Download Airway pressure release ventilation, biphasic positive

176
INTENSIVE CARE
Airway pressure release ventilation, biphasic
positive airway pressure
Alveolar ventilation is achieved by the time-cycled switching
between two levels of CPAP. Inspiratory and expiratory pressure levels and times are set independently and unhindered
spontaneous breathing is possible in any phase of the
mechanical ventilatory cycle. APRV as originally described
employs very high I : E ratios with a short expiratory time.
With BiPAP the I : E ratio and flow rates can be varied over
a wide range. BiPAP can also be patient-triggered. Changes
in ventilatory demand do not result in alterations in the
level of mechanical support. Peak airway pressures are minimized and theoretically the risk of barotrauma is reduced.
BiPAP is now widely used, especially during NIV (see
below).
Continuous positive airway pressure
CARDIORESPIRATORY EFFECTS OF CPAP
CPAP achieves for the spontaneously breathing patient what
PEEP does for the ventilated patient. Not only can it improve
oxygenation by the same mechanism, but lung mechanics
improve, breathing becomes easier, respiratory rate falls, VT
increases and both VC and inspiratory force have been shown
to improve (Feeley et al., 1975; Katz and Marks, 1985; Venus
et al., 1979).
The cause of the increased compliance can be appreciated
by referring to Figure 3.23 (Chapter 3), which shows the
compliance curve for the lung and chest wall combined.
Normally, tidal exchange takes place from the resting expiratory position at which the propensity for the lungs to collapse is exactly counterbalanced by the tendency of the chest
wall to expand. It can be seen that this is also the steepest
point on the curve, where relatively small changes in pressure produce a large change in volume (i.e. compliance is
greatest and the work of breathing least). As lung volume
falls, however, the curve becomes flatter (i.e. the lungs
become stiffer and the work of breathing increases). The
application of positive airway pressure re-expands the lungs
and compliance increases. If the lungs are overexpanded,
however, compliance will again fall.
Because mean intrathoracic pressure is lower with CPAP
than with IPPV plus PEEP, cardiovascular depression is
minimized (Simonneau et al., 1982; Venus et al., 1979), renal
perfusion is maintained and the incidence of barotrauma
may be reduced. Furthermore, heavy sedation is unnecessary. A circuit suitable for applying CPAP is shown in Figure
7.11a.
CLINICAL USE OF CPAP
CPAP can be used as the primary treatment in patients with
acute hypoxaemic respiratory failure who are not exhausted
and in whom alveolar ventilation is adequate (Venus et al.,
1979). The use of a tight-fitting facemask (Fig 7.11b) allows
the application of CPAP without the need for tracheal intubation or tracheostomy (Greenbaum et al., 1976). This technique has been used extensively in the belief that clinical
deterioration might be prevented, tracheal intubation could
be avoided and outcome might be improved. This approach
is supported by a study in which mask CPAP was shown to
produce early physiological improvement as well as to reduce
the need for intubation and mechanical ventilation in
patients with severe hypercapnic cardiogenic pulmonary
oedema (Bersten et al., 1991). CPAP is particularly useful in
the management of postoperative respiratory failure associated with basal lung collapse/consolidation (see Chapter 8)
and is indicated in the management of selected patients with
chest trauma who remain hypoxic despite adequate analgesia and high-flow oxygen. In a randomized, controlled trial,
however, application of CPAP by full facemask failed to
reduce the need for tracheal intubation or improve outcome
in patients with acute hypoxaemic, nonhypercapnic respiratory insufficiency due to acute lung injury (Delclaux et al.,
2000). Worryingly, in this study there were four cardiac
arrests in the CPAP group, compared to none in the standard
therapy group: three occurred at the time of intubation and
one when the mask was removed, precipitating extreme
hypoxia. It may be that mask CPAP is most useful when the
underlying cause of the respiratory failure is readily reversible, as is the case in cardiogenic pulmonary oedema or collapsed basal lung segments, but is less likely to be beneficial
when the lung pathology is progressive and/or slow to
resolve. Although there is some evidence that CPAP may
benefit patients with acute exacerbations of COPD, NIV is
now the treatment of choice in such patients (see below and
Chapter 8).
CPAP masks are uncomfortable to wear for prolonged
periods and the technique is only suitable for those who are
alert, able to clear secretions and protect their airway. Gastric
distension, vomiting and aspiration are a potential risk and
a nasogastric tube should be inserted in most patients receiving mask CPAP.
Nasal masks may be better tolerated and safer, but mouth
breathing renders them less effective. They are most useful
when applied at night in patients with obstructive sleep
apnoea and nocturnal hypoventilation (Jenkinson et al.,
1999). Recently helmets that surround the patient’s head and
neck have been introduced; these may be better tolerated
than masks.
CPAP is also useful for weaning patients from mechanical
ventilation since it prevents the alveolar collapse and hypoxia
that may otherwise occur (Feeley et al., 1975). In general,
therefore, patients who have required CPAP should not be
allowed to breathe spontaneously through an endotracheal
tube with ZEEP, but rather should be extubated directly
from 5 cm H2O PEEP. Once extubated, patients provide their
own ‘physiological PEEP’.
Non-invasive ventilation
NIV involves the provision of mechanical respiratory support
without invading the patient’s upper airway and can be delivered using either a nasal or full facial mask. Although nasal
masks are generally better tolerated they require a cooperative
Respiratory support
a
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Pressure
manometer
Water for
humidifier
Oxygen
analyser
Flow generator
on/off
Flow adjustment
O
Entrained
air
Inspired oxygen
adjustment
CPAP valve
Piped oxygen
b
Tight
fitting
mask
Harness
Fresh
gas
flow
Humidifier
CPAP valve (e.g. 7.5cmH2O)
Safety valve
(e.g. 15 cmH2O)
Expired gas
Fig. 7.11 (a) Circuit suitable for applying continuous positive airway pressure (CPAP) using a flow generator in a patient with an
endotracheal tube in place. (b) CPAP can be applied using a tight-fitting facemask.
patient who is able to maintain mouth closure to be effective.
In general alveolar ventilation, and consequently the reduction in PaCO2, is greater with full facemasks which are therefore
the most appropriate choice for patients with acute respiratory failure. Nasal plugs or ‘pillows’ can also be effective but
are often poorly tolerated. The application of non-invasive
positive-pressure ventilation via a ‘helmet’ is also feasible
but is not as effective in terms of carbon dioxide elimination
(Antonelli et al., 2004). Most modern ventilators can
provide NIV, but they are often unnecessarily complicated
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INTENSIVE CARE
and portable non-invasive devices are now widely used, both
on intensive care/high-dependency units and on respiratory
wards. The most popular ventilators for this purpose are those
providing BiPAP, which are simple to use, cheap and flexible.
Alternatively ACV, volume control or PSV, with or without
PEEP, can be delivered using a conventional mechanical
ventilator.
Successful application of NIV requires considerable
attention to detail, particularly in the first few hours, in order
to ensure that the mask is comfortable, with minimal leaks,
and that the ventilatory pattern is optimal (Brochard, 2000).
Achieving a good mask fit is crucial both for patient comfort
and to ensure effective ventilatory support; simply overtightening the headgear in an attempt to eliminate leaks is
uncomfortable for the patient and increases the risk of skin
damage. A better fit is obtained if dentures are left in place.
The patient’s response to NIV should be assessed by clinical evaluation, including patient comfort, conscious level,
chest wall motion, accessory muscle use, coordination of
respiratory effort with the ventilator, respiratory rate and
heart rate and by continuous pulse oximetry supplemented
by repeated blood gas analysis. The patient should be
reviewed regularly to optimize ventilator settings.
Institution of NIV can rest the respiratory muscles,
reduce respiratory acidosis and restlessness, improve clearance of secretions and re-expand collapsed lung segments.
NIV may be undertaken as a therapeutic trial, recognizing
that in the event of failure tracheal intubation will be
required, or as the ceiling of treatment in patients who are
not candidates for invasive respiratory support or admission
to intensive care.
Advantages of NIV include:
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avoidance of tracheal intubation and associated complications, especially VAP;
improved patient comfort and avoidance of sedation;
preservation of airway defence mechanisms;
spontaneous coughing and expectoration are not
hampered;
ventilatory assistance can be provided intermittently,
allowing normal eating, drinking and communication
(periods of NIV as short as 6–8 hours a day can be effective), as well as facilitating gradual weaning;
ventilation can be interrupted to allow administration of
nebulized medications, physiotherapy, coughing and
expectoration;
early mobilization is facilitated;
patient morale is maintained;
ventilatory support can be initiated at an earlier stage and
in some cases admission to intensive care can be
avoided.
Disadvantages include the absence of a route for endotracheal suction and the lack of airway protection. Complications include nasal bridge and facial ulceration, nasal
congestion and eye irritation. Air swallowing is common and
most patients receiving NIV should have a nasogastric tube
in place. More serious complications, such as aspiration of
gastric contents, cardiovascular instability and pneumothorax, are fortunately rare.
Contraindications to NIV include:
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recent facial or upper-airway surgery, burns or trauma;
fixed obstruction of the upper airway;
vomiting;
recent upper gastrointestinal surgery or bowel
obstruction;
uncooperative patient, coma, confusion or agitation;
inability to protect the airway or to cough and expectorate effectively;
copious respiratory secretions;
focal consolidation on chest X-ray;
life-threatening hypoxaemia;
haemodynamic instability;
severe comorbidity.
NIV is most clearly indicated in the management of selected
patients with an acute exacerbation of COPD who are not
severely hypoxaemic and in whom a respiratory acidosis persists despite maximum medical treatment and controlled
oxygen therapy. In this situation NIV has been used successfully both in intensive care/high-dependency units (Brochard
et al., 1990, 1995, Girou et al., 2000; Martin et al., 2000;
Meduri et al., 1991) and on general respiratory wards (Bott
et al., 1993; Plant et al., 2000, 2001, 2003). Taken together
these studies have demonstrated that in such patients appropriately applied NIV can reduce the need for tracheal intubation, the length of hospital stay, costs and the in-hospital
mortality rate, a conclusion supported by a meta-analysis
(Peter et al., 2002). The improved outcome with NIV seems
to be at least partly explained by a reduction in the incidence
of complications, especially VAP and other nosocomial infections (Brochard et al., 1995; Girou et al., 2000). Long-term
survival is sufficiently good to lend further support to the use
of NIV in this category of patient (Plant et al., 2001).
The role of NIV in the management of acute respiratory
failure unrelated to COPD is less clear, however (Peter et al.,
2002). In one study of patients with unresponsive acute
respiratory failure and severe hypoxaemia non-invasive
positive-pressure ventilation reduced the rate of serious
complications and, in survivors, the length of ICU stay
(Antonelli et al., 1998), whilst in another study the rate of
tracheal intubation was reduced (Martin et al., 2000),
although improved survival was only seen in those with
COPD. Similarly, Confalonieri et al. (1999) found that in
selected patients with acute respiratory failure caused by
community-acquired pneumonia NIV was associated with a
significant reduction in the rate of tracheal intubation and
duration of ICU stay. Again 2-month survival was only
improved in those with COPD. In a more recent study, the
use of NIV prevented intubation, reduced the incidence of
septic shock and improved survival in patients with severe
hypoxaemic respiratory failure, except in those with ARDS
(Ferrer et al., 2003b).
Respiratory support
Non-invasive PSV has also been shown to lead to more
rapid improvement in oxygenation and reduction in respiratory rate when compared to conventional oxygen therapy in
acute cardiogenic pulmonary oedema (Masip et al., 2000;
Nava et al., 2003; Park et al., 2004), although in some studies
outcome was not improved (Masip et al., 2000; Nava et al.,
2003) and it is unclear whether NIV/BiPAP is more efficacious
than CPAP in this condition (Park et al., 2004). Indeed, a
recent meta-analysis indicated that, compared with standard
therapy, CPAP reduces mortality in acute cardiogenic pulmonary oedema but there was only a suggestion of a trend towards
a reduction in mortality with bilevel NIPPV (Peter et al.,
2006). There is even some suggestion that CPAP or NIV may
reduce the need for tracheal intubation in selected patients
with severe asthma (Fernández et al., 2001), although currently this approach cannot be recommended. Finally NIV
may be particularly indicated in the management of immunocompromised patients with acute respiratory failure, in whom
the rates of tracheal intubation and serious complications
may be reduced and survival improved (Hilbert et al., 2001).
In conclusion, current evidence supports the use of NIV
in acute hypercapnic respiratory failure associated with
COPD, provided that the patient is not profoundly hypoxaemic, and suggests that the technique may also be useful in
selected patients with acute hypoxaemic respiratory failure
of various aetiologies. NIV may also be useful as an aid to
early extubation of patients who cannot sustain unsupported spontaneous ventilation (Girault et al., 1999),
although the influence of this approach on length of stay and
survival is unclear (Girault et al., 1999; Nava et al., 1998).
NIV is also the treatment of choice in decompensated ventilatory failure due to chest wall deformity or neuromuscular
disease and has been used successfully in decompensated
sleep apnoea (see Chapter 8).
Extracorporeal gas exchange
EXTRACORPOREAL MEMBRANE OXYGENATION (ECMO)
The technique of ECMO using venoarterial bypass was
introduced in an attempt to save the lives of some patients
who would otherwise inevitably have died from severe progressive acute respiratory failure. It was based on the premise
that the lungs of such patients would eventually recover
provided death from hypoxia could be avoided. This proved
not to be the case since in the majority of cases the lung
lesion continued to progress, culminating in irreversible pulmonary fibrosis. When a prospective randomized study
demonstrated that the mortality of patients with severe ARF
was similar whether or not ECMO was used (Zapol et al.,
1979), its use in adults was largely abandoned.
LOW-FREQUENCY POSITIVE-PRESSURE VENTILATION
WITH EXTRACORPOREAL CARBON DIOXIDE REMOVAL
(Gattinoni et al., 1980, 1986)
This technique uses a venovenous bypass, with low flows
of only 20–30% of the cardiac output, to remove carbon
dioxide. The lungs are ventilated at a very low frequency and
oxygenation is maintained by applying a positive pressure to
179
the airways and adjusting the FIO2. Although alveolar oxygen
concentrations may be high, the combination of normal
pulmonary perfusion and minimum ventilation may avoid
ventilator-associated lung injury, rest the lungs and provide
optimal conditions for lung healing.
The major complication of this technique is bleeding,
which is potentially fatal. The incidence of bleeding can be
reduced by using surface heparinized extracorporeal circuits
or by the simultaneous administration of aprotinin with
heparin. In general, however, extracorporeal techniques are
contraindicated in patients at risk of haemorrhage. In contrast to the use of ECMO, thromboembolic complications
have not been reported and the technique is simplified by
the use of percutaneous venous cannulation.
A later randomized controlled trial indicated that this
technique may not improve outcome, especially when compared to protocol-driven ventilatory strategies designed to
minimize levels of PEEP and FIO2 (Morris et al., 1994).
Nevertheless, some authorities remain convinced that,
when used by experienced teams in specialist centres, extracorporeal gas exchange can significantly reduce the mortality
associated with severe ARDS, and that further technical
improvements will facilitate the more widespread application of this technique.
DISCONTINUING AND WEANING
VENTILATORY SUPPORT
(MacIntyre et al., 2001)
In view of the numerous complications associated with invasive respiratory support it is important to discontinue
mechanical ventilation, and, if possible, to extubate the
patient as soon as possible. On the other hand, premature
attempts at weaning may be deleterious and can adversely
affect the morale of conscious patients; during weaning a
fine balance must be drawn between proceeding too quickly
and unnecessarily prolonging the process.
Following a prolonged period of mechanical respiratory
support ventilatory function may be compromised by:
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decreased central drive;
weakness and wasting of skeletal muscle and the
diaphragm;
critical-illness polyneuromyopathy (see Chapter 15);
pathological changes in muscle, including corticosteroidinduced myopathy (Douglass et al., 1992);
chest wall instability;
operative or traumatic damage to the phrenic nerves;
abdominal distension with upward displacement of the
diaphragm.
The role of respiratory muscle fatigue is poorly understood
but there is usually some persisting abnormality of gas
exchange and lung mechanics, including, for example,
dynamic hyperinflation. In patients who have been ventilated for any length of time, therefore, spontaneous respiration normally has to be resumed gradually.
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