| Square | Descending Ramp | Exponential Decay | |
|---|---|---|---|
| Mode | Volume Control | Volume Control | Pressure Control |
| Set by | Operator | Operator | Physics (Ο = R Γ C) |
| Peak pressure | Higher | Lower | Set (= Pinsp) |
| Flow at end-insp | Same as peak | Reduced (preset) | Near zero |
| Adjustable? | Yes β rate set | Yes β rate set | No β determined by patient |
Identify the problem, then choose your lever based on the current mode:
[1] Mireles-Cabodevila 2022 Β· [2] Chatburn 2014 Β· [3] Pham et al. 2017
One complete cycle: gas flows in (inspiration), then gas flows out (expiration). One in, one out β that's one breath. "A breath is one cycle of positive flow (inspiration) and negative flow (expiration)" [2].
Every breath has two decision points β who starts it and who stops it. These two answers determine whether a breath is spontaneous or mandatory.
| Patient | Ventilator | |
|---|---|---|
| Trigger (starts it) | Flow or pressure change from effort | Timer reaches set rate interval |
| Cycle (stops it) | Flow decays to % of peak | Set time or volume reached |
Spontaneous = Patient starts it AND patient stops it [2].
Mandatory = Ventilator starts it OR ventilator stops it β either one is enough [2].
A breath can be spontaneous AND assisted at the same time. "Assisted" simply means the ventilator adds pressure during inspiration β it says nothing about who triggered or cycled the breath. Pressure Support is the classic example: the patient triggers it, the patient cycles it (spontaneous), but the ventilator provides pressure throughout (assisted) [2].
Every breath requires pressure to overcome elastic load (lung stiffness) and resistive load (airway friction). The ventilator controls EITHER pressure OR volume/flow β but not both [1].
Every breath is mandatory. The ventilator decides when inspiration ends β every time, no exceptions. The patient can start breaths (trigger), but the machine always controls the off-switch (cycle).
Set rate = minimum. The patient can trigger above this rate, but every breath β whether patient-triggered or time-triggered β is ventilator-cycled [2].
A mix of mandatory and spontaneous breaths. The ventilator delivers mandatory breaths at a set rate, and the patient breathes freely (spontaneously) between them.
Set rate = maximum mandatory rate. Spontaneous breaths are patient-triggered AND patient-cycled; mandatory breaths are ventilator-cycled [2].
| Sub-type | How it works |
|---|---|
| IMV(1) | Mandatory always delivered at set rate. Spontaneous allowed between [1]. |
| IMV(2) | Spontaneous breaths can suppress (replace) mandatory breaths [1]. |
| IMV(3) | Spontaneous minute ventilation suppresses mandatory breaths [1]. |
| IMV(4) | Dual targeting creates spontaneous breaths within what appears to be CMV [1]. |
Every breath is spontaneous. The patient starts it AND stops it. The ventilator provides support (pressure) during inspiration but never overrides the patient's timing.
All breaths are patient-triggered AND patient-cycled (flow-cycled). The ventilator's role is purely supportive [2].
The targeting scheme describes the feedback logic the ventilator uses within or between breaths. A comma in the TAG means different targeting for mandatory vs. spontaneous breaths [2].
| TS | Name | Definition | |
|---|---|---|---|
| s | Set-point | Operator sets all parameters directly [2]. | COMMON |
| d | Dual | Switches between Volume Control and Pressure Control within a single breath [2]. | COMMON |
| a | Adaptive | Adjusts pressure between breaths to achieve a VT target [2]. | COMMON |
| o | Optimal | Minimizes or maximizes an overall performance metric [2]. | RARE |
| i | Intelligent | Uses AI: fuzzy logic, expert systems, neural networks [2]. | RARE |
[1] Mireles-Cabodevila 2022 Β· [2] Chatburn 2014
β’ Vasoactive trend β escalating vs. weaning/stable dose
β’ MAP stability over preceding hours, not a single snapshot
β’ Heart rate and rhythm trajectory
β’ FiOβ and PEEP trajectory β stable/weaning vs. escalating
β’ SpOβ trend and variability, not a single reading
β’ Ventilatory demand β is RR climbing? Air hunger?
β’ ABG trajectory vs. a single PaOβ value
β’ Lactate trend β clearing vs. rising
β’ ScvOβ trend if available
β’ End-organ perfusion signs β mentation, UOP, skin
β’ Improving, stable, or deteriorating?
β’ What changed in the last 4β12 hours?
β’ Sedation level and neurological engagement
β’ Team assessment and patient goals
The sections that follow address how ventilator settings can be optimized to support active patients once the clinical team has determined readiness.
"There are only 3 goals of mechanical ventilation (safety, comfort, and liberation)" [1].
Adapted from Figure 7, Mireles-Cabodevila et al. 2022 [1]
| TAG | Common Name | Why It Matters for Mobility | |
|---|---|---|---|
| PC-CSVs | Pressure Support | Patient controls rate, flow, and Ti | β BEST |
| PC-CMVs | Pressure Control | Consistent Pinsp, effort β more VT | β Good |
| VC-CMVs | Volume Control* | Flow is locked β can't match demand | β Avoid |
| PC-CMVa | PRVC | Vent βsupport as patient βeffort | β οΈ Counterproductive |
| PC-IMVs,s | SIMV+PS | Mandatory=time-cycled, Spont=flow-cycled | β οΈ Mixed |
*Servo-U "Volume Control" is classified as VC-IMVd,d (dual targeting), not VC-CMVs β see Comparator tab.
"Patient-ventilator dyssynchrony...occurring in about one-third of patients" [3]. During activity, increased drive and changing mechanics make dyssynchrony more likely β and more consequential.
Organized by what you observe at bedside β then what's likely happening:
Patient is working to trigger but the vent isn't responding fast enough, or delivered flow doesn't match demand.
The ventilator is still pushing air in after the patient wants to exhale. Uncomfortable and increases work of breathing.
Two breaths delivered in rapid succession. The first breath ended too early, so the patient immediately triggers a second.
High VT or high VΜE alarms firing because activity legitimately increases ventilatory demand.
Patient is clearly trying to inhale β accessory muscles active, tracheal tug β but the ventilator doesn't respond.
1. Is this new or pre-existing?
If it started with activity β likely increased demand exceeding current settings. If present before β underlying mode/parameter issue.
2. Is it the mode?
If in Volume Control or PRVC β the mode may be actively working against the patient (see Effort & Modes). Consider switching to Pressure Support or Pressure Control.
3. Is it the parameters?
Trigger sensitivity (too insensitive?), cycle threshold (too low in obstructive, too high in restrictive?), flow settings (demand exceeding supply?).
4. Communicate specifically
Instead of "they look uncomfortable" β "Patient appears to be double-triggering with activity β can we assess cycle threshold?" or "Patient showing signs of flow starvation β is there room to increase support or switch from Volume Control?"
The cycling mechanism determines whether the ventilator's timing can align with the patient's neural timing:
These are considerations for optimizing the chances of a successful mobility session β not absolute requirements. Clinical judgment always applies.
[1] Mireles-Cabodevila 2022 Β· [2] Chatburn 2014 Β· [3] Pham et al. 2017
Select a mode on either ventilator β the closest match auto-selects on the other.
| Hamilton | Servo-U | |
|---|---|---|
| CV | β | β |
| BS | β | β |
| TS1 | β | β |
| TS2 | β | β |
| TAG | β | β |
| IMV | β | β |