Weaning methods
·
T-piece
o
theoretic advantages:
§
simple
§
provides strenuous exercise to the ventilatory muscles
followed by periods of full ventilatory muscle rest;
this is theoretically the best way to condition the
inspiratory muscles
o
technique:
§
match PCO2 and tidal volume to the patient's baseline
prior to initiating weaning attempt
§
increase FiO2 by 10% at initiation of weaning
§
stop trial if :
·
respiratory rate
increases > 10
·
PCO2 rises > 5
·
pulse rises > 20
·
blood pressure rises >
20 mm systolic
§
attempt trials at least twice per day with a minimum of
1 hour between trials
§
extubation can be considered if the patient breathes on
their own for at least 2 hours without distress
o
disadvantages:
§
removes the internal resistance of the ventilator but
does not alter the resistance due to the endotracheal
tube
§
resistance is proportional to length and inversely
proportional to the 4th power of the radius
§
some examples of endotracheal tube resistances:
·
6.0 tube = 3.2
cm/liter/sec
·
7.0 tube = 2.5
cm/liter/sec
·
8.0 tube = 1.4
cm/liter/sec
·
9.0 tube = 0.5
cm/liter/sec
§
this added resistance can pose a great deal of extra
work for the patient over and above that of breathing
spontaneously without an endotracheal tube
·
SIMV
o
technique:
§
decrease rate of support by 2-4 breaths per trial as
tolerated
§
return to full support between trials
§
extubate when rate close to or equal to zero
o
theoretic advantages:
§
allows gradual switch from fully supported to fully
independent breaths
§
prevents patients from "fighting" the ventilator
§
reduces respiratory muscle fatigue
§
allows some normal venous return to the right heart due
to some negative pressure (spontaneous) breaths
o
problems:
§
work of breathing and oxygen consumption can increase as
much as 2-fold due to work necessary to activate the
demand valve
§
flow triggering results in 30-40% less inspiratory
effort than pressure triggering
·
pressure support
o
theoretic advantages:
§
augments a spontaneous breath with positive pressure
§
counteracts extra work of breathing due to resistance of
the tubing
§
in patients with volume overload or left ventricular
failure, rapid changing to spontaneous breathing (e.g.,
T-piece trials) can result in lung flooding and rapid
failure of weaning; pressure support avoids this by
gradually reducing positive pressure and thus permitting
gradual correction of fluid status by diuresis, etc.
o
technique:
§
start at a pressure support sufficient to provide a
tidal volume of 8-10 ml/kg (this will usually be about
equal to the plateau pressure while the patient is on
assist-control)
§
reduce support by 2-5 cm as tolerated
§
extubate when pressure support is equal to the
calculated resistance of the ventilator circuit (e.g..
endotracheal tube, tubing, expiratory valves, etc.);
this is usually about 5 cm
o
problems:
§
requires physician to make frequent ventilator setting
decisions
·
recent comparative
studies:
o
Brochard, 1994
§
randomized multicenter study of 456 patients
§
all patients initially underwent a 2 hour trial of
spontaneous breathing; if they had respiratory distress
(109 patients) then they were assigned to 1 of 3 weaning
modes:
·
T-piece
·
SIMV
·
pressure-support
§
outcome was measured as the percentage of patients
extubated by 21 days:
·
T-piece = 64%
·
SIMV = 60%
·
pressure-support = 90%
§
conclusion: pressure-support resulted in the most
patients extubated by 21 days
o
Esteban, 1995
§
randomized multicenter study of 546 patients
§
all patients initially underwent a 2 hour trial of
spontaneous breathing; if they had respiratory distress
(130 patients) then they were assigned to 1 of 4 weaning
modes:
·
IMV
·
pressure support
·
T-piece (2 or more
trials per day)
·
T-piece (once daily
trial)
§
outcome was measured by time until extubation:
·
IMV = 5 days
·
pressure support = 4
days
·
T-piece (2 or more/day)
= 3 days
·
T-piece (1/day) = 3 days
§
conclusion: T-piece trials result in the most rapid
successful extubation
·
Ely, 1996
o
randomized single
hospital study of 300 patients
o
control patients
underwent daily respiratory function screening
o
study patients underwent
daily respiratory function screening and underwent a
T-piece trial for up to 2 hours if the screening
criteria were met; if the patient satisfactorily
completed the trial, the attending physician was
notified
o
for the screening
criteria to be met:
§
PO2:FiO2 ratio > 200
§
PEEP => 5
§
cough reflex intact
§
f/Vt < 105
§
no continuous infusion of vasopressors or sedatives
o
time to extubation:
§
study group = 4.5 days
§
control group = 6 days
o
complication rate:
§
study group = 20%
§
control group = 41%
o
ICU costs:
§
study group = $15,740
§
control group = $20,890
o
conclusion: daily
screening for "weanability" results in shorter days on
the ventilator, fewer complications, and lower ICU costs
·
Girqult, 1999
o
randomized single
hospital study of 33 patients who failed a 2 hour
T-piece trial
o
patients were assigned
to early extubation with immediate institution of
non-invasive ventilation using pressure support or to
continued mechanical ventilation via an endotracheal
tube until a T-piece trial was successfully performed
o
There was no difference
in complication rates or survival but patients in the
study group had a significantly decreased duration of
mechanical ventilation
·
Henneman, 2001
o
124 patients had weaning
facilitated by a collaborative weaning flowsheet and
were compared to 77 patients weaned per the previous
usual practice of the ICU
o
Patients weaned using
the "weaning board" and "weaning flowsheet" came off of
the ventilator an average of 3.6 days faster than
patients weaned per the previous usual practice of the
ICU.
·
Krishnan, 2004
o
145 patients in an
academic medical center ICU staffed 24 hours per day
o
No differences in
weaning rate by physician-directed weaning versus
protocol-directed weaning.
o
Findings may relate to
the presence of 24-hour per day physician availability
therefore, protocol-driven weaning may still be
appropriate for ICUs that are not staffed 24 hours per
day.
·
writers personal
philosophy:
o
all patients should have
a weaning assessment to include f/Vt daily
o
SIMV is probably never
the best mode of weaning medical patients; it is
acceptable for uncomplicated post-operative patients
because of its simplicity and because almost any weaning
mode works well in these patients
o
T-piece weaning is
better (more rapid) for medical patients who are rapidly
improving
o
pressure-support is
better for some medical patients who are improving
slowing
o
the combination of
pressure-support + SIMV as a weaning mode is
unnecessarily confusing and adds little to
pressure-support alone
o
rapid weaning in
patients who fail an initial T-piece trial may be
warranted in COPD patients if extubated to full face
mask non-invasive ventilation (Nava, 1998).
Causes of failure to wean:
·
hypoxemia
o
diffuse pulmonary
disease
o
focal pulmonary disease
(pneumonia, etc.)
o
pulmonary edema
§
removal of positive pressure can increase preload and
lead to worsened heart failure, especially in patients
with diastolic dysfunction which should be suspected in
patients who become unexpectedly tachycardic and/or
hypertensive during weaning; these patients often
require a calcium channel blocker (verapamil or
diltiazem) or a beta blocker for control of diastolic
dysfunction before weaning will be successful
·
insufficient ventilatory
drive
o
response to pathologic
metabolic alkalosis
o
inadequately functioning
CNS respiratory drive center
§
sedatives
§
malnutrition
§
myxedema
§
primary CNS disease
·
excessive ventilatory
demands
o
excessive CO2 production
§
sepsis
§
agitation
§
fever
§
high carbohydrate intake
·
respiratory muscle
weakness
o
neuromuscular disease
o
malnutrition
o
drugs
§
neuromuscular blocking agents
§
corticosteroids
§
aminoglycosides
o
low magnesium,
potassium, phosphorus
·
excessive work of
breathing
o
airway obstruction
§
bronchospasm
§
secretions
§
bronchitis
§
kinked endotracheal tube
§
mechanical airway obstruction (tumor, bronchostenosis,
tracheo-bronchial malacia)
o
endotracheal tube too
small
o
Chest motion restriction
(bandages, positioning, etc.)
·
acid-base disorders
·
phrenic nerve injury
o
esp. following open
heart surgery
o
unilateral diaphragm
paralysis rarely symptomatic unless there is substantial
contralateral pulmonary disease
·
psychological factors
sleep deprivation
o
anxiety
o
depression
o
confusion
o
pain
