Effects of High Flow Nasal Cannula on Deadspace Reduction and Regional Distribution of Ventilation

Background and Significance Historically, oxygen nasal cannulae have been used in hypoxemic patients as a therapy to increase arterial oxygenation. In the neonatal community, oxygen flows >2 L/min and > 4 L/min in the pediatric patient have seldom been used due to the potential for excessive drying of the nasal mucosae and risk for developing hypothermia. It has been speculated that larger pediatric and adult patients are able to better tolerate flow rates in excess of 4 L/min because the large nasal mucosal surface area can support hydration of dry medical grade gases. High flow nasal cannula (HFNC) therapy is a form of respiratory support that provides flows that are believed to be in excess of a patient's spontaneous inspiratory and expiratory flow rates. In the last decade, a proliferation of heated-humidified high flow nasal cannula (HFNC) devices has been introduced into the clinical setting. These devices are intended to provide optimal heating and humidification of medical gases and regardless of flow setting. In the case of neonates, which has been the predominant patient population receiving HFNC, there clearly is no consensus on which flows constitute "high" in this population. If "high" is intended to mean a flow rate that exceeds spontaneous inspiratory flow rate then there is no reasonable clinical measurement to ascertain this relationship. Thus, the true definition of HFNC remains an elusive term and our protocol hopes to shed some light on this subject.

Our prospective randomized trial of the effects of 3 different oxygen nasal cannula flow rates (low, medium, and high) on respiratory rate, SPO2, transcutaneous CO2 and regional distribution of ventilation as measured by EIT will help clinicians define a range of HFNC flow rates in which deadspace washout occurs without positive distending pressure (low range) and in which deadspace washout occurs with positive pressure that creates regional distribution of ventilation changes.

C. Preliminary Studies One neonatal source defines HFNC as flows > 1 L/min and another defines flows >3 L/min as HFNC.1 Classification is further complicated in larger pediatric and adult patients where flows during HFNC have been reported to be in excess of those traditionally used with a standard nasal cannula ~ 6 L/min) and as high as 30-40 L/min.2 With the technologic ability to provide better heat and humidity, clinicians have found that HFNC may be able to support a larger fraction of patients that would otherwise require continuous positive airway pressure (CPAP), noninvasive ventilation (NIV), or invasive mechanical ventilation. There are several proposed mechanisms by which HFNC may provide greater respiratory assistance than standard oxygen delivery devices.

Flows that exceed the expiratory flow rate may provide "back-pressure" at the nasal airway opening during exhalation that is similar to nasal CPAP. Additionally, gases may provide a physiologic purging of carbon dioxide from the anatomic dead space via anatomic leak (nasal/oral airway). These effects are likely to vary based on the flow, minute ventilation, patient size, leak, and nasal airway opening/prong size relationship. There are currently three FDA approved HFNC systems. Clinical acceptance is related to the fact that HFNC is less expensive, simpler to operate and requires a less complicated airway interface than a standard CPAP or NIV device. Another proposed benefit is that HFNC prongs are generally less occlusive and may cause less nasal airway injury than CPAP prongs or a BiPAP mask. The widespread acceptance and use of this approach has been implemented with very little experimental data to support HFNC flow settings as a safe and effective option in all patients with hypoxic respiratory failure.

Electrical Impedance Tomography (EIT) Electrical impedance tomography capitalizes on changes in impendence in air-filled versus tissue-filled spaces to characterize and quantify regional distribution of lung volume at the bedside. This technology has been validated in animal3 and human4, 5 studies performed over the past decade at Boston Children's Hospital. The technology utilizes a series of 16 electrodes placed across the patient's chest (Figure 1). As small currents are passed between the electrodes, impedance is measured between and amongst the series. Through a complex interrogation and manipulation of these impedance values, a two-dimensional image is formed (Figure 2), and has been shown to correlate with clinical and radiographic changes in patients4. The ability to estimate lung volume and regional distribution of gas non-invasively and in real time may give us insight as to what mode of ventilation is more effective.