Nasopharyngeal tubes in pediatric anesthesia: Is the flow-dependent pressure drop across the tube suitable for calculating oropharyngeal pressure?

Abstract

Background: Nasopharyngeal tubes are useful in pediatric anesthesia for insufflating oxygen and anesthetics. During nasopharyngeal tube-anesthesia, gas insufflation provides some positive oropharyngeal pressure that differs from the proximal airway pressure owing to the flow-dependent pressure drop across the nasopharyngeal tube (ΔP_NPT ).

Aims: This study aimed to investigate whether ΔP_NPT could be used for calculating oropharyngeal pressure during nasopharyngeal tube-assisted anesthesia.

Methods: In a physical model of nasopharyngeal tube-anesthesia, using Rohrer's equation, we calculated ΔP_NPT for three nasopharyngeal tubes (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 proximal airway pressure minus ΔP_NPT . Calculated and measured oropharyngeal pressure couples of values were compared with the root mean square deviation to assess accuracy. We also investigated whether oropharyngeal pressure accuracy depends on the nasopharyngeal tube diameter, flow rate, gas composition, and leak size. Using ΔP_NPT charts, we tested whether ΔP_NPT calculation was feasible in clinical practice.

Results: When we tested small-diameter nasopharyngeal tubes at high-flow or high-peak inspiratory pressure, proximal airway pressure measurements markedly overestimated oropharyngeal pressure. Comparing measured and calculated maximum and minimum oropharyngeal pressure couples yielded root mean square deviations less than 0.5 cmH₂ O regardless of ventilatory modality, nasopharyngeal tube diameter, flow rate, gas composition, and leak size.

Conclusion: During nasopharyngeal tube-assisted anesthesia, proximal airway pressure readings on the anesthetic monitoring machine overestimate oropharyngeal pressure especially for smaller-diameter nasopharyngeal tubes and higher flow, and to a lesser extent for large leaks. Given the importance of calculating oropharyngeal pressure in guiding nasopharyngeal tube 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 anesthetic machine using the ΔP_NPT charts.

Keywords: Rohrer's equation; anesthesia; nasopharyngeal tube; oropharyngeal pressure; pediatrics; pressure drop.