Oxygen delivery through high-flow nasal cannulae increase end-expiratory lung volume and reduce respiratory rate in post-cardiac surgical patients

† In this study of patients after cardiac surgery, HFNC increased end-expiratory lung impedance, suggesting increased lung volumes and functional residual capacity. † Oxygenation improved and the benefits were greatest in patients with high BMIs. † Further data are required to assess the clinical significance of these data. Background. High-flow nasal cannulae (HFNCs) create positive oropharyngeal airway pressure, but it is unclear how their use affects lung volume. Electrical impedance tomography allows the assessment of changes in lung volume by measuring changes in lung impedance. Primary objectives were to investigate the effects of HFNC on airway pressure (Paw) and end-expiratory lung volume (EELV) and to identify any correlation between the two. Secondary objectives were to investigate the effects of HFNC on respiratory rate, dyspnoea, tidal volume, and oxygenation; and the interaction between BMI and EELV. Methods. Twenty patients prescribed HFNC post-cardiac surgery were investigated. Impedance measures, Paw, Pao2 /FIo2 ratio, respiratory rate, and modified Borg scores were recorded first on low-flow oxygen and then on HFNC. Results. A strong and significant correlation existed between Paw and end-expiratory lung impedance (EELI) (r¼0.7, P,0.001). Compared with low-flow oxygen, HFNC significantly increased EELI by 25.6% [95% confidence interval (CI) 24.3, 26.9] and Paw by 3.0 cm H2O (95% CI 2.4, 3.7). Respiratory rate reduced by 3.4 bpm (95% CI 1.7, 5.2) with HFNC use, tidal impedance variation increased by 10.5% (95% CI 6.1, 18.3), and Pao2 /FIo2 ratio improved by 30.6 mm Hg (95% CI 17.9, 43.3). A trend towards HFNC improving subjective dyspnoea scoring (P¼0.023) was found. Increases in EELI were significantly influenced by BMI, with larger increases associated with higher BMIs (P,0.001). Conclusions.ThisstudysuggeststhatHFNCsreducerespiratoryrateandimproveoxygenation byincreasingbothEELVandtidalvolumeandaremostbeneficialinpatientswithhigherBMIs.