Virtual Reality for Pulmonary Rehabilitation: Comprehensive Review

Background Pulmonary rehabilitation is a vital component of comprehensive care for patients with respiratory conditions, such as lung cancer, chronic obstructive pulmonary disease, and asthma, and those recovering from respiratory diseases like COVID-19. It aims to enhance patients’ functional ability and quality of life, and reduce symptoms, such as stress, anxiety, and chronic pain. Virtual reality is a novel technology that offers new opportunities for customized implementation and self-control of pulmonary rehabilitation through patient engagement. Objective This review focused on all types of virtual reality technologies (nonimmersive, semi-immersive, and fully immersive) that witnessed significant development and were released in the field of pulmonary rehabilitation, including breathing exercises, biofeedback systems, virtual environments for exercise, and educational models. Methods The review screened 7 electronic libraries from 2010 to 2023. The libraries were ACM Digital Library, Google Scholar, IEEE Xplore, MEDLINE, PubMed, Sage, and ScienceDirect. Thematic analysis was used as an additional methodology to classify our findings based on themes. The themes were virtual reality training, interaction, types of virtual environments, effectiveness, feasibility, design strategies, limitations, and future directions. Results A total of 2319 articles were identified, and after a detailed screening process, 32 studies were reviewed. Based on the findings of all the studies that were reviewed (29 with a positive label and 3 with a neutral label), virtual reality can be an effective solution for pulmonary rehabilitation in patients with lung cancer, chronic obstructive pulmonary disease, and asthma, and in individuals and children who are dealing with mental health–related disorders, such as anxiety. The outcomes indicated that virtual reality is a reliable and feasible solution for pulmonary rehabilitation. Interventions can provide immersive experiences to patients and offer tailored and engaging rehabilitation that promotes improved functional outcomes of pulmonary rehabilitation, breathing body awareness, and relaxation breathing techniques. Conclusions The identified studies on virtual reality in pulmonary rehabilitation showed that virtual reality holds great promise for improving the outcomes and experiences of patients. The immersive and interactive nature of virtual reality interventions offers a new dimension to traditional rehabilitation approaches, providing personalized exercises and addressing psychological well-being. However, additional research is needed to establish standardized protocols, identify the most effective strategies, and evaluate long-term benefits. As virtual reality technology continues to advance, it has the potential to revolutionize pulmonary rehabilitation and significantly improve the lives of patients with chronic lung diseases.


Background
Pulmonary rehabilitation focuses on breathing exercises that can help people with chronic lung diseases improve lung function and reduce symptoms of chest tightness, chronic cough, and wheezing.Apart from lung diseases, pulmonary rehabilitation is commonly used for treatments related to hypertension, chronic pain, and cardiovascular disorders, such as coronary artery issues, arrhythmia, and myocardial infarction [1,2].Breathing exercises may also offer effective and simple solutions for depressive and anxiety episodes, and other mental health-related disorders [3,4].
In recent years, virtual reality (VR) technology has been used in a wide variety of medical applications, including but not limited to areas involving the delivery of treatments for pulmonary diseases, such as asthma [5,6], chronic obstructive pulmonary disease (COPD), [7], and lung cancer [7], and mental health conditions, such as anxiety and stress-related disorders [5,8].This is because exploration around physiological signals collected in VR can offer holistic breathing guidance options to users and provide them with breathing benefits [8][9][10].Nowadays, individuals may explore and seek the assistance of professional coaches or use advanced devices that are specially designed for the purpose of breathing; however, these means often have a high cost and are time-consuming [5,7].VR technology, on the other hand, is becoming one of the most accessible and low-cost solutions in the health care domain for breathing interventions [8,11].Further, VR allows users to have full control over the environment they are exposed to.In combination with the use of biosensing technology, which provides acoustic, visual, and biofeedback guidance, VR users are offered the ability to consciously and self-effectively control and monitor their respiratory rate (RR) [2,5,12].
During the COVID-19 pandemic, patients' medical care, including admission to clinics and use of emergency services, was affected owing to the danger of contamination and the limitations of medical service resources [14].In this situation, clinical visits, nonurgent treatments, and nonearnest clinical issues, especially among vulnerable populations like people with pulmonary diseases, were initially interrupted and later resumed with a diminished scope [15].Doctors had to confront the quandary of who could be treated at clinical centers or at home, or who could be allocated to the set number of beds in intensive care units [16].New technologies helped support vulnerable populations during the pandemic, and VR helped overcome a variety of clinical challenges.This technology is quickly changing clinical training, patient therapies, and rehabilitation [14,15,17].The pandemic has changed the clinical framework and placed standard methods with virtual telemedicine and software systems to provide clinical benefits for alleviating the effects of COVID-19 [18].A significant part of the clinical framework involves rehabilitation, and it is significant owing to the pandemic period [19].This might be driven by telehealth stages, as with the use of VR.
The fundamental objective of pulmonary rehabilitation is to further develop the patient's psychophysical state [19].Regardless of restricted admittance to hospitals for rehabilitation owing to COVID-19, VR technology can be applied to this group of patients.It can provide extensive help in various areas, including patient management and clinical treatment, monitoring of patient progression in rehabilitation or assessment of changes, and evaluation and advancement of body function, exercise, and consecutive participation [16,19].Pulmonary rehabilitation is also beneficial after COVID-19 infection, even in patients who are recovering, who need assisted ventilation or oxygen therapy [20].Additionally, COVID-19 survivors experience stress, depression, and low quality of life, and the symptoms of dyspnea and fatigue can last more than 3 months after infection.Recent evidence has shown that pulmonary rehabilitation can alleviate these symptoms and can improve exercise performance, lung function, and quality of life in COVID-19 patients and survivors [21][22][23].
However, scientific studies examining and evaluating the opportunities and challenges of VR for breathing remain limited.In this literature review, we introduce state-of-the-art VR technologies relevant to pulmonary rehabilitation and breathing exercises by analyzing recent related articles on the subject.Owing to the lack of studies related to breathing gaming exercises, for this review, we only examined and evaluated 32 available studies from the last decade [5][6][7][8]10,.Evidence from related empirical and experimental studies that comprised several types of breathing exercises and patterns was systematically reviewed to address the following research questions: • Is VR an effective solution for breathing exercises?
• Which are the most common VR contents used for breathing exercises in gaming?
• How feasible is gamified biofeedback breathing VR for real-world deployment?
• What are the current barriers to biofeedback VR technologies?
Search terms: The following 3 queries were used in all the libraries since the aim was to cover any type of VR technology for breathing: 1. Virtual Reality AND Breath 2. Virtual Reality AND Pulmonary Rehabilitation 3. Virtual Reality AND Breathing games Search procedure: The search terms were used to examine the publication's title, abstract, and keywords.
Search results: The search results in Phase 1 can be seen in Table 1.

Phase 2: Publications Retrieved for Detailed Evaluation
First exclusion: All search results from Phase 1 were imported into Mendeley electronic library.Possible entries with wrong years were excluded (625 wrong-year entries were removed).This elimination decreased the number of papers to 1694.
Second exclusion: Duplicate papers, either extracted from one or more libraries, which either produced or concluded the same outcome, were removed from this review.Moreover, duplicate papers, extracted from one or more similar terms, which either produced or concluded the same outcome, were also removed.A total of 375 duplicate publications were removed, leaving 1319 different papers.
Third exclusion: The entries were narrowed down to original full papers that were written in English.We excluded papers that did not have the full text available (ie, we did not have access to the full text) and papers that were not original full papers, such as workshops, posters, speeches, reviews, magazine articles, and generally grey literature without formal peer review.As a result, 312 papers were excluded, leaving 1007 papers.

Phase 3: Final Exclusion
Since this review was focused on VR technologies related to breath and breathing exercise gaming, we excluded papers that examined and used other types of technologies that were not related to VR or to breathing exercise gaming.Moreover, research that was only related to breathing without the use of VR was excluded.In this phase and based on these conditions, any irrelevant studies that appeared in Phase 1 and were not removed in Phase 2 were excluded.However, these studies may appear in our findings since relevant words were contained in our research but did not match the specific technical content.Based on these conditions, we removed 376 studies unrelated to VR, 215 studies unrelated to breathing, and 284 studies unrelated to breathing exercise gaming.We ended up with 32 relevant papers (24 journal articles and 8 conference papers; there were no book chapters).The flowchart is presented in Figure 1.All relevant studies were downloaded for examination.

Phase 4: Data Gathering
All related information from the studies was extracted for examination.An Excel file was created, and the following data were extracted from each paper: sample size of the population studied, methodology, instruments, apparatus, VR content, VR interventions, types of biofeedback sensors, types of biosignals, VR feasibility, key findings, current VR limitations, and VR future directions.Moreover, we categorized each study based on the results as positive (+), negative (−), or neutral.

Phase 5: Data Analysis
The data gathered in Phase 4 were analyzed using descriptive statistics.Consequently, the literature was reviewed to support and enhance the additional knowledge provided by the study.Thematic analysis was used as an additional methodology to classify our findings based on themes.The following themes were considered: VR training, VR interaction, types of virtual environments (VEs), VR effectiveness, VR feasibility, VR design strategies, VR limitations, and VR future directions.Intercoder reliability was assessed between the researcher and research assistant.The Cohen kappa formula was used to calculate the similarity between the researcher and research assistant, and the similarity value was 0.89.

Self-Reported Data
As mentioned above, most of the studies enhanced conventional breathing training through VR interventions.To evaluate the effectiveness of the VR system, studies used several measures, which are presented in Table 3.All the reviewed studies (32/32) collected participants' demographic information (eg, age, sex, ethnicity, educational level, etc).Half of the reviewed studies (14/32) examined the effect of VR breathing training on mental health (ie, anxiety) [6,25,26,[28][29][30]32,34,36,37,[39][40][41].To assess the level of anxiety, the following protocols were used: State-Trait Anxiety Inventory (STAI; 3/32 studies) [25,40,44], Generalized Anxiety Disorder-7 (GAD-7; 2/32 studies) [27,49], Hospital Anxiety and Depression Scale (HADS; 2/32 studies) [42,50], Perceived Stress Scale (PSS; 2/32 studies) [28,39], and Edmonton symptom rating scale (1/32 studies) [50].The STAI is a 4-point Likert scale form, which contains 40 questions measuring pressure, worry, and anxiety [55,56].GAD-7 is a 7-question screener that assesses participants' psychological well-being status [27,57].The HADS is a 14-item scale that scores 0 to 3 for each item.The first 7 items relate to anxiety, and the remaining 7 relate to depression.A higher score is associated with greater anxiety and symptoms of depression [42].The PSS is a 14-item scale with 7 positive and 7 negative items rated on a 5-point Likert scale [58].The Edmonton symptom rating scale is a questionnaire used to rate the intensity of 9 common symptoms experienced by cancer patients, including pain, tiredness, nausea, depression, anxiety, drowsiness, appetite, well-being, and shortness of breath [59].Moreover, the Cognitive Interference Questionnaire (CIQ) was used to measure the performance, task-oriented worries, and off-task thoughts of participants through a 22-item questionnaire based on a 5-point scale [5,60].The Mini-Mental State Examination (MMSE; 2/32 studies) is a set of 11 questions to check if participants have cognitive impairments such as problems with thinking, communication, understanding, and memory [61].Lastly, the visual analog scale (VAS; 2/32 studies) [5,25] was used to evaluate participants' calmness and relaxation self-efficacy.The VAS is usually used for measuring pain on a 10-point Likert scale, but since the scale is easy to reflect, studies also use it to measure other emotional reflections as well [62].Some studies (9/32) examined the VR experience [6,10,31,33,38,42,[44][45][46] through self-reported questionnaires.The questionnaires included questions related to the participants' positive and negative emotions, game flow, engagement, ability, capacity, pressure that the said user was experiencing, and challenges that the user perceived [62,63].Two studies (2/32) also evaluated the motivation of the participants, using a multidimensional self-reported 7-point scale with a Likert-type format called the Intrinsic Motivation Inventory (IMI) [40,44].The IMI consists of 7 subscales that measure participants' experiences related to a target activity, such as interest/enjoyment, perceived competence, effort, value/usefulness, pressure/tension, perceived choice, and relatedness [64].The Just Noticeable Difference (JND) was used in a study (1/32) to stimulate the perception level of the user on system latency [46].Another study (1/32) used the Igroup Presence Questionnaire (IPQ) to assess the sense of presence in different VEs [48].Lastly, an instrument named the State Mindfulness Scale (SMS; 1/32) [25] was designed for mindfulness assessment with 21 questions self-reported on a 7-point Likert scale.In particular, the SMS counts the level of present-moment attention to and awareness of activity (mindful or mental) [65].

Biosignals and Physiological Data
Biosignals are physical signals that describe the state of human living.A wide variety of biosignals are regularly used in hospitals and in home monitoring.The most well-known include electrocardiography (ECG), electroencephalography (EEG), and photoplethysmography (PPG).Alternatively, physiological data consist of heart rate (HR), blood pressure, RR, etc.

VEs and Interventions
As mentioned above, one of the most important advantages of VR is its ability to provide the feeling of being immersed in a simulated environment [66].Even though the participants of VR technology acknowledge and recognize that the environment provided by VR is not real, they act as they would in a real environment.The reviewed studies focused on specific types of VEs for mental and anxiety rehabilitation and pulmonary rehabilitation.The study results for VEs are presented in Table 5.

Virtual environments Study
Tissue layers and cells are presented to the participant.Through different types of breathing, the participant diminishes the cancerous lung cells.

Abushakra et al [7], 2014
A room with a matched-gender virtual body lying on a couch.The virtual body is illuminated synchronously or asynchronously according to the patient's chest movements.

Betka et al [49], 2022
The natural environment of a beach scenery at sunset with palms, rocks, several light sources, and a campfire.

Blum et al [25], 2019
Natural environment with a landscape having hills, flowers, parts of trees, and rocks that change their color according to breathing.

Blum et al [10], 2020
The trainees must shoot hostile zombies while they leave the benign zombies unharmed.Brammer et al [41], 2021 There were 4 games: (1) Beat Saber, (2) Space Pirate Trainer, (3) Gorn, and (4) Final Approach.Charoensook et al [31], 2019 There were 3 games: (1) Popping balloons, (2) Car, and (3) Diving.van Delden et al [5], 2020 A dedicated space for meditation with peers and a virtual instructor with an hourglass to track time.There is a bonsai tree that grows to signify progress.The environment changes every few sessions, including weather (sunset or rainstorm).

Feinberg et al [47], 2022
A square arena in which a ball bounces with a randomly initialized direction and location.Gummidela et al [45]

Kluge et al [36], 2021
Fantasy Forest Environment: the participant walks in nature and relaxation sceneries that facilitate recovery from job stress.

Ladakis et al [37], 2021
There were 2 scenes: (1) nature scene with trees, grass, and flowers moving based on the user's breathing; and (2) elevator scene with a glass elevator outside of a large building in a city, with a height of 6 levels.

Mevlevioğlu et al [38], 2021
A parking garage with friendly and hostile human targets to shoot or not.Michela et al [44], 2022 Life Tree: a tree starts growing through inhalation and exhalation.Patibanda et al [34], 2017 Pulse Breath Water: ocean waves that change their movement according to breathing pace.Prpa et al [26], 2018 Calm Place: climate sequence that goes from dusk to noon with the appearance of a blue object in the middle of the virtual scene to guide the breathing exercise.

Quintero et al [32], 2019
Two types of virtual environments of nature, with elements such as trees, grass, flowers, and rocks.Rockstroh et al [39], 2021 A relaxed environment.Rodrigues et al [50], 2022 An underwater world in which children can move around freely and explore at their leisure.van Rooij et al [40], 2016 Virtual therapeutic garden.Rutkowski et al [42], 2021 A sailing boat moving backward and forward with the participant's breathing.Shih et al [24], 2019 A jellyfish moving up and down in an underwater environment.Soyka et al [28], 2016 An underwater world with 2 jellyfish and a growing glass sponge.

Tatzgern et al [48], 2022
A white cloud moving toward and away in the direction of the participant's mouth.Tinga et al [29], 2018 There were 2 games: (1) Balloon, where the participant could control the movement of a balloon through respiration and (2) Pilot, where the participant's breathing could control a flight's course.

Types of VEs for Mental and Anxiety Rehabilitation
Numerous studies (12/32) used nature scenes, such as scenes of beaches, forests, oceans, and mountains [10,25,26,28,29,33,34,[36][37][38][39][40].Below, we present 4 of the most impressive natural VEs.The first environment was a beach scenery at sunset with several dynamic parameters like lights and clouds, which shifted according to breathing to provide feedback to the participant (Figure 4) [25].If the participant's breathing was below the threshold, which was preset by the system, the breathing pace was considered to be correct.As a result of correct breathing, the sky turned clear, and the participant could enjoy a star-spangled sky.A campfire was also included in the said scenario as a dynamic object to make the participant feel more relaxed and to provide the participant with an indication of whether breathing was at the appropriate pace.The second environment was a dynamic scenario with a tree submerged in the middle of water (Figure 5) [34].The goal of this intervention was to help the participant to practice pursed-lip breathing.The participant was told to wear an unidentified HMD and sit in a comfortable position with the legs crossed.Before the start of the intervention, the participant was advised to go through a breathing exercise introduction.After that, the participant had to follow rhythmic breathing by inhaling and exhaling until the tree started to expand on inhalation and contract on exhalation.If the participant continued to breathe rhythmically, the tree started to bloom.If the breathing of the participant was nonrhythmic, the view of the participant started to blur in the monitor until the right rhythm was found.Leaves were also used to provide feedback to the participant on the breathing rhythm.More colorful leaves indicated that the participant was following the correct breathing rhythm.The last environment was an underwater experience for diaphragmatic breathing practiced with children with anxiety issues (Figure 7) [40].The participant was instantaneously informed of the state of breathing by a dynamic circle in the VE that expanded according to breathing.The system applied gravity to the participant's avatar if the lung capacity was more than half.The participant's breathing pace was able to determine the direction and magnitude of the force.When the participant inhaled, an upward force was applied, and when the participant exhaled, an extra forward force was applied so that the participant was able to dive into the deep ocean.The combination of slow and deep breathing allowed the participant to swim better and have more control in the game.The type of VR equipment was not reported in the study.

Types of VEs for Pulmonary Rehabilitation
Several studies (10/32) developed different VEs for breathing exercises to improve respiration [5,6,8,29,31,32,34,41,47,49].In particular, Prpa et al [26] generated a 3D element of a body of water (an ocean) (Figure 8).This minimal environment displayed the ocean and the sky through a variety of grayscale shades.A continuous breathing pattern allowed the participant to control the ocean environment.It started with a light grey sky and a stationary participant position above the ocean surface.When the participant found the breathing flow, the game continued with movements of ocean waves, which were based on the participant's breathing pace.The right breathing pattern was mapped to wave movement and musical rhythm.The sky changed from grey to black until the ocean was stationary again, as was seen by the participant at the beginning of the procedure, with the only difference being the color of the sky.
Another study developed a VR intervention for alleviating dyspnea in patients recovering from COVID-19 pneumonia (Figure 9) [49].Specifically, it created a room with a matched gender body lying on a couch.The goal was to illuminate the virtual body synchronously or asynchronously according to the patient's chest movements.A particularly interesting study used multi-user VR to simultaneously immerse two or more participants into an underwater world with jellyfish and a growing glass sponge (Figure 10) [33].The aim was to synchronize the breathing between the participants, and enhance the breathing awareness, breathing pace, and relation between the participants.In this environment, each participant's breath was represented by a jellyfish, which moved and glowed in such a way as to provide clear breathing feedback.As the participants synchronized their breathing, the glass sponge was structured to begin to grow and emit light.Interestingly, 2 of the reviewed studies used existing predeveloped systems for pulmonary rehabilitation [42,43].Rutkowski et al [42] used the virtual therapeutic garden game, released by the European Association of Psychotherapy (Figure 11).Initially, the garden appears grey (untidy and unkept), and the watering pot is on its side.Diaphragmatic exercises with resistance, prolonged exhalation exercises, and chest percussion activate the watering pot to water the garden.With each rehabilitation session and the correct conduct of fitness exercises, the garden becomes increasingly colorful and alive, symbolizing the process of gaining health through the rehabilitation sessions.Bubble Tower [43] investigated the development of mapping breathing techniques in VR gameplay mechanics (Figure 12).The whole platform offered a set of 8 mini-games, where Bubble Tower was designed to train the participant in the fundamentals of breathing techniques.The mini-games teach the participant basic breathing skills like long and strong breathing, and breathing strength control.These mini-games include stages where the participant is required to pop the bubble with a strong breath, blow candles in a room with a long breath, build up the momentum for a windmill to spin fast with a long breath, etc.An interesting study presented a VE based on the escape room game philosophy (Figure 13) [48].In the escape room scenario, the participant used different breathing pattern interactions to answer a sequence of tasks to escape from the room.The room had all the objects that the participant needed to escape.During the training, the participant interacted with different individual objects.The first task was to blow out candles on a cake, and then, the participant had to blow bullets through a blow tube toward a target.The third task included the movement of ships based on breathing force.The participant continued with inflating balloons and sorting them by resistance.After that, the participant had to reveal numbers on a mirror with the breath.The last task involved shooting with a toy gun by holding the breath.

Principal Findings
Based on all the studies that were reviewed (29 studies had a positive label and 3 had a neutral label), it is suggested that VR can be an effective solution for pulmonary rehabilitation among patients with lung cancer, patients with COPD, patients with asthma, and individuals and children who are dealing with mental health-related disorders such as anxiety.Overall, the results indicated that VR can enhance the functional outcomes of pulmonary rehabilitation, increase breathing body awareness, and improve relaxation techniques.In the COVID-19 crisis, evidence showed the need for VR technology adoption.VR pulmonary rehabilitation presents an opportunity for the safe and effective recovery of COVID-19 patients and survivors at home.This technology could be adopted on a large scale to further develop the well-being of individuals during pandemics like COVID-19 and could similarly advance autonomous medical care.In the reviewed studies, most of the VR systems included features of natural environments, like a beach, sky, and forest.Further, it is highly recommended for future studies to incorporate water elements (eg, bubbles) and undersea sceneries (eg, seabed, jellyfish, and sea plants) to enhance relaxation.Some systems involved the use of music and vibrant colors.As technology continues to advance and progress, it is expected that biofeedback in VR systems will have a vital role in the practice of breathing in both the medical setting and the real world.The reviewed studies proved the feasibility of implementing a VR system for pulmonary rehabilitation enhanced with biofeedback.Such a system can be a reliable solution to enhance participant training.An assortment of minimal-cost sensors and biofeedback frameworks can incorporate VR and provide exact and significant information from tasks in gamified biofeedback interventions for breathing.Regarding the adequacy of pulmonary rehabilitation, biofeedback VR innovations must bridge explicit obstacles like equipment assembly, participant population, experiment length, and breathing patterns.The boundless breathing direction in VR frameworks can cause issues in the legitimate decisions of breath-detection equipment.Accordingly, many specialists battle with equipment assembly, equipment incorporation, and its compatibility with VR systems.There have been hindrances in adopting accepted procedures of biofeedback VR in various populations requiring breathing training for different purposes, including overcoming mental health issues and stress pressure, and achieving overall health benefits.The restricted length of examinations can prevent the assessment of the longer-term impacts of pulmonary rehabilitation.
Most of the studies (14/32) examined the effectiveness of breathing through VR based on physiological data [6][7][8]24,25,29,35,36,[43][44][45][47][48][49].Overall, the review suggests that VR is a reliable and feasible solution for pulmonary rehabilitation.Specifically, one of the reviewed studies [8] revealed that VR rehabilitation can be a reliable solution to treat pneumonia.Moreover, it was found that participants who performed pulmonary rehabilitation through VR felt more confident compared to those who performed the training with face-to-face supervision from health care professionals [25].It was explained that embodied interactions through VR and biofeedback responses made the participants more aware of their inhalation and exhalation rhythm [28].Additionally, the reviewed studies about pulmonary rehabilitation in COVID-19 patients reported significant improvements in tiredness, shortness and comfort of breath, and vital signs, such as HR, RR, blood pressure, and SpO2 [48,49].A study comparing patients with COPD undergoing VR pulmonary rehabilitation and those not undergoing this rehabilitation found that VR rehabilitation was associated with high stress release and a sharp reduction in depressive episodes [41].The positive impact of VR on the enhancement of rehabilitation with different breathing patterns in emotional well-being has been documented by several studies (13/32), with most of these studies being focused on anxiety and stress monitoring [5,8,10,27,28,31,33,34,36,37,[39][40][41]43,49].The level of anxiety of participants was found to decrease within few minutes after using VR with biofeedback for controlling breathing.In particular, a study comparing induced anxiety between participants who used VR and those who did not use VR reported a reduction in the level of anxiety in those who used VR [39].In addition, a study documented that apart from the positive effect VR has on anxiety, it can also increase concentration and positively motivate participants [54].In particular, it was found that participants who performed pulmonary rehabilitation via VR were less reluctant to XSL • FO RenderX participate in the training activities.Anxiety levels have also been studied in young populations [39] and COVID-19 patients [49].VR was suggested to be an effective solution for children at risk for anxiety disorders [39] and was reported to be effective at improving anxiety and increasing the feeling of well-being in patients with COVID-19 [49].
A study reported that participants were able to perform paced breathing techniques without distraction [27].It suggested the use of vibrant, rich, and multi-dimensional VEs to deliver an effective and enjoyable VR experience.Another study mentioned that water manifestation is a key element to decrease anxiety.More specifically, it was reported that having water features can enhance stress management techniques and significantly expand HRV based on paced breathing [27,54].Two studies suggested that participants were able to more accurately control the pace of their breathing on adding biofeedback [28,30].Zafar et al [30] noticed that participants who were exposed to biofeedback systems were able to control their breathing more precisely as opposed to the traditional type of training.Correspondingly, participants of VR biofeedback systems scored higher in their subsequent stress test compared with the pretest.Furthermore, a study that examined the effectiveness of respiratory biofeedback during VR meditation by measuring EEG and ECG signals in a respiratory biofeedback state, control feedback stress state, and control no feedback state, proved that VR meditation is effective for relaxation and breathing exercises.The study findings suggest that if VR is used for meditation, no biofeedback equipment is needed to reduce arousal, providing a more affordable and less intrusive option to apply VR to relaxation exercises [28].In summary, biofeedback is recommended for the effective deployment of VR systems for pulmonary rehabilitation.The feasibility and findings of the studies are presented in Table 6.VR for home breathing therapy for stress Soyka et al [28], 2016 Positive Positive outcomes suggest that the system is functional.
VR for breath gaming to mediate physiological synchrony for social connection Desnoyers-Stewart et al [33], 2019

Positive
The system makes use of state-of-the-art breath-sensing techniques without specialized sensing hardware.
VR for respiratory pneumonia rehabilitation Tabor et al [8], 2020 Positive The latency perception threshold is higher for inexperienced participants than experienced participants.
VR for playing a music instrument with breath Tao et al [46], 2020

Limitations and Future Work
Even though the effectiveness of VR for pulmonary rehabilitation is well documented, several limitations have been reported in the reviewed studies.First, some of the reviewed studies measured the relaxing effect of VR through the use of psychophysiological responses, such as HR, and self-reported questionnaires.However, some of the studies suggested the need for additional instruments that explicitly assess different aspects of affect and mood [10,41].For example, a study suggested the collection of 2 different biosignals (PPG and ECG) for HR accuracy [10].Moreover, the study mentioned that a participant's respiration has limited validation and recommended the use of an additional belt sensor [10].It was suggested to measure stress levels using not only traditional reports, such as self-reports and the Stroop task, but also cortisol levels [41].Future studies should consider triangulating the physiological data with interviews and other qualitative data to provide a holistic assessment of the impact of biofeedback systems [29,44].Future studies can also investigate the factors that can enhance VR pulmonary rehabilitation to support existing relaxation and destress techniques, and this should be compared to traditional practices [27].
Second, the reviewed studies highlighted the limit of breathing guidance in VR systems, which can provoke complications and barriers in the correct choice of breathing sensing hardware [45,47].As a result, many researchers struggled with aspects of the hardware apparatus, like the weight of the hardware on the user's head in addition to the VR HMD [47], as well as with hardware integration and its compatibility with VR systems [45].In addition, most hardware equipment involved high-end solutions, and the cost for the equipment in most of the studies was between €300 and €1200 (US $322 and US $1289, respectively) [5,6,10,27,[29][30][31][32][33][36][37][38][39]45].Three studies [34,46,47] developed their own affordable equipment, for which the cost was approximately €70 (US $75).Future studies should provide clear guidelines for the effective apparatus as well as the cost of the system.As mentioned previously [66], moving to low-cost and accessible solutions will decrease the need for technical support.This suggests that participants will be able to have their own personalized devices, which could lead to an increased quality of life.
It was further suggested for future studies to build systems that include machine learning algorithms to empower participant rehabilitation.These kinds of systems can offer the advantage of higher relaxation levels.For this, it is recommended for model algorithms to automatically classify breathing states, compared with other sensors, and analyze different visual cues in VEs.Moreover, it was stated that machine learning methods should adapt logic modules to provide automatic adaptations in VEs [31].It is worth mentioning that Fast Fourier Transformation implementation presents a limitation compared with other XSL • FO RenderX methods of low-frequency detection, although it is fully functional.For future studies, it is therefore suggested to implement a wavelet transform method, which can provide higher resolution at low frequencies without requiring a larger window size [33].
To improve the systems even further, future studies should examine the effectiveness of biofeedback compared with proper control conditions in different groups of participants under different types of circumstances to determine exactly when and why biofeedback might not be preferable.For example, the effects of biofeedback in children, as examined in the study by van Rooij et al [40], could differ from the effects of biofeedback in adults, as examined by Tinga et al [29].In the study by Tinga et al [29], the reduction in arousal (on all outcome measures combined and HR specifically) was the largest in the control feedback placebo condition, indicating that respiratory biofeedback had no additional value in reducing arousal and was even less effective than the control feedback placebo.The above finding indicates no preference for respiratory biofeedback compared with control feedback placebo in lowering pain levels in participants with chronic back pain.
Third, a study suggested the extension of VR exposure time, since it was found that this might allow participants to develop their own strategies for producing respiration patterns [31].Expectedly, most studies suggested that a large sample is required to verify the trend of average HRV and other bioindicators [30,43], as well as to address difficulties in VR design [8,45].An enhanced sample size could have a positive impact on VR design since a wider set of participants can express their interests [48,49].Finally, it has been suggested for future studies to extend experiments to multi-session investigations to examine the longer-term effects of pulmonary rehabilitation among participants [25,30,33,49].

Conclusion
The future directions of biofeedback VR technologies hold huge potential for significant advancements in the pulmonary field, ushering in a new era of personalized and adaptive experiences, enhanced sensor technologies, integration with artificial intelligence and machine learning, gamification and immersive exercises with integration into telehealth, and remote monitoring.
One of the most exciting prospects of VR is the ability to deliver personalized and adaptive experiences through biofeedback VR technologies.VR applications can dynamically tailor experiences to patients based on their specific needs.Real-time integration of user responses and physiological data enables these applications to optimize the effectiveness of biofeedback interventions, ensuring that patients receive the most relevant and impactful feedback.Advancements in sensor technologies are another crucial area of development.Wearable sensors, such as biometric devices, provide real-time data on BR, HR, skin conductance, muscle tension, etc. Continued improvements in sensor miniaturization, wireless connectivity, and comfort will enhance the usability and reliability of biofeedback VR technologies, making them more accessible and user friendly.
The integration of artificial intelligence and machine learning presents exciting possibilities for biofeedback VR technologies.These algorithms can analyze massive amounts of biofeedback data, identify patterns, and provide personalized recommendations for stress reduction, relaxation breathing techniques, or breath performance enhancement.Machine learning can also help in monitoring progress, evaluating outcomes, and optimizing the effectiveness of biofeedback interventions.The cooperation between artificial intelligence, machine learning, and biofeedback VR technologies has the potential to unlock new levels of personalized and evidence-based interventions.Gamification and immersive experiences play crucial roles in engaging users and maximizing the benefits of biofeedback VR technologies.By incorporating game elements and designing interactive VEs, biofeedback VR applications can provide engaging and motivating experiences.
The reviewed studies showed that VR technology can be applied in various areas in the health field, such as stress management, anxiety disorders, pain management, phobia treatment, and rehabilitation.It is necessary to establish evidence-based practices and guidelines for the use of biofeedback VR technologies in health care through collaboration among researchers, health care professionals, and developers.The integration of biofeedback VR technologies in clinical settings will revolutionize the way doctors and researchers approach treatment, improving accessibility, reducing health care costs, and enhancing patient engagement and outcomes.
In the future, innovations in biofeedback VR technologies may also include developments in neurofeedback and brain-computer interfaces with the brain activity of patients.As technology continues to grow and VR progress is investigated, these future directions are poised to shape the field, leading to transformative applications and advancements in wellness, health care, education, etc.The continued exploration and integration of VR technologies and biofeedback technologies have the potential to revolutionize how we understand and enhance human performance, well-being, and quality of life in the future.

Figure 1 .
Figure 1.PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flowchart of the literature review.ACM: ACM Digital Library; SD: ScienceDirect; VR: virtual reality.

Figure 2 .
Figure 2. Images of assembled input devices for breathing.(A) The device used a microphone to detect exhalation, which was mapped to the virtual reality (VR) game.(B) The device mapped breathing techniques to the VR game.

Figure 3 .
Figure 3. Developed system.(A) The first iteration used a common medical oxygen mask, which did not seal the airflow paths sufficiently to be able to experience breathing resistance.Thus, a safety respirator mask was used.(B) Early design of the disk controlling resistance.

Figure 4 .
Figure 4. Screenshot of the beach virtual environment in its default state (A) and while exhaling (B).

Figure 5 .
Figure 5.A participant playing the tree game while wearing a breathing headset and a virtual reality head-mounted display.

Figure 6 .
Figure 6.ZenVR learning environment that includes a virtual teacher, an hourglass, a bonsai tree, and numbers to count inhalation and exhalation.

Figure 7 .
Figure 7. Underwater environment.Screenshots showing the virtual underwater world (left), and pictures of children playing (right).

Figure 8 .
Figure 8. Developed virtual environment.Phase 1 starts with a light grey sky at a stationary participant position above the ocean surface.The participant's breath activates the water element in the virtual environment in phases 2 and 3.

Figure 9 .
Figure 9.The virtual reality system developed for alleviating dyspnea.(A) Scheme showing the real posture of the patient and the biosignal devices.(B) Representation of the virtual body and how the body's luminosity changes according to the patient's chest movements.

Figure 10 .
Figure10.The multi-user virtual reality JeL system.(A) Two jellyfish agents and a growing glass sponge.The jellyfish respond directly to each user's breathing, while the sponge reflects the synchronization of their breath.HDM: head-mounted display; VR: virtual reality.

Figure 11 .
Figure 11.The virtual therapeutic garden game.(A) Initial stage of the game in grayscale.(B) Final stage of the game with the garden full of color.

Figure 12 .
Figure 12.The games in Bubble Tower.(A) Four games are presented.(B) Game users.

Figure 13 .
Figure 13.The virtual environment based on the escape room game philosophy.The large image shows the play area of all objects.The other images show some tasks from the game.
a ACM: ACM Digital Library.bSD: ScienceDirect.

Table 2 .
Virtual reality breathing studies: sample, study design, objective, and intervention.

Table 3 .
Virtual reality breathing studies: measures, training, and study duration.
• Qualitative data: Questionnaire about what happened in the game, what was liked the most, and what was liked the least • Quantitative data: VAS • Physiological data: Lung function VRE: 5-15 min; FES: 10 sessions × 25 min No Feinberg et al [47], 2022 • Qualitative data: Expert meditator interview and qualitative learning assessment (quality of experience) • Quantitative data: Quantitative learning assessment VRE: 5 min; FES: 20 structed questionnaire before and after the intervention, and IMI i • Physiological data: HRV j VRE: 3 min; FES: not reported Yes Patibanda et al [34], 2017 • Qualitative data: Formal Analysis of Gameplay • Physiological data: RR XSL • FO No • Quantitative data: Perception of Stress Questionnaire and HADS • Physiological data: Lung function and expiratory volume Rutkowski et al [42], 2021 VRE: 6 min; FES: 50 min Yes • Quantitative data: Self-reports based on 7-point Likert scales • Physiological data: Acoustic signal of respiration, inhalation duration, exhalation duration, breathing cycles, and HRV Shih et al [24], 2019 VRE: 10 min; FES: 25 min No • Quantitative data: PSS-10 • Physiological data: RR, HR, HRV, and blood pressure b RR: respiratory rate.c HR: heart rate.d VRE: virtual reality exposure.e FES: full experimental session.f VAS: visual analog scale.g STAI: State-Trait Anxiety Inventory.h VR: virtual reality.j HRV: heart rate variability.k PSS-10: 10-item Perceived Stress Scale.l HADS: Hospital Anxiety and Depression Scale.

Table 4 .
Virtual reality breathing studies: biofeedback sensors and interactive devices.

Table 5 .
Virtual reality breathing studies: virtual environments.

Table 6 .
Virtual reality breathing studies: feasibility and findings.