Physiology of High-Flow Nasal Cannula (HFNC)

PICU Doc On Call

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:

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References

Miller AG, Gentle MA, Tyler LM, Napolitano N. High-Flow Nasal Cannula in Pediatric Patients: A Survey of Clinical Practice. Respir Care 2018; 63:894.

Wraight TI, Ganu SS. High-flow nasal cannula use in a pediatric intensive care unit over 3 years. Crit Care Resusc 2015; 17:197.

Hutchings FA, Hilliard TN, Davis PJ. Heated humidified high-flow nasal cannula therapy in children. Arch Dis Child 2015; 100:571.

Lee JH, Rehder KJ, Williford L, et al. Use of high flow nasal cannula in critically ill infants, children, and adults: a critical review of the literature. Intensive Care Med 2013; 39:247.

Wing R, James C, Maranda LS, Armsby CC. Use of high-flow nasal cannula support in the emergency department reduces the need for intubation in pediatric acute respiratory insufficiency. Pediatr Emerg Care 2012; 28:1117.

Bressan S, Balzani M, Krauss B, et al. High-flow nasal cannula oxygen for bronchiolitis in a pediatric ward: a pilot study. Eur J Pediatr 2013; 172:1649.

Mayfield S, Bogossian F, O'Malley L, Schibler A. High-flow nasal cannula oxygen therapy for infants with bronchiolitis: pilot study. J Paediatr Child Health 2014; 50:373.

Kelly GS, Simon HK, Sturm JJ. High-flow nasal cannula use in children with respiratory distress in the emergency department: predicting the need for subsequent intubation. Pediatr Emerg Care 2013; 29:888.