High Flow, High Flow, it’s off to work we go. Let’s give some CPAP and… stop singing. OK, anyway, as a follow up to yesterday’s article on standard oxygen therapy I wanted to dive into High Flow. It entails the delivery of heated and humidified oxygen via special devices (eg, Vapotherm®) providing up to 8 L/min in infants and up to 40 L/min in children and adults. It is generally delivered via nasal cannula and though hard to assess he exact amount in vivo, results in the generation of positive end expiratory pressure, thus stenting open the airways. It is an advantageous modality in that it can deliver high FiO2 with lower flow rates that is more comfortable for the patient. The heated/humidified O2 also hydrates the mucosae, thus thinning secretions making them easier to suction. There are different machines and set-ups – but a common variation uses two different circuits – Low and High Flow. Before you get all mixed up let me clarify.

The low or high options should be chosen based on the age and size of the patient. In general, the “Low” option for High Flow Nasal Cannulae can deliver only 1-8 L/min and is mostly helpful for small babies. The “High” option can deliver 5-40 L/min and is used with pediatric or adult cannulae.

To answer this question of why HFNC is helpful let us delve into the research a bit shall we?  A good place to start would be with Lee, 2013 Intensive Care Med. They noted that regarding HFNC in children most studies look at neonates or bronchiolitis. These studies mostly demonstrate that CPAP is generated, but no predominant mechanism in reducing respiratory distress has been elucidated. Spentzas, 2009  J Intensive Care Med noted that 46 infants/children on HFNC had an improved comfort score, respiratory clinical scale, & O2 sats. They also noted CPAP of approx 4cm H2O generated via NP probe. In their study 5/46 patients ultimately required intubation. Arora, 2012 Pediatr Emerg Care agreed that positive airway pressure was being generated in a prospective study of 25 infants with bronchiolitis. Via probes placed in the pharynx pressure increased linearly up to flows of 6 L/min. in general they saw 0.45 cm H2O increase in pressure for each 1 L/min increase in flow. Not surprisingly, closed mouths lead to higher pressures. Milesi, 2013 Intensive Care Med conducted a prospective study of 21 infants admitted to PICU with bronchiolitis. They measured pharyngeal pressure at flows 1, 4, 6 & 7 L/min. Flow >2 L/kg/min was associated with >4cm H2O and improved respiratory effort. They also noted that >6 L/min was required to generate sufficient positive pressure throughout the respiratory cycle.

Ok, so it is clear that some measure of positive pressure is being generated by HFNC. So could this be helpful in an illness like bronchiolitis where mucous filled edematous airways lead to collapse of alveoli? Let’s examine three studies that addressed this issue.

McKiernan, 2010 J Pediatr

A Retrospective review of infants in the PICU with bronchiolitis. Before the availability of HFNC 23% were intubated, after 9%. They controlled for age, weight and RSV +/- status. Overall HFNC resulted in decreased respiratory rate and decreased ICU length of stay. Though there were fewer patients intubated after the arrival of HFNC, there is not definitive data to support a claim that HFNC was responsible for independently reducing the risk of intubation.

Schibler, 2011 Intensive Care Med

In a retrospective review 298 <2 year-olds admitted to the ICU they noted that 36 (12%) needed invasive ventilation. Of the subset of 167 (56%) with bronchiolitis only 6 (4%) needed invasive ventilation. High flow became available during the course of the study duration. The rate of intubation was 37% at the start and 7% at the end of the study. Though more patients got HFNC there was no statistically significant proof it reduced intubation risk independently.

Kelly, 2013 Pediatr Emerg Care

This was a retrospective review of 498 patients <2 years that received HFNC within 24h of ED triage. They noted the following proportion of diagnoses: Bronchiolitis = 46%, Pneumonia = 28%, Asthma = 8%. Overall intubation was required in 8% (42/498). The risk for intubation was greater if the pCO2 was >50mmHg (OR, 2.51; 95% CI, 1.06-5.98) or if the initial venous pH was <7.30 (OR, 2.53; 95% CI, 1.12-5.74). Having bronchiolitis as opposed to one of the other diagnoses appeared to be protective vs intubation (OR, 0.40; 95% CI, 0.17-0.96).

So can we safely conclude that HFNC reduces the risk of intubation in bronchiolitis? Not necessarily. First of all, the rate of intubation in bronchiolitis is declining overall. Second, though HFNC seems to improve some physiologic parameters and generates (admittedly a difficult to measure) distending airway pressure, there is no statistically significant proof that it prevents intubation. Furthermore, the FDA has approved it for the delivery of heated humidified air – not for providing PEEP or as an alternative to other methods of CPAP. So, essentially, if you are using it to provide positive pressure to a bronchiolitis you are doing so in an “off-label” fashion. Nevertheless, it is mostly safe – though case reports of air-leak syndrome have been published. One example coming from Hedge, 2013 Pediatrics. This case series described 3 patients with HFNC induced barotrauma. Consider their examples and recall whether or not you had a similar patient:

  • 12 month old male with RSV bronchiolitis developed a pneumothorax on day 5 of illness @ 8 L/min. he necessitated intubation x14 days.
  • 16 year-old male with CP developed a pneumomediastinum @ 20 L/min. he eventually died.
  • 22 month old toddler with a subdural hematoma secondary to non-accidental trauma developed a pneumothorax on 6 L/min thus necessitating a chest tube.

 

Still, most patients on HFNC do fine, though papers like the aforementioned one from Hedge should prompt you to continue to reassess patients after application of HFNC.

OK, so let’s wrap things up. HFNC allows you to give high FiO2 with lower flows and generates CPAP-like distending pressures that may be useful in bronchiolitics and other conditions prone to airway/alveolar collapse. You should use it in a patient with good respiratory drive that needs assistance with ventilation. It probably stimulates some neonates to breathe a la other methods of non-invasive respiratory support applied for periodic apnea. Use the “low flow” setup for babies, and the “high flow” set up for bigger children and adults and start high – for instance a flow of 10-20 L/min and adjust both the rate and the FiO2 to achieve sats >94% and better respiratory effort. You can use it in situations where you are worried that the baby may need intubation, but don’t use it in lieu of more definitive airway management in the patient with persistent apnea or cardiopulmonary compromise. A finally, thoguh you may have heard otherwise, it is important to remember the following point – at least until more robust studies have been performed.
[message_box title=”Take Home Point” color=”green”]There is no statistically significant proof that high flow prevents intubation in bronchiolitis[/message_box]