Target volume settings for home mechanical ventilation: great progress or just a gadget?

Home mechanical ventilation (HMV) is a well-established treatment option for patients with chronic hypercapnic respiratory failure, whereby non-invasive positive pressure ventilation (NPPV) serves as the predominant means of HMV delivery. 2 In general, there are two physiologicallydifferent modes of NPPV deliveries, volume-preset NPPV and pressure-preset NPPV. During volume-preset NPPV a fixed inspiratory volume (Vinsp) is set at the ventilator, while the inspiratory positive airway pressure (IPAP) varies depending on airway resistance. Conversely, Vinsp varies during pressure-preset NPPV, while IPAP remains fixed. The advantage of volume-preset NPPV is that Vinsp, and hence tidal volume, are relatively stable; however, this can lead to a breath-bybreath variation in IPAP levels that can become a burden for the patient, and the leakages that regularly occur during NPPV are not compensated for. In contrast, the Vinsp that is delivered during pressurepreset NPPV may be unstable due to increased airway resistance; however, given that the variation in IPAP is lower, this is often better tolerated by the patient. In addition, leak compensation is provided by pressure-preset NPPV, as shown by in vitro and in vivo studies. In addition, ventilators providing pressurepreset NPPV are cheaper. Thus, pressurepreset NPPV has become the predominant means of delivering HMV. Nevertheless, randomised controlled trials have shown that volumeand pressure-preset NPPV generally have comparable effects on improvements in blood gases, sleep quality and health-related quality of life (HRQL), 6 although pressure-preset NPPV is reportedly better tolerated due to fewer gastrointestinal side effects. However, clinicians should always weigh the advantages and disadvantages of the two different approaches on an individual patient basis. Recently, the so-called hybrid modes have been developed to overcome the disadvantages of volumeand pressurepreset NPPV, respectively. These modes seek to combine the advantages of the two classical modes, that is, the stability of Vinsp and the avoidance of cumbersome variations in breath-by-breath IPAP levels. This is achieved by setting a so-called ‘target volume’. Here, pressure-preset NPPV is primarily used, but instead of a fixed IPAP, a pressure range with minimal and maximal IPAP values is set at the ventilator. In response to the automatic calculations and adjustments made by the ventilator, IPAP can undergo a smooth transition within a preset range in order to reach the target volume. The most extensively studied hybrid mode is the Average Volume Assured Pressure Support (AVAPS) mode. An initial randomised-controlled cross-over trial suggested that the addition of AVAPS to pressure-preset NPPV resulted in better control of nocturnal ventilation compared with pure pressure-preset NPPV in patients with obesity hypoventilation syndrome (OHS). However, both HRQL, as assessed by the Severe Respiratory Insufficiency (SRI) questionnaire, 9 and sleep quality, as measured by polysomnography, showed similar improvements under each of the two modes. Subsequent trials that primarily included OHS and chronic obstructive pulmonary disease (COPD) patients showed an overall comparable outcome, although nocturnal hypoventilation was better controlled in some trials (table 1). However, in one study polysomnographic sleep quality was shown to be reduced during AVAPS compared with pure pressure-preset NPPV, probably as a result of considerably high target volume settings (table 1). However, observational periods were rather short in the previous trials, and in some cases patients were already familiar with pressure-preset NPPV prior to randomisation, thus indicating a selection bias. Therefore, the evidence for target volume setting remains inconclusive, and it is yet to be established whether these hybrid modes of ventilation have benefits that are clear and consistent enough to warrant official recommendations. Murphy and colleagues randomly allocated 50 NPPV-naive patients with severe OHS (body mass index 5067 kg/m) either to pure pressure-preset NPPV using pressure support ventilation or to AVAPS mode, with 46 patients completing the trial. After 3 months of treatment, daytime arterial partial pressure of carbon dioxide (primary outcome) improved comparably in both groups by a mean 0.6 kPa. In addition, improvements in HRQLdagain measured with the highly specific SRI dalso showed a similar improvement in both groups, while both treatment strategies resulted in equivalent degrees of weight loss (table 1); therefore, AVAPS was not advantageous. This study significantly adds to the current body of knowledge about target volume setting used for HMV for the following reasons. First, it is the largest randomised controlled trial investigating HMV with target volume settings, and second, the observational period is the longest implemented to date (table 1). Third, and most importantly, ventilator settings were individually adjusted according to a predefined protocol that included a nocturnal assessment period aimed at achieving optimal control of nocturnal ventilation. This protocol was a unique aspect of the study and held true for both modes, rather than just for the AVAPS settings. Therefore, patients could be discharged after optimal treatment had been established, with or without AVAPS. As a consequence, target volume settings did not provide any additional benefits compared with pressure-preset NPPV. In reality, clinicians are regularly confronted with new technical developments for HMV, whereby new modes and features are increasingly provided without any evidence of benefits or even without clear recommendations on how to adjust the settings. Target volume serves as an excellent example: despite the lack of evidence for any actual benefits, many new-generation ventilators provide target volume modes. Confusingly, the nomenclature and algorithms used for IPAP adaptation vary considerably among different types of ventilators and Department of Pneumology, Cologne-Merheim Hospital, Kliniken der Stadt Köln gGmbH, Witten/Herdecke University Hospital, Germany