Physiology of High-Flow Nasal Cannula (HFNC)

Today’s case presentation involves a 2-year-old girl who was previously healthy and was admitted to the Pediatric Intensive Care Unit (PICU) for acute respiratory distress characterized by increased work of breathing and wheezing.

A 2-year-old girl with acute respiratory distress due to RSV infection

• Presented with increased work of breathing, wheezing, and no fever
• Started on High Flow Nasal Cannula (HFNC) therapy in the PICU

Key Elements:

• Prodrome of URI symptoms
• Increased respiratory effort (nasal flaring, intercostal retractions, decreased lung base air entry)
• HFNC improved the work of breathing and oxygen saturation

Washout of Nasopharyngeal Dead Space:

• HFNC clears nasopharyngeal dead space, improving oxygen efficiency.
• Reduces re-breathing of CO2 from the anatomical dead space.
• Enhances ventilation efficiency and oxygenation.

Reduction in Upper Airway Resistance:

• HFNC reduces resistance in the upper airway.
• Delivers rapid gas flow matching or exceeding natural inhalation rate.
• Eases breathing, especially in neonates and infants with narrow airways.

Optimal Conditioning of Gas:

• HFNC delivers heated and humidified oxygen, matching the body's conditions.
• Reduces energy expenditure and risk of airway irritation
• More comfortable and effective compared to cold, dry air delivery

• HFNC generates minimal and variable PEEP.
• Amount of PEEP depends on factors like flow rate and cannula size
• Not as high or consistent as other respiratory support devices

• A 2022 CHEST study by Khemani et al. on children with bronchiolitis challenged the conventional understanding of HFNC's mechanisms.
• HFNC primarily reduces breathing effort but does not consistently increase lung volume (EELV) or tidal volume (VT).
• Reduction in the pressure rate product (PRP) indicates decreased breathing effort, but not significant alterations in EELV or VT.