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(Chest. 2006;130:815-821.)
© 2006 American College of Chest Physicians

Average Volume-Assured Pressure Support in Obesity Hypoventilation*

A Randomized Crossover Trial

Jan Hendrik Storre, MD{dagger}; Benjamin Seuthe{dagger}; René Fiechter, MD; Stavroula Milioglou; Michael Dreher, MD; Stephan Sorichter, MD and Wolfram Windisch, MD

Department of Pneumology, University Hospital Freiburg, Freiburg, Germany.

Correspondence to: Wolfram Windisch, MD, Department of Pneumology, University Hospital Freiburg, Killianstrasse 5, D-79106 Freiburg, Germany; e-mail: windisch{at}med1.ukl.uni-freiburg.de

Abstract

Background: Average volume-assured pressure support (AVAPS) has been introduced as a new additional mode for a bilevel pressure ventilation (BPV) device (BiPAP; Respironics; Murrysville, PA), but studies on the physiologic and clinical effects have not yet been performed. There is a particular need to better define the most efficient ventilatory treatment modality for patients with obesity hypoventilation syndrome (OHS).

Methods: In OHS patients who did not respond to therapy with continuous positive airway pressure, the effects of BPV with the spontaneous/timed (S/T) ventilation mode with and without AVAPS over 6 weeks on ventilation pattern, gas exchange, sleep quality, and health-related quality of life (HRQL) assessed by the severe respiratory insufficiency questionnaire (SRI) were prospectively investigated in a randomized crossover trial.

Results: Ten patients (mean [± SD] age, 53.5 ± 11.7 years; mean body mass index, 41.6 ± 12.1 kg/m2; mean FEV1/FVC ratio, 79.4 ± 6.5%; mean transcutaneous PCO2 [PtcCO2], 58 ± 12 mm Hg) were studied. PtcCO2 nonsignificantly decreased during nocturnal BPV-S/T by –5.6 ± 11.8 mm Hg (95% confidence interval [CI], –14.7 to 3.4 mm Hg; p = 0.188), but significantly decreased during BPV-S/T-AVAPS by –12.6 ± 12.2 mm Hg (95% CI, –22.0 to –3.2 mm Hg; p = 0.015). Pneumotachographic measurements revealed a higher individual variance of peak inspiratory pressure (p < 0.001) and a trend for lower leak volumes but also for higher tidal volumes during BPV-S/T-AVAPS. The SRI summary scale score improved from 63 ± 15 to 78 ± 14 during BPV-S/T (p = 0.004) and to 76 ± 16 during BPV-S/T-AVAPS (p = 0.014). Sleep quality and oxygen saturation also comparably improved following BPV-S/T and BPV-S/T-AVAPS.

Conclusion: BPV-S/T substantially improved oxygenation, sleep quality, and HRQL in patients with OHS. AVAPS provided additional benefits on ventilation quality, thus resulting in a more efficient decrease of PtcCO2. However, this did not provide further clinical benefits regarding sleep quality and HRQL.

Key Words: bilevel pressure ventilation • health-related quality of life • noninvasive positive-pressure ventilation • obesity hypoventilation syndrome • sleep

Noninvasive positive-pressure ventilation (NPPV) used for home mechanical ventilation (HMV) is a well-established and increasingly used therapeutic option for patients with chronic hypercapnic respiratory failure due to COPD, or thoracic rib cage or neuromuscular diseases.1234 NPPV therapy has also been successfully used in patients with obesity hypoventilation syndrome (OHS).567 Here, continuous positive airway pressure (CPAP), bilevel pressure ventilation (BPV), and volume-limited ventilation have been used in OHS patients, but randomized controlled trials comparing different ventilation modalities have yet not been performed.

Following the expansion of NPPV therapy over the past 15 years, the technique for the application of NPPV has been greatly refined. There has been a shift away from volume-limited ventilators to more comfortable and smaller pressure-limited ventilators.46 Recent randomized controlled trials89 have indicated that therapy with volume-limited and pressure-limited NPPV are comparably effective. However, while pressure-limited NPPV has been shown to be better tolerated by the patient due to the less varying peak inspiratory pressure (PIP),8 volume-limited NPPV provides greater stability of tidal volume (VT) in the face of varying patient effort, chest wall compliance, or airway resistance.10

There is increasing interest by manufacturers in combining the advantages of pressure-limited and volume-limited modes of ventilation into one ventilation mode. Hence, hybrid modes such as average volume-assured pressure support (AVAPS) have been developed to ensure a more consistent VT while delivering the comfort and advantages of pressure support ventilation. This new hybrid mode has been studied in intubated patients, suggesting the presence of a reduction in the muscle workload and an improvement of synchrony between the patient and the ventilator.11 Remarkably, no studies on the physiologic and clinical effects of this new mode have yet been performed in patients with chronic hypercapnic respiratory failure, although this feature is now available for routine clinical use. Such studies are needed in order to verify the postulated benefits of these ventilation modes. Therefore, the present study was aimed at prospectively investigating the physiologic and clinical effects of AVAPS in addition to the BPV-spontaneous/timed (S/T) ventilation mode vs BPV-S/T mode alone in OHS patients following a crossover design.

Materials and Methods

This study protocol was approved by the institutional review board for human studies of the Albert-Ludwig University, Freiburg, Germany, and was performed in accordance with the ethical standards laid down in 2000 by the Declaration of Helsinki. Informed written consent was obtained from all patients.

Patients
Clinically stable OHS patients with a body mass index of >30 kg/m2 and daytime hypercapnia (ie, PaCO2 ≥ 45 mm Hg) who had failed to respond to CPAP therapy were consecutively enrolled. No other cause for their chronic respiratory failure could be identified, and patients were naive to any ventilatory treatment. Patients who had evidence of acute respiratory failure (ie, patients with worsening symptoms during the last 2 weeks, a breathing frequency [fb] of >30 breaths/min, a pH of < 7.35) or signs of respiratory infections were excluded from the study. Patients who had been intubated during the last 3 months or had received any other form of ventilatory support prior to hospital admission were also excluded from the study.

Study Design
All modes of ventilation were provided by use of a BPV device (BiPAP Synchrony; Respironics Inc; Murrysville, PA). Patients who did not respond to CPAP therapy were randomly assigned to receive one of the two modes of BPV-S/T in a crossover design, one with AVAPS and one without AVAPS. A CPAP responder was defined as a patient who achieved a transcutaneous PCO2 (PtcCO2) level of < 45 mm Hg and a respiratory disturbance index (RDI) score of < 10 events per hour.

CPAP nonresponders were discharged from the hospital to home with therapy with BPV-S/T with or without AVAPS following randomization and were readmitted to the hospital after 6 weeks of HMV therapy. Baseline measurements were repeated, and patients were switched to receiving the complementary mode of BPV-S/T. Measurements were again performed after another period of 6 weeks of HMV therapy following hospital readmission.

Ventilator Settings
Ventilator settings were changed according to the patient’s daytime and nocturnal tolerance, and to maximally decrease PtcCO2 prior to polysomnography for the study, as follows: for BPV-S/T, inspiratory positive airway pressure (IPAP) was set at a level up to 20 mbar; expiratory positive airway pressure (EPAP) levels ranged 4 to 8 mbar; fb was set between 12 and 18 breaths/min, as tolerated; and the inspiratory to expiratory time was set to a ratio of 1:2.

The optional AVAPS mode is an additional feature that ensures a preset inspiratory VT during BPV-S/T. For this purpose, the expiratory volume (Vexp) of the patient (ie, the approximated VT) is calculated based on pneumotachographic inspiratory and expiratory flow measurements. A tolerance of ± 10% has been indicated for this calculation by the manufacturer. IPAP is then titrated during ventilation in steps of 1 mbar/min in order to achieve the preset VT. Therefore, an IPAP range that may be delivered during AVAPS is set rather than a fixed IPAP. In the present study, the maximal possible IPAP range was chosen in order to view the total spectrum of AVAPS so that the IPAP was set between EPAP and 30 mbar. AVAPS was set to 7 and 10 mL/kg, and was calculated to an ideal weight of 24 kg/m2, with the best setting regarding tolerance and maximal decrease of PtcCO2 being chosen.12 During both treatment periods, the settings for EPAP, respiratory rate, and inspiratory/expiratory ratio were kept at the same level. No patient received supplemental oxygen.

Measurements
Lung function parameters (Masterlab-Compact Labor; Jaeger; Hochberg, Germany) were measured at baseline. Daytime blood gas levels measured with the patient at rest were obtained from the arterialized earlobe and analyzed (AVL OMNI; Roche Diagnostics GmbH; Graz, Austria). Health-related quality of life (HRQL) was measured using the severe respiratory insufficiency questionnaire (SRI), which has been specifically designed to measure HRQL in patients receiving HMV.13

Full polysomnography (SIDAS GS; Heinen & Loewenstein; Bad Ems, Germany) and measurements of PtcCO2 (Tina TCM2; Radiometer; Copenhagen, Denmark) were performed according to published guidelines1415 during the night at baseline, and during therapy with CPAP, BPV-S/T, and BPV-S/T-AVAPS. Measurements of ventilation were made during therapy with both modes of BPV-S/T using a pneumotachograph (Ventrak Respiratory Monitoring System, model 1550; Novametrix Medical Systems; Wallingford, CT) that was placed between the mask and the exhalation system (Silentflow 2; Weinmann; Hamburg, Germany), which served as an intentional leak of the circuit. Measurements of volumes, pressures, and fb were performed every minute. Therefore, pneumotachographic measurements produced a mean value and the SD for every patient, indicating the individual variance of pneumotachographic measurements.

Statistical Analysis
Statistical analysis was performed using a statistical software package (Sigma-Stat, version 3.1; Systat Software, Inc; Point Richmond, CA). Data are presented as the mean ± SD after testing for normal distribution (Kolmogorov-Smirnov test). If the data were normally distributed, one-way repeated-measures analysis of variance was performed to compare values at baseline, and during therapy with CPAP, BPV-S/T, and BPV-S/T-AVAPS. Friedman repeated-measures analysis of variance on ranks was performed for nonnormally distributed data. Comparisons between measurements at two different times were performed using the paired t test if the data were normally distributed and using the Wilcoxon signed rank test if the data were not normally distributed. For normally distributed data, the 95% confidence interval (95% CI) was given. Correlation analysis was performed using the Pearson product moment correlation. Statistical significance was assumed with at a p value of < 0.05.

Results

One CPAP responder, one patient who was intolerant to any mask, and two patients who were intolerant to BPV who were discharged from the hospital using CPAP therapy were excluded from the study and were not analyzed further. The characteristics of the 10 patients who completed the study protocol are provided in Table 1 . The mean BMI was 41.6 ± 12.1 kg/m2 (range, 30.1 to 65.4 kg/m2). All patients had obstructive sleep apnea syndrome (OSAS) in addition to OHS. Based on patient comfort, six patients used the nasal mask (Profile Lite; Respironics) and four patients used the full facemask (Comfort full; Respironics) throughout the study.


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  		Table 1.. Demographic Data, Lung Function Parameters, and Daytime Blood Gas Levels at Baseline in Patients Who Completed the Study*

 
The mean CPAP level set was at 8.9 ± 1.0 mbar (range, 7 to 10 mbar). The mean IPAP setting was 17 ± 2 mbar for BPV-S/T. Pneumotachographic measurements during BPV-S/T therapy revealed a mean PIP of 15 ± 2 mbar, which was lower than the IPAP setting (p = 0.064). However, the IPAP setting and PIP were highly correlated during BPV-S/T therapy (r = 0.94; p < 0.001). The mean AVAPS setting (608 ± 134 mL [five patients at 7 mL/kg and five patients at 10 mL/kg]) was significantly correlated to PIP during BPV-S/T-AVAPS therapy (r = 0.71; p = 0.021). The mean PIP ranged from 11.8 to 17.8 mbar during BPV-S/T therapy and from 9.8 to 21.2 mbar during BPV-S/T-AVAPS therapy. The variance of PIP overnight was significantly higher during BPV-S/T-AVAPS therapy, as indicated by its higher individual SD (Fig 1 ). The mean EPAP was set at 6 ± 1 mbar during therapy with both BPV-S/T and BPV-S/T-AVAPS, and this was not different from pneumotachographic measurements made during therapy with positive end-expiratory pressure (Table 2 ).


Figure 1
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  		Figure 1.. SD indicating the variance of the PIP during nocturnal BPV-S/T and BPV-S/T-AVAPS therapy.

 

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  		Table 2.. Pneumotachygraphic Measurements During Nocturnal BPV-S/T and BPV-S/T-AVAPS Therapy*

 
Large leak volumes (Vleaks) were seen during both bilevel studies (Table 2). Vleak tended to be nonsignificantly lower, and Vexp tended to be higher during BPV-S/T-AVAPS therapy compared to BPV-S/T therapy. The mean respiratory rate of the ventilator was set at 14 ± 1 breaths/min both during BPV-S/T and BPV-S/T-AVAPS therapy, which was lower than the actual fb (p = 0.033 and 0.016, respectively).

At baseline, sleep architecture and gas exchange were severely disturbed (Table 3 ). During CPAP therapy, the arousal index (p = 0.003), RDI (p < 0.001), and arterial oxygen saturation (SaO2) [p = 0.007] improved significantly, but there was no increase in non-rapid eye movement (NREM) stage 3 and 4 sleep, and patients remained severely hypercapnic. Following the establishment of BPV-S/T therapy, NREM stage 3 and 4 sleep increased significantly when compared to baseline values (p = 0.005) and to NREM stage 3 and 4 sleep during CPAP therapy (p = 0.016). SaO2 during BPV-S/T therapy was improved compared to baseline values (p = 0.019) and was without any further benefit when compared to SaO2 during CPAP therapy.


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  		Table 3.. Polysomnography, Nocturnal PtcCO2, and Daytime Blood Gas Levels for Patients Receiving CPAP, BPV-S/T, and BPV-S/T-AVAPS Therapy*

 
Mean PtcCO2 nonsignificantly decreased during BPV-S/T therapy (decrease, –5.6 ± 11.8 mm Hg; 95% CI, –14.7 to 3.4 mm Hg; p = 0.188), but markedly and significantly decreased during BPV-S/T-AVAPS therapy compared to baseline values (–12.6 ± 12.2 mm Hg; 95% CI, –22.0 to –3.2 mm Hg; p = 0.015). PtcCO2 during BPV-S/T-AVAPS therapy was also lower when compared to values during BPV-S/T therapy (–6.9 ± 5.5 mm Hg; 95% CI, –10.8 to –3.0 mm Hg; p = 0.003) [Fig 2 ]. However, this reduction in PtcCO2 did not lead to further improvements in sleep quality compared to those with standard BPV-S/T therapy (Table 3). The RDI score was even higher during BPV-S/T-AVAPS therapy than during BPV-S/T therapy alone (p < 0.05) [Table 3]. Daytime PaCO2 and bicarbonate levels significantly improved following BPV-S/T-AVAPS therapy only (Table 3). HRQL comparably improved following the establishment of BPV-S/T and BPV-S/T-AVAPS therapy (Fig 3 , Table 4 ).


Figure 2
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  		Figure 2.. PtcCO2 during the night at baseline, and during therapy with CPAP, BPV-S/T, and BPV-S/T-AVAPS.

 

Figure 3
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  		Figure 3.. Summary scale of the SRI at baseline, and following therapy with BPV-S/T and BPV-S/T-AVAPS.

 

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  		Table 4.. HRQL Assessed by the SRI at Baseline, and Following BPV-S/T and BPV-S/T-AVAPS Therapy*

 
Discussion

In the present study, OHS patients not responding to CPAP therapy were randomly assigned to receive BPV-S/T or BPV-S/T-AVAPS therapy in a crossover design. The main finding was that sleep quality and gas exchange substantially improved during nocturnal BPV-S/T therapy compared to baseline, but patients remained hypercapnic overnight even after 6 weeks of HMV following the careful establishment of BPV-S/T therapy in the hospital. The addition of volume assurance (ie, AVAPS) to BPV-S/T therapy resulted in a significant decrease of PtcCO2, thus normalizing PtcCO2 during sleep.

This is the first crossover study that has investigated the physiologic and clinical effects of volume assurance modalities such as AVAPS when added to BPV-S/T therapy with the following findings. First, as expected, a relatively constant PIP was seen during BPV-S/T therapy. In contrast, the variance of PIP was markedly higher when AVAPS was added, suggesting a changing IPAP level aimed at volume assurance. Second, the inspiratory volume (Vinsp) delivered by the ventilator was nearly identical during BPV-S/T and BPV-S/T-AVAPS therapy. Despite this, the minimal assured VT (ie, the Vexp) tended to be higher, and the Vleak tended to be lower during BPV-S/T-AVAPS therapy. However, inspiratory leakage is likely to be overestimated by Vleak, which also includes expiratory leakage. Similarly, real VT is likely to be underestimated from Vexp alone due to expiratory leakage. Unfortunately, inspiratory and expiratory leakage could not be assessed separately. Therefore, the benefit of AVAPS could become more evident with regard to the real VT (ie, Vexp + expiratory leakage). Finally, when AVAPS is added to BPV-S/T therapy, these mechanisms resulted in a more efficient PtcCO2 decrease.

Previous work5671617181920 has shown that NPPV is capable of substantially decreasing PaCO2 in patients with OHS. However, it has been clearly pointed out in the most recent study7 that an IPAP of ≥ 20 mbar is usually poorly tolerated, and that a significant proportion of patients continue to be hypercapnic. Accordingly, the patients in the present study remained significantly hypercapnic during nocturnal BPV-S/T therapy even after six 6 weeks of HMV when AVAPS was not used. On the other hand, volume-limited ventilation has been suggested to be more efficient in decreasing PaCO2 in OHS patients by increasing PIP in order to deliver a fixed Vinsp.7 However, volume-limited ventilation may be poorly tolerated in a significant number of patients with OHS,518 and BPV devices have become the ventilators that are used most frequently in most centers.6716181920 Therefore, the present study provides evidence that BPV-S/T-AVAPS therapy combines the advantages of BPV and volume-limited ventilation by providing the most efficient ventilation with high tolerance.

Compared to previous studies,5671621 patients were younger and less hypoxemic. However, hypercapnia has been suggested to serve as a pertinent discriminating factor between patients with OSAS alone and those in whom OSAS is associated with OHS.2122 It is, therefore, conceivable that patients were assessed during an earlier stage of OHS. All OHS patients were in a stable state, but some patients in previous studies57 had experienced an acute exacerbation, as indicated by the lower pH values and clinical circumstances. Furthermore, airflow obstruction was evident in patients in other studies,61621 indicating that some of the individuals had COPD in addition to OHS.

A recent study23 has indicated that subjective sleepiness, as assessed by the Epworth sleepiness scale, and general aspects of HRQL, as assessed by the Medical Outcomes Study 36-item short form,2425 improved in OHS patients following treatment with CPAP.26 In line with this observation, the present study has shown that specific aspects of HRQL improved following the establishment of BPV-S/T therapy when a highly specific questionnaire, the SRI,13 was used. Compared to different historical cohorts receiving HMV (ie, patients with COPD, or neuromuscular or thoracic rib cage diseases), HRQL when assessed by the SRI was considerably higher in the OHS patients in the present study. The highest benefit was gained in the SRI anxiety subscale, indicating the importance of anxiety in patients with OHS.

It might be argued that the addition of AVAPS to BPV-S/T therapy has physiologic benefits as it more efficiently decreased PaCO2. Despite this, no clinical benefits could be documented, since sleep quality and HRQL did not further improve compared to the results from BPV-S/T therapy alone. However, other important outcome parameters such as survival or exacerbation rate were not assessed in the present study, and no long-term observations were provided. Another limitation of the present study was that the EPAP settings used may have been inadequate to maintain upper airway patency, thus contributing to a high residual RDI score. Therefore, the impact of AVAPS remains unclear if higher EPAP levels had been chosen. It also remains unclear why the RDI score was even higher when AVAPS was added to therapy. It might be also suggested that AVAPS titration with steps of 1 mbar/min is too slow to correct hypopnea. Therefore, AVAPS may improve ventilation independently from the ability to prevent hypopneas, since nocturnal and daytime PaCO2 values were significantly improved. There is clearly room for further studies to investigate the physiologic and clinical impacts of volume assurance such as AVAPS during BPV-S/T therapy.

In conclusion, BPV-S/T therapy substantially improves clinical parameters such as sleep quality and specific aspects of HRQL in patients with OHS. The addition of AVAPS to BPV-S/T provides beneficial physiologic improvements, resulting in a more efficient decrease of PtcCO2 compared to BPV-S/T therapy alone. This, however, did not provide further clinical benefits regarding sleep quality and HRQL in the present group of highly selected patients. Further studies including different patient cohorts and different ventilator settings are necessary in order to further define the benefit of volume assurance during pressure-limited NPPV therapy.

Footnotes

Abbreviations: AVAPS = average volume-assured pressure support; BPV = bilevel pressure ventilation; CI = confidence interval; CPAP = continuous positive airway pressure; EPAP = expiratory positive airway pressure; fb = breathing frequency; HMV = home mechanical ventilation; HRQL = health-related quality of life; IPAP = inspiratory positive airway pressure; NPPV = noninvasive positive-pressure ventilation; NREM = non-rapid eye movement; OHS = obesity hypoventilation syndrome; OSAS = obstructive sleep apnea syndrome; PtcCO2 = transcutaneous PCO2; PIP = peak inspiratory pressure; RDI = respiratory disturbance index; SaO2 = arterial oxygen saturation; SRI = severe respiratory insufficiency questionnaire; S/T = spontaneous/timed; Vexp = expiratory volume; Vinsp = inspiratory volume; Vleak = leak volume; VT = tidal volume

{dagger} These authors contributed equally to this work. Back

The study was supported by Heinen & Loewenstein, Bad Ems, Germany.

The authors have reported to the ACCP that the study design, the results, the interpretation of the findings or any other subject discussed in the article were not dependent on financial support.

Received for publication March 27, 2006. Accepted for publication June 13, 2006.

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