Page 721 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

540 PART 4: Pulmonary Disorders

Myocardial Ischemia

•
V O 2
Preload
Weaning Afterload MV O 2 > MQ • O 2
•
Contractility
Heart Rate

Congestive Heart
Failure

FIGURE 60-3. Pathophysiologic mechanisms of weaning-related ischemia and congestive heart failure.

pressure-volume curves or frequent, large esophageal or intravascular risk of postextubation failure. However, lack of a cuff leak does not abso-
pressure fluctuations during assisted ventilation may help to identify lutely predict extubation failure.
60
patients who are working hard on the ventilator (see Chap. 48). Empiric Mental status, airway protective mechanisms, ability to cough, and vol-
manipulation of tidal volumes, inspiratory flow rates, waveforms, ume of secretions also determine extubation outcome. Patients with cough
and triggering mechanisms may aid in improving patient-ventilator peak flows ≤60 L/min are 5 times as likely to fail a trial of extubation as
synchrony. Finally, in patients with obstructive lung disease, intrinsic those with cough peak flows >60 L/min. Those who cannot cough and
PEEP may significantly increase the work required to trigger ventilator- wet a white card placed 1 to 2 cm from the end of the open endotracheal
supported breaths. In selected patients, addition of applied PEEP to tube are 3 times as likely to fail. Those who require endotracheal suctioning
nearly match intrinsic PEEP can reduce this ventilator-induced load. 48 more than every 2 hours are also at increased risk of extubation failure.
68
We are reluctant to extubate patients if they have excessive or tenacious
Noninvasive Positive Pressure Ventilation to Speed Weaning: Some
patients who fail an SBT can be extubated nevertheless by using non- secretions and a weak cough or are not cooperative enough to aid in their
own pulmonary toilet (deep breathing and expectoration of secretions).
invasive positive pressure ventilation (NIPPV) as a bridge. The best
61
candidates are those with COPD who pass the SBT safety screen; have Patients with severe cerebral vascular accidents frequently present with this
constellation. It remains unclear as to whether early elective tracheostomy,
adequate cough and mentation; will not present a difficult reintubation;
and are good candidates for NIPPV (able to breathe spontaneously for to prevent aspiration and aid in pulmonary toilet, is superior to a trial of
extubation. When the cause of impaired airway protection is thought to
10 minutes and have no anatomic characteristics that preclude wearing
a mask). be reversible, treating and attempting a trial of extubation at a later time is
reasonable. Swallowing is abnormal in many patients following extubation,
Automated Weaning: The prospect of automated weaning is attractive. especially in those with neurological impairment, and is associated with
Modern ventilators contain sophisticated computers capable of being prolonged hospital stay. Accordingly, all patients, and especially those
69
programmed with sophisticated algorithms to adjust the ventilator in with altered mentation or stroke, should be carefully observed after extu-
ready response to a changing patient. Especially since delayed weaning is bation and formal swallowing assessment is advisable for many patients.
often due to clinicians’ failure to recognize readiness, automation could Postextubation stridor arising from upper airway edema is fairly
be effective, especially in settings where clinicians are not readily avail- common after extubation. Pulmonary edema can develop in some of
able around the clock. In fact, studies of automated weaning show that these patients, in part because large negative intrathoracic pressures
70
these ventilators do respond much more readily than clinicians, making during inspiration can dramatically increase left ventricular afterload.
many incremental changes. This automation, however, has not yet Nebulized racemic epinephrine and parenteral corticosteroids treating
62
translated into more expeditious liberation. 63 airway edema but have not been systematically studied. Heliox or mask
CPAP may be used to temporarily reduce upper airway resistance in
EXTUBATION selected patients who do not require immediate reintubation. 71
In contrast to its use as a bridge to extubation, NIPPV is probably not
There is an important distinction between liberation from mechani- effective in those who appear to be failing immediately after extubation.
72
cal ventilation and extubation. Once a patient has been liberated (has In 221 patients with a variety of illnesses randomized to NIPPV or rein-
passed the SBT), he does not need the ventilator. The endotracheal tube, tubation, ICU mortality was statistically higher (25 vs 14%) in the NIPPV
aside from providing the avenue for ventilation, allows removal of secre- group. There are some groups, particularly those with COPD or congestive
tions for patients whose airway protective mechanisms have been altered heart failure, in which NIPPV may be effective if used cautiously (especially
by disease. Accordingly, after a patient has passed an SBT, the physician precluding patients with airway incompetence or refractory tachypnea),
must determine whether the patient still needs the artificial airway. but further studies are required before this can be recommended routinely.
One consideration is whether the patient has an upper airway lesion
which could collapse to a critically small size after extubation. Patients at BREAKING THE RULES
risk include those who were initially intubated for upper airway stenosis
and stridor and those who have had a traumatic or prolonged intuba- Most experienced clinicians have treated patients whose SBTs suggested
64
tion. A number of studies have suggested that the ability to breathe that they would fail, but who were successfully liberated and extubated
around a deflated endotracheal tube cuff 65-67 or to produce a cuff leak nonetheless. When the numbers look bad but the patient looks good or
67
greater than 110 mL during volume-cycled ventilation predicts a low there is concern that the presence of the endotracheal tube is responsible

section04.indd 540 1/23/2015 2:20:48 PM