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.

Case Presentation

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

Physiology of HFNC

Mechanisms of Action

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

Debunking the PEEP Theory (Positive End-Expiratory Pressure)

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

Research Findings

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.

Physiological Effects

HR, RR, and SpO2 are key indicators of HFNC efficacy.
HR and RR should approach normal ranges for the child's age.
Improvement in SpO2 levels while maintaining or reducing FiO2 indicates a positive response.

Conclusion

HFNC is a valuable tool in pediatric care for alleviating respiratory distress.
Not a one-size-fits-all solution; vigilant monitoring and reassessment are crucial
Recognizing HFNC's mechanisms allows for optimized bedside application.

Closing Remarks: