Adjustable breathing resistance for laryngectomized patients: Proof of principle in a novel heat and moisture exchanger cassette.

BACKGROUND: Due to the heat and moisture exchanger's (HME) breathing resistance, laryngectomized patients cannot always use an (optimal) HME during physical exercise. We propose a novel HME cassette concept with adjustable "bypass," to provide adjustment between different breathing resistances within one device.
METHODS: Under standardized conditions, the resistance and humidification performance of a high resistance/high humidification HME (XM) foam in a cassette with and without bypass were compared to a lower resistance/lesser humidification HME (XF) foam in a closed cassette.
RESULTS: With a bypass in the cassette, the resistance and humidification performance of XM foam were similar to those of XF foam in the closed cassette. Compared to XM foam in the closed cassette, introducing the bypass resulted in a 40% resistance decrease, whereas humidification performance was maintained at 80% of the original value.
CONCLUSIONS: This HME cassette prototype allows adjustment between substantially different resistances while maintaining appropriate humidification performances.

INTRODUCTION

Heat and moisture exchangers (HMEs) are used as a standard treatment for pulmonary rehabilitation after a total laryngectomy. , , , , Normally, the upper airways condition (heat and humify) the inhaled air, but in laryngectomized patients the lungs are exposed to the dry and cold air during open stoma breathing. An HME covering the stoma can to some extent improve the pulmonary condition. The benefits of HME use have been underlined in many studies; it does not only improve the pulmonary functioning, such as a decrease in mucus production, coughing, and forced expectorations, but also the psychosocial functioning of laryngectomized patients. , , , , , , Laryngectomized patients are recommended to continuously use an HME with the highest possible humidification performance (the highest water exchange). ,

The humidification performance of the HME, and thus its benefits, rely mainly on the HME core material and cassette design. The HME core material often consists of a porous polymer foam impregnated with hygroscopic salt, which acts as a condensation and evaporation surface. , , Since the HME is a passive humidifier, its humidification performance can primarily be improved by increasing the width and height of the core material or decreasing the foam's pore size. Increase of width and height are limited by aesthetic considerations. Additionally, these performance improvements have a trade off with the HME's breathing resistance and consequently patient acceptance. To cater to the different patient needs and activity levels, multiple types of HMEs have been developed, which vary in resistance and performance. ,

Nevertheless, complete HME compliance has not yet been achieved in all laryngectomized patients. Laryngectomized patients discontinue their (high humidification performance) HME use due to the higher breathing resistance of the HME compared to open stoma breathing, especially periodically during physical activities. , , , , , , Other reasons for laryngectomized patients to discontinue their HME use, outside the scope of this study, include: adhesive related skin irritation, mucus problems or the HME's aesthetics. , , , , , Although physical exercise can sometimes be anticipated, changing between different HME types with varying breathing resistance is not always an option or requires additional effort and preparation. , As a result, some patients do not use any HME at all.

Patient compliance and comfort during different levels of physical activities could potentially be improved by providing one HME device that enables a quick and simple adjustment of the breathing resistance based on the patient's activity level. During rest, a laryngectomized patient can use the HME device with a higher resistance and humidification performance setting. Alternatively, during physical activities the HME device can be adjusted to decrease its resistance, while maintaining an appropriate humidification performance.

We propose a novel HME cassette concept with an adjustable “bypass” at its base. In this study, we designed and tested this adjustable HME cassette prototype to validate that it will result in substantially different breathing resistances with appropriate humidification performances for each level of activity.

MATERIALS AND METHODS

HME devices and prototype

In this study, we used two types of HME foams taken from the two most commonly used HMEs at the Netherlands Cancer Institute – Antoni van Leeuwenhoek: the Provox® XtraMoistTM HME (XM) and the Provox® XtraFlowTM HME (XF, both Atos Medical AB, Malmö, Sweden). An overview of the specifications of the Pressure Drop and Moisture Loss, and of the measurements of the Water Exchange of the XM and XF are given in Table 1. Water Exchange is a direct measure of the humidification performance. The XM is one of the highest performing commonly used HMEs. The XF is considered to be an HME with an “acceptable” breathing resistance by the majority of the laryngectomized patients, unable to (continuously) tolerate the higher breathing resistance of the XM. , However, the XF has a lesser humidification performance compared to the XM. The HME cassettes of the XM and XF are identical: the differences in breathing resistance and performance are due to the difference in core material (Figure 1).

TABLE 1

Specifications of the Moisture Loss and Pressure Drop values of the Provox XtraMoist (XM) and Provox XtraFlow (XF), as provided by the manufacturer (Atos Medical AB, Malmö, Sweden) in accordance with ISO 9360‐2:2001, and the humidification performance (Water Exchange) as reported by previous studies.

	Pressure Drop (Pa)			Moisture Loss a (mg/L)	Water Exchange (mg) Van den Boer et al. (2014a) 14	Water Exchange (mg) Van den Boer et al. (2014b) 21
HME	At 30 L/min	At 60 L/min	At 90 L/min	At V T = 1 L (AHamb‐ref = 0 mg/L)	At V T = 0.5 L (AHamb‐ref = 5 mg/L)	At V T = 0.5 L (AHamb‐ref = 5 mg/L)
Provox XtraMoist	70	240	480	21.5	3.61	3.63
Provox XtraFlow	40	130	290	24.0	2.89	1.95

Note: The pressure drop of the XF at a flow of 60 L/min is approximately 60% of that of the XM. The humidification performance (Water Exchange) of the XF shows relatively less decline: approximately 80% of that of the XM.

Abbreviations: AH, chosen reference value for ambient humidity; HME, heat and moisture exchanger; ISO, International Organization for Standardization; V, tidal volume.

The lower the moisture loss value, the better the HME's humidification performance.

FIGURE 1

The photo shows, from left to right, the original HME cassette of both the XF and XM with speaking valve (pink lid), the 3D‐printed (FormLabs, Form2) closed cassette with inserted XF foam and the 3D‐printed (FormLabs, Form2) cassette with bypass on the tracheal side, with inserted XM foam (note the difference in pore size between the two different foams). A speaking valve was not included in the 3D printed cassette designs to simplify the prototyping and to limit the scope of this proof of principle study to only the effect of the bypass. The thicker cylinder at the base of the 3D‐printed cassettes is used to connect them to the measurement set‐up (spirometer). HME, heat and moisture exchanger; XF, lower resistance/lesser humidification HME; XM, high resistance/high humidification HME [Color figure can be viewed at wileyonlinelibrary.com]

In this study, we use the pressure drop as a measure for resistance (in Appendix A, the mathematical relationship between pressure drop, flow and resistance can be found). Water Exchange, the amount of water an HME evaporates during inhalation and condensates during exhalation, is used as a measure of humidification performance.

The high breathing resistance of an HME can be reduced by introducing a relatively simple “bypass” in the HME cassette, or a simple hole in the HME foam (see Appendix A). A bypass functions as a “shortcut” for the airflow and will therefore decrease both resistance and humidification performance. Due to the almost quadratic relationship between flow and resistance (Appendix A), a bypass reduces the HME's breathing resistance considerably more than its humidification performance.

A bypass should be designed which can easily be opened or closed and does not interfere with the HME's speaking valve. Additionally, it is desirable that this specific bypass can modify an XM‐like HME into an HME with the properties comparable to an XF. Therefore, the following 3D‐printed (FormLabs, Form2) HME cassette designs were used as a prototype in this study: two simplified closed straight cylindrical cassettes without a speaking valve, Figure 2a,b (further on called the “closed cassette”‐type), and a similar cassette with an opened bypass at its tracheal side, Figure 2c (further on called the “cassette with bypass”‐type). The bypass consists of eight holes with a diameter of 4 mm, distributed evenly around the cassette's base, which can quickly and easily be opened or closed by adjusting a “twist‐ring” (compare Figure 2b and 2c, similar to the “twist‐ring”‐concept as seen on salt shakers, Figure 2d). This specific bypass configuration was chosen such that the resistance of the XM foam, when the bypass is opened, drops to the breathing resistance similar to the breathing resistance of an XF foam in the closed cassette. The dimensions of the cassettes were chosen such that the cassettes closely fitted the HME foams.

FIGURE 2

The two HME cassette types. A, Design of the closed cassette for the XF foam measurements. B, Design of the closed cassette for the XM foam measurements. The bypass on the tracheal side of the cassette is closed off with a “twist‐ring.” C, 3D‐design of the cassette with opened bypass for the XM foam measurements. The specific bypass consists of eight d = 4 mm holes at the base of the cassette and can be opened or closed by adjusting the “twist‐ring.” D, “Twist‐ring” concept as seen on salt shakers. The bar at the base and the two small holes at the top of the cassettes, intended for inserting a pin, keep the HME foam in place during the measurements. The thicker cylinder at the base of the 3D‐printed cassettes is used to connect them to the measurement set‐up (spirometer). HME, heat and moisture exchanger; XF, lower resistance/lesser humidification HME; XM, high resistance/high humidification HME

Equipment

The pressure drop (a measure of the HME's breathing resistance) of the HME devices was assessed with a digital pressure indicator (DPI 705, BHGE Druck, Houston, Texas) at different airflow rates of 30, 60, and 90 L/min in correspondence with the ISO standards (see Table 1), representing approximately breathing at rest and during light and strenuous exercise.

Performance measurements, measuring the HME's Water Exchange, were executed as validated by van den Boer et al. (2013 and 2014). , The measurement protocol was slightly adapted to fit the objectives of this study (see Study design). Summarizing, a healthy volunteer breathes through a spirometer set‐up with a standardized breathing pattern, with the HME device connected to a coupler on the other side of the spirometer (Flowhead MLT300 AD Instruments GmbH, Oxfordshire, United Kingdom). First, the HME is conditioned toward its equilibrium water saturation (duration of conditioning is determined separately for each HME). After this initial conditioning, a sequence of weight measurements is conducted, alternating at the end of an inhalation and the end of an exhalation, to determine the HME's Water Exchange. The weight changes of the HME device are measured using a microbalance (Sartorius MC210p, Göttingen, Germany). The HME foam is reconditioned for at least five breathing cycles between each weight measurement. During the measurement sequence, the ambient humidity and temperature of the room are recorded by a commercial humidity sensor (Testo BV, Almere, The Netherlands) to perform data normalization. At the start and end of each measurement sequence, the ambient humidity and temperature of the room is additionally monitored with a hygrometer (Philips Thermo + Hygro, Eindhoven, The Netherlands) and digital thermometer (ThermaLite Digital, E.T.I. Ltd., Worthings, UK) and the temperature of the volunteer is measured with an electronic ear thermometer (Braun WelchAllyn, Kaz Inc., Marlborough, Massachusetts). In this set‐up the volunteer functions as an “artificial lung”. The temperature of the volunteer is used for normalization (see Analysis). The volunteer was asked to breath in a fixed rectangular breathing pattern, which is guarded by the spirometer.

Study design

For this study, resistance (Pressure Drop) and humidification performance (Water Exchange) measurements were conducted for 10 XM foams (one batch, batch year: 2019) and 15 XF foams (three batches, batch years: 2017, 2018, and 2019) inside the two different cassette types: both the XF and XM foams in the closed cassette and the XM foams in the cassette with the bypass (Figure 2). All performance measurements were performed by one healthy volunteer (female, 27 years old, ML) for one breathing pattern under room climate conditions. A tidal volume (V ) of 1 L and target flow of 0.33 L/s was chosen, which was a comfortable breathing pattern for the volunteer and corresponds to the ISO standards (see Table 1). After initial conditioning of the HME foam, a sequence of 15 weight measurements was conducted (starting and ending with an exhalation). This resulted in 13 weight changes per HME since the first measurement was disregarded to account for differences in conditioning periods between the HME devices.

Analysis

All performance measurements were normalized to the reference ambient humidity of 5 mg/L and a reference humidity at the tracheal side of 32 mg/L (see Appendix B). An independent sample t test was conducted using IBM SPSS Statistics 25 (SPSS, Chicago, IL) to compare the average performances of the different HME devices.

RESULTS

An overview of the average resistance (Pressure Drop) and the humidification performance (Water Exchange) of all XF and XM foams in the two different HME cassette types are shown in Table 2 and Figure 3.

TABLE 2

Overview of the average resistance (pressure drop) and normalized humidification performance (water exchange) of the XM and XF foams in the two different cassette types.

HME device		Pressure Drop in Pa (SD)			Water Exchange in mg (SD)
HME foam type	HME cassette type	At 30 L/min	At 60 L/min	At 90 L/min	At V T = 1 L, F = 0.33 L/s, AHamb‐ref = 5 mg/L, and AHts = 32 mg/L
XM foam	Closed cassette	50 (2)	158 (7)	325 (13)	5.70 (0.42)
	Cassette with bypass	29 (1)	95 (5)	201 (11)	4.77 (0.40)
XF foam	Closed cassette	26 (1)	93 (3)	196 (4)	4.91 (0.35)

Note: The tidal volume (V ) and airflow rates of the pressure drop measurements correspond to the ISO standards (see Table 1). The different airflow rates of 30, 60, and 90 L/min represent approximately breathing at rest and during light and strenuous exercise. The SDs of the Water Exchange measurements of the HME devices are comparable to those previously reported by van den Boer et al. (2013). For the XF foam, a weighted mean and SD were calculated to represent the three different batches in equal proportion.

Abbreviations: AH, reference ambient humidity; AH, reference humidity at the tracheal side of the HME; F, flow; HME, heat and moisture exchanger; V, tidal volume; XF, lower resistance/lesser humidification HMESD, standard deviation; XM, high resistance/high humidification HME.

FIGURE 3

Resistance (Pressure Drop at 60 L/min) against normalized humidificationperformance (Water Exchange at V = 1 L) of the different HME devices. The horizontal and vertical error bars indicate the standard deviations from the average Resistance and Water Exchange, respectively. Abbreviations: HME, heat and moisture exchanger; XF, lower resistance/lesser humidification HME; XM, high resistance/high humidification HME; V, tidal volume [Color figure can be viewed at wileyonlinelibrary.com]

In the closed cassette, the average pressure drops and Water Exchange values of the XM foam are higher than that of the XF foam. When the bypass was introduced in the XM foam's cassette, the pressure drop of the XM foam decreased to a pressure drop similar to the XF foam in the closed cassette. The average Water Exchange of the XM foam in the cassette with bypass was slightly lower than the average Water Exchange of the XF foam in the closed cassette (not significant, P > .05). Compared to the XM foam in the closed cassette, the bypass resulted in pressure drop of approximately 60% the original pressure drop value, thus a 40% decrease in resistance, whereas the humidification performance was maintained at approximately 80% of the original Water Exchange value of the XM foam.

DISCUSSION

This proof of principle study shows that introducing a bypass in the base of an HME cassette can substantially decrease the resistance of a high resistance/high humidification HME (XM) foam to the lower breathing resistance of a lower resistance/lesser humidification HME (XF) foam in the closed cassette, while humidification performance stays at an acceptable level.

Intuitively, one would expect that creating holes in an HME cassette (which lets the air bypass the HME's foam) will decrease the HME's resistance and consequently its humidification performance to a level where the HME will become “useless” for the pulmonary rehabilitation of laryngectomized patients. However, both the theory stating the (almost) quadratic relationship between pressure and flow (Appendix A), as the results of this study indicate that a bypass will decrease the resistance much more than the humidification performance. Additionally, careful examination of existing HMEs shows that the cassettes already (coincidentally) have “bypasses” in their designs and still these HMEs have good Water Exchange values. For example, the Provox® Luna® HME (Atos Medical AB, Malmö, Sweden) clearly has two side openings acting as “bypasses.”

In this proof of principle study, we used a cassette without speaking valve. However, cassettes without a speaking valve are nowadays often not acceptable to patients with a voice prothesis. In Appendix B.4, Table B2, a comparison is made between the performance measurements found in this study (Table 2), with the humidification performance values of with the HMEs with speaking valve found by van den Boer et al. (2014a, 2014b) and the manufacturer's specifications (Table 1). , Additionally, unpublished experiments' results were included in Table B2, performed in the Netherlands Cancer Institute – Antoni van Leeuwenhoek during the past 3 years. The humidification performance results with and without speaking valve are very similar. Therefore, we predict that a final prototype with speaking valve will have a similar clinically acceptable humidification performance. The assessment of the user functionality and compliance, important device considerations for a final prototype with speaking valve, requires the support of a manufacturer and was outside the scope of this study. Such a study with laryngectomized patients, in which the effectiveness of the final prototype is evaluated, is recommended as the next step.

This proof of principle shows that an adjustable HME is feasible. Such an HME would have several important advantages. In the first place, it can be used by the laryngectomized patients to modify the breathing resistance, which eliminates the need to remove or switch HME types based on activity level. Even if the novel HME cassette is used solely on the lowest resistance setting, it still has a clinically acceptable humidification performance similar to an XF. If laryngectomized patients are not able or willing to switch HMEs, an adjustable HME enables a lower breathing resistance during physical activity and an optimal HME with a higher breathing resistance during nonstrenuous activities. Furthermore, since clinically acceptable breathing resistance does not only vary between physical activity levels but also between laryngectomized patients , , this novel HME cassette concept could also be employed to gradually train laryngectomized patient to a (higher) HME resistance over time (eg, by using the “twist‐ring” in an intermediate setting). Altogether, this might increase overall HME compliance and pulmonary rehabilitation in laryngectomized patients.

CONCLUSION

By introducing a bypass, this novel HME cassette prototype allows adjustment between substantially different HME resistances while maintaining appropriate humidification performances. The advantage of the specific bypass in the prototype is that it can easily be opened, closed or adjusted by the laryngectomized patient. This potentially facilitates physical exercise without changing or removing the HME and might therefore increase overall patient compliance.

Currently, this adjustable “bypass”‐principle is not yet available in any commercial HME cassette. We hope that this prototype will be developed further into an effective medical device.

CONFLICT OF INTEREST

Atos Medical AB had no role in the concept, study design, and drafting of this manuscript. The authors have no other funding, financial relationships, or conflicts of interest to disclose.

TABLE B1

Normalized input and verification data of different HMEs for the determination of the conversion from the normalized Water Exchange (WE) to Moisture Loss (ML).

All values in mg/L		WEVHME32;0	MLHME	MLHMEcalc	Ref. for WEHME
Year	HME type	@32; 0 mg/L (V T = 1 L)	@44; 0 mg/L (V T = 1 L)	@44; 0 mg/L (V T = 1 L)
2014	Hiflow	4.49	24.4	23.9	Van den Boer et al. (2014b)2
2014	Normal	4.39	23.7	23.9	Van den Boer et al. (2014b)2
2014	XtraFlow	4.49	24	23.9	Van den Boer et al. (2014b)2
2014	XtraMoist	7.58	21.5	22.1	Van den Boer et al. (2014b)2
2014	Hiflow	4.44	24.4	23.9	Van den Boer et al. (2014a)1
2014	Normal	5.37	23.7	23.4	Van den Boer et al. (2014a)1
2014	XtraFlow	5.92	24	23.1	Van den Boer et al. (2014a)1
2014	XtraMoist	7.09	21.5	22.4	Van den Boer et al. (2014a)1
2016	XtraFlow	5.01	24	23.6	a
2016	XtraMoist	6.79	21.5	22.6	a
2017	XtraFlow	4.41	24	23.9	a
2017	XtraMoist	5.58	21.5	23.3	a

Note: Normalized WEV () values at V = 1 L were calculated (see Appendix B.1 and B.2) from the values as measured by van den Boer et al. (2014a, 2014b).1,2 ML values were provided by the manufacturer (Atos Medical, Malmö, Sweden) in accordance with ISO 9360‐2:2001.3 ML was calculated from WE using Equation (21) and = 17.8 mg. For, abbreviations, see nomenclature in Appendix B.3.

Note: 1. van den Boer C, Muller SH, Vincent AD, van den Brekel MW, Hilgers FJ. Ex vivo assessment and validation of water exchange performance of 23 heat and moisture exchangers for laryngectomized patients. Respiratory Care. 2014; 59(8): 1161‐1171.

Note: 2. van den Boer C, Muller SH, Vincent AD, Züchner K, van den Brekel MWM, Hilgers FJM. Ex vivo water exchange performance and short‐term clinical feasibility assessment of newly developed heat and moisture exchangers for pulmonary rehabilitation after total laryngectomy. European Archives of Oto‐Rhino‐Laryngology. 2014;271(2):359‐366.

Note: 3. International Standards Organization. Anesthetic and respiratory equipment—heat and moisture exchangers (HMEs) for humidifying respired gases in humans. HMEs for use with tracheostomized patients having minimal tidal volume of 250 mL. Geneva: ISO; 9360‐2:2001.

The observations in 2016 and 2017 were made following the protocol of van den Boer (internal communication).

TABLE B2

Comparison of the data measured in this study with the Water Exchange and Moisture Loss values (in accordance with ISO 9360‐2:2001)6 of the HMEs.

All values in mg/L, at V T = 1 L	Water Exchange/V T, normalized to 32/5 mg/L					Moisture Loss, normalized to 44/0 mg/L
	This study	Van den Boer et al. (2014a)3	Van den Boer et al. (2014b)2	NKI‐AVL unpublished 2016	NKI‐AVL unpublished (averaged over 2016, 2017 and 2018)	This study, calculated	Atos Medical
Cassette foam	Narrow fitting, no speaking valve	with speaking valve	with speaking valve	with speaking valve	Narrow fitting, no speaking valve	Narrow fitting, no speaking valve	with speaking valve
XtraMoist	5.70	5.98	6.40	5.73	5.47	22.6	21.5
XtraFlow	4.91	4.99	3.79	4.23	n.a.	23.2	24.0

Note: All Water Exchange data from Van den Boer et al.2,3 were normalized to AH = 5 mg/L and AH32 mg/L (Appendix B.1) and converted to V = 1 L (Appendix B.2). Van den Boer et al. measured in volunteers, so the actual AH during the measurements was about 32 mg/L. However, they performed their data normalization with an AH of 44 mg/L (AH in the alveoli of the lungs). Using the appropriate AH value, only has a minor impact on the results of van den Boer et al. (2014a and 2014b); the WE increases with approximately 4% and the rating of HMEs stays the same.]) Moisture Loss data for our HME devices were determined using Equation (21) (Appendix B.3.4). For comparison with the ML values, the table shows WE/V values (at V = 1 L, numerically equal to WE).

Abbreviations: NKI‐AVL, Netherlands Cancer Institute – Antoni van Leeuwenhoek (Amsterdam, The Netherlands); also see nomenclature in Appendix B.

Note: 1. International Standards Organization. Anesthetic and respiratory equipment—heat and moisture exchangers (HMEs) for humidifying respired gases in humans. HMEs for use with tracheostomized patients having minimal tidal volume of 250 mL. Geneva: ISO; 9360‐2:2001.

Note: 2. van den Boer C, Muller SH, Vincent AD, Züchner K, van den Brekel MWM, Hilgers FJM. Ex vivo water exchange performance and short‐term clinical feasibility assessment of newly developed heat and moisture exchangers for pulmonary rehabilitation after total laryngectomy. European Archives of Oto‐Rhino‐Laryngology. 2014;271(2):359‐366.

Note: 3. van den Boer C, Muller SH, Vincent AD, van den Brekel MW, Hilgers FJ. Ex vivo assessment and validation of water exchange performance of 23 heat and moisture exchangers for laryngectomized patients. Respiratory Care. 2014; 59(8): 1161‐1171.