Ventilation Approach to ARDS


ARDS lungs are typically irregularly inflamed and highly vulnerable to atelectasis as well as barotrauma and volutrauma.
Their compliance is typically reduced, and their dead space increased. The standard of care for the ventilatory management
of patients with ARDS changed dramatically in 2000 with the publication of a large multicenter, randomized trial comparing
patients with ARDS initially ventilated with either the traditional tidal volume of 12 mL/kg or a lower TV of 6 mL/kg.
This trial was stopped early because the lower tidal volume was found to reduce mortality by an absolute 8.8% (P=0.007).
Notably, plasma interleukin 6 concentrations decreased in the low TV group relative to the high TV group (P <0.001),
suggesting a decrease in lung inflammation.7   (* Please note that Vt in ARDS patients is calculated from the an estimation of the
patient's weight based upon height to avoid overinflation in obese patients).


As a result of the study, the authors recommended initial ventilation of patients with ARDS with A/C (TV 6 mL/kg, Rate 12 , PEEP 5)
with goal pH = 7.25. The study noted that there is not current evidence to routinely recommend PEEP > than 5 cm H2O,
but, in appropriately monitored circumstances, it may be attempted. Intrinsic PEEP may occur in patients with ARDS at
high ventilatory rates and should be watched for and treated by reducing the rate of ventilation under direct observation
until plateau pressures decrease. The authors recommend a target plateau pressure of less than 30 cm H2 O.
Once a patient has been stabilized with adequate tidal volumes at a plateau pressure of less than 30 cm H2 O,
considering a trial of pressure-cycled ventilation is reasonable.

Recruitment Maneuvers

Recruitment maneuvers have been devised to increase the proportion of alveoli ventilated in ARDS.
Typically attempt short-term increased PEEP or volume is utilized to open occluded or collapsed alveoli.
In one study it was found that among ARDS patients undergoing whole-lung CT, applying 45 cm H2 O PEEP
recruited a mean of 13% new lung tissue.8 The National Heart, Lung, and Blood Institute ARDS Clinical Trial Network,
however, in a randomized comparison of high and low PEEP among 549 patients with ARDS, found no
statistical difference in the outcomes of death rates and time spent intubated.9

Small studies have evaluated the effects of prone positioning and kinetic therapy in ARDS/ALI.
Generally it has been found that both prone positioning and kinetic therapy have demonstrated similar
increases in oxygenation as compared to standard therapy administration using supine therapy.

Permissive Hypercapnia

Permissive hypercapnia is a ventilatory strategy that has used in the management of patients with ARDS
and COPD / asthma who would normally require dangerously high tidal volumes and airway pressures.
Although contraindicated in patients with head injury, cerebrovascular accident (CVA), elevated intracranial \
pressure, or cardiovascular instability, permissive hypercapnia allows decreased tidal volumes,
airway pressures, and respiratory rates. At this time evidence of mortality changes has not been
fully eveluated.

Noninvasive Ventilatory Stragies

Noninvasive ventilatory strategies have met with little success in the treatment of patients with ARDS.
The authors recommend great caution and close monitoring if noninvasive positive pressure ventilation
(NIPPV) is attempted among patients with ARDS. In trials of NIPPV among patients with undifferentiated
hypoxemia, the presence of pneumonia or ARDS was associated with significantly increased risk of failure.
Some subgroups of patients with ARDS may benefit from NIPPV; however, Antonelli et al demonstrated
greater success in applying noninvasive positive pressure ventilation to patients with simplified acute
physiology scores and higher PaO2/FiO2 ratios.

References:
http://emedicine.medscape.com/article/810126-overview
http://ccforum.com/content/9/S1/P108
Antonelli M, Conti G, Esquinas A, et al.  http://www.medscape.com/medline/abstract/17133177
Subpages (1): Quiz 1
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