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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_885_Библиотеки_им_академика_М_И_Перельмана.pdf
228 F. M. Pieracci, J. Haenel and M. Sawyer
Transitioning the work of breathing to the patient
• Ventilatory support modes vary from full support (e.g., assist control) to sup-
plemental oxygen only (e.g., T-piece for endotrachel tube or trach collar via
a tracheostomy tube) [Chapter 5-(ii)].
• Most ventilated critically ill surgical patients without underlying respiratory
disease do not require prolonged weaning. Rather, the necessity for mechanical ventilation is transient and related to the underlying acute pathology. Time
for substantial deconditioning of respiratory muscle/drive has not elapsed.
Therefore, stable patients may be rapidly transitioned to a partial support mode
and evaluated for extubation.
• Although this transition may occur quickly, it is contingent upon several
fators. Broadly speaking, the patient’s metabolic requirements (and thus CO2
production and ultimately required Ve) must be low enough to be managed
independently by the patient. Conversely, the patient’s mental status and neuromusclar condition must be sufficient to initiate spontaneous breaths and
maintain adeqaute V
• A patient’s ability to successfully sustain spontaneous ventilation depends
upon the mechanical load on the respiratory system, including:
{ Resistance
{ Elastance
{ Intrinsic PEEP
{ Respiratory muscle fatiuge
• The following represent relative contra-indications to transitioning a patient
to a partial ventilatory support mode:
{ Recent neuromuscular paralysis
{ Elevated intra-cranial pressure
{ Shock
{ FiO
{ Minute ventilation < 5 or > 15 L/min
> 50%
2
.
e
Determining successful transitioning
• Transitioning to a partial support mode will not be successful if the patient is
not able to generate sufficient respiratory effort and tidal volume to maintan
a normal Ve, or if the ventilatory requirements (CO2 elimination) of the underlying disease process are too great.
• Both hypoxemia and desaturation of arterial hemoglobin are infrequent and
late finding in failed transitioning.
Liberation from Mechanical Ventilation 229
• In general the following are indicative of failed transitioning:
{ Apnea
{ Respiratory rate > 30
{ Sustained heart rate > 20% baseline for > 5 minutes
{ Systolic blood pressure > 180 mm Hg or < 90 mm Hg
{ Hypercapnia
{ Anxiety/diaphoresis
{ Abdominal paradox
The myth of “minimal ventilator settings”
• One method of predicting successful liberation from mechanical ventilation
entails attempting to replicate as closely as possible the conditions that the
patient will face after extubation.
• This theory has led to the concept of “minimal ventilator settings,” which
typically refers to both a pressure support and PEEP of 5 cm H2O, which are
purported to apply only that pressure which negates the resistence of the tube
and replicates intrinsic PEEP.
• This reasoning is flawed for several reasons:
{ Upper airway edema and inflammation that develops in response to intu-
bation likely results in airway resistance which far exceeds that imposed
by a standard endotracheal tube.
{ The addition of as little as 5 mm H
inspiratory work by as much as 40%.
{ The concept of physiologic PEEP is not substantiated by data. The static
recoil pressure of the respiratory system is zero at end-expiration in a
healthy adult.
{ Furthermore, the addition of 5 cm H
work of breathing by as much as 40%.
• Intensivists may then be lulled into a false sense of security when observing
a patient breathing comfortably on “minimal ventilator settings.”
• Most ventilated patients can tolerate an approximately 50% increase in res-
piratory load following extubation. Thus, they are able to compensate for the
removal of both pressure support and PEEP.
• In the remainder of cases, and when any doubt as to the success of extubation
is raised, a trial of spontaneous breathing with the artificial airway still in
place (e.g., T-piece or trach collar) may be executed, thereby removing the
advantages of both pressure support and PEEP.
O of pressure support can decrease
2
O of PEEP can decrease inspiratory
2
230 F. M. Pieracci, J. Haenel and M. Sawyer
Extubation
• Both the intensive care literature and the anesthesiology literature have
exhaustive algorithims for intubation of the challenging airway. But there is
very scant knowledge guidelines for the extubation of these difficult patients.
• A similar level of vigilance is required for the safe extubation of the previ-
ously ventilated patient as there has been for the safe intubation of the ICU
patient.
• In fact, extubation should be considered more dangerous than intubation and
studies have shown greater complications during extubation.
• The ASA guidelines for management of the difficult airway do not give any
concrete guidelines for extubation.
• Traditional extubation criteria should be applied if possible and correlated to
the clinical picture of the patient. These include:
{ Following commands
{ Clear oropharynx with intact gag reflex
{ 5 second head lift/hand grasp
{ Vital capacity > = 10 ml/kg
{ NIF > 20 cm H
{ Tidal volume >6 cc/kg
{ Rapid shallow breathing index < 100
{ Arterial Blood gas showing adequate oxygenation
{ Positive “cuff leak”
• One should realize that the majority of ICU patients that are intubated are
actually intubated before they are brought to the ICU, hence a review of the
previous airway notes are essential before embarking on routine extubation.
• Extubation should be addressed with the plan already in place for possible
failure and urgent reintubation.
• It is best done during time periods when adequate staffing is availible.
• If trial extubation over an airway exchange catheter is planned, anesthesia
should be at bedside: this is an advanced technique that is not commonplace.
• Simply, extubation should include members of the ICU team, respiratory
therapy team and anesthesiology should be present when there is a possibility
of failure if the Airway Care Plan indicates a difficult airway.
O
2
The difficult to wean patient
A small subset of critically ill or injured surgical patients may require a formal
weaning plan.
Liberation from Mechanical Ventilation 231
Common scenarios for when a structured weaning plan is required include:
• Cervical spine injuries resulting in tetraplegia, particularly above C-5.
• Critical illness polyneuropathy complicating the primary admission diagnosis.
• COPD associated with post-operative acute on chronic respiratory failure.
• Persistent inflammation and immunosuppression related to MOF.
• Pneumonectomy /lobectomy associated with post-operative pneumonia.
Classification of the difficult to wean patient
Time to successful ventilator discontinuation starting with the first spontaneous
breathing trial (SBT) can be used to describe the degree of difficulty involved in
the weaning process.
• Simple weaning consists of patients who after one weaning attempt go on to
successful extubation. This group of patients represents ≈ 69% of all patients
and not surprisingly has a quoted mortality rate of 5%.
• Difficult weaning includes patients who fail three SBT or need seven days of
additional SBT to successfully discontinue ventilator support.
• Prolonged weaning includes patients who have failed three SBT or require > 7
days of attempts after the first SBT. As many as 15% of patients fall into the
prolonged category.
Patients classified as “prolonged weaning” are frequently transferred to long
term rehab facilities that offer formal weaning schedules over a period of weeks
to months.
Why my patient will not wean
There are essentially four reasons why a patient remains ventilator dependent.
• Cardiovascular instability
• Hypoxemia
• Psychological
• Imbalance between Demand versus Capability to breath
{ Cardiovascular instability may first present during a spontaneous breath-
ing trial as new onset atrial fibrillation, premature ventricular contractions
or signs of acute ischemic changes on the EKG. Alternatively, patients with
known chronic left heart failure may show evidence of acute pulmonary
edema when switching the patient from positive pressure ventilation to
spontaneous breathing.
232 F. M. Pieracci, J. Haenel and M. Sawyer
{ Hypoxemia is common in intubated patients and multifactorial in origin.
Initiation of spontaneous breathing without positive pressure promotes
posterior diaphragm movement but may result in worsening of ventilationperfusion matching due to dependent collapse. Ineffective secretion
clearance as a result of pain or weakness sets the stage for atelectasis and
increase work of breathing resulting in hypoxemia.
{ Psychological issues, while not frequent present a challenge when they do
occur. Patients with neuromuscular diagnosis’s (tetraplegia and myopathies), depression, delirium, or COPD may experience periods of intense
dyspnea during SBT’s that may result in the patient not being willing to
participate further. It is imperative that the patient feels comfortable with
the care providers who are performing the SBT and this combined with
judicious use of anxiolytics may result in better cooperation.
{ Demand versus Capability is responsible for the preponderance of all
significant delays in the ventilator discontinuation process.
Demand is quantitated by assessing the patient’s minute ventilation.
A minute volume consistently > 13 liters a minute may result from
sepsis (increased CO2 production, metabolic acidosis), pulmonary
dysfunction (increased dead space ventilation, decreased compliance,
or increased airway resistance), neurogenic or psychogenic causes.
The weak, elderly, malnourished patient is going to have a difficult
time maintaining this level of minute ventilation.
Capability is reflected by the patient’s drive to breath and muscle
performance i.e. respiratory rate, tidal volume, forced vital capacity,
negative inspiratory force and rapid shallow breathing index.
• Weakness versus fatigue: is your patient weak or fatigued?
Weakness is a decreased capacity of a rested muscle to perform a task.
Fatigue is a reversible decrease in the ability of a muscle (diaphragm in this case)
to contract caused by over-activity and will reverse with simple rest periods.
Once a patient has been identified as “ difficult to wean,” it is imperative to
formulate a weaning schedule. A couple of rules:
{ Post the plan for everyone to see
{ Do not change the plan until it is determined it is not working!
{ Document the patients progress daily
{ Communicate with everyone involved, especially the patient!
Liberation from Mechanical Ventilation 233
What are my options?
{ T-Piece trials
{ Pressure support trials
{ SIMV/pressure support trials
Although more labor intensive, T-Piece trials have advantages over the other
two options. A T-Piece trial permits periods of work and then rest, it is easy to
assess if the patient is actually making progress and the patient will notice that
they are actually doing better. It is imperative that the patient “rest” in between
each trial and rest at night. Resting is not guaranteed with pressure support and
should be done with a full ventilator support mode.
234 F. M. Pieracci, J. Haenel and M. Sawyer
Practical Algorithm(s)/Diagrams
Fig. 1. The Denver Health Medical Center pre-extubation airway assessment record.
Liberation from Mechanical Ventilation 235
Review of Current Literature with References
• Among respiratory variables, Yang and Tobin found the rapid shallow breath-
ing index (henceforth referred to as the “Tobin” index) to be the most accurate
predictor of successful extubation in an original cohort of 100 medical ICU
patients (New Engl J Med 1991; 324: 1445–1450).
• The most recent Cochrane Review of early vs. late tracheostomy concluded
that there was still insufficient evidence to favor one strategy over the other
(Cochrane Database Syst Rev 2012; 14; 3: CD007271).
• Girard et al. reported that a protocol of daily interruption of sedation and
assessment for appropriate liberation from mechanical ventilation resulting
in improved outcomes as compared to standard of care (Lancet 2008; 371:
126–134).
• Both the Society for Critical Care Medicine (www.sccm.org) and the
American College of Chest Physicians (www.chestnet.org) maintain updated
online guidelines for discontinuation of mechanical ventilation.
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Chapter 6-(v)
Acute Respiratory Distress Syndrome
Jeffrey L. Johnson, MD*
* Acute Care Surgeon, Denver Health Medical Center,
Associate Professor of Surgery, University of Colorado School of Medicine
Take Home Points
• Adult respiratory distress syndrome (ARDS) is the manifestation of an inflam-
matory injury to the Alveolar-Capillary interface. It has many underlying
causes, and is best thought of as a spectrum of disease.
• The clinical picture of ARDS includes hypoxemia refractory to oxygen,
diffuse patchy pulmonary infiltrates and decreased pulmonary compliance.
• The pathophysiology of ARDS includes noncardiogenic pulmonary edema,
infiltration of alveoli by inflammatory cells, loss of type II pneumocytes, and
impaired pulmonary vasomotor function.
• The injured lung in ARDS is markedly heterogeneous. Different zones of
alveoli can be characterized by four types: flooded/collapsed, recruitable,
open, and overdistended.
• Strategies to open alveoli and keep them participating in gas exchange
are pivotal in managing patients with severe ARDS. These may include
Contact information: MC 0206, 777 Bannock Street, Denver, CO 80204; Email: Jeffrey.
Johnson@dhha.org
237