The cornerstone of treatment
is to keep the PaO2 >60mmHg, without causing injury to the lungs with
excessive O2 or volutrauma.
Pressure control ventilation
is more versatile than volume control, although breaths should be volume
limited, to prevent stretch injury to the alveoli.
In general tidal volumes should not exceed 6ml/kg and plateau pressure
should not exceed 30cmH2O. However tidal volumes of 4ml/kg should be
delivered irrespective of airway pressure.
The management of patients
with respiratory failure goes beyond ventilation strategies, we must have
a holistic multisystem approach: I like to remind residents of the ABCDEFG
mnemonic.
A = Airway,
establish an patient airway, intubate as necessary.
B = Breathing,
commence mechanical ventilation and obtain an adequate minute volume to
maintain oxygen delivery.
C = Circulation:
blood pressure, pulse, intravascular volume – fluid
resuscitation and
vasopressors as necessary
D = Diagnosis, find
the underlying problem and control the source.
E = Empiric therapy,
for example antimicrobials for sepsis
FG = Feed the Gut, to
prevent villus atrophy and bacterial translocation
The principles of
mechanical ventilation are simple:
1.
Give enough
oxygen to keep the PaO2 over 60mmHg preferably, and over 50mmHg at the
very least.
2. Avoid
volutrauma and barotrauma, by keeping the tidal volumes in the 4-6ml/kg
range and the airway plateau pressure below 30 - 35cmH2O (the tidal
volume should not be less than 4ml/kg, irrespective of airway pressure).
The PaO2 is a function of
the FiO2, the PEEP level, the mean airway pressure and the minute
ventilation. The tidal volume, depending on what mode of ventilation is
used, is determined by the pressure control level (in pressure controlled
modes) or the tidal volume dialed up on the ventilator (in volume
controlled modes).
There is no clear evidence that any particular
mode or strategy improves outcome in ALI, except for controlling tidal
volumes and airway pressures. What follows is a suggested starting
strategy:
1. Start with a high FiO2 (use the same FiO2 on the patient following
intubation as before).
2. Set the CPAP/PEEP level – if the patient has a P/F ratio of 200-300 start
with CPAP/PEEP of 5cmH2O, if the P/F ratio is <200, use a CPAP/PEEP of
10cmH2O.
3. For inspiratory support, use a decelerating flow pattern, with a
tidal volume of 5-6ml/kg, of if pressure control is being used, a
pressure limit which gives a tidal volume of 5-6ml/kg (1).
Please see tutorials on
ventilator strategy.
It is important to note that ARDS is a disease of altered
lung compliance. This is reduced due to the presence of large quantities
of extravascular lung water. However, chest wall compliance may also be
low - in patients who are edematous, have had massive fluid resuscitation
or have abdominal hypertension. In this situation, the chamber in which
the lungs are inflating (the chest), bears more resemblance to a brick
wall than a rib cage with muscles. Higher inflation pressures are required
to inflate the lungs in these circumstances and
higher PEEP is required
to maintain FRC.
The choice of mode of
ventilation is institution specific. The majority of intensive care units
in the United States continue to use volume controlled modes of
ventilation to treat ARDS. Severe hypoxemia is managed by increasing mean
airway pressure by escalating levels of PEEP and rapid respiratory rates.
The logic behind increasing mean airway pressure is that much of the
ventilation perfusion mismatch contributing to hypoxia occurs at end
expiration (click here for more information).
Although the majority cases can be managed in this way, more versatile
modes are available, under the pressure control umbrella.
Pressure control modes have
the advantage of allowing us manipulate the mean airway pressure by
prolonging inspiration, and this may improve oxygenation without
increasing peak or plateau pressures . In addition,
pressure control may improve gas distribution at the end of inspiration,
particularly where different lung units have different resistance patterns
(ALI is, after all, a heterogeneous process).
The drawback of prolonging inspiration, and, in effect, inverting the I:E
ratio (2;3), is that the patient may experience a lot of discomfort, and
requires deep sedation. Further, incomplete expiration tends to reduce
CO2 elimination, and the patient will develop “permissive hypercapnia”
and respiratory acidosis. As we now know that ventilator induced lung
injury causes much more trouble than respiratory acidosis, we do not
consider the latter to be a major problem (4). Newer pressure
control modes such as BiLevel / Airway Pressure Release ventilation have been developed to address the problem
of patient discomfort in inverse ratio ventilation; with some success.
References
(1) Ventilation with lower tidal volumes as compared with traditional
tidal volumes for acute lung injury and the acute respiratory distress
syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med
2000; 342(18):1301-1308.
(2) Armstrong BW, Jr., MacIntyre NR. Pressure-controlled, inverse ratio
ventilation that avoids air trapping in the adult respiratory distress
syndrome. Crit Care Med 1995; 23(2):279-285.
(3) Tharratt RS, Allen RP, Albertson TE. Pressure controlled inverse ratio
ventilation in severe adult respiratory failure. Chest 1988;
94(4):755-762.
(4) Lewandowski K. Permissive hypercapnia in ARDS: just do it? Intensive
Care Med 1996; 22(3):179-181.
Copyright Patrick Neligan
2001-2002 |
