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396 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

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volume, flow, time and dual control (such as used in the effort is not acknowledged by the ventilator. CMV may
mode pressure regulated volume control [PRVC]). Breath also be called volume-controlled ventilation (VCV) or
sequencing refers to the sequence of mandatory and pressure-controlled ventilation (PCV) depending on the
spontaneous breath. A spontaneous breath is one during target (volume or pressure) variable. VCV requires clini-
which inspiration is both started (triggered) and stopped cian selection of the frequency, PEEP, FiO 2 , tidal volume,
(cycled) by the patient. Spontaneous breaths may be flow waveform, peak inspiratory flow and either the
assisted, as with pressure support, or unassisted. Manda- inspiratory time or I : E ratio. PCV requires clinician selec-
tory breaths are either triggered or cycled by the ventila- tion of rate, PEEP, FiO 2 , inspiratory pressure, as opposed
tor. 145 A complete mode description should include: (1) to tidal volume, and inspiratory time or I : E ratio depend-
the control variable; (2) the breath sequence; and (3) the ing on the ventilator type. Peak inspiratory flow and the
targeting scheme (limit variable). flow waveform are manipulated by the ventilator, to
achieve the clinician-selected inspiratory pressure within
Pressure Control vs Volume Control the set inspiratory time. The inability to breathe sponta-
Traditionally, clinicians have favoured volume control neously during CMV contributes to diaphragm muscle
due to the ability to regulate minute ventilation (VE) dysfunction and atrophy which may result in difficulty
154
and carbon dioxide (CO 2 ) elimination with straight- weaning from the ventilator.
forward manipulation of ventilation. 146 Volume control
provides consistent tidal volume delivery, independent Synchronised Intermittent
of the patient’s lung mechanics. A disadvantage of Mandatory Ventilation
volume control, however, is the lack of control over peak Synchronised intermittent mandatory ventilation (SIMV)
airway pressure that changes in response to altered com- delivers breaths at a set frequency (rate), and can be either
pliance and resistance. Elevated peak airway pressures pressure- or volume-targeted. Setting of the ventilator is
may cause alveolar overdistension, barotrauma and similar to setting VCV or PCV. The availability of patient
haemodynamic effects such as reduced venous return, triggering with SIMV facilitates provision of gas flow in
cardiac output, hypotension and thus decreased organ recognition of a patient’s spontaneous effort. SIMV uses
perfusion. 147 Clinicians need to carefully monitor ventila- a timing window to deliver mandatory breaths in syn-
tion to avoid injurious pressures. In volume control the chrony with patient inspiratory effort. Additional spon-
116
peak airway pressure is achieved at the end of inspira- taneous breaths occurring outside of the timing window
tion, and only for a short duration, therefore distribution may be assisted with pressure support to augment the
of gas may not be optimised and shearing stress can patient’s spontaneous effort to a pre-set pressure level.
occur. 148
Pressure control allows ventilator control over the peak Assist Control
inspiratory pressure and inspiratory time. Clinicians are In assist control (A/C,) the patient can trigger the ventila-
required to monitor minute ventilation and gas exchange tor, however, unlike SIMV, every patient-initiated breath
due to the lack of a guaranteed tidal volume and pos- is assisted to the same clinician-determined tidal volume
sible changes in respiratory compliance and resistance. (A/C [VC]) or inspiratory pressure (A/C [PC]). All breaths
The variable and decelerating inspiratory gas flow pattern are cycled by the ventilator irrespective of being patient-
of pressure control enables rapid alveolar filling and or ventilator-triggered. In the absence of spontaneous
more even gas distribution compared to the constant breathing, A/C resembles CMV.
flow pattern that may be used with volume control.
This decelerating flow pattern results in improved Pressure Support Ventilation
gas exchange, decreased work of breathing and preven- Pressure support ventilation (PSV) is a spontaneous
tion of overdistension in healthy alveoli. 149-152 During mode of ventilation in which the patient initiates and
pressure control, the set inspiratory pressure is achieved cycles all breaths, with support of the patient’s inspiratory
at the beginning of the inspiratory cycle and maintained effort by the ventilator using rapid acceleration of flow to
for the set inspiratory time. This promotes recruitment achieve a preset level of inspiratory pressure. Unlike CMV,
of alveoli with high opening pressures and long SIMV or A/C, there is no setting of ventilator breaths in
time-constants.
this mode. PSV is usually employed with positive end
expiratory pressure (PEEP) which maintains partial infla-
COMMONLY EMPLOYED MODES tion of alveoli during the expiratory phase to promote
OF VENTILATORS alveolar recruitment and oxygenation.
Contemporary ventilators now provide a range of modes
to facilitate mechanical ventilation. Modes of mechanical Continuous Positive Airway Pressure
ventilation are described in Table 15.7. Continuous positive airway pressure (CPAP) is one set
baseline positive pressure applied throughout the inspira-
Controlled Mandatory Ventilation tory and expiratory phase. In this spontaneous breathing
Controlled mandatory ventilation (CMV) is a mandatory mode, unlike PSV, no additional positive pressure is pro-
mode, and is the original and most basic mode of ventila- vided to the patient during inspiration. Due to nomen-
tion. 153 CMV delivers all breaths at a clinician-determined clature used on some ventilator models, PSV is frequently
set frequency (rate) and the patient’s spontaneous misrepresented as CPAP.