1. Introduction
Complications related to preterm birth are the leading cause of death in children under five years of age. According to data of the World Health Organization (WHO), every year an estimated 15 million babies are born preterm, i.e., before the first 37 weeks of pregnancy. These babies present complications due to the immaturity of their lungs, resulting in a lack of pulmonary surfactant in the respiratory tract and the appearance of the so-called Respiratory Distress Syndrome (RDS) of the newborn. This natural substance, generated from week 25 of pregnancy approximately, plays an important role in increasing the pulmonary compliance and preventing the collapse of the lung at the end of the expiration.
The surfactant replacement therapy employed currently consists of the instillation of exogenous surfactant and the application of mechanical ventilation. Even though this procedure provides good results, it is an invasive technique that may lead to deep lung and cerebral injuries, even with short terms of mechanical ventilation [
1,
2]. For that reason, over the last few years, research has been focused on the development of less invasive surfactant administration (LISA) techniques, by which natural surfactant can be delivered without the need of tracheal intubation [
3]. Despite the recent advances in the development of new synthetic surfactants, as the lucinactant, the guidelines still recommend the application of animal-derived ones [
4]. A survival advantage was noted when using a porcine lung surfactant dosage of 2.5 mL/kg with respect to 1.25 mL/kg of bovine lung surfactant, although it is uncertain if this was a consequence of the dose difference or of the composition of each type of surfactant [
5].
Within the different LISA techniques, aerosolization emerges as a promising choice to alleviate the effects of the RDS in preterm babies. However, many issues have still to be overcome. Four clinical trials [
6,
7,
8,
9] showed the safety and feasibility of this technique, but only the work of Jorch et al. [
7] provided an improvement of respiratory parameters and the avoidance of mechanical ventilation and endotracheal intubation. Pillow and Minocchieri [
10] are currently working on a randomized controlled trial where the infant receive aerosolized surfactant in the first hour of life. Their goal is to analyze the need for intubation in the first 72 h of life and the duration of mechanical ventilation in this period. The main issue to overcome with the aerosolized surfactants is the small proportion reaching the lungs. Kohler et al. [
11] made a comparison with jet and ultrasonic nebulizers obtaining a lung deposition not higher than 1% of the initial dose.
Perfluorocarbons (PFCs) have shown great potential to treat pulmonary failure in neonatal, pediatric, and adult animal models, reaching clinical trials. PFCs have been investigated in the setting of acute pulmonary failure because of their low surface tension (surfactant action), a relatively high solubility of respiratory gases, and a high density. Kandler et al. [
12] studied the effect of aerosolized PFC in a surfactant-depleted piglet model, showing an improvement of oxygenation and pulmonary gas exchange. The subsequent work of von der Hart et al. [
13] supported these results and confirmed the suitability of different PFCs for aerosol treatment. Burkhardt et al. [
14] confirmed that the administration of an emulsion of surfactant and PFC contributes to obtain a more homogenous distribution and an improvement in oxygenation. Murgia et al. [
15,
16] studied the suitability of intratracheal inhalation catheters to produce the aerosolization of surfactant or PFCs during different mechanical ventilation strategies. Goikoetxea et al. [
17] analyzed the feasibility of delivering aerosolized surfactant and PFCs beyond the third generation of branching in a preterm infant airway model by means of an inhalation catheter. Recently, Syedain et al. [
18] developed a novel aerosol generator to deliver surfactant in preterm infants with a low air flow, ideal to prevent potential lung injuries.
The definition of numerical models by means of Computational Fluid Dynamics (CFD) tools provides a helpful methodology to analyze several health issues. Oldham et al. [
19] studied the influence of parameters such as the breathing rate and the particle size on particle deposition in the respiratory tract. Kleinstreuer et al. [
20] made a review of inhaled toxic aerosols in cigarette smoke, marking the importance of CFD simulation models as a promising research tool to study all physical mechanisms involved in the deposition and transport of toxins in models of the respiratory system. Koombua et al. [
21] explored the influence of airway wall elasticity analyzing two cases, rigid and flexible wall, respectively. It was observed that the pressure within the airways was affected but not the air flow velocity or the wall shear stress. Feng and Kleinstreuer [
22] studied the deposition, interaction, and transport of particles in triple bifurcations, by means of the dense discrete phase model and the discrete element method. Recently, Elcner et al. [
23] developed a CFD model validated with experimental results of the inspiratory airflow in a model from the throat to the fourth generation of the respiratory tract.
Nevertheless, most of the CFD literature is based on the geometries of the adult airways. The smaller size of the preterm infants’ airways and the difference of breathing conditions affect parameters such as the air flow velocity and particle transport and, therefore, do not make it possible to extrapolate the results obtained from adult airways. In the treatment of respiratory diseases, Longest et al. [
24] created a CFD numerical model to evaluate the deposition patterns in the airways of a four-year-old child under healthy and constricted conditions. Liu et al. [
25] studied, with a CFD model, the airflow patterns and the deposition of particles in the first three generations of a pediatric upper respiratory tract. Their results showed higher velocity fields, deposition rates, and impaction numbers in comparison with those observed in adults. De Jongh et al. [
26] obtained, by means of computed tomography, the geometry of the upper airways of a nine-month-old child to calculate the deposition of microparticles and compared them with experimental results. The aerosolization of surfactant was also analyzed with a CFD model by Goikoetxea et al. [
27] using an inhalation catheter with minimal manipulation of the airways.
In this article, an experimental and numerical model of the aerosol produced with an inhalation catheter has been developed. Two perfluorocarbons compounds are analyzed, PFD and FC75, at driving pressures of 4–5 bar, and the numerical results are compared with experimental data.
2. Experimental Setup
The experimental model to produce the aerosol was carried out with an inhalation catheter (IC), as shown in
Figure 1. It consists of a central lumen, where the liquid to be aerosolized is delivered, and six outer lumens where compressed air is dispensed. The diameter of these lumens becomes smaller as it approximates to the catheter tip, resulting in an increase of the air velocity. This high air velocity, along with the closeness of the lumens, leads to the aerosolization of the fluid. In the present study, two different perfluorocarbons (PFC), perfluorodecalin (PFD; density = 1.95 g/mL) and FC75 (density = 1.78 g/mL) were used to produce aerosols with driving pressures of 4–5 bar.
The aerosol parameters were measured by an Aerodynamic Particle Sizer (APS) spectrometer, which provides high-resolution measurements of aerosol particles from 0.5 to 20 µm. It uses a patented double-crested optical system for unmatched sizing accuracy and, by means of two laser beams, it generates a signal when a particle passes through them, as illustrated in
Figure 2. The acceleration of particles, that will be smaller for bigger particles because of their larger inertia, is obtained from the time between the peaks of the signal, known as time-of-flight. Then, with the sphere calibration stored in the spectrometer memory, the APS converts each time-of-flight measurement to its corresponding aerodynamic particle diameter. For a particle, the aerodynamic diameter is described as the diameter of a spherical particle with a density of a water droplet (1000 kg/m
3) that has the same settling velocity as the particle.
Figure 3 illustrates the experimental setup used to measure the aerosol properties on the basis of the study of Aramendia et al. [
28]. A pressure regulator was used to provide compressed air to both the liquid chamber and the connection directed to the outer lumens of the IC. The distance between the catheter tip of the IC and the inlet nozzle of the APS was checked in order to obtain the average 1000 particles/cm
3 particle concentration recommended by the manufacturer. The APS measures and classifies the particles in four events according to their aerodynamic diameter. The first one groups the particles with a diameter smaller than 0.5 µm, the second one classifies the particles that are in the spectrometer measuring range from 0.5 to 20 µm, the third event considers those particles that cross the laser beams at the same time, and the last event catches particles bigger than 20 µm. Thus, in order to get accurate measurements, it is important to get the vast majority of the particles classified in the second group. All the information and parameters captured by the APS were recorded and stored by the software associated with the hardware.
3. Experimental Results
The compounds PFD and FC75 were tested at driving pressures of 4–5 bar, while varying the distance between the APS nozzle and the catheter tip. The results are summarized in
Table 1,
Table 2,
Table 3 and
Table 4. A sample time of 10 s was set, recording five samples for each pressure and distance. The optimal distance was achieved taking into account three criteria. Firstly, that most of the particles measured were classified in the first and second event of the APS, ensuring that the particle coincidence or recirculating particles did not affect the accuracy of the measurements; secondly, that the particle concentration was within the recommended range according to the APS manufacturer guideline; finally, that a minimum of five samples were obtained in this previous conditions for each pressure and compound. It was found that the optimal distance was 60 mm and 52 mm for PFD and FC75, respectively. Initially, for FC75, measurements were taken at higher distances, however, a very low particle concentration was observed that could be explained by the high volatility of the compound. Additionally, measurements at 30 mm and 46 mm were taken for the PFD compound to compare the results at different distances.
Three parameters were analyzed to study the aerosol behavior. Firstly, we analyzed the geometric standard deviation (GSD), which is an indicator of the spread of an aerodynamic particle size distribution. Values below 1.25 denote an aerosol formed by particles of approximately the same size, whereas values above 1.25 indicate an aerosol made up of particles with different sizes. Another important parameter is the mass median aerodynamic diameter (MMAD), which measures the aerodynamic diameter at which 50% of the aerosol mass will be present in particles below this value. Finally, the geometric particle diameter (D
g) was analyzed. This value can be obtained with the aerodynamic diameter (D
a) measured with the APS by the expression given by Equation (1):
where
ρ is the density of the PFC,
ρ = 1.95 g/cm
3 for PFD and
ρ = 1.78 g/cm
3 for FC75, respectively, and
ρ0 is the unit density, 1 g/cm
3.
These experimental results were used subsequently to validate the numerical model. Furthermore, measurements at the nozzle level (h = 0) were taken to define the initial conditions of the injectors that will represent the population of the particles in the numerical model. The average particle concentration measured at this point was higher than the recommended. However, the vast majority of them were grouped in the second event, which measures the particles within the range of the APS from 0.5 to 20 µm.
The PFD and FC75 aerosolization rate (AR) was measured by means of the Equation (2).
where 0 correspond to the instant before and
t to the instant after the aerosolization pulse,
mcam is the mass of PFC within the liquid chamber,
ρ is the density of the PFC, and
tpulse is the time of the aerosolization pulse.
An AR of 0.272 mL/min and 1.066 mL/min was obtained with a pressure of 4 bar for PFD and FC75, respectively. Similarly, the AR corresponds to 0.303 mL/min and 1.313 mL/min for a pressure of 5 bar. As it was expected, an increase of the pressure resulted in higher AR values.
5. Numerical Results and Experimental Validation
Firstly, the Navier–Stokes and continuity equations were solved for the continuous phase in steady state. The results of the converged solution were saved and subsequently used as the initial condition for the transient simulation of both phases, i.e., the airflow and the parcels representing the discrete phase.
The solution was considered converged with a three-order-of-magnitude drop in the numerical residuals. Furthermore, an axial velocity variation of 8.7 × 10
−3% in the last 1000 iterations confirmed the stability of the solution, as shown in
Figure 8. A Courant number of 50 was defined, and the under-relaxation factor of the two-way coupling model was decreased to 0.1.
The solution of the continuous phase is illustrated in
Figure 9 with the airflow velocity fields.
Figure 9a,b corresponds to the velocity field with a driving pressure of 4 bar and of 5 bar, respectively. As expected, the largest values of the axial velocity were observed close to the catheter tip with 391 m/s and 455 m/s for pressures of 4 and 5 bar, respectively. The velocity in the injectors’ position was 247 m/s and 293 m/s for 4 and 5 bar of pressure, respectively. These values were checked in order to define the initial velocity condition of the droplets in the injector points. The high air velocity values obtained confirm the importance of considering the airflow as a compressible flow in this application.
The solution of the discrete phase is presented in
Figure 10 with the formation of the aerosol cone. The solution was taken 0.06 s after the release of particles from the injectors. The transitory condition of the aerosol flow changes the distribution of particles with time. For that reason, it was necessary to track a representative sample of 0.01 s in order to study the numerical results.
The axial velocity results with a pressure of 4 bar were monitored at several distances from the catheter tip and validated with the experimental results obtained by Goikoetxea et al. [
37]. The turbulence model along with the mesh refinement generated close to the catheter tip led to an accurate prediction of the airflow velocity values, as shown in
Figure 11.
The results of the cumulative mass distribution as a function of the droplet size (
Ydg) are presented in
Figure 12 and
Figure 13 for PFD and FC75 compounds, respectively, and were compared with the experimental data obtained by means of the APS. Some differences were found between the numerical and the experimental results. The collision and coalescence of the particles may be the cause of those deviations. In the numerical model, which considered the breakup of particles with the TAB breakup model but not the coalescence, a higher cumulative mass fraction was obtained for particles above 3 µm for the PFD compound at both pressures and for each distance. For the FC75 compound, with a pressure of 4 bar, the cumulative mass fraction was higher for particles below 5 µm and considerable lower for particles above 10 µm, whereas with a driving pressure of 5 bar there was a different pattern in the experimental results, and the cumulative mass fraction for particles above 10 µm was considerable higher.
6. Conclusions
This article presents the generation of a PFC aerosol by means of an inhalation catheter and its main parameters measured with an Aerodynamic Particle Sizer (APS) and CFD tools. Different cases were studied for PFD and FC75 varying the driving pressure of the compressed air between 4 and 5 bar. The results were measured at the optimal distances between the APS nozzle and the catheter tip and at some additional distances in the case of PFD. The GSD values varied from 1.61 to 2.04, leading to the formation of heterodisperse aerosols. The aerodynamic diameter (Da) provided values that were within the recommended range values (1–5 µm) and, although the MMAD results were between 5–10 µm, we must take into account that the goal of this study was the generation of an aerosol beyond the nasopharyngeal area, avoiding in that way the deposition of these bigger particles.
Subsequently, the numerical model was introduced in order to simulate the generation of the PFD and FC75 aerosols with CFD techniques. These numerical simulations were validated with the axial velocity results obtained experimentally with a hot-wire anemometer, confirming their accuracy at different downstream positions. The differences in the cumulative mass distribution of the aerosol particles between the numerical and experimental procedure suggest that the collision and coalescence of the PFC droplets play an important role during the formation of the aerosol. Similar behavior could be seen for PFD at both driving pressures, whereas for FC75, the cumulative mass fraction for particles above 10 µm was considerable higher with a driving pressure of 5 bar.
The numerical study presented in this work could be a preliminary tool to facilitate parametric studies of optimal inhalators in preterm babies with RDS.