Continuous positive airway pressure (CPAP) ventilation requires constant positive airway pressure delivered throughout the whole respiratory cycle. Continuous positive airway pressure increases lung functional residual capacity decreasing shunt, improves oxygenation, and reduces work of breathing and oxygen (O2) consumption.1 Scientific literature recommends noninvasive CPAP in case of acute cardiogenic pulmonary edema, adult respiratory distress syndrome, posttraumatic acute respiratory failure, and postoperative hypoxemia.1-3 Moreover, CPAP seems to be useful to complete weaning after extubation.4 Currently, noninvasive CPAP, together with high-flow nasal cannula, represents the first-line treatment choice for hypoxemic respiratory failure in immunocompromised patients.5,6
Noninvasive CPAP has been usually delivered via facial mask, but during the early 1990s, a new interface named “helmet” was developed for this treatment.7-9 This patient interface is made of a clear plastic hood on a hard-plastic ring with a multisize silicon-polyvinyl chloride soft collar, to fit a wide range of necks' dimensions. Skin breakdowns are the main complications of noninvasive ventilation (NIV) delivered with face mask.10,11 The use of a helmet can eliminate these problems because it has no contact points on the patients' face. Moreover, a reduction in gastric distension due to air swallowing has been reported in CPAP delivered through helmet, when compared with facial mask CPAP.12 In south Europe (i.e., Italy and Spain), the helmet has been extensively used, especially in intensive care unit (ICU) settings, mainly for hypoxemic respiratory failure and acute cardiogenic pulmonary edema.13,14 Evidence has shown discomfort to be a major cause of NIV failure, because patients undergoing noninvasive CPAP treatment cannot be heavily sedated.15,16 A careful helmet management is fundamental to minimize patient's claustrophobia and preserve his/her full visual contact and communication with health care providers and relatives.17-19 Studies comparing helmets' and facial masks' effectiveness for delivering NIV reported noise as a barrier to the use of a helmet.20,21 Noise can contribute to patient's discomfort during the ICU stay,22-24 and noise exposure during helmet CPAP can be underestimated among the factors that influence the patient well-being.
Cavaliere and colleagues21 reported on helmet CPAP and systems that employ the Venturi effect (using air entrainment windows, affecting the concentration of O2 through high- flow rates) and determined noisiness worsening patient's discomfort and affecting ear function. In their study, they tested different gas source systems to inflate the helmet, with a maximum gas flow of 45 L/min.21 Other bench studies reported that noise intensities generated by the neonatal helmet CPAP, with a gas flow between 8 and 12 L/min, were significantly higher than those registered while using a conventional neonatal CPAP system.25 The authors underline that helmet's noise intensity depends on the gas flow rate. All the aforementioned studies reported noise levels inside the helmet during CPAP delivered through gas flow rates lower than 50 L/min. However, despite the helmet size, the total amount of inlet gas flow should not be lower than 50 L/min.8 Literature has clearly shown that carbon dioxide rebreathing occurs inside the helmet with gas flow rates lower than 50 L/min.9 Especially in a hypoxemic patient with adult respiratory distress syndrome, a gas flow rate higher than 50 L/min is crucial to reduce patient's respiratory rate and minimize CO2 rebreathing inside the helmet.26
Because of the lack of data about noise levels associated to helmet CPAP delivered through gas flow rates higher than 50 L/min, we performed a randomized crossover laboratory study on healthy subjects. The primary objective of this study was to compare noise level to which healthy subjects are exposed during helmet CPAP, with gas flow rates of 60 and 80 L/min, through different gas source systems and different breathing respiratory circuit configurations. The secondary objectives were to evaluate if a heat and moisture exchanger (HME) applied to the inspiratory port of the helmet could help to reduce noise levels and improve the subjects' comfort during helmet CPAP treatment.
MATERIALS AND METHODS
Sample, Equipment, and Setup
Ten healthy voluntary subjects (aged 25–50 years; median, 30 years; four females and six males) without knowledge or previous experience of NIV were included in this study. The exclusion criteria were as follows: positive medical history for ear, nose, or throat diseases.
The study protocol expected that subjects underwent CPAP with a helmet (Dimar CPAP 500/9390; Dimar S.r.l., Medolla, Italy) with a positive end-expiratory pressure level of 5 cm H2O (DEAS positive end-expiratory pressure valve, 03986 NS) in a quiet room.
They received in random order the following: 2 different gas flow rates (60 and 80 L/min), generated by 3 flow generator systems, commonly used in emergency room and/or intensive care settings, and 3 different breathing circuit systems.
The tested CPAP systems were as follows (Figure 1): (A) Venturi system (Starvent ; StarMed, Mirandola, Italy) including a flow generator that produced an O2/air mixture by the Venturi effect, (B) single box with four high-flow flowmeters (SF4; Flow Meter S.p.A., Levate, Italy) powered by medical compressed air and O2, and (C) Venturi system with monitor and electronic control of faction of inspired oxygen (Fio2) and gas flow (DimAIR; Dimar S.r.l.).
Furthermore, we tested 3 different breathing circuit configuration setups to connect the studied gas sources to the helmet: (1) spiral inner surface tube (100 cm, ref 290/5035, DAR; Medtronic S.p.A., Mirandola, Italy); (2) smooth inner surface tube (100 cm, ref 286/5068, DAR; Medtronic S.p.A.); (3) smooth inner surface tube (100 cm, ref 286/5068, DAR; Medtronic S.p.A.) combined with an HME filter (Inter-Therm filter, ref. 1341197S; Intersurgical S.p.A., Mirandola, Italy) placed on the ambient air supply port of the Venturi system (only for gas sources A and C) (Figure 2A). The placement of an HME filter on the air port of Venturi system was included in the tests of this study to verify if it could reduce the noise induced by the Venturi generator.
Furthermore, all the measurements derived from the different combinations of gas sources and breathing circuits setup were obtained with and without an HME filter applied to the inspiratory port of the helmet (Inter-Therm filter, ref. 1341197S; Intersurgical S.p.A.) (Figure 2B). The filter was inserted in an attempt to decrease the noise induced by gas that flowed through the circuit. During the experiment, all the volunteers, lying in Fowler's position on an intensive care bed, were not informed about different sequences of the study steps performed. All volunteers performed the 32 different study combinations.
Noise measurements were performed at different gas flow rates: 60 and 80 L/min. Each volunteer underwent all the combinations among CPAP systems and ventilator circuits for 10 minutes in random sequence. The intensity of the noise to which the subjects were exposed was measured with a sound-level meter MLM02 (Tack Life; Shenzhen Temie Technology Co., Ltd., Shenzhen, China). The sound level range was between 30 and 130 decibels (dB) (sensibility 0.1 dB), and the frequency range was between 31.5 Hz and 8 kHz. An A-weighting filter was used to better reflect the frequency response of the human ear; consequently, sound pressure level was given in Decibel A. The microphone was inserted on the accessory side entry connector of the helmet. While the function “max” was selected, the sound-level meter displayed the maximum sound intensity registered. Two intervals of 1 minute were taken into account, and the mean between the 2 maximum sound intensities was recorded. For each investigated step of the study, we also assessed the background noise in the room at the subject's bedside (feet zone of the bed). The noise perceived by the subjects was determined at the end of each 5-minute test with a visual analog scale (VAS) that ranged from 0 (absence of noise) to 10 (an overpowering noise totally impairing interaction with the environment).
Sound data were expressed as mean and SD. Visual analog scale data were expressed as median (interquartile range). Sound-level meter measurements were analyzed by 2-way analysis of variance for repeated measurements. The Student t test was used for analysis of sound-level measurements and VAS scores.
This study was conducted according to the ethical standards included in the 1964 Declaration of Helsinki. The study was approved by the ethical committee of our institution (University of Milan-Bicocca, Italy). In accordance with national regulations, written informed consent was obtained from each enrolled subject.
Background noise in the room before the study ranged from 36.1 to 38.0 dBA. The noise level inside the helmet, during the study, ranged between 76 ± 4 and 117 ± 1 dBA. The gas source and the gas flow affected the noise level inside and outside the helmet in every combination (P < .001). The different “circuit setup” did not reach statistical significance for noise level inside the helmet (P = .244), but affected the noise level outside, especially when a Venturi system was used (P < .001). The presence of an HME filter placed at the junction between the inspiratory circuit and the helmet significantly decreased the noise intensity inside the helmet (mean dBA without HME, 99.56 ± 13.30 vs 92.26 ± 10.72 with HME; P < .001) and outside (mean dBA without HME, 68.16 ± 12.05 vs 64.97 ± 12.17 with HME; P < .001) in all the combinations. The measurements are shown in the Table. Figure 3 summarizes the noise levels inside the helmet at every investigated condition, with and without HME filter placed on the inlet gas flow port of the helmet.
Overall, subjects judged that noise inside the helmet was lower when an HME filter on the inspiratory inlet gas port was in place (without HME median 6 [interquartile range 4–7] vs 7 [5–8] with HME, P < .001). However, the difference between VAS values did not always reach statistical significance in every investigated combination of gas flow rate, gas source, and circuit setup. Figure 4 summarizes the VAS levels recorded during each step of the study.
Noise intensity inside the helmet during our study ranged between 76 ± 4 and 117 ± 1 dBA. We found that CPAP gas source systems differ in terms of noisiness, and the noise was mainly due to gas flow level. Our study results are different from other studies that investigated noise levels during helmet NIV. For example, in neonatal CPAP, it has been demonstrated that the noise intensity increases with increasing flow rate, showing difference between 70.0 and 73.5 dB, depending on the flow rate (8 and 12 L/min, respectively), but the used gas flow rates were lower than 50 L/min.25 Our data showed in adult healthy subjects that the noise intensity was affected by gas flow rates over 50 L/min. Noise levels recorded in this study always exceeded the upper limit recommended by the Environmental Protection Agency/World Health Organization (WHO), for ICU environment.27 The WHO recommendations limit the ICU noise levels between 45 and 60 dB during daytime and 35 dB during nighttime. Furthermore, WHO suggests that nighttime noise peaks (loud noises) should not exceed the threshold of 40 dB. Monitor and ventilator alarms, staff conversations, and telephones account for almost all of the sounds peaks.28-30 Inside the helmet, the noise exposure is much more intense than the ICU noise. Noise could increase patient discomfort, especially during prolonged and nocturnal CPAP treatments. Sounds peaks inside the helmets may potentially cause sleep disruption. However, recent studies have reported that sleep deprivation in ICU is multifactorial and that noise is responsible for only a limited proportion of arousals and awakenings.31-33
In our healthy subjects, in all different investigated combinations, noise levels were higher than 70 dBA, and subjects' comfort was significantly lower without using an HME on the inlet gas port. Using an HME in helmet's inspiratory circuit can reduce the intensity of the noise generated at different gas flow rates. This is a simple and inexpensive idea to decrease noise exposure, especially in prolonged use of helmet CPAP. Our results reinforced previous data published by Trevisanuto and colleagues.25 On the other hand, some authors have not found any relationship between the use of HME and sound levels, but in these studies, the gas flow was lower than 50 L/min.20,21,34 Hernández-Molina and colleagues34 found that the effect of HME during neonatal helmet CPAP attenuates sound pressure at a gas flow rate of 20 L/min, but when the flow rate was set at 40 L/min, the diffuser filter increased sound level rather than attenuating it. The noise during helmet CPAP is affected by many factors. The Venturi effect (systems A and C) causes more gas turbulence than independent flowmeters for O2 and air (system B). We found a difference between the 2 investigated Venturi systems. System A was a standard Venturi system and uses the Venturi effect to entrain room air and generate a high gas output flow. A supplementary O2 source located downstream from the air-entrainment valve can be connected to increase Fio2. Tables provided by the manufacturers indicate, for each level of CPAP, the O2 flows needed to provide a specific total air flow and Fio2. System C was a new Venturi system, and it has been designed to be used inside ICU wards. This system combines a Venturi generator and an electronic system to regulate Fio2 and gas flow rates. The results of our study show that these new Venturi generators produce less noise compared with traditional medical air and O2 flowmeters, and their use in ICUs and general wards should be implemented. Traditional Venturi systems should be used only for transporting patients and in emergency rooms, as suggested by previous research.20,21
We investigated the role of smooth inner surface tubes in reducing the noise inside the helmet. Tubes with smooth inner surfaces reduced the noise inside the helmet, but the differences were not always statistically significant. We also tested the role of using an HME on the Venturi system inlet air port and found no difference in affecting the noise inside the helmet. The use of HME on the Venturi system air port and smooth tubes could have a role regarding environmental noise. This reduction of noise is crucial if patients are hospitalized in nonindividual rooms, to avoid other patients' discomfort.
Finally, such difference in our study results, approximately 10 dB, may falsely appear small because of the unit of measurement utilized. The dB is a logarithmic unit used to measure sound intensity, but it is not easily figured by subjects unfamiliar with sound intensity measurements. The noise levels associated with helmet CPAP with system A, with 60 and 80 L/min, without using an HME were approximately 110 dBA. These levels were comparable to the noise caused by an outboard motor, or a motorcycle, or a farm tractor. This decibel effect is 8 times louder than 70 dB. The noise levels with helmet CPAP and system C, with using an HME and tubes with smooth inner surface, were comparable to a passenger car at 100 km/h (77 dB), living room music (76 dB), radio or TV audio, or a vacuum cleaner (70 dB).
This study has some important limitations. First, there was a small number of subjects for the sample, and subjective evaluation of the noise was performed by healthy volunteers in a quiet environment and may not correspond to noisiness perceived by critically ill patients in an ICU.
Second, we measured the sound intensity in the helmet and not that perceived by the inner ear.
Third, we have not tested intensive care ventilators with a continuous flow generation function (>50 L/min), as recently suggested by Grieco and colleagues.35 Unfortunately, in ICU laboratory, where the study was carried out, none of the present mechanical ventilators had the aforementioned option.
We observed that sound pressure levels during the administration of helmet CPAP with gas flow greater than 50 L/min exceed the limits currently recommended by WHO (<45 dBA), and noise levels reached values between 76 and 110 dBA. We found that sound pressure levels and subjects' comfort are related to the gas source, the gas flow rate, and the use of an HME at the junction of the inspiratory branch of the helmet. In order to reduce noise, traditional Venturi systems should be used only during patient's transport or in emergency rooms.
RECOMENDATIONS FOR PRACTICE
During helmet CPAP, higher gas flow rates are necessary to maintain a relatively low inspiratory CO2 concentration. For successful helmet CPAP treatment, gas source devices must deliver sufficiently high gas flow rates to maintain airway pressure throughout the breathing cycle. A minimum gas flow rate of 60 L/min is needed to guarantee a constant CPAP level also in presence of high inspiratory demand. The use of helmet CPAP and systems that utilize the Venturi effect increases noisiness with the potential increasing of patients' discomfort. When administering helmet CPAP with a gas flow up to 50 L/min, an HME placed on the helmet inlet gas port should be used to reduce noise inside the helmet and to improve patients' comfort. Devices such as HME filters, single box high-flow flowmeters, new-generation Venturi systems, and tubes with smooth inner surface decrease noise inside and outside the helmet, therefore reducing discomfort. Because of their noisiness, traditional Venturi systems should be reserved only to time-limited settings such as the transport of patients undergoing helmet CPAP or during stay in emergency rooms.
1. Luo Y, Luo Y, Li Y, et al. Helmet CPAP versus oxygen therapy in hypoxemic acute respiratory failure: a meta-analysis of randomized controlled trials. Yonsei Med J
. 2016;57:936–941. doi:10.3349/ymj.2016.57.4.936.
2. Ferreyra GP, Baussano I, Squadrone V, et al. Continuous positive airway pressure for treatment of respiratory complications after abdominal surgery: a systematic review and meta-analysis. Ann Surg
3. Patel BK, Wolfe KS, Pohlman AS, Hall JB, Kress JP. Effect of noninvasive ventilation delivered by helmet vs face mask on the rate of endotracheal intubation in patients with acute respiratory distress syndrome: a randomized clinical trial. JAMA
. 2016;315(22):2435–2441. doi:10.1001/jama.2016.6338.
4. Bellani G, Laffey JG, Pham T, et al. Epidemiology, patterns of care, and mortality for patients with acute respiratory distress syndrome in intensive care units in 50 countries. JAMA
. 2016;315:788–800. doi:10.1001/jama.2016.0291.
5. Principi T, Pantanetti S, Catani F, et al. Noninvasive continuous positive airway pressure delivered by helmet in hematological malignancy patients with hypoxemic acute respiratory failure. Intensive Care Med
6. Bellani G, Patroniti N, Greco M, Foti G, Pesenti A. The use of helmets to deliver non-invasive continuous positive airway pressure in hypoxemic acute respiratory failure. Minerva Anestesiol
7. Patroniti N, Foti G, Manfio A, Coppo A, Bellani G, Pesenti A. Head helmet versus face mask for non-invasive continuous positive airway pressure: a physiological study. Intensive Care Med
8. Taccone P, Hess D, Caironi P, Bigatello LM. Continuous positive airway pressure delivered with a “helmet”: effects on carbon dioxide rebreathing. Crit Care Med
9. Patroniti N, Saini M, Zanella A, Isgrò S, Pesenti A. Danger of helmet continuous positive airway pressure during failure of fresh gas source supply. Intensive Care Med
10. Pisani L, Carlucci A, Nava S. Interfaces for noninvasive mechanical ventilation: technical aspects and efficiency. Minerva Anestesiol
11. Bambi S, Peris A, Esquinas AM. Pressure ulcers caused by masks during noninvasive ventilation. Am J Crit Care
. 2016;25(1):6. doi:10.4037/ajcc2016906.
12. Antonelli M, Conti G, Pelosi P, et al. New treatment of acute hypoxemic respiratory failure: noninvasive pressure support ventilation delivered by helmet—a pilot controlled trial. Crit Care Med
13. Crimi C, Noto A, Princi P, Esquinas A, Nava S. A European survey of noninvasive ventilation practices. Eur Respir J
. 2010;36(2):362–369. doi:10.1183/09031936.00123509.
14. Crimi C, Noto A, Princi P, Nava S. Survey of non-invasive ventilation practices: a snapshot of Italian practice. Minerva Anestesiol
15. Hilbert G, Clouzeau B, Nam Bui H, Vargas F. Sedation during non-invasive ventilation. Minerva Anestesiol
16. Liu J, Duan J, Bai L, Zhou L. Noninvasive ventilation intolerance: characteristics, predictors, and outcomes. Respir Care
. 2016;61(3):277–284. doi:10.4187/respcare.04220.
17. Lucchini A, Valsecchi D, Elli S, et al. The comfort of patients ventilated with the helmet bundle. Assist Inferm Ric
18. Bambi S, Mati E, De Felippis C, Lucchini A. Noninvasive ventilation: open issues for nursing research. Acta Biomed. 2017;88(1S):32–39. doi: 10.23750/abm.v88i1 -S.6282.
19. Lucchini A, Elli S, Bambi S, et al. How different helmet fixing options could affect patients' pain experience during helmet–continuous positive airway pressure [published online November 20, 2018]. Nurs Crit Care
. 2019;24(6):369–374. doi:10.1111/nicc.12399.
20. Cavaliere F, Conti G, Costa R, Proietti R, Sciuto A, Masieri S. Noise
exposure during noninvasive ventilation with a helmet, a nasal mask, and a facial mask. Intensive Care Med
21. Cavaliere F, Conti G, Costa R, et al. Exposure to noise
during continuous positive airway pressure: influence of interfaces and delivery systems. Acta Anaesthesiol Scand
22. Tegnestedt C, Günther A, Reichard A, et al. Levels and sources of sound in the intensive care unit—an observational study of three room types. Acta Anaesthesiol Scand
. 2013;57:1041–1050. doi:10.1111/aas.12138.
23. Johansson L, Lindahl B, Knutsson S, Ögren M, Persson Waye K, Ringdal M. Evaluation of a sound environment intervention in an ICU: a feasibility study. Aust Crit Care
. 2018 Mar;31(2):59–70. doi:10.1016/j.aucc.2017.04.001.
24. Alsulami G, Rice AM, Kidd L. Prospective repeated assessment of self-reported sleep quality and sleep disruptive factors in the intensive care unit: acceptability of daily assessment of sleep quality. BMJ Open
. 2019;9(6):e029957. doi:10.1136/bmjopen-2019-029957.
25. Trevisanuto D, Camiletti L, Udilano A, Doglioni N, Zanardo V. Noise
levels during neonatal helmet CPAP. Arch Dis Child Fetal Neonatal Ed
. 2008 Sep;93(5):F396–F397. doi:10.1136/adc.2008.140715.
26. Grieco DL, Menga LS, Eleuteri D, Antonelli M. Patient self-inflicted lung injury: implications for acute hypoxemic respiratory failure and ARDS patients on non-invasive support [published online March 12, 2019]. Minerva Anestesiol
. 2019;85(9):1014–1023. doi:10.23736/S0375-9393.19.13418-9.
27. World Health Organization. Guidelines for community noise
. In: Berglund B, Lindvall T, Schwela DH, eds. HO Expert Taskforce Meeting Held in London, United Kingdom in April 1999. Geneva, Switzerland: World Health Organization; 1999. http://whqlibdoc.who.int/hq/1999/a68672.pdf
28. Tainter CR, Levine AR, Quraishi SA, et al. Noise
levels in surgical ICUs are consistently above recommended standards. Crit Care Med
29. Hamilton DK, Shepley MM. Creating Therapeutic Environments for Critical Care. Design for Critical Care: An Evidence-Based Approach
. Amsterdam, the Netherlands: Elsevier/Architectural Press; 2010:153–193.
30. Johansson L, Bergbom I, Waye KP, Ryherd E, Lindahl B. The sound environment in an ICU patient room—a content analysis of sound levels and patient experiences. Intensive Crit Care Nurs
31. Bihari S, Doug McEvoy R, Matheson E, Kim S, Woodman RJ, Bersten AD. Factors affecting sleep quality of patients in intensive care unit. J Clin Sleep Med
. 2012;8:301–307. doi:10.5664/jcsm.1920.
32. Ding Q, Redeker NS, Pisani MA, Yaggi HK, Knauert MP. Factors influencing patients' sleep in the intensive care unit: perceptions of patients and clinical staff. Am J Crit Care
33. Mattiussi E, Danielis M, Venuti L, Vidoni M, Palese A. Sleep deprivation determinants as perceived by intensive care unit patients: Findings from a systematic review, meta-summary and meta-synthesis. Intensive Crit Care Nurs
. 2019;53:43–53. doi:10.1016/j.iccn.2019.03.006.
34. Hernández-Molina R, Fernández-Zacarías F, Benavente-Fernández I, Jiménez-Gómez G, Lubián-López S. Effect of filters on the noise
generated by continuous positive airway pressure delivered via a helmet. Noise Health
. 2017;19(86):20–23. doi:10.4103/1463-1741.199237.
35. Grieco DL, Biancone M, Maviglia R, Antonelli M. A new strategy to deliver helmet CPAP to critically ill patients: the possible role of ICU ventilators. Minerva Anestesiol