Mechanical Ventilation Flashcards
What are the indications for mechanical ventilation (both invasive or non-invasive)?
- Respiratory failure (or impending)
- Inadequate oxygenation
- Inadequate ventilation
- Elevated work of breathing - Airway protection
- Upper airway obstruction (tracheal intubation)
When to choose between NIV and invasive ventilation?
- Ability to protect airway
- Type of respiratory failure
- Hypoxaemic -Trial NIV, if not improved after 1-2 hours to consider intubation
- APO - NIV - Haemodynamic instability - intubation
- Previous failed NIV attempts
- Condition worsened on NIV
What is the difference between CPAP and BiPAP?
Continuous positive airway pressure (CPAP)
- Elevates baseline expiratory pressure that patient spontaneously breathes from (PEEP) with NO additional inspiratory assistance
- End expiratory airway pressure maintained at set CPAP level
Benefits:
1. Maintains alveolar recruitment
2. Stents open the upper airway
Useful in: ADHF, OSA, atelectasis
Bilevel positive airway pressure (BiPAP)
- Inspiratory pressure support (IPAP)
- End expiratory pressure (PEEP, called EPAP)
Useful in: hypoventilation, need to unload work of breathing (asthma, COPD)
Pressure control and volume control
Pressure control - ventilatory pressure assistance (set), flow is provided to meet inspiratory demand, with patient controlling tidal volume (can be small or large) (variable)
Tidal volume (Vt) depends on:
1. Lung and chest wall impedance
2. Set inspiratory airway pressure
3. Patient effort
Better tolerated than volume control
Volume control - Vt (set), flow pattern, maximum flow and inspiratory time (set), inspiratory airway pressure allowed to vary (variable)
Better lung protection (tidal volume limitation)
High patient-ventilator asynchrony if patient is able to actively trigger ventilator
Mechanical ventilation refrain strategy
Consider high flow nasal cannula (HFNC) in hypoxaemic respiratory failure
- Heated, humidified oxygen at high flow (40-60L/min), provides small PEEP and flushes CO2 from upper airway
- Reduces work of breathing
- Rapid gas velocity provides stable FiO2 - limits air entrapment
Same caution for NIV exists with HFNC
- If not responding or worsening within 1-2 hours, to consider intubation
Ventilator waveforms morphology
Pressure-Time Scalar
In PC: pressure controlled and constant throughout inspiration, not affected by patient respiratory mechanics or amplitude
In VC: shape changes according to:
- Pmus (insp. muscle pressure): high -> downward deformation and upward concavity
- Airway resistance: high -> high peak pressure
- Compliance: low -> high peak pressure
(Distinguish resistance and compliance with Pplat)
Flow-Time Scalar
In PC: shape changes according to:
- Pmus: strong -> sinusoidal waveform, increased peak inspiratory flow
- Airway resistance: high -> reduced peak inspiratory flow, decreased slope of waveform, fail to reach baseline before expiration
- Compliance: high elasticity (low compliance) -> short time of lung filling/emptying -> early baseline
In VC: flow controlled and fixed, unaffected by patient respiratory mechanics or amplitude
What is patient-ventilator asynchrony?
Patient’s respiratory efforts and support from ventilator not in synchrony
The greater control the ventilator exerts over a patient’s ventilator pattern, the greater the likelihood for asynchrony.
Types of asynchrony:
1. Flow asynchrony - inspiratory effort demand greater flow than provided
2. Trigger asynchrony - inspiratory activation not coordinated with ventilator
> delayed trigger, missed trigger, double trigger, reverse trigger
3. Cycle asynchrony - patient end inspiration and ventilator ending not in sync
4. Mode asynchrony - mode causes asynchrony
What is the commonest type of asynchrony?
- Trigger asynchrony, particularly missed triggering
- Cycle asynchrony - in pressure ventilation
What is trigger asynchrony and its subtypes?
Inability to sequentially trigger ventilator-delivered breath (discoordination)
Subtypes:
1. Delayed trigger
2. Missed trigger
3. Double trigger
4. Reverse trigger
What is missed triggering?
Inspiratory effort (which is ineffective) fails to trigger mechanical ventilation
- Occurs when patient’s inspiratory effort starts before exhalation reaches functional residual capacity (FRC)
- Gas still trapped in airway by PEEP
- Patient’s effort insufficient to overcome PEEP, ventilator fails to respond
–> Increase work of breathing
What is delayed triggering?
Long delay time (>100ms) between patient development of negative airway pressure and ventilator responding with delivered breath
What is double triggering?
Activation of second ventilator-delivered breath before exhalation of previous breath is complete
- Cause increased tidal volume (and peak airway pressure in volume control)
-> High transpulmonary pressure and driving pressure, possible doubling of tidal volume
-> Potentially causes lung injury
Causes of double triggering:
1. Inadequate mechanical inspiratory flow
2. Short inspiratory time
3. Reduced compliance
What is reverse triggering?
Spontaneous breath stacked on top of controlled or double triggered breath
Breath initiated by ventilator (time-triggered), then neural inspiration occurs, extending into expiratory phase
- Pressure waveform returning towards baseline without maintaining plateau.
- Upward distortion of expiratory flow-time curve (inspiratory effort impedes expiratory flow)
May result in doubling of tidal volumes (from set Vt)
-> lung injury
What is flow asynchrony?
Ventilator delivered gas flow less than patient’s inspiratory flow demand
-> Increasing workload, high transpulmonary pressure
-> potentially causes lung injury
Occurs in modes when inspiratory flow is fixed
What is cycle asynchrony?
Patient’s inspiratory time and ventilator’s inspiratory time differs in length or out of phase.
Commonly in pressure ventilation (PS, PC, P assist)
Correction by adjusting inspiratory termination criteria (in PS) or inspiratory time (in PC)
