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Ventilation and Oxygenation Management 393

Ventilation is commonly commenced on a high FiO 2 ‘auto-triggering’ is triggering by the ventilator in the
setting, but as noted earlier, consideration is given to absence of spontaneous inspiratory effort.
the risks of oxygen toxicity which include disruption
to the alveolar-capillary membrane and fibrosis of the Inspiratory Time and Inspiratory :
alveolar wall. 119 Expiratory Ratio
The total time available for each mandatory breath is
Tidal Volume determined by the set frequency. The total breath time
Tidal volume (V T ) is the volume, measured in mL, of comprises the inspiratory and expiratory time which can
each breath. The V T is calculated using the patient’s be expressed as a ratio (I : E). In normal spontaneous
ideal body weight using height and gender-specific breathing, expiratory time is approximately twice as long
120
tables to achieve 6–8 mL/kg (see Table 15.6). Strong as the inspiratory time (1 : 2 ratio). Gas flow also influ-
evidence indicates a mortality benefit for using 6 mL/ ences inspiratory time, with higher gas flows resulting in
kg in patients with acute respiratory distress syndrome decreased time to achieve the target V T . The I : E ratio can
121
(ARDS). Some evidence also indicates 6 mL/kg as be manipulated to create an inverse relationship (1 : 1,
a target for patients without ARDS or acute lung injury 2 : 1, 4 : 1) with the goal of increased mean airway pres-
(ALI). 122,123 While further studies are required, clinicians sure resulting in alveolar recruitment and improved oxy-
should consider aiming for 6–8 mL/kg in all ventilated genation. Inverse ratios are more frequently applied with
patients. pressure control ventilation as application in volume
control can result in increased risk of barotrauma due to
Respiratory Rate peak and plateau airway pressure variation. 128
Mandatory frequency or respiratory rate (f, RR) is set with
consideration of the patient’s own respiratory effort, Inspiratory Flow and Flow Pattern
anticipated ventilatory requirements and the effect on the The flow rate refers to the speed of gas and is measured
I : E ratio. Use of high doses of sedation with or without in litres per minute (L/min). Generally, inspiratory flow
neuromuscular blockade requires setting a mandatory is delivered at speeds of 30–60 L/min. Higher flow rates
rate that facilitates adequate gas exchange and meets oxy- cause gas to become more turbulent and result in
genation requirements. A lower frequency can be set for increased peak airway pressures. Lower flow rates result
a patient able to breathe spontaneously in modes such as in laminar flow, an increased inspiratory time, improved
synchronised intermittent mandatory ventilation (SIMV) distribution of gas, and lower peak airway pressures. 129
and assist control (A/C) (see below) to enable spontane- The flow of inspiratory gas can be delivered in three
ous triggering. Physiologically normal respiratory rates styles: constant or square wave, decelerating ramp and
are 12–20 breaths per minute. Patients with hypoxaemic sinusoidal pattern (see Figure 15.4). In a constant flow
respiratory failure generally breathe 20–30 breaths per pattern, the peak flow is achieved at the beginning of
minute. 124 inspiration and is held constant throughout the inspira-
tory phase. This may result in higher peak airway pres-
Triggering of Inspiration sures. Using a decelerating ramp, the gas flow is highest
Depending on the mode of ventilation, breaths are trig- at the beginning of inspiration and tapers throughout the
gered by the ventilator or patient in various sequences. A inspiratory phase. Sinusoidal gas flow resembles sponta-
breath may be triggered by the ventilator in response to neous ventilation.
time in modes with clinician-determined set frequency Pressure Support
such as CMV, and in A/C and SIMV in the absence of
spontaneous effort. Patient triggering requires the ventila- When triggered by the patient, the ventilator delivers flow
tor to sense the patient’s inspiratory effort. Most modern to achieve the clinician-determined set pressure support.
generation ventilators now use flow triggering, as evi- The flow is variable, depending on the patient demand.
dence indicates that flow triggering may be more respon- The V T achieved with pressure support is dependent on
125
sive to patient effort than pressure triggering. Pressure chest and lung compliance as well as airway and ventila-
triggering requires the patient to create a negative pres- tor resistance. Pressure support is generally set at
sure within the ventilator circuit for long enough to 5–20 cmH 2 O. Increasing the level of pressure support
enable the ventilator to sense the effort and commence will result in increased V T , and improvements in gas
flow of gas. Flow triggering requires a predetermined flow exchange if compliance and resistance remain constant.
of gas, usually 5–10 L/min, referred to as the bias (or
base) flow, that travels continuously through the ventila- Positive End Expiratory Pressure
tor circuit. When the patient makes an inspiratory effort, Positive end expiratory pressure (PEEP) is the pressure
they divert flow that is sensed by the ventilator. If the flow applied at the end of the expiratory cycle to prevent alveo-
diversion reaches a clinician-determined set value, a lar collapse. PEEP increases residual lung volume thereby
breath is initiated. 126 The flow trigger is usually set at recruiting collapsed alveoli, improving V/Q match and
1–3 L/min (1 L/min represents less patient effort and enhancing movement of fluid out of the alveoli. 130,131
3 L/min represents greater patient effort). Despite PEEP was originally introduced by Ashbaugh and col-
advances in ventilator technology, various studies leagues 132 in the 1960s as a technique for treating refrac-
continue to identify missed patient triggers that contri- tory hypoxaemia in patients with ARDS. Animal studies
bute to patient–ventilator asynchrony. 127 Conversely, suggest ventilator-associated lung injury (VALI) may be