Do Breath Types and Modes Matter?

A mode of positive-pressure mechanical ventilation is a predetermined pattern of patient-ventilator interactions controlled by the ventilator and can include a variety of different breath types (eg, volume control, pressure control, and pressure support, all of which may include feedback control mechanisms such as adaptive pressure control).¹ In this issue of Respiratory Care, Gibbs et al² report a pilot study to see if 2 commonly used modes of ventilatory support (volume control–continuous mandatory ventilation [VC-CMV] and adaptive pressure control–CMV [APC-CMV]) could be effectively randomized and implemented in preparation for a larger comparative trial. The results demonstrate the feasibility of such a trial, but I'm left concerned about the design and value of such a trial.

VC-CMV and APC-CMV both have similar minute ventilation, PEEP, and F_IO2 capabilities. VC-CMV has straightforward controls with clinicians required to set tidal volume and inspiratory flow (flow/volume targeted breaths). Because pressure is the main dependent variable, monitoring/alarm settings with VC-CMV are straightforward and based on airway pressure. VC-CMV gives a high level of clinician control over ventilation, but it may be difficult to synchronize fixed inspiratory flows with patient effort. VC-CMV is still the most commonly used mode around the world.³

In contrast, APC-CMV provides pressure targeted breaths, much like pressure control ventilation.^1,4 However, with APC-CMV, the inspiratory pressure is not clinician-set but rather determined by a feedback mechanism based upon a clinician-set desired tidal volume. This provides a pressure-limited, variable-flow breath allowing some patient control of flow and the ultimate tidal volume. Monitoring/alarm settings with APC-CMV are less straightforward than VC-CMV as both volume and pressure limits must be set.

Observational studies have shown that pressure targeted breaths improve patient comfort.⁵ Moreover, there are theoretical reasons why varying (rather than fixed) inspiratory flows may enhance gas mixing. On the other hand, there are potential downsides. Because patient effort impacts flow and volume delivery, a tidal volume setting below what the patient desires or an acute increase in effort from pain or anxiety could result in an inappropriately low applied inspiratory pressure. In addition, changes in lung mechanics (eg, compliance or resistance changes lowering tidal volume) may not be recognized quickly as the feedback mechanism will adjust inspiratory pressure to restore the tidal volume. Finally, as noted in the Gibbs study,² delivered volumes with APC-CMV sometimes exceed the desired tidal volume.

If a trial comparing VC-CMV to APC-CMV is to be undertaken, it should be done with the goal of determining whether important clinical differences related to breath type and mode exist. To this end, study design will be critical. First and foremost, it must be remembered that a mode encompasses more than just an “on-off” switch—modes require setting a number of important breath delivery parameters that ultimately determine safety and efficacy. Comparing modes without consideration for settings will be difficult to interpret.

It is well established that in ARDS (and likely other forms of respiratory failure) ventilator-induced lung injury from excessive tidal volumes (ie, > 8 mL/kg ideal body weight) and end-inspiratory transpulmonary distending pressures (ie, chest wall–adjusted plateau pressures > 30 cm H₂O) is a major driver of mortality, duration of the need for ventilatory support, and length of stay.⁶ It follows that studies comparing modes really need to assure that settings used are consistent with these guidelines. Settings should not be at “clinician discretion” as in the Gibbs study² but rather protocolized to assure comparable, evidence-based management goals in both arms. Only then can the role of breath type and mode be isolated and its impact determined.

One might speculate that if both VC-CMV and APC-CMV deliver volumes and pressures according to evidence-based guidelines, mortality differences would be unlikely. However, other clinically relevant outcomes could be important. First, the ease and speed in achieving and maintaining desired parameters could be mode-dependent and should be assessed. In addition, perhaps one mode could be shown to provide even “safer” settings (eg, lower F_IO2, pressure, volume requirements) despite both modes providing appropriate evidence-based support. Second, the incidence of mode failure (ie, need to permanently switch to another mode) might be driven largely by clinician perception but should be quantified by some objective documentation. Third, sedation use would be an additional useful surrogate for patient comfort and synchrony. Fourth, unscheduled clinician time (eg, alarm responses, setting adjustments, surprises [unexpected events]) should be captured. Finally, time to initiate spontaneous breathing trials/discontinuation assessments needs to be documented.

At the end of the day, I suspect both VC-CMV and APC-CMV will be shown in a properly designed trial to provide safe and effective support with similar ultimate outcomes. However, there may be minor differences between them that may be useful in specific patients. That said, the default mode chosen for a given patient should probably be the one with which the clinician (and the clinical team) is most comfortable. Moreover, a thorough understanding of the different features of alternate modes is required before changing.