Pressure recordings for monitoring tidal breathing and optimizing settings for nasal high-flow therapy
Rutger Hebbink is a PhD student in the department Engineering Fluid Dynamics. (Co)Supervisors dr.ir. R. Hagmeijer from the faculty of Engineering Technology, prof.dr. P.J. Wijkstra and dr. M.L. Duiverman from the University of Groningen.
Chronic obstructive pulmonary disease (COPD) is a slowly progressing respiratory disease, characterized by persistent respiratory symptoms and airflow limitation. Patients may suffer from acute worsening of symptoms called exacerbations. Severe exacerbations require costly hospitalization, increase the vulnerability to new exacerbations, and the recovery can be slow or even incomplete. The management of COPD therefore mainly aims at the prevention of exacerbations, but early and reliable prediction of exacerbations is difficult. The gold standard for assessing airflow limitation, and thereby diagnosing COPD and grading the severity of COPD, is spirometry. As spirometry may not be the perfect predictor of exacerbations and not easy to do at home, tidal spirometry, which measures restful instead of forced (maximal) breathing, may be an alternative. However, the lack of an accurate and easy-to-use measurement method restricts the current use of tidal spirometry, such that it is also fairly unknown.
Severe COPD exacerbations are commonly treated with non-invasive ventilation (NIV), but nasal high-flow therapy (NHFT) was rapidly adopted as an alternative due to its high patient comfort and easy application. A major problem, however, is the lack of clinical guidelines. A better understanding of the washout effect (clearance of anatomical dead space) and the generated pressures during NHFT in relation to the nasal cannula size and the NHFT flow rate can contribute to these guidelines. In addition patient monitoring is required to optimize settings to the individual.
The work of this thesis consists of two parts. The first part focuses on the development of a measurement method for using tidal breathing profiles to monitor respiratory health, which might be applied to early detect exacerbations. The second part focuses on measurements during NHFT, which may be used to improve the treatment of exacerbations.
First of all, a novel auto-calibrating method is presented that allows for the reconstruction of scaled tidal flow-volume curves from pressure recordings obtained via a nasal cannula. The method was experimentally validated by feeding realistic healthy and unhealthy breathing patterns to anatomically correct 3D printed upper airways of an adult and an infant. Comparison of the reconstructed flow-volume curves and the actual flow-volume curves demonstrates a very high level of accuracy.
Secondly, the relation between tidal and forced spirometry is investigated to facilitate the clinical use of tidal flow-volume curves. Both tidal (novel method) and forced (gold standard) spirometry is measured in a group of healthy subjects and patients with a variety of lung diseases. The Tiffeneau-Pinelli index is used to characterize the forced spirometry test. Based on these measurements, a set of reference flow-volume curves is established. Approaches are then introduced to (a) obtain the 'expected shape of tidal flow-volume curves' for a given value of the Tiffeneau-Pinelli index, and to (b) obtain the 'expected Tiffeneau-Pinelli index' for a given shape of the tidal flow-volume curve.
The shape of tidal flow-volume curves in different patients with similar values of the Tiffeneau-Pinelli index appears to vary considerably, but the average shape of these tidal curves, called the expected shape, varies in a characteristic way with varying index, leading to an objective ranking of tidal curves. Focusing on the expiratory part of the tidal curves with decreasing index, the peak flow rate shifts from the middle to the left, and the remaining part transforms from convex to concave. The expected Tiffeneau-Pinelli index can be used to classify tidal flow-volume curves based on their shape. The clinical value of the expected index in the prediction of exacerbations remains to be established.
Next, by combined use of the lattice Boltzmann method (LBM) and a large eddy scale (LES) model, the flow during a realistic, transient, breathing profile in an adult upper airway is revealed. Pressure data is compared to experiments in a 3D print of the same geometry. At peak inhalation and exhalation the simulations and experiments show some deviations in regions of turbulent flow, but on average simulations and experiments are in good agreement. This shows that combined LBM/LES is an adequate tool for simulating flows in the human nasal airway. The method may therefore be applied to reveal the flow dynamics of nasal high-flow therapy (NHFT), but the higher Reynolds numbers increase the required computational effort.
The washout of anatomical dead space and the provision of positive end-expiratory pressure (PEEP) during NHFT are studied by measuring the pressure distribution in 3D printed upper airway geometries of an adult, an asymmetric infant, and a symmetric infant. It appears that, without respiration, the NHFT jet penetrates into or slightly beyond the nasal cavity, depending on airway anatomy, but hardly depending on cannula size or NHFT flow rate. PEEP is approximately proportional to the square of the flow rate. The level of PEEP generated further depends on the inner diameter of the cannula (dynamic pressure of the NHFT jet) and the outer diameter of the cannula in relation to the diameter of the nares (occlusion rate). A high nostril occlusion rate is shown to largely increase inspiratory and expiratory resistance.