Page 719 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

538 PART 4: Pulmonary Disorders

limiting fluid boluses to patients likely to benefit from it (see Chap. 34), variation occurring in unstructured care systems. Whether achieved by
and using diuretics and renal replacement therapy before hypervolemia protocol or by individual clinician perseverance, we believe that patients
is excessive. can be liberated from mechanical ventilation more expeditiously if they
Inspiratory muscles suffer atrophy and contractile dysfunction dur- are screened on a daily basis.
ing critical illness and mechanical ventilation. 13,32 This VIDD is seen
early, progresses quickly, and is associated with prolonged ventilation ■ THE SPONTANEOUS BREATHING TRIAL (SBT)
and death. In animal models, ventilator modes that maintain active Pressure support, continuous positive airway pressure (CPAP), and
contraction (assisted, rather than controlled, modes) largely prevent T-piece trials are the most common methods used to test readiness for
VIDD. These findings suggest that controlled modes should be avoided, liberation from mechanical ventilation. Strong evidence is lacking to
when possible. support one approach over the others. An advantage of T-piece trials is
■ STEP THREE: RECOGNIZE READINESS TO BREATHE simplicity, but some patients failing T-piece can safely be extubated.
38
Most intensivists prefer 5 to 7 cm H O pressure-support because this
2
Weaning implies gradual, rather than rapid, withdrawal of ventilatory maintains the monitoring and alarming functions of the ventilator; this
assistance. This word suggests that the ventilator is beneficial or nurtur- degree of ventilator assistance does not generally produce false nega-
ing and that the mechanism for successfully separating the patient is to tives (ie, passing the SBT does not lead to excessive extubation failures;
gradually adjust the machine. Liberation more accurately describes the although patients with primary neuromuscular disease may be an
process by which most patients are freed from the ventilator. For many, exception); PEEP can be continued; and most large mechanical ventila-
this is as simple as recognizing that the ventilator is no longer needed. tion trials have employed this approach.
In patients who cannot breathe independently, liberation will only be In preparation for the SBT, sedatives and narcotics should be discon-
possible after the patient is treated and recovers. tinued several hours beforehand to reduce the likelihood of inadequate
drive to breathe. Coordinating the sedative interruption and SBT
CAN THE PATIENT BREATHE WITHOUT improves success, reducing time on the ventilator and even long-term
mortality. Especially when SBTs are conducted by protocol, a safety
28
THE VENTILATOR? screen is necessary to reduce risk and select patients most likely to
■ WEANING PARAMETERS benefit. Typical safety screens require hemodynamic stability; adequate
oxygenation on an acceptable PEEP; some spontaneous breathing effort;
Historically “weaning parameters” were used to predict patients’ ability and absence of agitation, cardiac ischemia, or intracranial hypertension
to breathe without the ventilator. However, despite decades of research, (Fig. 60-2). In individual circumstances and with appropriate monitor-
no weaning parameter has predictive accuracy sufficient to be used ing, an SBT can be conducted despite higher than nominal levels of
exclusively to make liberation decisions. Moreover, the question PEEP or while patients are still requiring vasoactive infusions for shock,
33
can be answered directly with a trial of spontaneous breathing (SBT). since the ventilator may be a more noxious intervention than norepi-
Accordingly, we do not use weaning parameters routinely to make lib- nephrine, for example.
eration decisions. 34 The first SBT need be only 30 minutes since extending the trial
39
The use of interdisciplinary weaning teams 4,35,36 or respiratory therapist- longer does not enhance the clinician’s ability to assess readiness for
driven protocols may expedite successful liberation by actively address- extubation. The proper duration of subsequent SBTs in those who fail
ing this question each day. Other studies have applied very different has not been studied (30-120 minutes are generally used). Thus the
algorithms to achieve significant reductions in duration of ventilation. 10,37 available data suggest that patients should be considered for a trial of
However, they all have one thing in common: they substitute a program extubation after a successful trial (30-120 minutes) of either T-piece,
of daily systematic scrutiny of readiness for breathing for the individual CPAP, or pressure support of 5 to 7 cm H O.
2
Daily interrogation of readiness
1. Hemodynamically stable
2. Pa /Fi >120 on PEEP 5 cm H O
2
O 2
O 2
PS 7 cm H O, CPAP = 5 cm H O or T-piece
2
2
breathing for 0.5–2.0 H on 50% Fi
O 2

1. Respiratory rate <35/min Define mechanism of failure
2. Heart rate increment <20/min No 1. Rapid shallow breathing/ CO 2
3. SBP increment <20 mm Hg - Treat elevated loads
4. ABG without acute acidosis or - Treat reasons for reduced capacity
Pa O 2 < 60 mm Hg 2. Hypoxemia (Pa /Fi <120)
O 2
O 2
- ?Atelectasis-due to weakness
Yes Weak cough- or secretions.
?Sedative effect, CNS/PNS event - Acute pulmonary edema-
1. Cough PF >60 L/min No
2. ETT suction <Q2° Excess secretions- diurese, preload/afterload
Treat infections, aerosol RX
3. Cuff leak if appropriate reduction if appropriate.
Upper airway obstruction-
Yes 48° steroids
Trial of extubation
FIGURE 60-2. A simple bedside algorithm for liberating patients from mechanical ventilation and performing a trial of extubation.

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