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 PresentationA 2-year-old girl with acute respiratory distress due to RSV infectionPresented with increased work of breathing, wheezing, and no feverStarted on High Flow Nasal Cannula (HFNC) therapy in the PICUKey Elements:Prodrome of URI symptomsIncreased respiratory effort (nasal flaring, intercostal retractions, decreased lung base air entry)HFNC improved the work of breathing and oxygen saturationPhysiology of HFNCMechanisms of ActionWashout 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 irritationMore comfortable and effective compared to cold, dry air deliveryDebunking the PEEP Theory (Positive End-Expiratory Pressure) HFNC generates minimal and variable PEEP.Amount of PEEP depends on factors like flow rate and cannula sizeNot as high or consistent as other respiratory support devicesResearch FindingsA 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 EffectsHR, 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.ConclusionHFNC is a valuable tool in pediatric care for alleviating respiratory distress.Not a one-size-fits-all solution; vigilant monitoring and reassessment are crucialRecognizing HFNC's mechanisms allows for optimized bedside application.Closing Remarks:Subscribe, share feedback, and leave a review on the podcast.Visit picudoconcall.org for more episodes and management cards.Hosted by Dr. Pradip Kamat and Dr. Rahul DamaniaReferencesMiller 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.