Ventilator Management
Modes of Mechanical Ventilation: Pressure and Volume
In volume targeted: ventilator delivers a certain volume of gas in a set time
In pressure targeted: ventilator delivers a certain level of pressure for a set time
Both pressure and volume targeted modes are TIME CYCLED.
The time cycle is controlled through setting the inspiratory time. Exhalation is
passive
Flow Cycling: Inspiration ends when flow gets to be a certain percent of max. This
only occurs in patient triggered modes
Minute ventilation: RR x Vt. Desired minute ventilation to match patient’s
metabolic needs is usually 3-4x BSA. Can be estimated at 100ml/kg. Normally 57L/min. This will also increase 9% for each degree C above normal.
Tidal Volume: volume with each spontaneous breath normally 5-7ml/kg.
Calculated based on IBW. Normally start at 8-10ml/kg. For ARDS or patients with
poor compliance, 4-6ml/kg.
PEEP: positive end expiratory pressure. Pressure set to keep lungs from collapsing
at end of expiratory phase so that pressure does not drop to zero.
Besides the mode of ventilation used, there are 3 other concepts to understand:
1. Control Variable: what triggers, limits and cycles the breath
a. Triggers: time, patient effort
b. Targets: pressure, volume
c. Cycles: time
2. Breath Sequence: full vs partial support vs spontaneous
a. Full: Ventilator delivers all breaths at full support -- CMV
b. Partial: ventilator delivers set breath at full support and patient can
trigger extra breaths at variable pressure/volume limits -- SIMV
c. Spontaneous: patient regulates rate and volume for breath
3. Target Feedback: response of ventilator to lung and patient -- CPAP/PS
a. PRVC
b. SIMV with autoflow: automatic flow adjustment to get set tidal
volume at lowest possible pressure. Pressure and flow automatically
adjusted based on resistance and compliance
Ventilator Concepts to Understand:
1. Peak Inspiratory Pressure (PIP): measure of large airway resistance
2. Plateau Pressure: measure of pressure at alveolar level. Can assess by doing
inspiratory hold – when flow is at zero, measure of the pressure.
3. Autopeep: PEEP generated when next breath started before patient finishes
previous breath. Can be measured (along with volume trapped) by doing an
expiratory hold on the vent.
4. Mean airway pressure: Calculated automatically by the ventilator.
(PIP-PEEP) xTi/Ttotal +PEEP
CMV: control mode ventilation. Can be volume or pressure targeted.
- triggered by time and patient
- flow can be constant in volume targeted modes; descending in pressure
targeted
- time cycled
SIMV: synchronized intermittent mandatory ventilation. Set volume or pressure
targeted breaths. Patient triggered breaths can be of any volume/pressure (up to
limits set)
- patient and time triggered
- volume/pressure targeted
- time cycled
Pressure Support: delivers an inspiratory pressure above the set level of PEEP.
Often set with CPAP (continuous positive airway pressure) which acts like PEEP.
Can be added in modes where patient can assist with breaths
- patient triggered
- pressure targeted
- flow cycled
APRV (airway pressure release ventilation): Set a Phigh (high CPAP) and Plow
(low CPAP, usually 0) and Thigh (Inspiratory time) and Tlow (expiratory time).
- time triggered, pressure targeted, time cycled
- Tlow is most important to prevent derecruitment
- Patient able to breathe spontaneously at Phigh
Other Variables that are set on Ventilator
PEEP: Positive pressure set at end of exhalation. Prevents premature alveoli
collapse and promotes oxygenation
FiO2: fraction of inspired oxygen
- try to keep <50% as higher levels of oxygen can lead to nitrogen washout in
lungs, free radical formation and rebound atelectasis
- can lower to 21%
Inspiratory Time: control variable of respiratory cycle.
- normal I:E ratio is 1:2 to 1:3 (inspiratory time 25-33% of total cycle time)
- cycle time = Itime + Etime
- frequency of breathing = 60/cycle time