Nasopharyngeal tubes in paediatric anaesthesia: is the flow-dependent pressure drop across the tube suitable for calculating oropharyngeal pressure?

Background: Nasopharyngeal tubes (NPTs) are useful in paediatric anaesthesia for insufflating oxygen and anaesthetics. During NPT-anaesthesia, gas insufflation provides some positive oropharyngeal pressure that differs from the proximal airway pressure (Paw) owing to the flow-dependent pressure drop across the NPT (ΔPNPT ). Aims: This study aimed to investigate whether ΔPNPT could be used for calculating oropharyngeal pressure during NPT-assisted anaesthesia. Methods: In a physical model of NPT-anaesthesia, using Rohrer's equation, we calculated ΔPNPT for three NPTs (3.5, 4.0, and 5.0, mm inner diameter) under oxygen and several sevoflurane in oxygen combinations in two ventilatory scenarios (continuous positive airway pressure and intermittent positive pressure ventilation). We then calculated oropharyngeal pressure as Paw minus ΔPNPT . Calculated and measured oropharyngeal pressure couples of values were compared with the root mean square deviation (RMSD) to assess accuracy. We also investigated whether oropharyngeal pressure accuracy depends on the NPT diameter, flow rate, gas composition, and leak size. Using ΔPNPT charts, we tested whether ΔPNPT calculation was feasible in clinical practice. Results: When we tested small-diameter NPTs at high flow or high peak inspiratory pressure, Paw measurements markedly overestimated oropharyngeal pressure. Comparing measured and calculated maximum and minimum oropharyngeal pressure couples yielded RMSDs less than 0.5 cmH2 O regardless of ventilatory modality, NPT diameter, flow rate, gas composition, and leak size. Conclusion: During NPT-assisted anaesthesia, Paw readings on the anaesthetic monitoring machine overestimate oropharyngeal pressure especially for smaller-diameter NPTs and higher flow, and to a lesser extent for large leaks. Given the importance of calculating oropharyngeal pressure in guiding NPT-ventilation in clinical practice, we propose an accurate calculation using Rohrer's equation method, or approximating oropharyngeal pressure from flow and pressure readings on the anaesthetic machine using the ΔPNPT charts.