Slideshow on Mechanical Ventilation .pdf

Mechanical ventilation basics

2. Trigger

MV breaths are triggered in one of four possible ways: (a) time, (b) pressure, (c)flow, or (d) neural sensing.

Time is the trigger for Controlled breaths, meaning that a breath will be delivered after a fixed time from the prior
breath, defined by the set respiratory rate or frequency (time = 60/frequency).

In contrast, Assisted breaths rely on the ventilator to detect patient effort. Most commonly, effort is sensed
through a fall in pressure at the airway opening (pressure trigger) or by detecting a difference in the bias flow
comparing the inspiratory and expiratory limbs of the ventilator circuit (flow triggering).

Neural sensing, in which the integrated diaphragmatic electromyographic (EMG) signal is used to initiate the
breath, improves neuro-ventilatory coupling, especially in patients with severe airway obstruction and in children.

3. Target

Ventilator breaths are conventionally classified based on the targeted variable: pressure-targeted breaths vs
flow-targeted breaths.

In pressure-targeted breaths, the clinician sets and the ventilator produces a controlled pressure (PI)throughout
inspiration.

In contrast, during flow-targeted breaths, the clinician sets and the ventilator delivers a controlled inspiratory flow
rate.

Generally, is constant throughout the breath(constant flow or square wave) but can also decelerate or vary
sinusoidally.

4. Passively ventilated patient on flow- (A) and pressure (B)-targeted mode.

A, Flow remains constant and volumerises linearly throughout inspiration, while pressure is determined by the
respiratory system mechanical properties.

B, The pressure remains constant, being the independent variable, while the flow and volume are determined by
the respiratory system mechanical properties.

5. Cycle

The cycle variable is the factor that terminates inspiration.

There are four possible cycle variables: (1) time, (2) flow, (3) volume, and (4) neural sensing.


PC breaths are terminated by time (inspiratory time, TI), often set between 0.5 and 1.0 seconds.

Flow is the cycle variable for PS breaths, which terminate when flow falls to a specified fraction of the initial
inspiratory flow or, on some ventilators, below an absolute threshold of flow. Often, a value near 30% of initial
flow is chosen to signal the ventilator to switch off.

In patients with severe airflow obstruction, this can lead to very long ventilator TI (longer than neural TI) and
patient-ventilator dyssynchrony (PVD). For these patients, synchrony can be improved by raising the cycle
threshold to 50% or even higher.

The third means to cycle from inspiration to expiration is to rely on delivered volume, typical of flow-targeted
breaths.


Finally, inspiration can be cycled off by using the declining signal from the diaphragm EMG.

6. Volume Assist Control Ventilation

time- or effort-triggered, flow-targeted, and volume-cycled


Volume assist control ventilation (VACV) mode allows both VC and VA breaths.


In this mode the physician sets the tidal volume, inspiratory flow rate, and respiratory frequency.


For the passively ventilated patient, a set tidal volume is delivered with each VC breath at a specified frequency.

In the actively breathing patient, extra breaths can be triggered (VA), each at the same VT and inspiratory flow
rate as VC breaths: this will change the I:E ratio and TE.


When respiratory drive is high, or when neural TI exceeds the ventilator TI, patients may trigger a second breath
(rarely a third) which is superimposed on the initial breath.

This phenomenon, called double triggering, is especially common during lung-protective ventilation of the acute
respiratory distress syndrome (ARDS), and risks overdistension of lung or dynamic hyperinflation.

7. Volume assist control ventilation.

As in most flow- targeted modes, flow remains constant and volume rises linearly throughout inspiration, while
pressure is determined by the respiratory system mechanical properties.

The figure represents a passively ventilated patient; the VT, TI, and RR set by the clinician determine the I:E ratio
and flow rate.

8. Volume-Synchronized Intermittent Mandatory Ventilation

time- or effort-triggered; flow- or pressure-targeted; and time- or flow-cycled.

Volume-synchronized intermittent mandatory ventilation (V-SIMV) entails VC, VA, PS, and SB.



In the passive patient, it behaves the same way as VACV.

In actively breathing patients, if the triggering effort occurs close to the next scheduled mandatory
breath (defined interval or synchronization window), the ventilator delivers a VA breath and resets the
interval until the next time-cycled breath.

If the triggering effort occurs before the synchronization window, a PS or SB follows (unsynchronized
breath)

This ventilator mode has been used traditionally as a weaning strategy, although its use is decreasing,
perhaps because it appears to prolong ventilator liberation.

9. Volume-synchronized, intermittent, mandatory ventilation (V-SIMV).

The different breath types delivered by V-SIMV mode in an actively breathing patient are shown: controlled
breath (VC), followed by two unsynchronized spontaneous breaths (SB) and a synchronized-assisted breath (VA).

Notice that the VA breath is composed of the same flow rate and volume as the VC breath but pressures are lower
in accord with the patient’s inspiratory effort.

Here, the unsynchronized breaths are SB, but these can also be pressure support breaths.

10. Pressure Assist-Control Ventilation

time- or effort-triggered, pressure-targeted, and time-cycled

This mode consists of both PA and PC breaths

The respiratory rate (f), PI, and TI are set by the clinician

For the passive patient without airway obstruction and sufficiently long inspiratory time, PI and alveolar pressure
(Palv) equilibrate at end inspiration (signaled by zero flow), and the tidal volume is predictable.

Among patients with airflow obstruction or short TI, there will not be equilibration between PI and the alveolar
pressure, flow does not fall to zero at end-inspiration, and VT will be smaller than that predicted.

11. Pressure assist control ventilation.

Pressure is constant throughout inspiration. The inspiratory flow falls throughout the breath as the alveolar
pressure rises.

In this patient, the first breath is controlled (PC), while the next three are assisted (PA).

The downward deflection of the pressure curve at the beginning of inspiration represents the patient’s inspiratory
effort triggering the breath (arrow). Note the subtle differences in inspiratory flow during the PA breaths related to
variable patient effort

12. Pressure Support Ventilation

effort-triggered, pressure-targeted, and flow-cycled


The patient must trigger the ventilator to deliver a breath, so this mode cannot be applied to passive patients.

The clinician sets PI and (optionally) the flow threshold for cycling the breath off: rate and minute ventilation
depend on the patient’s drive.

13. Pressure support.

As is typical of most pressure-targeted modes, the inspiratory flow falls throughout the breath as the alveolar
pressure rises (because of elastic recoil and falling inspiratory effort). ,

14. Pressure-synchronized Intermittent Mandatory Ventilation


The combination of PC, PA, PS, and SB makes up pressure SIMV.

The clinician sets rate, PI (PC and PA breath-related PI is set independently of the PI for PS breaths), and TI.

This mode is therefore effort- or time-triggered, pressure-targeted, and both time-cycled (PC and PA breaths)
and flow-cycled (PS breaths).

When the patient is passive, pressure SIMV simply becomes pressure control mode.

When the patient triggers a breath in a brief window before a planned time-cycled PC breath, the breath
becomes a PA breath. If patient effort precedes this window, the breath is instead PS or SB depending on
whether the clinician has enabled PS.

15. Pressure-synchronized, intermittent, mandatory ventilation (SIMV).

Similar to volume-SIMV, several breath types can be delivered by this mode in an actively breathing patient:
a controlled breath (PC) is followed by two unsynchronized spontaneous breaths (SB) and a synchronized
assisted breath (PA). PA breaths have the same pressure profile and TI as PC breaths.

For unsynchronized breaths, pressure support is set at zero

16. Pressure-Regulated Volume Control and Volume Support Ventilation


Although the name pressure-regulated volume control (PRVC) suggests that it targets flow (or volume), in fact,
PRVC is a pressure-targeted mode allowing only PC and PA breaths.

In this mode, the clinician sets an initial PI, TI, frequency, and a tidal volume goal. The ventilator automatically and
continually adjusts PI to achieve the desired VT. Therefore, if respiratory system mechanical properties or patient
effort changes, so will PI. This is one means of controlling tidal volume for patients with ARDS


A similar mode is volume support ventilation (VSV) which uses PS breaths also to achieve a target tidal volume.
Because it uses PS rather than PC and PA breaths, VSV differs from PRVC in that TI and frequency are not
controlled by the clinician.

17. Pressure-regulated volume control.

In this passively ventilated patient, the clinician switched the target VT from 800 to 500 mL. Appreciate how the
PI is adjusted by the ventilator and decreases gradually to deliver the new target VT.

18. Airway Pressure Release Ventilation


PC, PS, and SB are combined in airway pressure release ventilation (APRV)

time- and effort-triggered, pressure-targeted, and time- and flow-cycled

This mode is characterized especially by a very long TI.

The clinician enters frequency, TLOW (which is simply TE), PHIGH (PI), and PLOW (PEEP).

TLOW is kept brief in order to produce auto-positive end-expiratory pressure (autoPEEP)

THIGH (TI) is determined mathematically as a consequence of TLOW and frequency (generally about 10
breaths/min).

PHIGH is often set near the plateau airway pressure determined during VACV breaths.

PLOW can be set at zero, adjusted based on oxygenation, titrated until VT falls to 6 mL/kg predicted body weight
(PBW).

19. In the absence of spontaneous respiratory efforts, APRV is identical to PACV (with inspiratory and expiratory times
inverted—this is a form of inverse ratio ventilation).


The point of APRV, however, is to facilitate spontaneous breathing. This mode allows unrestricted spontaneous
ventilation throughout therespiratory cycle, both during THIGH and TLOW, and these can be PS or SB.

This mode was created as a more comfortable means for delivering inverse ratio ventilation and is often used in
severe ARDS to improve oxygenation.

There are three downsides to APRV:

(a) tidal volume often exceeds values thought to be lung-protective even when it is used with protocols designed
to target low volume ventilation

(b) derecruitment during TLOW could produce atelectrauma

(c) spontaneous breathing in early, severe ARDS may worsen outcomes.

20. Airway pressure release ventilation in an actively breathing patient.

The patient can trigger spontaneous breaths in combination with this mode during THIGH or TLOW. TLOW
is typically quite short in order to produce autoPEEP

21. Neurally Adjusted Ventilatory Assist


This novel mode provides a pressure targeted breath in proportion to the electrical activity of the diaphragm
(EAdi), measured through microelectrodes built into a nasogastric tube and placed at the level of the
diaphragmatic crura.

NAVA is electrically triggered, pressure-targeted, and electrically cycled. The timing and intensity of ventilatory
assist is determined by the timing and intensity of the diaphragmatic electrical activity

NAVA requires correct positioning of the electrode sensors, normal diaphragmatic anatomy, and intact ventilatory
drive and breathing reflexes.

The ventilator also has a safety mechanism: it switches to PACV or PSV if EAdi is not detected.


NAVA is thought to produce better neuro-ventilatory coupling than other conventional modes, decreasing the
prevalence of ventilator asynchrony.

Two clinical trials showed an increase in ventilator free days with NAVA in comparison with conventional modes
among difficult to wean patients and in those with acute hypoxic respiratory failure with mild ARDS