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Ventilation and Oxygenation Management 401

alveoli or failure to recruit. 140 Once the recruitment
800 manoeuvre is terminated, derecruitment may occur
rapidly. Serious adverse effects have been noted during
Volume (mL) 600 High Raw pulmonary pressures resulting in reductions in venous
the use of RMs due to increased intrathoracic and intra-
return and cardiac output, and cardiac arrest and increased
400
(Solid line)
184,188
risk of barotrauma.
200 High Frequency Oscillatory Ventilation
High frequency oscillatory ventilation (HFOV) requires a
specialised ventilator and manipulation of four variables:
10 20 30 40 50 mean airway pressure (cmH 2 O), frequency (Hz), inspira-
Pressure (cm H 2 O) tory time, and amplitude (or power [ΔP]). 189 Alveolar
overdistension is limited through the use of sub-deadspace
*Dashed line depicts normal Raw tidal volumes whereas cyclic collapse of alveoli is pre-
129
FIGURE 15.10 Pressure–volume loop representing resistance changes. vented by maintenance of high end-expiratory lung
pressures. 190,191 High frequency (between 3 and 15 Hz)
oscillations at extremely fast rates (300–420 breaths/
min) create pressure waves enabling CO 2 elimina-
tion. 133,192 Oxygenation is facilitated through application
between the lungs and the ventilator circuit. Decreased of a constant mean airway pressure via the bias flow (rate
compliance requires greater pressure to achieve V T and is of fresh gas). 192,193 In adults, recommendations for the
reflected in a flattened P–V loop. 180 The area between the initiation of HFOV state mean airway pressure should be
loops represents the resistance to inspiration and expira- set 5 cmH 2 O above the peak airway pressure achieved
tion, known as hysteresis. As resistance increases, less V T with conventional ventilation. 194 The recommended fre-
is delivered resulting in a shorter and wider loop; con- quency range is 3–10 Hz with 5 Hz conventionally used
versely, as resistance decreases, a longer, wider loop is to initiate HFOV. Inspiratory time is set at 33% and the
generated (see Figure 15.10). 181 amplitude setting is determined by adequate CO 2 elimi-
nation. 133 Increased CO 2 elimination is achieved by low-
Flow–volume loops ering the frequency and increasing the amplitude.
Flow–volume loops recorded during positive pressure Until recently, HFOV was considered a rescue mode for
ventilation depict inspiration above the baseline and adult patients with acute respiratory distress syndrome
expiration below it. These loops are useful in determining (ARDS) experiencing refractory hypoxaemia and failing
response to bronchodilators and examining changes in conventional ventilation. 195,196 HFOV has been evaluated
airway resistance. in patients in early-onset ARDS and has been found to
improve oxygenation and to be well tolerated. 197 While
MANAGEMENT OF REFRACTORY HYPOXAEMIA further studies are required, these data suggest HFOV can
Refractory hypoxaemia may require strategies in addition be implemented in early ARDS.
121
to conventional lung-protective mechanical ventilation.
These include recruitment manoeuvres, high frequency Extracorporeal Membrane Oxygenation
oscillatory ventilation, extracorporeal membrane oxygen- Extracorporeal membrane oxygenation (ECMO) improves
ation and nitric oxide. total body oxygenation using an external (extracorporeal)
oxygenator, while allowing intrinsic recovery of lung
Recruitment Manoeuvres pathophysiology. Indications for ECMO include acute
Recruitment manoeuvres (RMs) refer to brief application severe cardiac or respiratory failure such as severe ARDS
198
of high levels of PEEP to raise the transpulmonary pres- and refractory shock. Bleeding as a complication of
sure to levels higher than achieved during tidal ventila- anticoagulation is a major risk of ECMO, with cerebral
199
tion with the goals of opening collapsed alveoli, recruiting bleeds being the most catastrophic. Another serious
slow opening alveoli, preventing alveolar derecruitment, complication is limb ischaemia when the femoral artery
and reducing shearing stress. 182-184 The most common is used.
RM is elevation of PEEP to achieve a peak pressure of ECMO consists of three key components:
40 cmH 2 O for a sustained period of 40 sec, although
studies report peak pressure elevations ranging from 25– 1. a blood pump (either a simple roller or centrifugal
50 cmH 2 O for durations ranging from 20–40 sec. 185 The force pump)
best method in terms of pressure, duration and frequency 2. a membrane oxygenator (bubble, membrane or
have yet to be determined. 186 Recruitment manoeuvres in hollow fibre)
humans have not produced consistent results in clinical 3. a countercurrent heat exchanger, where the blood
studies, 184,187 with a recent systematic review demonstrat- is exposed to warmed water circulating within
ing no mortality benefit despite transient increases in metal tubes.
oxygenation. 185 Effective recruitment may be difficult to In addition, essential safety features include bubble detec-
assess with the potential for either overdistension of tors that detect gas in the arterial line and shut the pump