Show Summary Details
Page of

Ventilator Control Parameters 

Ventilator Control Parameters
Chapter:
Ventilator Control Parameters
Author(s):

Yuan Lei

DOI:
10.1093/med/9780198784975.003.0009
Page of

PRINTED FROM OXFORD MEDICINE ONLINE (www.oxfordmedicine.com). © Oxford University Press, 2021. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Medicine Online for personal use (for details see Privacy Policy and Legal Notice).

date: 06 December 2021

9.1 Controls in general

9.1.1 Definition

For mechanical ventilation, an operator must specify a ventilation mode. However, this in itself is not enough for a ventilator to function. The operator must also set the controls or ventilator control parameters. Fig. 9.1 shows a typical ventilator control window.


Fig. 9.1 A control window on the HAMILTON-G5 ventilator.

Fig. 9.1 A control window on the HAMILTON-G5 ventilator.

A control parameter defines a specific aspect of ventilator operation. For most controls, an operator quantitatively defines the magnitude (e.g. tidal volume). Less frequently the operator selects from one of several options provided (e.g. flow pattern in volume modes).

Every ventilation mode has a unique set of controls, depending on the mechanical breath type or types specific to the mode.

During mechanical ventilation, the operator should periodically check the mode and controls, and modify them when clinically necessary. Optimal mechanical ventilation is possible only when both mode and controls are appropriately set.

9.1.2 Terminology

As with other areas of mechanical ventilation, the terminology of ventilator controls is not globally standardized. It may be helpful to group the names according to their basic functions (Table 9.1).

Table 9.1 Non-standardized terminology for ventilator controls

Group

Controls

Unit

Hamilton Medical

Dräger

Covidien/PB

Maquet

GE

CareFusion

GALILEO/Hamilton G5

Evita XL/V500

840

SERVO-i

Care-Station

AVEA

Triggering

Mandatory rate

b/min

Rate

F/RR

F

RR

Rate

Rate

Pressure trigger

cmH2O

P-trigger

PSENS

Trig. Pressure (setting < 0)

Trigger

Pres Trig

Flow trigger

L/min

Flow trigger

(Flow) Trigger sensitivity

V’SENS

Trigg. Flow (setting > 0)

Trigger

Flow Trig

Base flow

L/min

Bias flow

Bias flow

Bias Flow

Cycling

Inspiratory pause

% or s

Pause

Insp. pause*

TPL

Tpause

Insp. pause

Insp Pause

Inspiratory time

s

Ti

Tinsp or/Ti

Ti

Ti

  • Tinsp

  • (including pause)

Insp Time

I:E ratio

I:E

I:E ratio

I:E

I:E

Percent inspiratory time

%

%Ti

Peak inspiratory flow

L/min

Peak flow

Flow

V’MAX

Peak Flow

Flow cycle

%

ETS

ESENS

Inspiratory cycle-off

End Flow

Flow Cycle or PSV cycle

Maximum inspiratory time

s

TImax

¯ T I SPONT

PSV Tmax

Expiratory time

s

TE

Limiting

Tidal volume

ml

VT

VT/VT

Tidal volume

VT

TV

Volume

Target tidal volume

ml

Vtarget

VTi

Target volume

Tidal Volume

TV

Volume

Pressure control

cmH2O

Pcontrol

Pinsp

PI

PC

Pinsp

Insp Pres

Pressure support

cmH2O

Psupport

PASB/Psupp

PSUPP

PS

Psupp

PSV

Inspiratory pressure limit

cmH2O

Pmax

Pmax

Volume limit

ml

Vol Limit

Inspiratory profile

Rise time

ms*

Pramp

Ramp/Slope

P%

T inspiratory rise

  • Rise time

  • PSV rise time

Insp Rise or PSV Rise

Flow pattern

Flow pattern

Flow pattern

— (square)

— (square)

Waveform

Baseline

Positive expiratory end pressure

cmH2O

PEEP

PEEP

PEEP

PEEP

PEEP

PEEP

Oxygen

FiO2

%

FiO2

FiO2

O2%

O2conc.

FiO2

%O2

Biphasic modes only

PEEP high time

s

Thigh

Thigh

TH

THigh

Thigh

Time High

PEEP low time

s

Tlow

Tlow

TL

TPEEP

Tlow

Time Low

PEEP High

cmH2O

Phigh

Phigh

PEEPH

PHigh

Phigh

Pres High

PEEP Low

cmH2O

Plow or PEEP

Plow

PEEPL

PEEP

Plow

Pres Low

Note: The terms in bold are primary control parameters.

9.1.3 A mode and its controls

You may notice that most controls seem to be related to those essential variables that we described in Chapter 7. Indeed they are related. For, as we know already, a ventilator mode is characterized by its mechanical breath types, which, in turn, are characterized by five essential variables. We also learned that an essential variable may have one or more mechanisms.

Ventilator controls can be either primary or secondary. Primary controls are directly related to essential variables. The sole exception is FiO2 (fraction of inspired oxygen). Secondary controls are used to define additional aspects of ventilator operation.

In some ventilators, a control, primary or secondary, may be set at the factory. If so, the control is not available on the ventilator. For instance, in Dräger Evita ventilators, flow cycling is fixed at 25%, so, you will not find a flow cycle control on these ventilators.

We know now that every mode has its unique set of controls. Can we roughly estimate the number of controls in a specific mode? Yes, we can: the number is directly related to the mechanical breath types in this mode.

Let’s take a look at the volume SIMV (synchronized intermittent mandatory ventilation) mode with its three breath types, volume control breath, volume assist/control breath, and pressure support breath. The essential variables for these breath types are given in Table 9.2.

Table 9.2 How many controls does the volume SIMV mode have?

Type

Control

Volume control

Volume assist

Pressure support

Number

Triggering

Time triggering (Rate)

1

Patient tripper (pressure/flow)

2

Cycling

Time cycling (Ti or I:E ratio)

3

Flow cycle

4

Limiting/control

Tidal volume

5

Inspiratory pressure (Pressure support)

6

Inspiratory pressure (pressure control)

Target tidal volume (adaptive limiting/control)

Baseline

Baseline pressure (PEEP)

7

Oxygen

FiO2

8

Secondary

Rise time for pressure support breath

9

Flow pattern for volume breaths

10

So, volume SIMV should have approximately ten controls. Of them, eight are primary controls, and two are secondary controls.

With this method, we can determine the controls for most common ventilation modes. Table 9.3 lists the number of controls commonly found in these modes. As you see, most ventilation modes have six to ten controls.

Table 9.3 Estimated number of controls in common ventilation modes

Assist/control modes

SIMV modes

Support modes

Biphasic modes

Volume A/C mode

7

Volume SIMV mode

10

Pressure support mode

6

BiPAP mode

9

Pressure A/C mode

7

Pressure SIMV mode

10

Volume support mode

6

APRV mode

9

Adaptive A/C mode

7

Adaptive SIMV mode

10

This method, however, does not fully apply to two special cases:

  • Advanced adaptive modes, such as ASV (adaptive support ventilation) or PAV (proportional assist ventilation).

  • TRC (tube resistance compensation), which is not regarded as a ventilation mode.

  • FiO2, which is not related to any essential variable, but which is present in all ventilation modes.

9.2 Common individual controls

9.2.1 Common controls for triggering

Common triggering controls include the following.

Rate for time triggering

  • Terms: Mandatory rate, frequency (f), respiratory rate (RR)

  • Unit: Breaths per minute (b/min)

  • Mechanism: The set rate determines the duration of control breaths (breath cycle time). Refer to section 7.2.1.

  • Remarks: The rate setting is required in all modes except for pressure support and volume support. The ranges allowed differ in adult, paediatric, and neonatal patients. If rate and pressure/flow triggering settings coexist in a mode, the set rate serves as the backup. The set rate takes over only when no valid patient triggering is detected. Therefore, the actual or monitored total rate can be higher, but not lower, than the set rate. In SIMV modes, the set rate is also called the SIMV rate. It can be set much lower than in assist/control modes.

Pressure trigger type and sensitivity

  • Terms: P-trigger, Psens, Trigg. pressure, Pres. Trig

  • Unit: Centimetres of water (cmH2O)

  • Mechanism: When the airway or circuit pressure drops below the set sensitivity threshold during the triggering window, the ventilator immediately starts inspiratory gas delivery. Refer to section 7.2.1.

  • Remarks: You can make the pressure trigger more or less sensitive through the sensitivity setting; the smaller the absolute value of the setting, the more sensitive the triggering. Not all ventilators provide pressure triggering. Note that the pressure trigger responds to the pneumatic signals not only from the patient’s inspiratory efforts, but also possibly from such artefacts as a gas leak or circuit rainout.

In general, pressure triggering is harder for the patient than flow triggering.

Flow trigger type and sensitivity

  • Terms: Flow trigger, V′sens, Trigg. flow, Flow Trig.

  • Unit: Litres per minute (L/min)

  • Mechanism: When the inspiratory airway flow meets or exceeds the set sensitivity threshold during the triggering window, the ventilator immediately starts inspiratory gas delivery. Refer to section 7.2.1.

  • Remarks: You can make the flow trigger more or less sensitive through the sensitivity setting. The smaller the absolute value of the setting, the more sensitive the triggering. Note that like the pressure trigger, the flow trigger responds to the pneumatic signals not only from the patient’s inspiratory efforts, but also possibly from such artefacts as a gas leak or circuit rainout.

In general, flow triggering is slightly easier for the patient than pressure triggering.

Flow triggering is based on a special technical feature called base flow, which is activated during late expiration.

9.2.2 Common controls for cycling

Common cycling controls include time cycling and flow cycling. Time cycling has several mechanisms, such as inspiratory time (Ti); inspiration to expiration ratio (I:E ratio); and peak flow.

Inspiratory time

  • Terms: Ti, Tinsp, Insp Time

  • Unit: Seconds (s)

  • Mechanism: Ti is a time cycling control. The ventilator switches to expiration when the set Ti is over.

  • Remarks: Ti is the simplest way to define inspiratory time. It applies to all breath types. Breath cycle time is the sum of inspiratory time and expiratory time. After rate and Ti are set, the expiratory time (Te) is given. With a given rate, an increased Ti causes a decreased Te, and vice versa.

I:E ratio

  • Terms: I:E ratio, I:E

  • Unit: None

  • Mechanism: I:E ratio is another way to define time cycling. The relationship between inspiratory time and expiratory time is expressed as a ratio. In clinical practice the commonly used I:E ratios include: 1:1, 1:2, 1:3, and 1:4. The assigned inspiratory portion is on the left of the colon, and the assigned expiratory portion is on the right. The I:E ratio control applies to all breath types.

  • Remarks: This control influences both inspiratory time and expiratory time at any given rate or breath cycle time.

Peak inspiratory flow

  • Terms: Peak flow, V′MAX

  • Unit: Litres per minute (L/min)

  • Mechanism: Peak flow is yet another way to define time cycling. Its application is limited to volume control and volume assist breaths with a constant inspiratory flow. In these breaths, tidal volume is the product of inspiratory time and peak inspiratory flow (i.e. VT = Ti × peak flow). At a given tidal volume, an increased peak flow causes a decreased Ti, and vice versa.

  • Remarks: The use of peak flow is limited to volume breaths with constant flow. This mechanism may be confusing, because the relationship between peak flow and Ti is not obvious. Over time, peak flow is losing ground, so that newer ventilators may not implement this mechanism at all.

Flow cycle

  • Terms: Flow cycle, ETS (for expiratory trigger sensitivity), ESENS, Inspiratory cycle-off, End flow, PSV cycle

  • Unit: Percent (%)

  • Mechanism: As its name implies, flow cycle is a cycling mechanism based on the inspiratory flow. During inspiration, inspiratory flow typically rises quickly to its peak and then gradually drops to zero. This peak is considered to be 100%. The ventilator switches to expiration when inspiratory flow drops to a defined threshold, such as 5%, 25%, or 50%. The flow cycle control is available only for pressure support breaths or adaptive support breaths.

  • Remarks: The flow cycle control lets the operator influence the Ti of spontaneous breaths. If set properly, it improves patient comfort in active patients.

The flow cycle mechanism may fail if the control is set very low and a significant gas leak is present.

9.2.3 Common controls for targeting

The controls in this category define the size of a mechanical breath. They are tidal volume, inspiratory pressure, and target tidal volume.

Tidal volume

  • Terms: VT, tidal volume, Volume

  • Unit: Millilitres (ml)

  • Mechanism: Tidal volume is the intended volume of gas that a ventilator delivers during inspiration. It relates exclusively to volume breaths (i.e. mechanical breaths with volume controlling). Typically the tidal volume is delivered at a constant flow rate during inspiration.

  • Remarks: When using volume modes, pay attention to circuit compliance. This consumes a part of the gas volume that the ventilator delivers, so the patient receives less volume than what the ventilator actually delivers. Refer to section 5.3.4 for details.

In some ventilators, you may also need to set the flow pattern. Refer to the section 7.2.3.

If the ventilator system has a gas leak, the patient may receive a tidal volume that is much less than the set tidal volume.

Pressure control

  • Terms: Pcontrol, PINSP, PI, PC, Insp Pres

  • Unit: Centimetres of water (cmH2O)

  • Mechanism: This control defines the inspiratory pressure above positive end-expiratory pressure (PEEP) in pressure control and pressure assist breaths. Inspiratory pressure generates the desired tidal volume.

  • Remarks: Pressure control is relative to PEEP. Pressure control and PEEP together make the peak airway pressure. This is very different from PEEP high in biphasic modes.

A secondary control, rise time, may be used with pressure control.

Pressure support

  • Terms: Psupport, PSUPP, PSV, PS

  • Unit: Centimetres of water (cmH2O)

  • Mechanism: This control defines the inspiratory pressure above PEEP in pressure support breaths. Inspiratory pressure generates the desired tidal volume.

  • Remarks: Pressure support and pressure control have the same mechanism, yet are used in different pressure breath types.

Target tidal volume

  • Terms: Vtarget, Target volume, TV, Volume

  • Unit: Millilitres (ml)

  • Mechanism: Target tidal volume is used exclusively in adaptive breaths (i.e. adaptive control breaths, adaptive assist breaths, and adaptive support breaths). The ventilator automatically regulates the inspiratory pressure to match the resultant tidal volume to the target. Refer to section 7.2.3 for details.

  • Remarks: Target tidal volume is used exclusively in adaptive modes, while tidal volume is used in volume modes.

9.2.4 Common controls for baseline pressure

PEEP (positive end-expiratory pressure)

  • Terms: PEEP

  • Unit: Centimetres of water (cmH2O)

  • Mechanism: PEEP defines the intended baseline pressure. Positive airway pressure is applied intermittently above the baseline. This control applies to all ventilation modes. Refer to section 7.2.5.

  • Remarks: PEEP can strongly influence oxygenation.

PEEP can keep the alveoli open and restore a decreased functional residual capacity (FRC). A moderate PEEP (3–5 cmH2O) is considered beneficial for all ventilated patients. A relatively high PEEP may be needed to treat patients with restrictive lung diseases, such as acute respiratory distress (ARDS) or acute lung injury. PEEP can be set to zero, but zero PEEP (known as ZEEP) should be avoided.

In a pressure support breath, a patient can breathe spontaneously at PEEP without pressure support.

A stable PEEP is critical for pressure triggering.

9.2.5 Common controls for oxygenation

Fraction of inspired oxygen (FiO2)

  • Terms: FiO2, O2%, %O2, O2 conc., Oxygen

  • Unit: Percent (%)

  • Mechanism: FiO2 defines the intended oxygen concentration in the inspiratory gas. It can be set from 21% to 100%. FiO2 is available in all ventilation modes.

  • Remarks: Obviously, FiO2 relies on the oxygen supply, which can greatly affect oxygenation.

9.2.6 Common secondary controls

Rise time

  • Terms: Rise time, Pramp, Slope, P%, T inspiratory rise, Insp Rise

  • Unit: Percent (%), seconds (s), or milliseconds (ms)

  • Mechanism: Rise time defines the intended speed at which the circuit or airway is pressurized (Fig. 9.2). It applies to pressure breaths and adaptive breaths, but not volume breaths.

  • Remarks: Not all ventilators have a rise time control, because this control is fixed in some ventilators.


Fig. 9.2 By adjusting the rise time setting, the operator influences the speed at which inspiratory pressure increases.

Fig. 9.2 By adjusting the rise time setting, the operator influences the speed at which inspiratory pressure increases.

Rise time has value mainly for active patients, especially those who have a strong drive. If set appropriately, it can improve patient comfort.

A super-fast rise time can cause pressure overshooting, an unwanted complication of mechanical ventilation.

Flow pattern

  • Unit: None

  • Mechanism: The flow pattern control defines the intended profile of inspiratory flow. It applies exclusively to volume breaths. The most common pattern is square, also known as ‘constant flow’. Other possible flow patterns are decelerating, accelerating, and sine (Fig. 9.3).

  • Remarks: Many ventilators do not have this control, because they use a single pattern, the square pattern.


Fig. 9.3 Volume breaths have four common inspiratory flow patterns.

Fig. 9.3 Volume breaths have four common inspiratory flow patterns.

9.2.7 Common controls for biphasic modes

The key feature of biphasic modes is the automatic alteration of the baseline pressure. Four controls are typically set to define this alteration: high PEEP, low PEEP, PEEP high time, and PEEP low time (Fig. 9.4).


Fig. 9.4 Biphasic modes have four special controls to define automatic alternation of baseline pressure.

Fig. 9.4 Biphasic modes have four special controls to define automatic alternation of baseline pressure.

PEEP high

  • Terms: P high, Phigh, PH, PHIGH, Pres High

  • Unit: Centimetres of water (cmH2O)

  • Mechanism: PEEP high defines the intended high level of PEEP.

  • Remarks: Another term for PEEP high is IPAP (inspiratory positive airway pressure).

PEEP at a high level is still PEEP. Like PEEP, it is relative to atmospheric pressure. This differs from inspiratory pressure, which is relative to PEEP.

The patient can breathe spontaneously at (high) PEEP.

PEEP low

  • Terms: P low, Plow, PL, PLOW, Pres Low

  • Unit: Centimetres of water (cmH2O)

  • Mechanism: PEEP low defines the intended low level of PEEP.

  • Remarks: Another term for PEEP low is EPAP (expiratory positive airway pressure).

PEEP low is comparable to PEEP in conventional or adaptive ventilation modes. It is relative to atmospheric pressure.

T high

  • Terms: T high, Thigh, TH, Thigh, Time High

  • Unit: Seconds (s)

  • Mechanism: PEEP high time defines the intended duration of PEEP high.

  • Remarks: Typically PEEP high time is relatively short in BiPAP (bilevel positive airway pressure) mode and quite long in APRV (airway pressure release ventilation) mode.

T low

  • Terms: T low, Tlow, TL, Tlow, Time Low

  • Unit: Seconds (s)

  • Mechanism: PEEP low time defines the intended duration of low PEEP.

  • Remarks: Typically PEEP low time is short in BiPAP mode and very short in APRV mode for pressure release purposes.