Synchronized Intermittent Mandatory Ventilation (SIMV)

Regardless of your exposure to ventilation, at some point during your education or career, the mode Synchronized Intermittent Mandatory Ventilation (SIMV) will be mentioned. This mode is not discussed fully in this book, as it is not currently considered “best practice” for mechanical ventilation.

SIMV can be a challenge to understand, as it is usually a combination of control, assisted and spontaneous breaths. This mode is not used to a large degree in most Intensive Care Units due to large amounts of research that have identified inherent issues in asynchrony and increased length of ICU stays.

As much as it may be tempting to not discuss SIMV at all, it is still discussed in most ventilation textbooks and taught in some health professions as a current ventilation strategy. If you would like to learn more about the initial theory that prompted the rise of SIMV, as well as the disadvantages and current research identifying the flaws inherent with the mode in its current form, read on…

A History Lesson

Historically, when being weaned from the ventilator, patients would be woken up while on full control ventilation, sedation would be weaned and as soon as patients roused enough to start fighting the ventilator, they were extubated. There were no supportive modes, and weaning was not as well understood. As you may imagine, this abrupt approach suffered from major issues ranging from the effects of muscle wastage to patient PTSD.

In the 1970s, a weaning mode was hypothesized that would allow for a steady decrease in the mandatory rate with time in between for the patient to breathe spontaneously on their own. This mode was termed Intermittent Mandatory Ventilation, and it became the gold standard of ventilation from 1980s-2000s. Clinicians would set a minimum mandatory rate, and above that, the patient would be able to spontaneously breathe whatever volume they pulled off the bias flow. The mandatory rate would be steadily decreased until the patient was breathing completely spontaneously. This gradual transition to spontaneous breathing was hypothesized to increase the patient’s work of breathing slowly in a controlled step-by-step manner.

It is important to understand that this mode was not synchronized with the patient at all. If the mandatory rate was set at 10bpm, every 6 seconds, the ventilator would deliver a full control breath as set by the clinician regardless on whether the patient was halfway through a breath already or if they needed it or not. The ventilator was totally “blind” to the patient.

Researchers and clinicians alike soon saw the asynchronies and breath-stacking inherent with this mode and came up with a solution to synchronize the ventilator-delivered control breaths with the patient. They programmed an approximate 0.5 second window around the scheduled mandatory breath that would allow any patient triggered breath in the window to be a fully assisted controlled breath. If no patient triggered breath was sensed, a full control breath was delivered. Thus “Synchronized” Intermittent Mandatory Ventilation, or SIMV, was born.

In recent decades, continued research has highlighted issues with weaning using SIMV. Studies started to show that sequential decreasing of the mandatory rate was not allowing the patient to gradually overtake their work of breathing. Instead, studies were pointing to the inability of the body to adapt its effort based on the amount of support available. In other words, the patient’s work of breathing was based on how hard they worked with the unsupported spontaneous breaths. These spontaneous breaths were not supported in any way. Patients would breathe without any pressure support—having to overcome the resistance of the ETT on their own. Therefore, even though every few breaths was fully supported, this was not able to unload the respiratory muscles. Even fully supported breaths showed the same high WOB levels, as the neurological drive to breathe was expecting the work of breathing of the un-supported spontaneous breaths and not able to adapt to the variable support levels.

Ventilator manufacturers again attempted to fix these issues by adding additional support to SIMV. Pressure support was added to the spontaneous breaths to attempt to unload the respiratory fatigue that was being seen. Current SIMV modes all now operate as a true mixed mode—oftentimes flipping between controlled and supported modes like PCV and PSV based on the time window set by the minimal rate. But again, the mode still has its struggles. No benefits have been identified in using SIMV over standard control modes followed by a transition to spontaneous modes.

Adding a further nail in the coffin, current trending in medical research is focusing the direct correlation between high rates of ventilator asynchrony and increased ventilator days. SIMV remains a main culprit, with a high incidence of asynchrony and multiple studies identifying a potential delay in the weaning process and increased ventilator days.

Every second that a patient is on a ventilator needs to be treated as incredibly important. We have already seen in this chapter how impactful being on a ventilator can be for a patient. Using a mode that can potentially delay weaning or cause dysfunction is not necessary if there is another option that can potentially work as well with fewer disadvantages.

Though once the most widely used mode of ventilation, SIMV use has dropped off precipitously in current critical care practice. Some studies have shown that SIMV use by experienced clinicians with careful patient selection can result in safe and effective use. It is still used in neonatal populations as well as post-operatively with lingering sedation.

Though SIMV is not commonly used, the basic principle has not been abandoned. Derivatives of this mode can be seen in specialty modes of many forms. And, as we have already learned, the field of mechanical ventilation is always evolving. Future developments could possibly bring this mode back to the cutting edge of medicine—though more likely it will be rebranded with a different name.