The Evolution of Intermittent Mandatory Ventilation: Update and Implications for Home Care

Introduction

Recently this Journal published a paper we wrote about how to apply the taxonomy for modes of ventilation¹ to portable ventilators used for noninvasive ventilation.² The mode taxonomy has 3 main components: (1) the control variable, pressure or volume; (2) the breath sequence, continuous mandatory ventilation (CMV), intermittent mandatory ventilation (IMV), and continuous spontaneous ventilation (CSV); and (3) the targeting schemes applied to mandatory and spontaneous breaths.¹ Such a taxonomy is just as important for understanding generic modes versus brand name modes as the taxonomy that distinguishes generic drugs from brand name drugs.³ Just like a drug taxonomy that changes over time, the mode taxonomy must evolve as the underlying technology of mechanical ventilators becomes more complicated. The purpose of this letter is to illustrate this phenomenon.

A New Type of Intermittent Mandatory Ventilation

The ventilator mode taxonomy relies on the concept of the breath sequence to provide a key characteristic for classifying a mode. The taxonomy defines 2 types of breaths: spontaneous breaths are those for which inspiration is both triggered (started) and cycled (stopped) by the patient. Mandatory breaths are anything else: patient triggered but machine cycled, machine triggered but patient cycled, or machine triggered and machine cycled. Variables used by ventilator design engineers for patient triggering and cycling commonly include pressure and flow and less commonly the electrical activity of the diaphragm (as used in neurally-adjusted ventilatory assist), volume, and chest-wall impedance. These are all variables that reflect the way that the patient’s respiratory system mechanics affect the variables in the equation of motion for the respiratory system and hence signal the ventilator to start or stop inspiration independently from any settings on the ventilator that would start or stop inspiration. In contrast, the variable used by the ventilator for machine triggering is time in the form of a preset breath rate that sets the ventilatory period (period = 1/breath rate). The variables used for machine cycling are volume (ie, preset tidal volume) or time in the form of a preset inspiratory time.

IMV, as one may imagine, creates many different ways to trigger and cycle breaths because it is defined as a mode for which spontaneous breaths may appear between mandatory breaths. We define IMV in this particular way to avoid confusion with modes of pressure control CMV on ventilators with active exhalation valves that allow spontaneous breaths to be superimposed on mandatory breaths⁴ (ie, the appearance of spontaneous breaths in this case should not turn CMV into IMV). These different ways to configure IMV have led to an evolution of the breath sequence into several different types. In a recent paper, we described this evolution as primarily the attempt to allow spontaneous breaths to suppress mandatory breaths.⁵ Design engineers have developed various ways to do this because it serves the goal of comfort.⁶ Suppression of mandatory breaths by spontaneous breaths serves the goal of comfort because spontaneous breaths maximize the synchrony between patient demand and ventilator response due the fact that the patient triggers and cycles inspiration and thus retains substantial control over the timing of the breath (ie, rate and inspiratory time). In other words, the patient’s brain generates a “neural” breath rate and inspiratory time that the ventilator must match with minimal error for maximum clinical patient-ventilator synchrony.⁷

Our previous paper described 4 different kinds of IMV⁵ that were used to classify noninvasive modes of ventilation.² However, since then, we have identified a fifth type of IMV that seems to appear only on portable ventilators. To review briefly, IMV(1) is the original form for which mandatory breaths are delivered at the preset rate regardless of any spontaneous breath rate. IMV(2) allows spontaneous breaths to suppress mandatory breaths if their rate is higher than the set mandatory rate (or in practice if the period between the last mandatory breath and the first spontaneous breath is less than the preset mandatory period). IMV(3) allows spontaneous breaths to suppress mandatory breaths if the minute ventilation due to spontaneous breaths is larger than the preset mandatory minute ventilation (ie, the product of preset mandatory breath rate and tidal volume). IMV(4) occurs with certain types of dual targeting⁸ whereby an individual breath that was scheduled to be mandatory (ie, volume controlled and time cycled) is turned into a spontaneous breath (ie, pressure controlled and flow cycled) if the patient’s inspiratory effort is large enough. The fifth type of IMV, IMV(5), results from the addition of a new setting to conventional Pressure Support (or Spontaneous, S) modes found on some portable ventilators. This setting is the minimum inspiratory time (T_{I min}) which must be set > 0. As a result, a breath that was intended to be spontaneous (ie, patient triggered, and patient cycled) becomes a mandatory breath if the inspiratory effort is too short (ie, the neural inspiratory time is less than T_{I min}). Thus, the machine assures a minimal duration of breath delivery and thereby reduces the risk of insufficient tidal volume. Inspiratory time may be dangerously curtailed during a spontaneous breath, particularly among patients with neuromuscular or restrictive disease. Thus, IMV(5) serves to provide safety under these circumstances.⁹ We must note that IMV(5) can be available without a set backup rate, of which clinicians should be aware. The 5 types of IMV are summarized in Table 1.

Table 1.

Types of Intermittent Mandatory Ventilation

The Rationale for Defining IMV(5) as Shown in Table 1 Is as Follows

Virtually all modes that are classified as CSV have a T_{I max} setting as a safety backup in the event that the normal patient cycling variable fails. For example, ventilators with pressure support allow the operator to set T_{I max} so that the breath will cycle off by time in the event that flow cannot decay to the flow cycle threshold in the case of a mask leak. If such an event occurs, technically the breath would be classified as mandatory (due the machine cycling). However, the mode taxonomy ignores this because it is not the normal function of pressure support and because it would be confusing and counterproductive to classify this mode as a form of IMV.

In contrast, the T_{I min} setting was designed for a specific purpose to occur as needed during normal ventilation. For example, the operator’s manual for the ResMed Stellar 100 and Stellar 150 ventilator (ResMed, San Diego, California) explains it this way: “For some patients whose inspiratory effort or flow are weak and insufficient, T_{I min} prevents the premature cycling to EPAP (expiratory positive airway pressure). Premature cycling to EPAP can result in insufficiently supported breaths.” What this means is that the period set by bounds on the inspiratory time (ie, T_{I max} − T_{I min}) represents what we could call a “spontaneous breath window.” If the patient’s neural inspiratory time falls within this window, a spontaneous breath results (due to patient triggering and cycling), mandatory breaths are suppressed, and the goal of comfort is served. If the patient’s neural inspiratory time is < T_{I min}, then the breath is cycled off by time (ie, at T_{I min}) and is a mandatory breath by virtue of the machine cycling, and hypoventilation is potentially avoided, and the goal of safety is served. Note that technically in this case the goal of safety has overridden the goal of comfort because cycling inspiration after the cessation of inspiratory effort is a discordance issue called late cycling.⁷ This is a clinically relevant advantage. But note also that if T_{I max} is set too short (ie, shorter than the patient’s neural inspiratory time) then its normal function of serving the goal of safety (ie, preventing prolonged inspiration, hypoventilation, and potential hemodynamic compromise) is changed. It now becomes a comfort problem by creating the discordance called early cycling, which could also cause a safety problem because the tidal volume will no doubt be less than that the patient’s ventilatory drive demands.

Targeting Schemes for NIV

In addition to the breath sequence, we should say something about the targeting schemes used by NIV and auto-titrating CPAP devices. Farré, et al¹⁰ have written an excellent description of “black box” technology represented by NIV devices. For auto-titrating CPAP devices in particular, they describe the aims of the control algorithms (ie, targeting schemes) as 1) to detect breathing events; 2) classify events as artifacts (eg, cough, swallowing, mouth breathing) or pathologic breathing events (eg, apnea, hypopnea, snoring, flow limitation); 3) assess air leaks; and 4) decide how to control the CPAP level applied to the patient. They also point out that given the wide variability in performance of these devices, the actual performance of a given device will depend on the specific implementation of “...its intelligence”. Finally, they note that noninvasive ventilators are technologically more complex than CPAP devices. Unfortunately, classifying the exact targeting scheme used by NIV devices is difficult because the operator’s manuals generally provide only vague descriptions of device performance. Even the patents upon which they are based are mostly useless because the technological details they describe are seldom, if ever, linked to specific device models. In light of this, at present, we choose to classify any NIV device that automatically adjusts airway pressure (or backup mandatory breath rate) based on the types of algorithms described above as having intelligent targeting schemes (ie,using the tools of artificial intelligence such as rule-based expert system).

Conclusions

Having said all this, we must now update two of the modes we classified in our paper on the taxonomy of modes on portable ventilators used for NIV.² The updated mode classifications are shown in Table 2.

Table 2.

Updated Portable Ventilator Modes Classifications