High flow nasal cannula therapy in children: working principles and treatment failure predictors

High Flow Nasal Cannula (HFNC) delivers high flowrates of a heated air/oxygen fresh gas breathing mixture, in an open system, at the exact amount of fraction inspired oxygen, and at the optimum hydration level. By definition, due to high flow rates, higher than 2 L/min, it produces a wash out of the anatomic dead space and the End-tidal Carbon dioxide (EtCO2), and augments thus effective alveolar ventilation at the same rate of minute ventilation, helping reduce partial arterial pressure of Carbon dioxide (PaCO2) levels. Although depending on mouth closure and the relative size of the nasal cannula prongs related to nares, it produces a minimum Positive End Expiratory Pressure (PEEP) level, which is especially helpful in keeping unstable alveoli open, recruiting lung volume, and increasing the functional residual capacity. It reduces respiratory resistance and the high work of breathing which is a common feature in patients with respiratory failure. But its most important characteristics are the ease of implementation and good patient tolerance. It has emerged as a promising support mode in the last decade, and its use is being continuously expanded. Although it started from neonates, it expanded to children and adults, and tested in all causes of acute hypoxemic respiratory failure, especially in bronchiolitis, and in post-extubation respiratory failure as well, starting from Emergency Department (ED), Pediatric Ward (PW), Pediatric Intensive Care Unit (PICU), and during transportation. Comparisons and meta-analyses, although not of equal modalities, have shown that it is definitely better than Standard Oxygen Therapy (SOT), and rather inferior to Continuous Positive Airway Pressure (CPAP). The aim of the present study is to explore the HFNC position in the timeline of recommendations for mechanical ventilation in critically ill children. We present a review on HFNC literature evidence in patients aged 1 month to 18 years, focusing on its mechanism of action, clinical effects, and timely recognition of treatment failure predictors.