Nomenclature
The nomenclature used on different ventilators is often confusing. It is,
therefore, important to consult the appropriate instruction manual to determine
what a particular mode actually does. Where possible we have used generic names
for modes.
Control mode ventilation
- time preset pattern of inspiratory flow that delivers a set tidal volume
- when time preset pattern of inspiratory pressure is delivered = PCV
- patient initiated breaths not possible
- this mode is not found on modern ICU ventilators
Synchronized intermittent mandatory ventilation
- patient can breathe spontaneously but also receives a set number of
mechanical breaths
- ventilator assisted breaths are synchronized with the patients' breathing to
prevent the possibility of a mechanical breath on top of a spontaneous breath
(see fig 5a & 5b)
- advantages over CMV thought to be: improved weaning, ventilator synchrony and
intrapulmonary gas distribution; minimization of sedation and prevention of
respiratory muscle atrophy
- no proven advantage over T-piece trials in weaning
- SIMV breaths often fails to unload respiratory muscles and impose same work as
intercurrent spontaneous breaths. ie at the same overall level of ventilator
support, effort is more or less independent of whether the breath is assisted or
not. This is probably due to the inability of the respiratory system to adjust
inspiratory effort on a breath to breath basis.
– by varying the difference between the preset rate and the SIMV rate it is
possible to manipulate the I:E ratio in apnoeic patients. This is because the
cycle time and thus the absolute inspiratory time is determined by the preset
rate not the SIMV rate. This feature of SIMV may be useful in patients with
severe asthma/COPD.
Inverse ratio ventilation
- increases mean airway pressure and therefore oxygenation by prolonging
inspiratory time
- usually pressure-controlled but can be volume controlled
- in pressure-controlled mode allows increased mean airway pressures without
increased peak pressures, theoretically improving oxygenation without increasing
the incidence of barotrauma. However reduced expiratory time predisposes to
inadequate emptying of lung regions with long expiratory time constants
resulting in air trapping and intrinsic PEEP. This may lead to overdistension
and rupture of alveoli. 26% incidence of barotrauma has been reported with PC-IRV
which is similar to incidence of barotrauma reported with CMV
- like extrinsic PEEP, intrinsic PEEP increases alveolar recruitment and thus
improves oxygenation. However, can decrease delivered volumes in
pressure-limited ventilation and can increase peak airway pressure in
volume-limited ventilation. Decrease in delivered volumes may be compensated for
by an improvement in ventilation-perfusion matching and thus decreased dead
space
- exact mechanism by which IRV improves oxygenation not clear
- although widely used there is little data to support its superiority over
other modes of ventilation. Primarily used in acute lung injury when toxic
levels of inspired oxygen required using conventional modes of ventilation
- "unnatural" respiratory timing necessitates sedation
- greater haemodynamic effect due to increased mean intrathoracic pressure
Airway pressure release ventilation
- like a variable level of CPAP
- upper level of airway pressure chosen to provide a mean airway pressure that
prevents hypoxia
– CO2 elimination occurs by:
- spontaneous patient ventilation
- intermittent reduction of airway pressure (to a lower or zero level of
CPAP) allowing for gas release from the lungs
- total ventilation is a function of effectiveness of spontaneous
ventilation; frequency, duration and pressure difference of each pressure
release; and intrinsic impedance of patients lung and chest
- peak airway pressure never exceeds preset upper level. Theoretically reduces
morbidity due to peak airway pressure elevations
- resembles IRV in that peak airway pressures are lowered while mean pressures
are increased by prolonged inspiratory times and short expiratory time decreases
potential for airway closure
- unlike IRV allows patient to breath spontaneously with little sedation
- exact role not clear at present
Inspiratory flow waveforms
- little evidence to suggest that any one waveform superior for any given
condition
- decelerating waveform associated with improved distribution of ventilation in
patients with chronic airflow limitation but also results in higher mean airway
pressure and therefore predisposes to potentially adverse haemodynamic effects
PEEP & CPAP
- PEEP applied by placing an expiratory resistance in circuit. May be
spring-loaded valve, weighted valve, under-water column, venturi valve,
electronically controlled scissor valve or pressure actuated solenoid valve
- threshold resistor mechanism preferred (minimal resistance to flow above
opening pressure). Minimizes expiratory work and barotrauma during coughing or
straining
- CPAP circuit may use demand valve or continuous fresh gas flow. Demand valve
may be flow triggered or pressure triggered. Advantage of demand valve system is
that ventilator monitoring is still available but work of breathing is increased
by need to trigger valve. Respiratory work imposed by circuit and demand valve.
Older ventilators imposed respiratory work similar to normal work of breathing.
Modern ventilators: relatively low work. Newer ventilators have flow-by system
- in patients with dynamic hyperinflation and auto-PEEP, extrinsic PEEP may
reduce respiratory work during spontaneous or patient initiated breaths. Due to
reduction in threshold work
– recruits partially collapsed alveoli reducing shunt
– redistributes intra-alveolar pulmonary oedema from the alveolar to the
interstitial compartment but doesn’t decrease total lung water
| 
