Fig. 77.1
Upper panel A: dual-control “within-a-breath” mode. The figure shows that, if the target volume cannot be reached by the pressure ventilation mode, the ventilator switches from pressure to volume control. The flow changes from decelerated to fixed (b) and inspiratory pressure passively increases (a). Target volume (Vt) is adjusted according to the measured inspiratory volume (Vti). Ti max = preset maximum inspiratory time. (Reproduced with permission from ResMed). Lower panel B: Dual-control “breath-to-breath” mode. The figure shows that if the tidal volume is not reached, the pressure progressively increases from breath to breath within a preset pressure range until the desired target volume is obtained (Reproduced with permission from Philips Respironics)
The iVAPS (intelligent volume-assured pressure support) is a dual-control mode in which the target is not tidal volume (like PSV-VT and AVAPS) but alveolar ventilation. It is an autotitrating mode of ventilation that has appeared on the market in recent years. While taking into account a predicted anatomical dead space, it varies respiratory rate and pressure support within preset limits to maintain a target alveolar ventilation. If ventilation falls below the target, the ventilator responds to the change quicker than the previous dual-control ventilators, increasing pressure support at a higher rate.
In dual-control ventilators, the target tidal volume must always be set. In addition, depending on the ventilator algorithm, minimal and maximal pressure, inspiratory flow rate, or inspiratory time may be required to set. There are no official recommendations about which target tidal volume should be set, and in most centers, indications provided by manufacturers are adopted. According to these indications, in most studies, dealing with patients with different diseases, the minimal tidal volume was set at about 8 ml/kg of ideal weight, with a range between 7 and 12. However, no comparative studies between settings have been performed. Both in the within-a-breath and in the breath-to-breath modes, if the preset minimal target volume is too low, it may lead to an increase in respiratory rate, which may cause an increase in the work of breathing. Instead, if the set tidal volume is too high, the ventilator may raise the pressure support and possibly cause barotrauma or hemodynamic compromise, or lead to the appearance of intrinsic positive end-expiratory pressure (PEEP).
In the breath-to-breath mode, the pressure range is manually or automatically set. If the maximum set pressure limit is too high it could not only cause barotrauma but also an undesired increase in the delivered volume. On the other hand, if the minimal preset inspiratory pressure is too low, the drop in pressure that follows leaks may cause hypoventilation.
In the within-a-breath mode, inspiratory flow rate or time is set. If the minimal inspiratory flow rate is set too low, lung inflation may be slow, with a late switch from pressure to volume control and an undesired prolongation of inspiratory time. An inspiratory time that is too long can cause expiratory asynchrony.
77.2.1 Consequences of Leaks on Efficacy of Ventilation
An important issue to address is the behavior of volume target mode in the presence of unintentional leaks. Contal et al. [4] tested seven home ventilators on a bench model adapted to simulate leaks. They found that tidal volume provided by the software in the presence of leaks was underestimated by all devices, and that, for most devices, bias increased with higher insufflation pressures. Similarly, Lujàn et al. [5] evaluated the reliability of the tidal volume provided by five ventilators in a bench test with simulated leaks. They found that all the tested ventilators underestimated tidal volume, and that the higher the leak, the higher the difference between the estimated and the actual tidal volume. By contrast, Sogo et al. [6], who evaluated the same ventilators in similar conditions, found that inspiratory leaks were followed by overestimates of tidal volume by most of ventilators. They also found that, in the presence of expiratory leaks, the behaviors of each of the tested ventilators were different; three ventilators underestimated and one ventilator overestimated the tidal volume. Independently from these contrasting results, which may be the result of different study designs and protocols, it is important to emphasize that, when large leaks occur, data provided by the ventilator software may not always be reliable. In clinical practice, where the behavior of leaks is often erratic, this may translate into a difficult assessment of patients’ ventilation. Inclusion of algorithms that calculate the pressure loss as a function of the flow exiting the ventilator may increase the reliability of tidal volume estimation.
Tidal volume estimation by dual-control ventilators during air leaks may vary with circuit configuration. Khirani et al. [7] studied the ability of seven home dual-mode ventilators to maintain the preset minimal target volume during leaks with different circuit configurations. They reported that all studied ventilators with a single-limb circuit were able to maintain the minimal preset tidal volume during leaks; only one ventilator overcompensated during prominent leaks. By contrast, with the exception of one ventilator model, all the studied ventilators with a single circuit and an expiratory valve, or with a double circuit, failed to maintain the minimal tidal volume during leaks, because they misinterpreted leaks as an increase in tidal volume and therefore decreased their inspiratory pressure to the minimal preset level. Similar results were reported by Carlucci et al. [8]. However, Lujan et al. [9] have shown that the introduction of a random leak also influences the performance of commercial ventilators with a single-limb circuit, and that inspiratory leaks decrease the delivered tidal volume, with an important clinical impact in terms of hypoventilation. Thus, an effective estimation of ventilation during leaks may depend on the circuit and the configuration of the ventilator and may vary with the ventilator model. The loss of accuracy in estimated tidal volume may also cause patient-ventilator asynchrony, although this point has not been specifically addressed in most of the previously cited studies.
77.2.2 Clinical Trials on Dual-Mode Ventilators (Table 77.1)
Table 77.1
Major trials on Dual-mode NIV available in the literature
Authors | Ventilator | Setting | Patients | Duration | Outcomes with target mode |
|---|---|---|---|---|---|
Storre et al. (2006) | Synchrony Cross-over | Fixed PS vs AVAPS 7–10 ml/kg IBW Pressure max 30 cmH2O | 10 stable OHS | 6 weeks | Improved nocturnal gas exchange; no variation in sleep quality and HRQL |
Janssen et al. (2009) | Synchrony Cross-over | Fixed PS vs AVAPS 8–10 ml/kg AB Pressure max 30 cmH2O | 12 stable OHS | 2 nights | Improved nocturnal gas exchange; decreased comfort, subjective and objective sleep quality |
Crisafulli et al. (2009) | Synchrony Cross-over | Fixed PS vs AVAPS 8 ml/kg IBW Pressure max 30 cmH2O | 9 stable COPD | 2 weeks | No variation in diurnal gas exchange and comfort Better subjective sleep efficiency |
Jaye et al. (2009) | AutoVPAP Cross-over | Fixed PS vs iVAPS Autotitrating Pressure max 21 cmH2O | 20 stable NMD | 2 months | No differences in nocturnal gas exchange and sleep quality |
Ambrogio et al. (2009) | Synchrony Cross-over | Fixed PS versus AVAPS 8 ml/kg IBW or 110 % of baseline VT Pressure max 30 cmH2O | 28 stable CRF | 2 nights | No differences in nocturnal gas exchange and sleep quality; greater minute ventilation |
Crescimanno et al. (2011) | Idea Ultra Cross-over | Fixed PS versus PSV-VG 8–10 ml/kg IBW Pressure max 20 cmH2O | 28 stable NMD | 2 nights | No differences in nocturnal gas exchange, comfort and subjective sleep quality; more asynchrony |
Murphy et al. (2012) | Synchrony Parallel group | Fixed PS versus AVAPS 8–10 ml/kg IBW Pressure max 22 cmH2O | 46 stable Super OHS | 3 months | No differences in daytime gas exchanges |
Briones Claudett et al. (2013)
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