HIGH FLOW: More than O’s

Heated Humidity

The alterations from cool, dry gas to the mucosa and clearance, may cause adverse effects with respiratory failure patients. Conditioning inspired gas with both heat and water vapor is known to improve mucociliary function while minimizing bronchoconstrictions. This in turn minimizes airway resistance, generally caused by oxygen therapy and easing the patient’s work of breathing. Conditioned gas is associated with less atelectasis, thus increasing the ventilation/perfusion ratio, resulting in improved oxygenation. 

Flow

HFNC attempts to meet the inspiratory flow demands that are seen in respiratory distress patients. Peak inspiratory flow (PIF) rates of 60-120 LPM are common, rendering a 15 LPM non-rebreather inadequate to meet demands. High flow of adequately heated and humidified gas is considered to have a number of physiological effects. High flow washes out carbon dioxide in anatomical dead space while having less mechanical dead space than other devices. The reduction of dead space is thought to be a key factor in improving alveolar minute ventilation with HFNC. Although the underlying diseases differed in studies, breathing frequencies decreased across the board with the application of HFNC. 

HFNC decreases the amount of air that is entrained, allowing for lower FiO2 to be used with therapy. During HFNC, actual FIO2 was close to calculated (predicted) FIO2, while measuring hypopharyngeal oxygraphy, capnography, and pressure.  Ritchie et al found that during nose breathing at rest, >30 L/min, measured FIO2 was close to delivered FIO2. 

HFNC ability to reduce the WOB and improve thoraco-abdominal synchrony is in accord with the growing appreciation for the importance of ventilation-induced lung injury and load-induced diaphragm injury in spontaneously breathing patients with AHRF. The strain applied to the lung by the strenuous exertions of the respiratory muscles and the potentially injurious respiratory load applied to the diaphragm may result in injury to the patient before intubation. A number of physiological effects of HFNC seem to intervene on both of these mechanisms to prevent injury and accelerate recovery for the patient. A prospective ED RCT found HFNC improved dyspnea and patient comfort compared with conventional oxygen therapy, while another study found reduced need for escalation of oxygen therapy within the first 24hrs. With minimal adverse events compared to NIV, HFNC makes its case as an intervention for patient WOB issues.