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)
Strategies to reduce asynchrony
Volume ventilation
1. Higher peak inspiratory flow rate
2. Shorter inspiratory time
3. Decelerating flow waveform
Pressure ventilation
1. Adjust inspiratory time, flow acceleration, driving pressure
2. Adjust expiratory termination criteria
Lung protective mechanical ventilation (LPMV)
Ventilatory support that minimises lung injury risk
(Originally for ARDS, now applied to all mechanical ventilation)
Strategies:
1. Tidal volume (Vt) 4-8mL/kg (of predicted body weight)
2. Plateau pressure (Pplat) < 28 cm H20
3. Driving pressure < 15 cm H2O
4. Lowest PEEP to sustain lung open at end exhalation
5. FiO2 maintaining PaO2 55-80mmHg or sats 88-95%
What is the physiological tidal volume (Vt or TV) ?
Why should a small tidal volume be set when ventilating a patient?
6mL/kg predicted body weight (PBW) at rest
Acutely ill patients have reduced lung volume, thus tidal volume is < 6mL/kg PBW with minute ventilation sustained by increase in RR
Ventilation aim: 4-8 mL/kg PBW
At time of extubation, patient might demand larger TV up to 10mL/kg PBW
What is plateau pressure (PPlat) ?
Pplat is the end inspiratory equilibration pressure (mean peak alveolar pressure)
- Estimation of mean maximum end inspiratory transpulmonary pressure
In sedated, paralysed patient, perform inspiratory hold
- Air will spontaneous flow out until pressure equilibrates with alveolar pressure
In controlled ventilation, PPlat is ALWAYS greater than end inspiratory transpulmonary pressure (in assisted ventilation, active inspiratory effort by patient may reduce it)
PPlat aim: < 28 cm H2O
- Reduces incidence of ventilator induced lung injury
Poor chest compliance require higher PPlat:
1. Morbidly obese
2. Stiff chest wall
3. Abdominal compartment syndrome
Measurement of oesophageal pressure helps to determine transpulmonary pressure - reduce risk of induced lung injury
What is driving pressure?
Driving pressure = PPlat - PEEP
- Amount of pressure needed to sustain volume of gas in patient’s lungs
- Expression of lung compliance when at a constant tidal volume
Driving pressure > 15 cmH20 increases mortality
What is positive end expiratory pressure (PEEP)?
Elevation of end expiratory pressure to specific level above atmosphere
- To sustain open alveolar units at end expiration
Aim to set PEEP at levels (1-4 cmH2O) that sustains positive end expiratory pulmonary pressures
- If transpulmonary pressure turns negative, lung will collapse
Lung recruitment manoeuver
Application of higher than normal pressure for short period of time to open lung units that are normally closed at end inspiration
- Pressure control ventilation (PCV)
- Fi 100%
- Pressure control 15 cm H2O
- Inspiratory time 3s
- Rate 10/min - Increase PEEP 3-5cm H2O every 5 breaths until maximum PIP achieved
- Maximum applied PEEP between 25-35 cm H2O dependent upon targeted maximum PIP
- Maximum PIP between 40-50 cm H2O based on haemodynamic stability - Once at maximum, continue for 1 minute
- Then decremental PEEP trial and insure positive end expiratory transpulmonary pressure (see separate card)
Decremental Best Compliance PEEP Trial
- Volume control ventilation (VCV)
- PEEP 20-25
- Vt 4-6mL/kg PBW
- inspiratory time 0.6-0.8s
- RR (20-30/min) at rate that DOES NOT cause autoPEEP - Measure dynamic compliance (about 30-45s for compliance to stabilise once PEEP set)
- Decremental Decrease PEEP by 2 cmH2O each time and reassess dynamic compliance until clear pattern indicating best compliance PEEP
- Initially, compliance increase as PEEP is decreased
- But when lung begins to derecruit, compliance will decrease
- Once obvious that compliance is decrreasing, stop trial - Recruit lung and set PEEP at best compliant PLUS 2cm H2O
(Usually best compliance decremental PEEP underestimates best oxygenation decremental PEEP by 2 cm H2O, thus the addition)
FiO2 in ventilation
RCT: normoxia better survival than hyperoxia
Target: FiO2 to maintain resultant PaO2 55-80mmHg or SpO2 88-95%
Transpulmonary pressure (Ptp)
Ptp is pressure across respiratory system during ventilation
- Measured during spontaneous unsupported breathing or during mechanical ventilation
Theory: Airway opening pressure minus pleural pressure
Practical: Pplat - PEEP - oesophageal pressure
Airway opening pressure: end inspiration as PPlat
Pleural pressure: end expiration as PEEP and minus oesophageal pressure
Alveolar stress and strain
Alveolar stress estimated by transpulmonary pressure (higher Ptp, higher stress)
Target: 15-20 cm H2O
(Still unknown maximum sustainable stress without causing lung injury)
Alveolar strain is the volume of deformation of lung by addition of lung volume
- All volume increase above baseline FRC causes alveolar strain (larger TV larger strain)
- Repeated opening and collapse of alveolar opening also contribute to strain and stress
Prone positioning ventilation
- Recruits lung
- Improves VQ match
- Allows drainage of secretions
Indications:
1. Severe hypoxaemia PFR < 100mmHg AND
2. LMPV strategy failed
Complications and risks:
1. Accidental dislodge of ETT, central catheters
2. Haemodynamic instability
3. Brachial plexus injury
4. Pressure ulcers
5. Increased intracranial pressure and intraocular pressure - ocular complications
6. Vomiting and high aspirates
