الفهرس 
Anatomical & Physiological considerations of respiratory systemOnly 14 pages are availabe for public view
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Abstract
The respiratory system is both remarkable and complicated.
Knowledge of pulmonary anatomy provides a sound foundation
for understanding the complex processes of respiration ( Pierce,
2006).
Over the past decades, the respiratory physiology research has
greatly improved our understanding of the respiratory system.
Successful mechanical ventilation requires a basic understanding
of respiratory physiology and ventilator mechanics ( Davenport et
al.,2010).
Mechanical ventilation is a commonly used technique in the
intensive care unit (ICU) (Slutsky and Brochard, 2006).
Many factors affect the decision to begin mechanical
ventilation. Because no mode of mechanical ventilation can cure a
disease process, the patient should have a correctable underlying
problem that can be resolved with the support of mechanical
ventilation. This intervention should not be started without
thoughtful consideration because intubation and positive-pressure
ventilation are not without potentially harmful effects. Mechanical
ventilation is indicated when the patient’s spontaneous ventilation
is inadequate to sustain life ( Byrd et al., 2013).
The evidence base for the use of non-invasive ventilation (NIV)
in a number of different clinical situations has increased greatly in
recent years; however, there remain a number of contraindications
to its use. despite increasing interest in its use, NIV should not be considered as a replacement for MV and should not delay
intubation and MV in those patients who fail to respond to or
deteriorate on NIV (McNeill and Glossop, 2012).
Patient-ventilator asynchrony, defined as a mismatch between
the patient’s neural inspiratory time and the ventilator’s
insufflation time (Thille et al., 2008).
Given the multiple interactions between the patient’s innate
control of breathing and ventilator operation, it is not surprising
that patient-ventilator synchrony is more often the exception than
the rule (Branson, 2011).
The goal of patient-ventilator synchrony is to have the various
parts of ventilator-assisted breathing coincide with the patient’s
intrinsic breathing pattern (De Wit, 2011).
Patient-ventilator interaction is influenced by factors related to
the patient and factors related to the ventilator. Patient-related
factors such as respiratory center output, respiratory mechanics,
disease states or conditions and endotracheal tube type or size
influence the patient-ventilator interaction (Mellott et al., 2009).
Synchronous patient-ventilator interaction requires a ventilator
to be sensitive to respiratory efforts and responsive to airflow
demand. Two major factors contributing to PVD are ventilator
triggering (signal opens inspiratory valve) and cycling (signal
opens expiratory valve at end inspiration) (Racca et al., 2005).
Patient-ventilator asynchrony is common, and its prevalence
depends on numerous factors, including timing and duration of observation; detection technique; patient population; type of
asynchrony; ventilation mode and settings (eg, trigger, flow, and
cycle criteria); and confounding factors (eg, state of wakefulness,
sedation) (Epstein, 2011 ).
Monitoring of patient-ventilator interactions at the bedside is an
integral part of caring for the critically ill patient. Caring for the
mechanically ventilated patient involves examining the impact of
patient breathing and behavior on ventilator settings, and vice
versa (MacIntyre and Branson, 2009).
Accurate assessment of patient-ventilator interactions and work
of breathing (WOB) requires invasive measurements of pleural
pressure and/or respiratory muscle electromyogram. Use of an
esophageal balloon, which permits determination of pleural
pressure, and respiratory muscle electromyograms have been used
to measure a variety of patient-ventilator interactions and to
compute WOB. However, these devices are not used during
routine patient care and clinicians must rely on physical
examination of the patient as well as visual inspection of
waveforms to assess for patient-ventilator synchrony and
asynchrony. Visual inspection of waveforms has been shown to
correlate well with esophageal-balloon readings, but is not without
error (Spahija et al., 2010).
It is important for clinicians not to assume that ventilator
settings are optimal for the patient. Rather, clinicians must
evaluate the patient and response to ventilator settings before drawing conclusions about patient-ventilator synchrony. The
patient is the focus point and the clinician must adjust the
mechanical ventilator to meet the patient’s ventilatory
requirements. The goal is to have the “right tool for the right job,”
and clinicians must not assume that one “tool” (ie, set of ventilator
parameters) satisfies the needs of different patients. It is only after
careful observation of the patient and examination of ventilator
waveforms that clinicians should assume the patient and ventilator
are synchronous. When a patient appears uncomfortable, physical
examination and evaluation of ventilator waveforms are the first
steps in the management of the patient (de Wit, 2011).
Ventilator graphics are available on almost all current
mechanical ventilators and have been available for evaluating the
patient-ventilator interface. Both direct and anecdotal evidence,
however, suggests that bedside use of ventilators’ graphics
capabilities is widely underutilized and standard approaches or
guidelines for graphics interpretation are often lacking. The use of
guidelines, standards and protocols for assessing and treating
disease states improves patient outcomes. Similarly, a standard
approach to analysis of ventilator waveforms should improve
Evaluation of patient-ventilator synchrony (Nilsestuen and
Hargett, 2005).
Dividing the breath into 4 phases, simplify the analysis of
patient-ventilator asynchrony.
Trigger Asynchrony (Phase 1):
Trigger asynchrony is fairly easy to identify on the flow and
pressure waveforms. The patient’s missed trigger often appear as a
sudden movement of the expiratory flow waveform toward the
zero baseline (convexity) and a concomitant DROP in the airway
pressure waveform toward baseline (concavity). In Doubletriggering
the airway pressure waveforms indicate inadequate flow
(concave or dished out pressure waveform) and the continuation
of patient effort, leading to double-triggering. In Autotriggering
the absence of the initial pressure DROP below end-expiratory
pressure is indicative of autotriggering. (Nilsestuen and Hargett,
2005).
Flow Asynchrony (Phase 2):
Evaluating Flow Asynchrony during Volume-Controlled
Ventilation. 1. Inadequate flow is evidenced by an inward
buckling on the initial part of the ascending limb of the pressure
waveform 2. To evaluate flow adequacy, the clinician needs to be
familiar with the normal, relaxed, pressure waveforms associated
with the various flow options. 3. Careful evaluation of the
pressure waveform may reveal over-distention ( Hasan, 2010).
Pressure ventilation with variable flow.
Flow starvation during PSV results in a loss of the usual peak on
the flow-time waveform. On the pressure–time scalar, such
increased efforts are also evident during the triggering phase, as deep negative deflections that precede the machine assisted
breaths ( Hasan, 2010).
Evaluating Rise Time.
1. Rise time can be evaluated by observing the pressure and
flow waveforms. 2. Usually the pressure waveform should achieve
near target pressure early in the inspiratory cycle. Short (fast) rise
time may cause a pressure spike near the beginning of the pressure
waveform. 3. The flow waveform should smoothly reach the peak
flow and then appropriately decrease. 4. In general, faster rise
time should reduce patient WOB (Gonzales et al., 2013).
Termination Asynchrony (Phase 3):
Delayed Termination. Delayed breath-termination is evidenced
by a pressure spike at the end of the breath and rapid decrease in
the flow, which can cause subsequent failed trigger attempts
(Correger et al., 2012).
Premature Termination. Premature breath-termination can be
identified on the pressure waveform, which will show a posttermination
concavity (indicating continued patient effort) and on
the flow waveform, which will show rapid deceleration and a
convex pattern in the expiratory flow waveform, which indicates
continued patient effort (Georgopoulos et al., 2006).
Expiratory Asynchrony (Phase 4):
The major consideration during the expiratory phase is the
presence or creation of auto-PEEP, which can be caused by
insufficient expiratory time or by asynchrony created during the flow or termination phases. 1. Routinely check for auto-PEEP by
inspecting the end of the expiratory flow waveform and by
measuring auto-PEEP (most commonly performed at the bedside
with the expiratory-hold technique) ( Hasan, 2010).
To date, detection of asynchrony has required well trained
clinicians at the bedside. More recently, several investigators have
explored the use of machine learning and pattern recognition to
automatically detect asynchrony (Branson et al., 2013 ).
Patient-ventilator asynchrony is associated with adverse effects,
including ineffective ventilation, impaired gas exchange, lung
overdistention, increased/wasted WOB and patient discomfort. Far
more common than previously recognized, it also predisposes to
respiratory muscle dysfunction and other complications, leads to
excessive use of sedation, interferes with weaning, prolonged
mechanical ventilation, longer ICU and hospital stay and possibly
higher mortality. Whether asynchrony is a marker of poor
prognosis or causes these adverse outcomes remains to be
determined (Pierson, 2011).
Assisted/supported modes of mechanical ventilation offer
significant advantages over controlled modes in terms of
ventilator muscle function/recovery and patient comfort (and
sedation needs). Assisted/supported ventilation is designed to
interact with patient muscle activity and “share” the work of
breathing. If properly done, assisted/supported ventilation
facilitates ventilatory muscle recovery and generally requires less sedation. For this to occur, however, the ventilator’s flow and
pressure delivery must synchronize with patient effort during all
three phases of breath delivery: breath initiation, flow delivery and
breath termination (Gilstrap and MacIntyre, 2013).
In applying conventional ventilation modes, several design
features can be used to improve patient-ventilator synchrony
(MacIntyre, 2011).
Optimizing Breath Triggering:
The clinician should choose the trigger sensor (flow vs pressure)
that is most sensitive and responsive to patient effort (Sassoon,
2011).
In the setting of intrinsic PEEP trigger dyssynchrony, there are
several clinical strategies. First, clinicians should try reducing the
intrinsic PEEP as much as possible by reducing minute ventilation
(e.g., reduce set rate, reduce set PI, reduce set VT, reduce
ventilation needs driving patient efforts), lengthening the
expiratory time, or improving airway mechanics. In addition, the
triggering load from intrinsic PEEP can be reduced by applying
70–80% of measured intrinsic PEEP as circuit PEEP (Chen et al.,
2008).
Managing an extra-triggering phenomenon depends on the
cause. Ventilator autotriggering can be managed with a careful
search for reversible causes (e.g., water in the circuit, small leaks)
and/or adjustments to the trigger sensitivity settings. Managing
entrainment effects can be more problematic as this phenomenon is less well studied. Additional sedation seems counterproductive
as entrainment is associated with the use of heavy sedation.
Conceptually, a reduction in sedation and mandatory breath
delivery might be useful, but this has not been studied (Chen et
al., 2008).
Optimizing Flow Delivery:
With a fixed-flow breath, because the size of the VT can impact
synchrony, the VT itself can be adjusted (Kahn et al., 2005).
In addition to VT magnitude, the magnitude and shape
(sinusoidal vs square vs decelerating) of the flow can be adjusted
to enhance synchrony. Inspiratory-time adjustments can also be
made with the application of an inspiratory pause. These
manipulations are usually applied with a trial-and-error approach
while constantly monitoring the airway pressure graphic and
patient comfort (Chiumello et al., 2002).
Pressure-targeted breaths may offer synchrony advantages over
flow-targeted breaths. This is because pressure targeting allows
the ventilator to deliver whatever flow is needed to attain the set
pressure target. Flow thus varies with patient effort and this
feature has been shown in many clinical studies to thereby
enhance flow synchrony (Yang et al., 2007).
The pressure-targeted breath also has several features that can
enhance synchrony. One is adjustment of the rate of pressure rise
to the target. A way to titrate this adjustment is simply to look at the airway pressure graphic and try to create a smooth square
wave of pressure (Gilstrap and MacIntyre, 2013).
Optimizing Breath Cycling:
Achieving breath cycling synchrony involves delivery of an
appropriate VT in accordance with patient demands and matching
of neural and machine TI. With flow–volume targeting, adjusting
the VT and machine TI is relatively straightforward as these are set
independent variables that produce the machine TI. With pressure
targeting, adjusting the VT and machine TI is more complex and
involves the interactions of applied PI, respiratory system
mechanics, patient effort and cycling criteria. Altering any of
these parameters often results in changes in others. In general,
higher PI settings, better mechanics, increased effort and longer
cycling criteria settings (higher set TI in pressure assist control
ventilation, lower expiratory flow criteria with PSV) extend the
machine TI. The pressure rise time in PSV can also affect machine
TI depending on its effects on the resulting patient ventilator drive
and its impact on peak flow and the flow cycling criteria (Gentile,
2011).
The Distribution of Ventilator Breaths May Affect Synchrony:
It would seem that the best clinical strategy is to provide as
consistent a breath pattern as possible (ie, avoiding SIMV and
supplying the fewest possible controlled backup breaths with
assist-control modes). This is the most direct approach to stabilizing respiratory drive, which, conceptually, makes it easier
to synchronize the ventilator and the patient (MacIntyre, 2011).
It is important to point out that evidence-based reviews of the
weaning process, done over the last 20 years, have never shown
an advantage from gradual reduction in support, versus simply
providing stable support and doing daily spontaneous breathing
trials. Thus, the notion of protocolized or automated reduction in
inspiratory pressure in between spontaneous breathing trials must
be challenged as a desirable strategy. It may be that, if one does
daily spontaneous breathing trials, the level of support in between
the trials is irrelevant, so aggressive pressure-reduction strategies
may only increase the need for sedation from asynchrony
(MacIntyre, 2011).
Ventilator manufacturers have developed new ventilation modes
that target both pressure and volume and have added adjuncts to
pressure-targeted ventilation that are designed to improve
synchrony: rise time and breath-termination (ie, cycle) criteria.
Some ventilator manufacturers have tried to automate these
functions to ensure that gas delivery changes as patient demand
changes, but despite these new options, asynchrony is still a major
problem (Kacmarek, 2011).
Proportional assist ventilation (PAV) and neurally adjusted
ventilatory support (NAVA), both of these modes are designed to
improve patient-ventilator synchrony. Although each controls gas
delivery by a different method, both are designed primarily to respond to changes in the patient’s ventilatory demand and to
decrease patient effort. PAV is controlled by changes in the
patient’s work of breathing and NAVA is controlled by changes in
the electromyographic (EMG) activity of the diaphragm. It is
critical to understand that with both PAV and NAVA the clinician
does not set pressure, flow, volume or time: all those variables are
under the complete control of the patient. Essentially, the
ventilator proportionally unloads patient effort, in both PAV and
NAVA, based on the setting of ventilator work: proportion (%)
unloaded by the ventilator (PAV), or cm H2O pressure applied per
millivolt of diaphragmatic EMG activity (NAVA) (Kacmarek,
2011).
Airway pressure release ventilation (APRV) is a mode of
mechanical ventilation that is best described as a partial
ventilatory support and is based on the open lung concept. The
primary goals of this mode were to provide both safety and
comfort: safety in that adequate or superior ventilatory support is
provided without dangerously high applied pressures. While
comfort in that unrestricted spontaneous breathing would be
allowed, which is a feature unavailable in conventional ventilatory
modes, thus minimizing patient-ventilator asynchrony
(Porhomayon et al., 2010).
Sedation has become an important part of critical care practice
in minimizing patient discomfort and agitation during mechanical
ventilation. Pain, delirium and anxiety must all be assessed and treated to reduce the critically ill patient’s agitation. A multimodal
approach to the treatment of pain appears to be the best approach
(Bennett and Hurford, 2011).
Historically, benzodiazepines have been the first line of therapy
in the ICU setting for sedating mechanically ventilated patients.
Typical benzodiazepines used in the ICU are midazolam and
lorazepam (Jacobi et al., 2002).
Preventing delirium by identifying and modifying risk factors is
the best treatment. The key to successful management of delirium
is prompt recognition utilizing a validated screening tool; then
efforts should be made at identifying the etiology and
pharmacotherapy, if appropriate (Otter et al., 2005).
Achieving an adequate state of sedation and ventilator
synchrony in some critically ill patients may require the use of
anesthetics such as propofol, dexmedetomidine and NMBAs
(Pohlman et al., 2008).
As a general guideline, the use of NMBAs should be
uncommon, the dose and duration minimized and guided by
peripheral neuromuscular monitoring and adequate sedation and
analgesia must be provided (Papazian et al., 2010).
Nonpharmacological, complementary therapies may be an
important alternative or adjunct to pharmacological interventions
to treat Patients receiving mechanical ventilation symptoms such
as pain, anxiety, agitation and lack of sleep while in the intensive
care unit (Tracy and Chlan, 2011).
Noninvasive ventilation (NIV) has received considerable
academic and clinical attention in recent years. There is arguably
more evidence to support the use of NIV than any other practice
related to the care of patients with acute respiratory failure. It is
unrealistic to expect NIV to avoid intubation in all patients. There
are a number of potential causes of NIV failure. Some of these
failures potentially relate to asynchrony, although the relationship
between asynchrony and NIV failure has not been well studied.
Good NIV tolerance has been associated with success of NIV and
improved comfort has been associated with better synchrony
(Hess, 2011).
For acute-care applications there is value in being able to use the
same ventilator for invasive ventilation, where leaks are usually
absent, and for NIV, where leaks are always present. Leak
compensation (control of trigger, cycle and flow delivery) in
critical-care ventilators is in its infancy. Probably this will
improve as engineers develop more sophisticated algorithms for
leak detection and compensation. Improvements in the design of
interfaces will reduce the amount of leak. Clinicians will better fit
the interface with experience in the application of NIV. With the
combination of improved ventilator performance in the setting of
leaks, better interfaces and increased clinician experience,
problems related to leaks should improve (Hess, 2012).
The ability to detect ineffective trigger efforts noninvasively and
automatically during NIV is attractive. This could conceivably prompt clinicians of the presence of asynchrony and the need to
make ventilator adjustments, such as PEEP to counter-balance
auto-PEEP. It could also be used to assess patient response to
efforts to improve synchrony. However, additional evaluation is
necessary before such systems are ready for incorporation into
clinical practice (Antonelli et al., 2013).
With an understanding of potential causes of asynchrony during
NIV, a number of strategies can be implemented to address this
clinical problem. These strategies are a combination of ventilator
adjustments and treatment of the underlying disease process
(Hess, 2011).
Ventilator adjustments to improve synchrony should be
systematically applied, and each intervention assessed by patient
examination, the patient’s subjective reports of dyspnea and
comfort, and careful assessment of the ventilator waveforms. For
many patients receiving NIV, the underlying physiology is
dynamic and there may be moment-by-moment changes in
respiratory drive. This may require frequent manipulations of
ventilator settings by clinicians, particularly when the patient is at
an unstable point in the disease process (Hess, 2011).
Neurally adjusted ventilatory assist (NAVA) and proportional
assist ventilation (PAV) are modes intended to improve patientventilator
synchrony. NAVA and PAV have been reported to
improve synchrony during NIV (Kacmarek, 2011).
While sedation represents the mainstay of treatment in the care
of patients suffering from end-of-life respiratory failure, it can be
a dangerous if improperly employed, especially during NIV. Since
lack of cooperation is a well-recognised cause of NIV failure,
sedation could theoretically improve patient–ventilator
interaction; however, if the altered mental status of a fighting or
agitated patient is the direct consequence of hypoxia and
hypercapnia, then sedation would only cause an already unstable
clinical situation to deteriorate (Nava et al., 2009).
A reasonable choice, at present, is to administer very small
doses of opioids (fentanyl and morphine) but only in cases where
a moderate blunting of respiratory drive is highly desirable.
Among the benzodiazepines, a small dose of lorazepam may be
recommended since it has the advantage of fewer drug
interactions compared with diazepam and midazolam (Nava et al.,
2009).