Supl79-02-b-ingles p65

Jornal de Pediatria - Vol.79, Supl.2, 2003 

 S153

of sufficient PEEP or both (Figure 3). However, strategies

that apply sufficient PEEP while avoiding alveolar over

distension can prevent the generation of pro-inflammatory

mediators (biotrauma) that may adversely affect the

progression of the pulmonary lesion,

17,20

 as well as damage

remote organs if these substances were to enter into

circulation.

21

 Despite the protective role of PEEP having

been systematically documented in laboratory studies, the

North American multi-center clinical trial of patients treated

with a high pulmonary expiratory volume and low FiO

2

compared with patients treated with a low pulmonary

expiratory volume and high FiO

2

 was recently terminated

due to futility after the inclusion of 550 patients.

22

ARDS who develop significant hypercapnia when protective

ventilation is initiated (elevated PEEP with limited Vt) are

promptly started on high frequency oscillatory ventilation.

Ventilation mode

Modern conventional mechanical ventilators offer an

increasing array of ventilation modes for use in patients

with ARDS. Conceptually, however, most ventilation modes

used in ARDS are similar in that they are cycled by time and

limited by volume or pressure. A mode that is cycled by time

and limited by volume implies that the cycle (inspiration

and expiration) is controlled by time (inspiratory time and

breath rate), and that during the inspiratory phase of the

cycle a certain pre-determined volume is administered. A

mode that is cycled by time and limited by pressure implies

that the cycle (inspiration and expiration) is controlled by

time (inspiratory time and breath rate), and that during the

inspiratory phase of the cycle a certain pre-determined

pressure is administered. In volume-limited ventilation, the

Vt administered during each inspiration generates a certain

airway pressure (which is measured and controlled in current

ventilators). Similarly, in pressure-limited ventilation, the

application of a specific pressure gradient between the

ventilator and the airway results in the generation of a

certain Vt that can be measured and controlled. Regardless

of the ventilation mode used, it is important to emphasize

that no one conventional ventilation mode has been shown

to be clinically superior to another in the management of

patients with ARDS, as long as the principles of protective

ventilation are respected.

Considering that precise Vt control is a very important

factor in ARDS support, time-cycled volume-limited modes

are preferred by the most of intensive care specialists

nowadays. In time-cycled volume-limited ventilation

(controlled, assist-controlled, intermittent mandatory or

intermittent mandatory with pressure support) the operator

defines the exact Vt to be administered by each mandatory

ventilator cycle. The pressure measurements generated by

this set volume at the end of inspiration (dynamic) or after

a pause (static or plateau pressure) are indicators of

pulmonary compliance in ARDS. A peak inspiratory pressure

which increases over time for a fixed volume generally

indicates worsening compliance. In an analogous manner,

a reduction in peak inspiratory pressure generally indicates

an improvement in compliance. Volume-limited ventilation

traditionally generates a triangular pressure waveform, in

contrast with the rectangular waveform of pressure-limited

ventilation (Figure 4). As the area under the pressure curve

reflects mean airway pressure, volume-limited modes

(triangular waveforms) generally have a slightly lower

mean airway pressure than pressure-limited modes

(rectangular waveform). Modern ventilators like the Servo

300, however, offer a mode known as pressure regulated

volume control (PRVC), in which the shape of the pressure

waveform of this volume-limited mode is similar to the

rectangular format of the pressure-limited mode. As such,

Acute respiratory distress syndrome – Rotta AT 

et alii

Figure 3 - Comparison of PEEP and tidal volume (Vt) among

different randomized controlled studies of reduced

tidal volume strategies in ARDS

Amato

5

19

18

15

Stewart

Brochard

ARDS Network

0

5

5

5

5

0

0

0

10

PEEP (cm H O)

2

VT (ml/kg)

VT (ml/kg)

VT (ml/kg)

VT (ml/kg)

PEEP (cm H O)

2

PEEP (cm H O)

2

PEEP (cm H O)

2

10

10

10

10

10

10

10

20

15

15

15

15

20

20

20

In clinical practice, pediatric patients with ARDS should

be ventilated with PEEP that is capable of maintaining

adequate pulmonary volume at the end of expiration. This

value is generally above 8 cm H

2

O and below 20 cm H

2

O,

other than in exceptional cases. Positive end-expiratory

pressure should be progressively increased (in 2 to 3 cm

H

2

O increments) to optimize oxygenation (saturation

between 90 and 95% with FiO

2

< 0.5) and pulmonary

inflation checked with chest radiographs or computerized

tomography. Patients with severe anasarca or other restrictive

lesions of the chest (circumferential burns), as well as

patients with excessive abdominal pressure, may require

higher PEEP levels.

A strategy that limits Vt while at the same time applies

an ideal PEEP, generally results in a reduction in minute

volume and hypercapnia, even when the respiratory rate is

increased.

17

 Strategies with permissive hypercapnia

(controlled hypoventilation), in which an elevation in PaCO

2

up to approximately 80 torr is accepted as long as pH is kept

above 7.25, are well tolerated by adults

23

 but may have

adverse effects

24

 and have not been adequately tested with

children. In our clinical practice, pediatric patients with

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