Measurement of intraoral pressure in preterm and term infants during non-invasive ventilation

The number of premature babies is almost 10% of all newborns and continues to rise. The lungs in these children are not yet fully developed, which is why many of them, especially the very immature children (< 28 weeks of pregnancy), require some form of respiratory support during their stay in intensive care. There are various methods that are used regularly for this purpose. Invasive and non-invasive forms of ventilation are used. Invasive forms of ventilation are more often associated with bronchopulmonary dysplasia (BPD) resulting from oxygen toxicity and above all from baro- and volumotrauma than non-invasive forms of ventilation. These include the application of continuous positive airway pressure (nCPAP) and high-flow nasal cannula therapy (HFT). While nCPAP was able to significantly reduce the rate of BPD, HFT also reduced the frequency of mucosal injuries and necrosis of the nose and of pneumothoraces. Therefore, if possible, HFT is preferably used, it is also better tolerated by the patients and their parents because of the more flexible hose systems and the smaller prongs. Although HFT is widely used around the world, its application is not standardized. The biggest problem is that the applied pressure cannot be set or read directly. To an extent that has not yet been clearly defined, the applied pressure is mainly influenced by the set flow, but also by other factors such as a leak in the nose, the size of the child's face, and the weight and maturity of the child. Therefore, the aim of the present prospective study was to measure the pharyngeal pressure and to investigate the factors influencing the pressure in HFT in more detail. In addition, a possibility was to be found to find the most accurate possible prediction for everyday clinical use. For this purpose, pressure measurements in the pharynx were carried out on 24 children in the period from January to July 2021. This was done using a suction probe commonly used in neonatology, which was connected to a computer via a pressure gauge. This setup made it possible to carry out real-time data acquisition. During the measurements, the length of the suction catheter, the position of the mouth (open or closed) and the set flow were noted continuously. In addition, other factors such as birth weight, the current week of pregnancy, the dimensions of the face, the size of the ventilation end piece (prongs) and the size of the nostril were noted. The latter was determined in millimeter increments using a self-designed measurement model, since no suitable method existed for this until now. The measurements showed that the pressure in the pharynx measured just before the entrance to the larynx correlated highly significantly with the applied flow. The outer diameter of the prongs was also positively related to pressure, and the air leak around the prongs, the distance between ear and nose and ear and chin, age and respiratory rate were negatively related to pressure. Other factors that, unlike other previous studies, did not show a significant association were birth weight and weight at the time of measurement, and gestational age at birth. However, further statistical analyzes such as multiple regression were not possible due to the structure of the data and the small number of subjects, which reduces the statistical significance of the study. The results showed that flow was the determining factor on the pressure achieved under HFT. A formula could be derived with which the pressure could be estimated: pressure [mbar] = 0.483 * flow [0.375-0.538]. Even if an exact prediction of the pressure with the help of a multiple linear regression was not possible and other significant factors could not be included, the formula could be converted into the following rule of thumb for everyday use: Pressure [mbar] = 0.5*flow [l/ minutes]. Even if this calculation does not enable an exact prediction of the pressure, the setting of the applied pressure can be estimated in everyday clinical practice. This is especially true when trying to achieve comparable pressure when switching from another form of ventilation such as nCPAP to HFT. It could also be shown that a minimum flow of 3 l/min is required for the HFT in order to be able to achieve a pressure of at least 1 mbar. Further studies to predict the applied pressure should be designed by optimizing the measurement method in such a way that as many patients as possible are included. In addition, as shown in the current study, special attention should be paid to the leakage between prong and nostril and the influence of maturity and weight should be verified.