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High Flow Nasal Cannula Therapy

Posted on Tue, Jul 07, 2015
High Flow Nasal Cannula Therapy

Best practice medicine for the treatment of newborn babies

by Alfred Sacchetti, Jr., M.D., FACEP

Why don’t newborn infants suffocate in the delivery room?

 That’s not exactly a difficult question – they breathe. They take a breath in, move air that contains oxygen into their lungs and then exhale their carbon dioxide… pretty basic sixth-grade biology.

 Well, that’s not exactly true. An infant’s, in particular a newborn's, head is proportionally much bigger than an adult's. In fact, it is so big that the anatomic dead space in the head and upper airway of a young infant is larger than the tidal volume of their lungs which means that when children take a breath in they are only rebreathing their exhaled air. No fresh supply of oxygen comes in and they retain all of their exhaled carbon dioxide. Not a good model for long-term survival past about five minutes.

 So why doesn't a newborn asphyxiate? The trick is in their respiratory rate. In order to adequately ventilate, they need to mix the exhaled air in their upper airway dead space with the new air they are breathing in. They do this by using their elevated respirations to agitate the air in the posterior pharynx above the larynx to create vortices that mix their inspired and expired gasses. In this way, they wash out their carbon dioxide (CO2) and create a source of higher oxygen to bring into their alveoli. So what does that have to do with emergency medicine?
 
In older children and adults, that upper airway anatomic dead space still exists. In an average adult, about 150 millileters (ml) or about a third of exhaled air is rebreathed. Since adult tidal volumes are in the range of 450-500 ml, this rebreathing is generally not a problem. However, in patients with respiratory compromises, taking in 150 ml of oxygen-poor, CO2 latent gas with each breath can lead to hypoxia and hypercarbia. This issue is even more significant if there is lung pathology that leads to a physiologic dead space on top of the anatomic dead space.

 So what if you could turn that dead space into usable ventilation space? Adding 150 ml of functional gas could effectively increase respiratory efficiency 50 percent. For pediatric patients, there is an even greater effect since they are already trying to increase their dead space mixing.

 High flow nasal cannula (HFNC) therapy accomplishes exactly that. By using specially designed nasal cannula, these devices can deliver flow rates up to 60 liters a minute, creating vortices in the supraglottic anatomic dead space. Unlike the physiologic mixing produced by infants, the flow rates in these systems do not simply blend the inspired and expired gasses – they wash them out completely. In essence, the rebreathed air is replaced by gasses with zero carbon dioxide and any oxygen level required. Inspired oxygen concentrations can be set anywhere from 21 percent to 100 percent and, because the flow rates provided in HFNC systems exceed those generated by a normal inspiration, whatever is set for the fraction of inspired oxygen (FiO2) is exactly what the patient receives – there is no dilution with room air.

As expected, such high flow rates can rapidly dry out the entire bronchial tree. To manage this dryness, the inhaled gas is super saturated with water vapor. To permit this degree of humidification, the inhaled gas is heated by the delivery system. This is why some systems are referred to as high flow high humidity nasal cannula.

The earliest clinical applications of HFNC were in pediatric patients, particularly in premature neonates. Subsequently, it has been shown to be very effective in infants with bronchiolitis and pneumonia. In bronchiolitis, the high humidity has resulted in loosening of secretions leading to increased clearing of the lower airways, in addition to the system’s effects on anatomic dead space.

 For adults, HFCN has proven effective in chronic obstructive pulmonary disease (COPD) patients, particularly those with carbon dioxide retention. Other uses have been for asthma, pneumonia and pulmonary fibrosis. HFCN produces a minimal, positive airway effect, so while it can help with oxygenation in congestive heart failure patients, it does not provide the pressure benefits of bilevel positive airway pressure (BiPAP) that help treat the failure itself.

In applying HFNC, the flow rate is selected first and then the FiO2. Like BiPAP, initial settings are only a starting point and both the flow and FiO2 are adjusted in accordance to the patient’s clinical course. “Set the flows then the O’s” is the tacky but accurate recommendation on how to begin HFNC therapy. For neonates and infants < 10 kg flow rates are generally set around four liter/minute and titrated up to a max of one to two liters per kilogram per minute. For adults, flows are started at 25 liters per minute (lpm) and titrated to max of 60 lpm. Higher starting points may be selected in patients with greater respiratory distress. At Our Lady of Lourdes Medical Center, instructions to the respiratory therapist for HFNC are to set the flow at a specific rate and then titrate the oxygen to achieve a specific pulse oximetry reading. The respiratory therapist then adjusts the flow and FiO2 as they monitor the patients, keeping the clinicians informed of their changes. So for the average COPD exacerbation, the order might read "HFNC at 40 liters per minute titrate FiO2 to oxygen saturation of 92 percent." For a six-month-old infant with respiratory syncytial virus (RSV), the order might read "HFNC at eight liters per minute titrate oxygen to 100 percent.”

 There is some relatively significant literature on the use of HFNC in neonates, but clinical studies of emergency department (E.D.) applications of HFNC are just beginning. Anecdotal E.D. experiences by those clinicians using HFNC have been extremely positive in patients of all ages, although the results of prospective studies have yet to be fully reported.

 

Alfred Sacchetti, M.D., FACEP, is EmCare’s Chief of Emergency Services for Our Lady of Lourdes Medical Center in Camden, N.J. Dr. Sacchetti also acts as a Medical Research Director for EmCare’s North Division. He received his medical education and training at the Medical College of Pennsylvania. Dr. Sacchetti is the recipient of Tthe 2013 Genesis Cup, EmCare’s recognition for innovation in healthcare, and leads the emergency medicine “hot topics” session at EmCare’s annual leadership conference.

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