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Lung mechanics
last updated in December 2008
- CT scans of chest early in the course of the disease, with the patient
supine, show striking asymmetry of lung involvement. Dependent posterior regions
preferentially infiltrated, consolidated or collapsed. Anterior areas often
normally or even excessively aerated during mechanical ventilation.
- non-homogeneity is probably not the result of non-homogeneous involvement
of the lung. There is a homogeneous alteration in the vascular permeability of
the lung with accumulation of oedema in all lung regions. Distribution of
collapse and consolidation due to a combination of:
- alveolar flooding
- compression by the heart
- compression by the weight of oedematous lungs
- pressure of intra-abdominal contents
- infiltrates movable to some extent so that a simple shift to prone position is
sufficient to cause substantial inversion of the pattern
- non homogeneous reduction in lung compliance of lung
- parts of the lung have normal or near normal compliance
- other parts have
greatly reduced compliance
- majority of the delivered tidal
volume being delivered to the part of the lung with normal compliance
particularly if inspiratory time is short. (If inspiratory time is prolonged
there may be redistribution to the less compliant areas.)
- since perfusion of the
poorly compliant areas is not proportionally reduced shunting is increased.
The only ventilatory strategy that has been shown to improve the outcome of
patients with ARDS or ALI is the ARDSnet
mechanical ventilation study. In the latter recruitment was stopped prematurely because of better outcome in the
low tidal volume (6 ml/kg predicted body weight) group.
Tidal volumes, inspiratory pressure and respiratory rate
- important to avoid overdistension of alveoli in the relatively normal parts
of lung
- adjust tidal volume and inspiratory pressure to produce tidal volume of
4-8 ml/kg predicted body weight and inspiratory plateau pressure of <30
cmH2O
- calculate predicted body weight from height and sex or use tables
- adjust respiratory rate to maintain minute ventilation but check for gas
trapping and auto PEEP
- initially aim for a tidal volume of 8 ml/kg predicted body weight and then
gradually reduce the target tidal volume to 6 ml/kg over the next few hours
(to give time for some metabolic compensation for respiratory acidosis)
- instead of aiming for a target Pco2 aim to
keep pH > 7.25. However, both neurologic and haemodynamic side effects
increase as Pco2 rises and so it is probably advisable to not to
allow Pco2 to rise above 20 kPa. Contraindications to acute
hypercapnia:
- hypoxia
- intracranial pathology
- relative contraindications include: right ventricular dysfunction (in
these patients hypercarbia tends to increase pulmonary hypertension),
congestive cardiac failure, coronary artery disease, arrhythmia,
hypovolaemia, beta blockade
- ? give bicarbonate to speed extracellular pH correction (intracellular
correction occurs in few hours)
- click here for ventilation algorithms
Inspired oxygen concentration
- high oxygen concentrations are toxic to the lung. The consensus view is
that the inspired oxygen concentration should not exceed 0.5-0.6 if possible
- aim for arterial saturation of 88-94%
Positive end-expiratory pressure
- positive end-expiratory pressure (PEEP) prevents alveolar collapse.
This reduces shunting, improving oxygenation, and should reduce shear
injury.
- current view is that PEEP should be set to maximize compliance and
oxygenation. In theory it may be preferable to perform a decremental PEEP
trial
- during incremental PEEP trials compliance depends not only on the
balance between alveolar re-expansion in dependent areas and alveolar
overdistension in non-dependent areas due to PEEP, but also on tidal
alveolar recruitment
- with decremental PEEP trials the lung is near maximally re-expanded at
the start of the trial, particularly if a recruitment manoeuvre is
carried out first. As a result there is little/no tidal recruitment and
the level of PEEP associated with the maximal compliance should be the
level that produces the optimal balance between recruitment and
overdistension
- ALVEOLI study
shows no difference in outcome in patients ventilated with
low tidal volumes and high PEEP compared to low tidal volumes and low PEEP.
But the patients in the high PEEP group were older. Also PEEP was set
according to preset combinations of FiO2 and not adjusted
according to lung mechanics in individual patients
- EXPRESS
study showed reduction in duration of mechanical ventilation, duration
of organ failure, better compliance, better oxygenation and lesser need for
adjunctive therapies if patients were given PEEP to reach plateau pressure
of 28 to 30 cm H2O. There was no
significant difference in mortality.
- LOV study
showed reduction in rate of refractory hypoxaemia, death with refractory
hypoxaemia and lesser need for rescue therapies if patients received higher
PEEP with plateau pressure lower than 40 cm H20
and recruitment manoeuvers.
-
Oesophageal pressure guided mechanical ventilation was found to improve
oxygenation, respiratory system compliance as compared with ventilation
according to ARDSnet recommendation. In this study, the PEEP was adjusted
according to the transpulmonary pressure estimated from oesophageal pressure
catheter. The
target of
the transpulmonary pressure was between 0 to 10 cm water. Unfortunately,
the survival benefits at 28-day (17% versus 39%) and 180-day (27% versus
45%) mortality did not reached statistical significance.
Recruitment manoeuvres
- designed to improve oxygenation and reduce shear injury by re-opening
alveoli
- thought to reduce shear injury at the interface between collapsed and open
alveoli
- no clear consensus as to best method of recruiting lung. A number of strategies
have been tried
- should probably not be carried out routinely but may be useful following
desaturation, disconnection from ventilator circuit and suctioning
Ventilator mode
- at equivalent inspiratory times PCV has the advantage of higher mean airway
pressure compared to constant flow volume controlled ventilation or the addition
of inspiratory pause. Decelerating flow volume controlled ventilation has a
similar airway pressure profile to PCV but with constant volume delivery
- ARDSnet study used volume preset assist control
- pressure regulated volume control has the theoretical advantage of
limiting both inspiratory pressure and tidal volume
- bi-level positive airway pressure has the advantage that the patient can
breath spontaneously. This has theoretical advantage that movement of the
dorsal part of the diaphragm is greater during spontaneous breaths.
Theoretical disadvantage is that spontaneous breaths at higher pressure
level may result in very large lung volumes
High frequency oscillation
- lung inflated and kept open with very low tidal volumes
- should result in minimal shear injury
- MOAT2 study showed a trend to decreased mortality in patients ventilated
using high frequency oscillation but the control group received a relatively
high tidal volume (~10 ml/kg)
Inverse ratio ventilation
- I:E ratio > 1
- can improve oxygenation in patients who remain hypoxic despite PEEP. However
not all patients benefit from this strategy and as yet it is not possible to
predict an individual patient's response
- although it is known that IRV results in re-opening of collapsed alveoli it is
not clear how it achieves this. It may be that reducing the expiratory time
results in auto-PEEP or that the increased inspiratory time allows greater
mixing time and therefore greater homogeneity of ventilation
- muscle relaxants often required when I:E ratios of > 2:1 are used
- small risk of causing haemodynamic compromise
Prone position
- improves oxygenation in 50-70% of patients with ARDS
- leads to more even distribution of ventilation and perfusion and thus reduces
shunt. Ventilation to dorsal regions improves when prone but position has little
effect on perfusion which continues to go preferentially to the dorsal region.
- once alveoli are re-opened in the prone position they may be kept open by
PEEP, resulting in persistence of improvements in in oxygenation when the
patient is turned supine again
- not all patients improve on turning prone. Reason for this variable response
is not known
- recent randomized controlled trial showed no outcome benefit from prone
ventilation. May be due to the relatively short periods of time for which
patients were kept prone. Post hoc subgroup analysis suggested a reduction
in 10 day mortality in
those with lowest PaO2/FiO2 ratios, highest severity
of illness and highest tidal volumes. However improvement in mortality did
not persist beyond ICU discharge
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