Page 426 - ACCCN's Critical Care Nursing

Ventilation and Oxygenation Management 403

improved bronchial secretion drainage, limitation of Current Recommendations
colonisation of distal lung, decreased atelectasis and No ventilation strategy is more lung-protective than the
increased alveolar recruitment but may increase spread timely and appropriate discontinuation of mechanical
of pathogens in the lung and may increase the risk of ventilation. Weaning refers to the transition from ventila-
aspiration. 220-223 229
tory support to spontaneous breathing. Evidence based
116
Prone positioning results in changes to the distribution consensus guidelines published for weaning in 2001
230
of ventilation and pulmonary blood flow. Pleural pres- and 2007 emphasise the importance of preventing
sures are lower in non-dependent regions and higher in unnecessary delays in the weaning process, early recogni-
dependent regions due to gravitational forces, the weight tion of a patient’s ability for spontaneous breathing and
of the overlying lung and mismatch between the local the use of a systematic method to identify the potential
physical structures of the lung and chest wall. 224 The for extubation.
weight of the overlying lung increases in ARDS due to
parenchymal oedema and fluid within the alveoli. 225 This Weaning Predictors
gradient in pleural pressures means transpulmonary pres- Clinician judgement regarding prediction of weaning
sure is higher in non-dependent lung regions, compared readiness is known to be imperfect, with unnecessary
to dependent regions. 225 Perfusion also increases from prolongation of ventilation or high rates of reintuba-
231
previously nondependent to dependent lung regions tion as resultant consequences, both of which are associ-
resulting in optimal matching of ventilation and perfu- ated with adverse outcomes. 232,233 An evidence based
sion to promote gas exchange. review evaluating over 50 objective physiological mea-
Pleural pressure in the dependent dorsal regions in the surements for determining readiness for weaning and
supine position can result in airway closure, atelectasis extubation found most had only a modest relationship
and hypoxaemia. 224 The difference in pleural pressures with weaning outcome; no single factor or combination
234
from non-dependent and dependent lung regions is of factors demonstrating superior accuracy. Of all pre-
greater in the supine compared to the prone position. In dictors studied, the respiratory frequency to tidal volume
235
the supine position, the heart and abdominal contents ratio (f/V T ) appears to be most accurate. However inclu-
also compress lung bases and decrease FRC, whereas in sion of the f/V T as part of a weaning protocol was found
prone positioning, the weight of these structures are lifted in one randomised study to increase, as opposed to
236
from the lung. decrease, the duration of weaning. At present, consen-
sus guidelines do not recommend routine inclusion of
230
The benefits of prone positioning continue to be debated. weaning predictors.
Although oxygenation improves in 70–80% of patients
turned from supine to prone, 226 a mortality benefit Weaning Methods
has not been shown in all trials. The most recent sys-
tematic review and meta-analysis 227 confirms a reduction Various studies have attempted to identify the best
in mortality in patients with severe baseline hypox- weaning method. Two of the most frequently-cited studies
aemia (PaO 2 /FiO 2 ratio <100 mmHg), but reported this have produced conflicting results. Brochard and col-
237
benefit was not present in patients with less-severe leagues compared PSV, T piece trial and SIMV, and
hypoxaemia. Other benefits of prone positioning dem- concluded that PSV reduced the duration of mechanical
onstrated were improved oxygenation and decreased ventilation compared with the other methods. Esteban
238
rates of VAP. Adverse events related to prone position- and colleagues compared PSV, T piece trials, CPAP and
ing were increased risk of decubitus ulcer formation, progressive reduction of SIMV support, and found a once-
endotracheal obstruction and accidental line or tube daily T piece trial led to extubation three times more
dislodgement. quickly than SIMV and nearly twice as quickly as PSV.
Failure to produce consistent results favouring a single
Implementing prone positioning requires forward plan- weaning style suggests it is not the mode that is important
ning to ensure eye care and protection, mouth care, but rather the application of a systematic process. 239
wound dressings, and tracheal suction are attended to
before positioning the patient prone. Intravenous lines, Spontaneous breathing trials
electrocardiogram leads, urinary catheter drainage, chest Spontaneous breathing trials (SBTs) incorporate a focused
drains and ostomy bags need to be secured and reposi- assessment of a patient’s capacity to breathe prior to extu-
tioned appropriately once the patient is positioned. 228 bation 240 and are recommended as the major diagnostic
Prone positioning can be achieved by manual handling test to determine extubation readiness. SBTs can be
230
of the patient, requiring up to five staff, although conducted using either a T piece or low levels of pressure
commercial devices are available that facilitate the turning support and should need to last only 30 minutes. 242
241
and positioning of the patient. 228
This method of weaning is uncommon in the ANZ setting,
in contrast to international findings. 65,66
WEANING FROM THE VENTILATOR
Weaning traditionally occurs via clinician-directed adjust- Protocols
ments to the level of support provided by the ventilator, Implementation of various organisational strategies
culminating in a spontaneous breathing trial comprising such as weaning teams and non-physician-led weaning
either low level pressure support or a T piece trial. protocols may assist in the timely recognition of weaning