Airway Resistance in Pulmonary Dynamics

Welcome to PICU Doc On Call, A Podcast Dedicated to Current and Aspiring Intensivists.

I'm Pradip Kamat and I'm Rahul Damania. We are coming to you from Children's Healthcare of Atlanta - Emory University School of Medicine.

Welcome to our Episode a 15 mo F with respiratory distress and runny nose.

Here's the case:

A 15 mo F presents to the ED with cough, runny nose, and increased work of breathing. Her mother states that the patient has had these symptoms for the past three days, however, the work of breathing progressed. The patient has had 2 fevers during this course, with the highest 101F. She says that her 3 yo cousin who she visited for the holidays had similar symptoms. Mother notes decreased PO and wet diapers. The patient presented to the ED with the following vital signs: T 38.5C, HR 155, BP 70/48 (MAP 50), RR 48, 92% on RA. The patient on the exam was noted to be tachypneic with abdominal retractions, grunting, and nasal flaring. The patient was nasally suctioned and initiated on 12 L 40% of HFNC. The patient was then transferred to the PICU for further management.

To summarize key elements from this case, this patient has:

• Increased work of breathing indicates respiratory distress.
• She has a prodrome of symptoms that worsened prior to presentation
• And a sick contact.
• All of which brings up a concern for acute respiratory failure requiring non-invasive positive pressure ventilation in the form of HFNC.
• Let's transition into some history and physical exam components of this case?

1. What are key history features in this child who presents with respiratory distress & URI sx?

• Usually, children under the age of two with bronchiolitis will present with cough, respiratory distress, and crackles on lung exam.
• The crackles indicate atelectatic alveoli that are filled with fluid which occurs due to inflammatory processes in the lung triggered by respiratory viruses.
• Respiratory distress, increased work of breathing, respiratory rate, and oxygenation all can change rapidly with crying, coughing, and agitation.

1. Are there some red-flag symptoms or physical exam components in a child with acute respiratory distress which you could highlight?

• That is a great question. We really want to highlight the distinction between respiratory distress and respiratory failure.
• Children with respiratory failure in our case may have issues with oxygenation or ventilation as well as increased work of breathing that necessitates higher levels of respiratory support like HFNC.
• In a 2003 Journal of Pediatrics study, infants who were most severely affected with bronchiolitis were born prematurely, <12 weeks of age, or who have underlying cardiopulmonary disease or immunodeficiency. These children are at risk for apnea and respiratory failure which may require escalation to mechanical ventilation.
• Finally, Infants with bronchiolitis may have difficulty maintaining adequate hydration because of increased fluid needs and metabolic demand. Remember these children will have increased insensible losses due to fever and tachypnea, as well as decreased oral intake related to their systemic illness.

To continue with our case, the patient's labs were consistent with:

• Mild hyper NA 149
• All other electrolytes were within normal limits.
• The patient had a respiratory viral panel which was positive for Rhino/Entero and RSV. Her COVID PCR was negative.
• A CXR was performed and showed alveolar airspace disease consistent with I would like to highlight an important point, with the exception of otitis media, a secondary bacterial infection is uncommon among infants and young children with bronchiolitis. In a nine-year prospective study of 565 children (<3 yo) hospitalized with documented RSV infection published in the Journal of Pediatrics, subsequent bacterial pneumonia was present in only 0.9 percent of these. 

Yes, Rahul, that is a great point. The risk of secondary bacterial pneumonia is increased among children who require admission to the intensive care unit, particularly those who require intubation.

Ok to summarize, we have:

• A 15 mo F who presented with URI symptoms and respiratory distress was admitted to the PICU with Rhino/Entero, & RSV+ bronchiolitis with concurrent community-acquired PNA. We would like to focus the rest of this podcast on discussing the use of HFNC, its principles of action, and the data surrounding its use in the PICU.
• Before we get into this topic, let’s start with a short multiple-choice question:
• A 13 mo ex-34 week infant presents to the ED with increased work of breathing, tachypnea, and hyperthermia. The patient is on a home 1/8 L nasal cannula and has no echocardiographic evidence of pulmonary hypertension on prior follow-up. HFNC is initiated at 1.5 L per kg. Which of the following responses best describes the MOA of HFNC?
• A. Increased nasopharyngeal dead space
• B. Decreased humidification of gas
• C. Negative distending pressure
• D. Reduction in upper airway resistance.

The correct answer here is D. Reduction in upper airway resistance. By providing gas flows that match or exceed spontaneous inspiratory flow rates, HFNC minimizes inspiratory resistance across the nasopharynx. The resultant reduction in work of breathing has been demonstrated in studies in neonates and infants by measuring diaphragmatic electrical activity and respiratory plethysmography.

Rahul, what does the literature say regarding positive distending pressure with the use of HFNC?

The data is definitely mixed but leans towards not HFNC not providing clinically significant PEEP. In a study of infants with bronchiolitis published in 2013 in Intensive Care Medicine, a flow rate of 2 L/kg per minute resulted in mean pharyngeal pressures >4 cm H2O as measured by transesophageal probes and improved breathing.

Subsequent studies have documented a difference in increased pharyngeal pressure during HFNC when the mouth is closed compared with when it is open. So if you are going to use HFNC to promote distending pressure concurrent use of a pacifier may be helpful in achieving the full benefit of HFNC.

To summarize key principles of how HFNC let’s review some respiratory physiology:

1. Rahul, what is Dead Space?
2. Dead space is the volume of air that is inhaled that does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused.
3. This means that not all the air in each breath is available for the exchange of oxygen and carbon dioxide.
4. HFNC creates a washout of nasopharyngeal dead space and creates a richly oxygenated reservoir of air. This reserve in the upper airway is what the patient draws from with each breath, minimizing the entrainment of room air and also decreasing the amount of CO2 in the anatomic zone of the respiratory tree.
5. What are key concepts related to Airway Resistance in Pulmonary Dynamics?
6. West Physiology defines airway resistance as the change in transpulmonary pressure needed to produce a unit flow of gas through the airways of the lung.
7. More simply put, it is the pressure difference between the mouth and alveoli of the lung, divided by airflow. Bronchiolitis creates a decrease in airflow thus increasing airway resistance. As HFNC increases flow, i.e. the denominator of our equation, It reduces resistance in the airway tree.
8. By providing gas flows that match or exceed spontaneous inspiratory flow rates, HFNC minimizes inspiratory resistance across the nasopharynx.
9. In a study published in 2009 in Respiratory Care, it was hypothesized that the resultant reduction in airway resistance which high flow provides the decrease in WOB. This was especially studied by measuring infant diaphragmatic electrical activity.

Rahul, what is the last major mechanism of a high-flow nasal cannula?

1. HFNC reduces the energy expenditure required by the body to condition air. It does this by delivering heated and humidified gas. This also promotes less bronchospasm which would occur with the delivery of cold air.

Pradip, in your experience, what are disease states we see in the PICU that are most amenable to HFNC?

1. Many causes of respiratory distress can respond to the use of HFNC. In this state, I would recommend that providers start HFNC, and then ensure close monitoring and frequent reassessment of these patients to ensure response as well as the need to escalate respiratory support.
2. Conditions for which HFNC can be used to include conditions such as Asthma, Tracheomalacia, Apneic Oxygenation, and Bronchiolitis.

HFNC should not delay advanced airway management in a patient deemed to require immediate endotracheal intubation. This may include patients with acutely impaired mental status, risk of aspiration, or other needs for airway protection

Yes, thank you for highlighting this, HFNC should be avoided in patients who have facial anomalies that preclude appropriate nasal cannula fit (like choanal atresia). Children who have active vomiting, bowel obstruction, or even sensory issues which may create Agitation may be some relative contraindications for HFNC. Lastly, I would also not delay escalation in invasive respiratory support especially if the patient does not have a significant change in hemodynamic (such as a decrease in HR) or oxygenation parameters after about 4 hrs on HFNC therapy.

Finally, HFNC oxygen therapy is considered an aerosol-generating procedure. Thus, appropriate infection control precautions are required when it is being administered to patients with unknown or positive coronavirus disease 2019.

This concludes our episode on bronchiolitis and HFNC. We hope you found value in our short, case-based podcast. We welcome you to share your feedback, subscribe & place a review on our podcast! Please visit our website picudoconcall.org which showcases our episodes as well as our Doc on Call management cards. PICU Doc on Call is co-hosted by myself Dr. Pradip Kamat and Dr. Rahul Damania. Stay tuned for our next episode! Thank you!