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Mechanical ventilation

Up Mechanical ventilation Pathology Radiology

ARDSnet studies
Lung recruitment

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.

Ventilatory strategies

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

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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