Mechanical Ventilation

Question 1

Definitions
What are the Ventilator phase variables?
Define Trigger Cycling and Limit

Answer 1

=>Ventilator Phase Variables:
These are parameters which control phases of a mechanical breath.
=>Three phase variables include:
Cycling · Triggering · Limit

TLC
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Question 2

Trigger

Answer 2

=>Triggering controls Initiation of inspiration

=>Types:
->Time Triggered:
Breath delivered after preset interval (e.g. in mandatory/controlled modes).
->Patient Triggered:
Ventilator senses patient effort and initiates inspiration. This can be:

• Fllow triggered[Drop in baseline flow (e.g. 2–3 L/min)->More sensitive, less WOB.]
• Pressure/ Volume triggered->[Drop in circuit pressure (e.g. 1–2 cmH₂O below baseline)]
• Neural trigger (NAVA):
  Uses diaphragmatic EMG (EAdi signal) to initiate breath.
  
  • Excellent synchrony, least WOB; not usable without intact diaphragm.
  • Only integrated in SERVO ventilators
• Shape signal triggering-

->Flow trigger m/c used in pt triggered modes.

->Volume and pressure triggers may increase the WOB, not used in modern day ventilators.

Question 3

NAVA
Advantages & Disadvantages

Answer 3

Neurally Adjusted Ventilatory Assist (NAVA) – additional mode

⸻
=>Advantages
* Diaphragmatic EMGs can be continuously monitored->Useful in impairments like
GBS / MG.
* Pt. has more control over amount of support recieved. Flow & pressure levels adjusted every 16 ms to suit patient demand.
* Least patient effort required; maximal comfort.
* Many reach weaning days earlier; ventilator can “learn” as they can tolerate decreased support.
* Eliminates problem of trigger / flow asynchrony -> No DHI
* Less diaphragm “disuse atrophy.”

⸻

Disadvantages
* Not useful if respiratory drive not intact, e.g.
 – Excessive sedation
 – Neuromuscular blockade
* Relies on accuracy of NG position → can be altered in critically ill.
* NG access may be contraindicated (esophageal trauma, varices, etc.)
* No evidence that it improves outcomes.

Question 4

Shape Signal Triggering

Answer 4

->Manufacturer-specific principle
->Patient’s inspiratory effort is predicted by analysing the inspiratory flow waveform, by superimposing a distortion waveform on top of the patient’s own waveform.
* Requires less effort → hence more patient comfort.
* Not proven in any studies, however.

Question 5

Cycling

Answer 5

→ Controls initiation of expiration
Cycling Variable
->Measured during inspiration phase.
->Determines how & when ventilator will cycle from inspiration to expiration.
->When the set parameter for this variable is achieved → ventilator will open expiratory valve.

Question 6

Clinical Notes
Inappropriate cycling and consequences

Answer 6

=>Inappropriate cycling → asynchrony:
->Premature cycling: Breath ends before patient finishes inspiration → double-triggering, high WOB.
->Delayed cycling: Breath persists beyond patient effort → air trapping, discomfort.
->Adjust by changing inspiratory time, flow-termination %, or sensitivity.

Question 7

Typical ventilator breath cycling methods

Answer 7

Types:
=>Time-cycled:
Inspiration ends after preset inspiratory time (e.g. mandatory modes).
=>Volume-cycled:
Ends when preset volume delivered (e.g. volume control ventilation-VCV).
=>Pressure-cycled:
Ends when preset pressure reached (rare in modern ICU modes).

=>Flow-cycled:
Ends when inspiratory flow decays to a preset fraction (e.g. 25%) of peak inspiratory flow (typical for pressure support modes).

=>Neural-cycled (NAVA):
Ends when EAdi signal decreases to a set percentage of peak EMG activity.

Question 8

Limit Variable

Answer 8

=>Definition:
->The Limit variable determines the maximum value that a controlled parameter (pressure, flow, or volume) is allowed to reach during inspiration.

->It restricts the variable but does not terminate inspiration.

->Inspiration continues as long as the other cycling condition (e.g. time, flow, or volume) has not been met.

Question 9

comparision table of variables in different modes

Answer 9

Answer 10

Question 11

Limit vs alarm

Answer 11