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 (ΔPNPT). Aims This study aimed to investigate whether ΔPNPT 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 ΔPNPT 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 ΔPNPT. 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 ΔPNPT charts, we tested whether ΔPNPT 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 cmH2O 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 ΔPNPT charts.

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.

| INTRODUC TI ON
In adults and children, among the many possible ways to manage the upper airway, one involves placing an endotracheal tube in the oropharynx (nasopharyngeal tube, NPT). The NPT has proved useful to deliver continuous positive airway pressure (CPAP) in premature infants 1 and to facilitate intermittent positive pressure ventilation (IPPV) in several emergency conditions. 2 In anesthetized spontaneously breathing adult patients, a customized nasopharyngeal airway, the nasal trumpet, was used to facilitate elective and semi-elective fiber optic intubation. 3 In this setting, the nasal trumpet helped to establish a patent airway and deliver positive pressure ventilation, without impeding fiber optic tracheal intubation. In pediatrics, a similar experience with the NPT comes from Holm-Knudsen and colleagues, who used this technique to help fiber optic intubation in small children with a difficult airway (ie, Pierre Robin sequence, Treacher Collins, and similar syndromes). 4 A procedure that can deliver oxygen, anesthetics and some positive pressure is essential during spontaneously breathing anesthesia. Preserving a balance between adequate sedation and effective ventilation is challenging for anesthesiologists caring for children. Young children have efficacious protective reflexes that require deep sedation for suppression. Children are particularly susceptible to upper airway obstruction because they have smalldiameter airways and a high incidence of tonsillar and adenoidal hypertrophy, which increases resistance to flow. Because the upper airway consists of soft tissue and during inspiration is kept patent by pharyngeal airway muscle dilation, drugs that reduce muscle activity can reduce airway patency, often at the velopharyngeal level, and, thus, increase upper airway resistance. 5 Hence, many anesthesiologists resort to intubation even for procedures that require only minutes to complete.
To avoid unnecessary intubation, an NPT can be placed with the tip just above the larynx and positive pressure can be applied at the oropharynx level. Setting flow to a target oropharyngeal pressure or having reliable pressure readings upon increasing the flow rate is difficult during NPT-assisted anesthesia partly owing to gas leakage through the mouth, but, also, because the resistance offered by the small NPT diameter makes the airway pressure measured at the proximal end of the tube (proximal airway pressure, Paw) higher than the pressure measured at the distal end (Pdist or oropharyngeal pressure). This pressure difference, that is, the flow-dependent pressure drop across the NPT (ΔP NPT ), depends on flow characteristics (rate, direction, and acceleration), gas composition and the tube diameter, length, curvature, and material. 6,7 As an alternative to the troublesome dedicated pressure catheter system, oropharyngeal pressure can be calculated from the Paw reduced by the ΔP NPT (oropharyngeal pressure = Paw − ΔP NPT ).
This mathematical construct has been successfully used for calculating pressure downstream an endotracheal tube (intra-tracheal pressure). 8 No published study has to date attempted to calculate oropharyngeal pressure during NPT-assisted anesthesia from the ΔP NPT . If this method proved effective, it would provide the basis for implementing anesthetic machines with algorithms for calculating oropharyngeal pressure during NPT ventilation. It would also allow physicians to know what pharyngeal pressure they are actually delivering during CPAP or IPPV when changing the flow rate or pressure.

K E Y W O R D S
anesthesia, nasopharyngeal tube, oropharyngeal pressure, pediatrics, pressure drop, Rohrer's equation

What is already known about the topic
• In anesthetized spontaneously breathing children, a nasopharyngeal tube connected to a flow-inflating bag can help deliver oxygen and anesthetics and improve airway patency by providing positive oropharyngeal pressure.
• In intubated children, the pressure gradient across endotracheal tubes is conventionally calculated to determine the distal pressure, a variable that is clinically more relevant than the proximal airway pressure because it identifies the force applied to biological tissue.

What new information this study adds
• In children under nasopharyngeal tube-anesthesia, the pressure drop across the tube is suitable for calculating oropharyngeal pressure using Rohrer's equation and its coefficients (K1 and K2).
• Rohrer's coefficient K2 increases under sevoflurane anesthesia and increases more for smaller than for larger tubes.
• Assessing oropharyngeal pressure will allow anesthesiologists, who use nasopharyngeal tube-anesthesia, to estimate the real pressure applied to the airway, a measure that unlike the proximal airway pressure reading is unaffected by small-diameter tube size.
In this study, we aimed to investigate whether characterizing the ΔP NPT for various tube sizes under various gas compositions would enable us to calculate oropharyngeal pressure from Paw and flow measurements in a physical model of NPT-assisted anesthesia. We assessed the accuracy of this method by comparing measured and calculated oropharyngeal pressure values and verified its feasibility in young children undergoing NPT-assisted anesthesia for diagnostic endoscopy.

| Experimental study
The method we used for calculating oropharyngeal pressure from

| Accuracy of calculated oropharyngeal pressure
To determine the accuracy of calculated oropharyngeal pressure, we calculated and directly measured oropharyngeal pressure in the simulated clinical scenarios previously described.
Oropharyngeal pressure was directly measured in the pharynx (1)

| Clinical studies
The study received hospital institutional review board approval.
Informed consent was obtained from parents or guardians.
Oropharyngeal pressure, Paw, and flow data were obtained from patients younger than 4 years undergoing NPT-assisted anesthesia for digestive or airway endoscopy. Five patients received inhalation anesthesia with 4% sevoflurane in pure oxygen (8% at induction).

| Analysis
We used the goodness-of-fit test (MATLAB ® software, version   Figure S1). The fitted curves yielded different slopes, and slopes steepened as the NPT diameters decreased and sevoflurane percentages increased.
Accordingly, Rohrer's coefficients K 2 increased with decreasing tube size and increasing sevoflurane concentrations (Table 1) (p < .0001 ANOVA).  (Table S1). Similarly, Paw mean values increased with increasing flow and sevoflurane (Table S1). Because the flow generator we used for CPAP was not "ideal," leaks slightly increased flow through the NPT and thus increased the pressure drop (Table S1).

| ΔP
During NPT-simulated anesthesia in the IPPV modality, ΔP NPT mean values increased with increasing Paw (15 cmH 2 O vs 25 cmH 2 O) and the smaller NPT sizes; large leaks also markedly increased ΔP NPT (Table S2).
3.1.3 | Oropharyngeal pressure changes related to NTP size, flow, sevoflurane, Paw, and leaks in the simulated clinical scenarios In our flow-regulated CPAP system, mean oropharyngeal pressure values increased with flow and sevoflurane concentrations, but were unaffected by NPT size (Table S1). During IPPV, oropharyngeal pressure increased with increasing NPT size and Paw, whereas it decreased with increasing leak size (Table S2).

| Oropharyngeal pressure calculation accuracy (RMSD)
Oropharyngeal pressure calculated in the clinical scenarios using the coefficients K 2 and K 1 generated using V′ exper approximated the measured oropharyngeal pressure well, as demonstrated by satisfactory RMSDs (<0.5 cmH 2 O) between calculated and measured maximum and minimum values during CPAP or IPPV (Tables S1 and   S2). Oropharyngeal pressure calculation accuracy slightly decreased with increasing flow and sevoflurane concentrations although RMSDs remained below 0.5 cmH 2 O (Tables S1 and S2).

| Clinical studies
In the five infants undergoing NPT-assisted anesthesia for endoscopy with 4%sevo/O 2 (8% at induction), the calculated and

| DISCUSS ION
These results show that the stepwise approach we used for characterizing ΔP NPT makes it easy to calculate oropharyngeal pressure in a clinical scenario of NPT-anesthesia using Rohrer's equation and its coefficients. This method proves accurate regardless of NPT size, flow, Paw, leak size, and gas composition. We also confirmed the method's feasibility in children undergoing NPT-assisted anesthesia for endoscopic procedures. In our study, K 2 increased by up to 50% at higher sevoflurane concentrations. Increased NPT resistance related to increased K 2 explains why Paw increased with sevoflurane, although a higher CPAP-expiratory valve resistance caused by sevoflurane may have contributed. Unexpectedly, oropharyngeal pressure also increased. If we assume that an ideal channel between the pharynx and the mouth allows flow heading toward the mouth to escape, we understand why a denser gas composition by exerting higher resistance increases the proximal pressure, that is, oropharyngeal pressure.

| Mathematical model
Our clinical scenario of NPT-anesthesia proved particularly suit-

| Clinical considerations
As pediatric procedural sedations continue to increase in number, Other than providing an exploratory glance at the research value of our oropharyngeal pressure calculation method, we try to offer physicians some clinical clues that could help them to determine oropharyngeal pressure in their daily practice using the measurements they have on hand. A common way to deliver oxy- can be plotted to estimate the mean pressure drop.
Following this practice, our residents are no longer worried about using high Paw values during NPT ventilation in emergency situations especially when the tube diameter is small, because they know that ΔP NPT is probably high and the resulting oropharyngeal pressure will therefore be far below the harm threshold. Similarly, when anesthesia is induced in difficult-to-intubate patients after carefully titrating sevoflurane in oxygen to maintain spontaneous respiration, having specific sevoflurane pressure-flow curves could help determine ΔP NPT more precisely using its homologous curve ( Figure S1). When using sevoflurane, the low flow we use to reduce the scavenging problem often elicits barely appreciable pharyngeal pressure probably unable to split the airway. In this situation, airway patency is better maintained using the jaw thrust maneuver throughout the procedure.
This study has limitations. We investigated few subjects Finally, our method's reliability depends on NPT patency. If the tube is obstructed, Paw becomes far higher than expected for the delivered flow, so that this discrepancy may be seen as a warning sign for NPT obstruction.
In conclusion, during NPT-assisted anesthesia, Paw readings on the anesthetic monitoring machine overestimate oropharyngeal pressure especially for smaller-diameter NPTs and higher flow, and to a lesser extent for large leaks. Given its clinical importance in guiding NPT ventilation in young children, we propose an accurate method for calculating oropharyngeal pressure using Rohrer's equation, or approximating clinically monitored flow and pressure values using ΔP NPT -flow relationship charts.

F I G U R E 5
A simulated infant undergoing nasopharyngeal tube (NPT)-continuous positive airway pressure (CPAP) with a 3.5 mm tube, through an anesthesia bag with the expiratory valve kept almost closed and connected to an anesthetic machine. Flow is set at 10 L/min. Paw is read on the screen. ΔPNPT is assessed by drawing a vertical line from 10 L/min on the X-axis to intercept the curve and then a horizontal line on the Y-axis shows ΔPNPT. Oropharyngeal pressure (OPP) is calculated as Paw minus ΔPNPT