Ramb - Janeiro

onseca

and

arvalho

ssoc

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2017; 63(1):1-3

were associated with the outcomes; two were related to the

patient, APACHE III or SAPS (RR = 1.36) and arterial pH

(RR = 0.73), and the other two were related to mechanical

ventilation parameters, FiO

(RR = 1.24) and driving pres-

sure (OR = 1.42). However, only exposure to high driving

pressure during the first days of mechanical ventilation was

strongly associated with a fixed and permanent risk over

the first 60 days after randomization, thus satisfying the

proportional hazards hypothesis. By analyzing the driving

pressure more closely, the authors were able to demonstrate

that with a fixed PEEP, increased driving pressure leads to

increased mortality. On the other hand, when we progres-

sively increase PEEP and maintain the driving pressure

fixed, there is no effect on mortality, but if we decrease the

driving pressure, we also reduce mortality. Thus, the asso-

ciation of driving pressure with mortality was evidenced.

Thus, an association between driving pressure and mortal-

ity was evident. Furthermore, in a series of additional ana-

lyzes, Amato et al. showed that in patients receiving pro-

tective mechanical ventilation (

plateau

pressure ≤ 30 cmH

and TV ≤ 7 mL/kg for an ideal weight) a driving pressure

of less than 13 cmH

0 generates less mortality, while a

plateau

pressure > or ≤ 26 cmH

0 or TV > or ≤ 7 mL/kg for

the ideal body weight has no effect on mortality.

If we consider these new evidences relevant and apply

them to the bedside, the main ways of manipulating ven-

tilatory parameters to benefit patients are to reduce tidal

volume and adjust PEEP level. The effect of decreasing

tidal volume has already been pointed out in the ARDSNet

Tidal Volume trial,

which showed that the group of pa-

tients receiving low tidal volumes had lower mortality. Not

coincidentally, this group also had a lower driving pressure,

with a difference of -8.8 cmH

O compared to the group of

higher tidal volumes. The three largest randomized con-

trolled trials comparing high versus low PEEP in patients

with ARDS failed to demonstrate benefit in relation to the

mortality of elevated PEEP. What probably happened was

that the changes in driving pressure in these studies

21-23

were not important enough, -2.8, -0.2, +0.2, respectively, to

produce a difference in mortality. And this, in turn, may

have occurred because in each patient the high PEEP had

conflicting consequences: either it reduced VILI by decreas-

ing atelectrauma, or it increased it as a result of overdis-

tension. Given the heterogeneity of the lung with ARDS,

“recruitability” is difficult to anticipate, and so adjusting

PEEP is of paramount importance.

Thus, the first step is to determine whether the patient

has recruitable lungs. Grasso et al. demonstrated that in

patients with recruitable lungs, the use of high PEEP caus-

es the PaO

/FiO

ratio to increase and, in contrast, it does

not change in patients with non-recruitable lungs.

Next,

we must determine the optimal PEEP. At this moment, the

findings of Amato et al. are of fundamental importance.

The adjustment of optimal PEEP can be performed by

determining the PEEP resulting in lower driving pressure,

preferably less than 14 cmH

This can be done by in-

creasing or decreasing PEEP by 4 cmH

O at a time, and

measuring the driving pressure at each pressure level. In-

creases or decreases smaller than this can cause problems

in the signal/noise ratio. These small additions or decreas-

es in PEEP should be made until the driving pressure

reaches its lowest value. If this point is exceeded, the driv-

ing pressure increases again. Also in favor of the driving

pressure, there is no need for special equipment to measure

it, the pressure can be observed directly from the mechan-

ical ventilation device without paralyzing the patient, who

only has to be relaxed during exhalation of the gases so

that the measured

plateau

pressure is more reliable.

Recently, Gattinoni et al.

presented an interesting

proposal that energy supplied per unit of time (“power”)

is an entity that unifies most of the parameters used in

mechanical ventilation, in addition to the relevant forces

involved in the genesis of VILI, providing us with a “com-

posite index” that can be transposed into clinical practice,

as it evaluates the relative contribution of adjustable

components at the bedside (TV, frequency, PEEP,

∆

airway

pressure, I:E ration, flow).

One should emphasize that because ARDS is distin-

guished by the heterogeneity of pulmonary involvement

as well as by conflicting responses to the therapies imple-

mented, such as maintaining high PEEPs, evaluation by

means of images of the adequacy of mechanical ventilation

is fundamental. The gold standard for mechanical venti-

lation monitoring in ARDS, however, is computed to-

mography, despite its disadvantages due to risks related

to the transport of patients and their excessive exposure

to radiation, which reduces its applicability.

On the oth-

er hand, there is electrical impedance tomography, which

seems to be a very appropriate alternative, providing con-

tinuous monitoring in real time, without the need for

radiation or patient transport. This test reliably shows

changes in lung volume and tidal volume.

1,26

The strategies of protective ventilation are effectively a

breakthrough. They allow the reduction of mortality despite

the partial understanding of how this benefit is achieved.

The additional understanding provided by Amato et al.

solved an old issue while simultaneously providing key ele-

ments for reducing mortality and allowing us to establish

a strategy for optimum PEEP adjustment, friendly to the

professionals and which does not require special equipment.