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Ventilation and Oxygenation Management 395

Unbel Flow Wave Pattern Description
(Rectangular, Peak flow rate is delivered immedately at the
square) onset of inspiration, maintained throughout
the inspiratory phase, and abruptly terminated
at the onset of expiration.
Common default pattern with volume-targeted
modes.

Sinusoidal Inspiratory flow rate gradually accelerates to
peak flow and then tapers off.
Believed to mimic spontaneous inspiratory
patterns.
May increase PIP (peak inspiratory pressure).
Accelerating Flow gradually accelerates in a linear fashion
(ascending to the set peak flow rate.
ramp)

Decelerating Flow is at peak at onset of inspiration and
(descending gradually decelerates throughout inspiratory
ramp) phase.

Flow ceases and ventilator cycles to expiratory
phase when flow decays to a percentage of
peak flow, usually 25% but varies by ventilator
model. Terminal flow criteria may be adjustable
in some newer ventilators.

Rapid intial flow raises mean airway pressure
and may assist in alveolar recruitment.
May improve the distribution of gases when
there is inhomogeneity of alveolar ventilation.
Decreases dead space, increases arterial
oxygen tension, and reduces PIP.

FIGURE 15.4 Standard mechanical ventilator flow-wave patterns.
129

Expiratory Sensitivity
Expiratory sensitivity describes the percentage of decay in Practice tip
peak flow reached during the inspiratory phase that
signals the ventilator to cycle to expiration for spontane- The P insp setting reflects a different value on different ventila-
ous breaths. In some ventilator models this is pre- tors. P insp equals total pressure including PEEP on some ventila-
determined at 25%, whilst other ventilator models tors and inspiratory pressure above PEEP on others. Use the
allow clinician selection. Premature termination of a pressure–time scalar to confirm.
breath will increase inspiratory muscle workload
whereas delayed breath termination increases expiratory
muscle load. 143
VENTILATOR MODES
Peak Airway Pressure The mode of ventilation describes inspiratory phase vari-
Airway pressures vary across the respiratory cycle, and are ables; how the ventilator controls pressure, volume, and
measured by the ventilator’s airway pressure gauge. A flow during a breath; as well as describing how breaths
number of pressures are identifiable (e.g. peak inspira- are sequenced. All breaths have trigger, limit and cycle
tory, end-expiratory). The airway pressure (P aw ) is an inspiratory phase variables. 144 Each breath is triggered
important parameter in assessing respiratory compliance (started) either by the patient or by the ventilator. During
and patient–ventilator synchrony, and will vary depend- inspiration, the breath is limited to a set target of pres-
ing on V T , RR, ventilator flow pattern, dynamic compli- sure, volume, or flow. This target cannot be exceeded
ance and airway resistance. In pressure-targeted modes during each breath. At the end of inspiration, the cycling
the peak inspiratory pressure is equivalent to the P insp . In variable determines the end of the inspiratory phase.
volume-targeted modes the peak inspiratory pressure is Again this variable may be pressure, flow, volume, or
determined by the set V T and patient compliance and time. Gas delivery during each breath is described by the
resistance. control variable. There are five control variables: pressure,