Spontaneous breathing modes, which include synchronized intermittent mechanical ventilation (SIMV) and various implementations of pressure support (PS), allow for the patient to breathe spontaneously either in between machine breaths with or without support or “augmentation” for that breath.  SIMV allows for spontaneous breaths with pressure support that augments or supports each spontaneous breath.  PS is an inspiratory pressure that is initiated when the patient takes a spontaneous breath.

In PS ventilation, each time a patient initiates a breath, the ventilator delivers a set level of pressure to assist the patient as they breathe in. The patient controls the frequency and volume of their breaths, allowing them to take long, deep breaths or shorter, more rapid ones as needed. The only constant is the set level of pressure provided by the ventilator, which will not change unless adjusted by a clinician.

The primary setting in PS ventilation that the clinician must determine is the amount of pressure support. This level should be sufficient to keep the patient’s work of breathing within normal limits. If the patient’s condition worsens and they are unable to maintain adequate breathing effort, they may be considered “failing” and could require additional support or a switch back to a controlled ventilation mode.

In PS, you will directly set the following:

• Pressure support (PS)
• PEEP (every ventilator, every time)
• FiO2 (every ventilator, every time)

In PS, you will observe but not directly set the following:

• Tidal volume (Vt)
• Inspiratory flow or I-time (V or TI)*
• RR (patient must trigger all breaths)
• Peak inspiratory pressure (PIP or Ppeak)
• I:E

Patients can take air in at whatever speed they want and end the breath when they want to. The mechanical ventilator will sense when inspiration and expiration start^[1].

Synchronized Intermittent-Mandatory Ventilation (SIMV)^[3]

This mode guarantees a certain number of breaths, but unlike AC/VC, patient breaths are partially their own, reducing the risk of hyperinflation or alkalosis. Mandatory breaths are synchronized to coincide with spontaneous respirations. Disadvantages of SIMV are increased work of breathing and a tendency to reduce cardiac output, which may prolong ventilator dependency. The addition of pressure support on top of spontaneous breaths can reduce some of the work of breathing.

In SIMV, you will directly set the following:

• Respiratory rate (RR or f)
• Tidal volume (Vt) or Inspiratory pressure (PC)
• PEEP (every ventilator, every mode)
• FiO2 (every ventilator, every mode)
• Pressure support (PS)

In SIMV, you will observe but not directly set the following:

• Peak inspiratory pressure (PIP)
• Inspiratory time (I-time) would not be set if the ventilator lets you set flow. In this case, observe I-time.

The ventilator establishes a time window, equal to the total cycle time for each respiratory cycle established by the set respiratory rate. At the start of each window, the ventilator assesses whether the patient is making an inspiratory effort. If the ventilator does not feel any inspiratory effort, it will provide a controlled breath. If the ventilator feels an inspiratory effort at the start of the window, it will provide an assisted breath. If the patient breathes spontaneously after the obligatory or assisted breath has been given, the patient will receive spontaneous type breaths for the remainder of the respiratory cycle.

Therefore, for mandatory or assisted breaths, the operator can choose to control the breaths either by pressure or flow/volume. For spontaneous breaths the support is controlled by setting a support pressure.

An advantage of using SIMV mode is that it allows the patient to breathe spontaneously (with supportive pressure).

Despite this advantage, these are disadvantages of using SIMV mode. Here are some disadvantages of using SIMV mode:

• The mode was designed to improve removal or weaning of a patient from a ventilator. This has subsequently been shown to be an error, and some form of a spontaneous breathing trial with spontaneous breath assistance or support has been found to be superior.
• It may lead to increased work of breathing if controlled and spontaneous breaths are not synchronized (frequently observed when the patient suffers from an increased respiratory rate).