first parameter that should be decreased during weaning from HFNC?

A) The Flow | B) The FiO₂ | C) Temperature | D) Flow and FiO₂ at the same time

4. What rule should the size of the nasal cannula follow?

A) Do not occupy more than 50% of the nostril | B) Completely seal the nostril | C) Size doesn't matter | D) The largest possible to maximize PEEP

5. What is an ABSOLUTE contraindication for HFNC?

A) Severe bronchiolitis | B) Tachycardia | C) pH 7.26 | D) Choanal atresia

6. A patient with HFNC improves his work of breathing, but his SpO₂ remains at 89% with FiO₂ 60%. What is the next step?

A) Increase Flow to 2.5 L/kg/min | B) Intubate the patient | C) Increase FiO₂ | D) Lower the temperature

7. Which of the following is NOT a direct benefit of humidification and gas heating?

A) Greater comfort for the patient | B) Improvement of mucociliary clearance | C) Prevention of nosebleeds | D) Decrease in dead space

8. What finding 60 minutes after starting HFNC would indicate a probable failure of the therapy?

A) The patient falls asleep | B) Decrease in Heart Rate | C) FiO₂ requirement of 50% | D) Increased indrawing and progressive acidosis

9. Why is HFNC particularly useful in bronchiolitis?

A) Administer a nebulized antibiotic | B) Attacks the multifactorial pathophysiology of the disease | C) Cools inflamed airways | D) Forces the opening of bronchioles with high pressure

10. What is "dead space flushing"?

A) Clear secretions from the nose | B) Increase PEEP to open alveoli | C) "Sweep" CO₂ from the upper airway | D) Humidify the air so that it does not irritate

Detailed Answers

1. Correct Answer: B) Nasopharyngeal dead space lavage

Explanation: Although all mechanisms are important, the washing (or sweeping) of CO₂-rich air from the nasopharyngeal dead space is the main contributor to the reduction in the work of breathing. By cleaning this area, each breath the patient takes has a higher oxygen concentration and less CO₂, making ventilation much more efficient. The patient does not need to breathe as fast or as deeply to achieve the same gas exchange. The other options are incorrect because FiO₂ treats hypoxemia, not exertion, HFNC heats (not cools) the gas, and the PEEP it generates is low level, not 20 cmH₂O.

2. Correct Answer: C) Flow 16 L/min (2 L/kg/min), FiO₂ 50%, Temp 37°C

Explanation: This option represents a complete and aggressive therapeutic initiation, suitable for moderate difficulty. A high flow (2 L/kg/min) is provided to treat respiratory work, a reasonable initial FiO₂ (50%) to correct hypoxemia (which will later be adjusted) and the ideal physiological temperature (37°C) for comfort and ciliary function. The other options are incorrect due to insufficient flow (A), inappropriate temperature (B), or being low flow instead of high flow (D).

3. Correct Answer: B) The FiO₂

Explanation: Weaning follows a logical order: first the problem of oxygenation (hypoxemia) is solved and then the problem of respiratory mechanics (respiratory work). Therefore, the first parameter to remove is FiO₂. Once the patient can maintain adequate saturation with room air (FiO₂ 21%), it means that their oxygenation problem is resolved. Only then do we begin to remove the flow support, which is what helps with the effort of breathing.

4. Correct Answer: A) Do not occupy more than 50% of the diameter of the nostril

Explanation: This is a fundamental safety rule. The cannula should not seal the nostril. There must be free space around it to allow escape of expired flow and excess inspired flow. If the cannula is too large, the system becomes a closed circuit, increasing the pressure in the airway in an uncontrolled manner with the risk of barotrauma (e.g. pneumothorax).

5. Correct Answer: D) Choanal atresia

Explanation: HFNC works by introducing flow through the nose. If the back of the nose is blocked (choanal atresia), flow cannot pass to the lungs, making the therapy useless and potentially dangerous by pressurizing the nasal cavity. The other options are indications (bronchiolitis), signs of the disease that are expected to improve (tachycardia), or a finding that could improve with therapy (mild acidosis).

6. Correct Answer: C) Increase FiO₂

Explanation: The scenario describes a dissociated problem: the work of breathing has improved (Flow is working), but oxygenation is insufficient (FiO₂ is too low). The correct action is to treat the specific problem, which is hypoxemia, by increasing FiO₂. Increasing the flow further would not be the first option, since the WOB is controlled. Intubation is premature without first optimizing NIV. Temperature does not affect oxygenation.

7. Correct Answer: D) Decrease in dead space

Explanation: Heat and humidity are crucial for comfort, tolerance, and ciliary function (mucus clearance). However, the decrease or washing of dead space is a direct mechanical effect of the high flow of gas, not its thermal properties.

8. Correct Answer: D) Increased indrawing and progressive acidosis

Explanation: Therapy failure is defined by clinical deterioration despite adequate support. An increase in the work of breathing (drawing) and a worsening of gas exchange (progressive respiratory acidosis) are unequivocal signs that HFNC is not sufficient and support should be escalated. The other options are signs of success (sleep, lowering HR) or a normal therapeutic adjustment (need FiO₂ 50%).

9. Correct Answer: B) Attacks the multifactorial pathophysiology of the disease

Explanation: Bronchiolitis causes inflammation, edema, and massive mucus production, which greatly increases the work of breathing. HFNC is ideal because its mechanisms of action counteract these problems: the flow reduces the WOB and washes away the dead space, while the heat and humidity fluidize the secretions and improve their expulsion.

10. Correct Answer: C) "Sweep" the CO₂ from the upper airway

Explanation: The "dead space" is the volume of the airway that does not participate in gas exchange (pharynx, trachea). At the end of expiration, this area is filled with air rich in CO₂. The continuous flow of the HFNC "sweeps" or "washes" this air, so that the next inspiration starts from a reservoir richer in oxygen, improving ventilatory efficiency.

Franklin, D., et al. (2018). A randomized trial of high-flow oxygen therapy in infants with bronchiolitis. New England Journal of Medicine, 378(12), 1121-1131.
Nishimura, M. (2016). High-flow nasal cannula oxygen therapy in adults: physiological benefits, indication, clinical benefits, and adverse effects. Respiratory care, 61(4), 529-541.
Franklin, D., et al. (2021). Effect of Early High-Flow Nasal Cannula Therapy on the Need for Mechanical Ventilation in Infants With Bronchiolitis: A Randomized Clinical Trial (PARIS-2). The Lancet, 397(10282), 1341-1351.
Mayfield, S., et al. (2014). High-flow nasal cannula therapy for acute respiratory failure in children. Cochrane Database of Systematic Reviews, (3).
Lodeserto, F. J., et al. (2018). High-flow nasal cannula in the pediatric emergency department and its utility in viral bronchiolitis. Pediatric emergency care, 34(8), 580-583.