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Inspiratory and expiratory muscle aids are devices and
techniques that involve the manual or mechanical application
of forces to the body or intermittent pressure changes
to the airway to assist inspiratory or expiratory muscle
function. The most important inspiratory aid is to receive
air under pressure when one inhales (intermittent positive
pressure ventilation or IPPV). The most important expiratory
aid is to have a negative pressure (vacuum) applied to
the airway via the nose and mouth when one coughs along
with a manual thrust to the belly to further increase
cough flows. No one should receive supplemental
oxygen, bronchodilators, or other medications as an
alternative to normalizing blood oxygen levels by normalizing
lung ventilation. Using oxygen rather than assisted
ventilation results in worsening of carbon dioxide retention
and inevitably results in respiratory failure.
Manually Assisted Coughing
Illness and death in people with generalized weakness
is almost always due to respiratory difficulty that
occurs because of a weak cough. Breathing (inspiratory),
expiratory, and throat (bulbar) muscles are needed for
effective coughing. The latter are predominantly the
abdominal muscles. Clearing airway secretions can be
a continual problem but it most often occurs during
chest infections and following general anesthesia for
surgery for any reason.
Peak cough flows (PCF) most often exceed 160 liters
per minute to be effective for coughing up airway debris.
PCF are increased by manually assisted coughing. If
the vital capacity is less than 1.5 liters, insufflating
or air stacking to the maximum insufflation capacity
(MIC) becomes crucial to optimize cough flows. Once
the person is insufflated to the MIC, an abdominal thrust
is timed to the cough to increase the flow. Techniques
of manually assisted coughing involve different hand
and arm placements for thrusts. A belly thrust with
one hand while applying counterpressure to the chest
with the other arm and hand further increases assisted
PCF for 20% of people.
Manually assisted coughing requires a cooperative patient,
good coordination between the patient and caregiver,
and adequate physical effort and often frequent application
by the caregiver. It is usually ineffective in the presence
of significant back deformity. Abdominal compressions
should not be used for one and one-half hours following
a meal, however, chest compressions can be used to augment
PCF. The inability to generate over 160 liters per minute
of assisted PCF despite having a vital capacity or MIC
greater than 1 liter usually indicates fixed upper airway
obstruction or severe throat muscle weakness and airway
collapse during coughing attempts. Vocal cord adhesions
or paralysis may have resulted from a previous translaryngeal
intubation or tracheostomy. Some lesions, especially
the presence of obstructing granulation tissue, can
be corrected surgically. When inadequate, the most effective
alternative for generating optimal PCF and clearing
airway secretions is the use of MI-E.
Mechanical Insufflation-Exsufflation
(MI-E)
Mechanical Cough Assist devices (J. H. Emerson Co.,
Cambridge, MA) deliver deep insufflations followed immediately
by deep exsufflations. The insufflation and exsufflation
pressures and delivery times are independently adjustable.
Except after a meal, an abdominal thrust is applied
in conjunction with the exsufflation. MI-E can be provided
via an oral-nasal interface, a simple mouthpiece, or
via an invasive airway tube like a tracheostomy tube.
When delivered via the latter, the cuff, when present,
should be inflated.
The Cough-Assist can be manually or automatically cycled.
Manual cycling facilitates caregiver-user coordination
of inspiration and expiration with insufflation and
exsufflation, but it requires hands to deliver an abdominal
thrust, to hold the mask on the patient, and to cycle
the machine. One treatment consists of about five cycles
of MI-E followed by a short period of normal breathing
or ventilator use to avoid hyperventilation. Insufflation
and exsufflation pressures are almost always from +35
to +60 cm H2O to -35 to -60 cm H2O. Most patients use
35 to 45 cm H2O pressures insufflations and exsufflations.
In experimental models, +40 to -40 cm H2O pressures
have been shown to provide maximum forced deflation
volumes and flows (see (3)further information). Multiple
treatments are given in one sitting until no further
secretions are expulsed and any secretion or mucus induced
oxygen desaturations are reversed. Use can be required
as frequently as every few minutes around the clock
during chest colds. Although no medications are usually
required for effective MI-E in people with weak muscles,
liquefaction of sputum using heated aerosol treatments
may facilitate exsufflation when secretions are inspissated.
Whether used via the nose or mouth or via invasive
indwelling airway tubes, routine airway suctioning misses
the left lung about 90% of the time so that 80% of pneumonias
are in the left lung. MI-E via an airway tube provides
the same exsufflation flows in both left and right airways
without the discomfort or airway trauma of tracheal
suctioning and it can be effective when suctioning isn't.
Patients invariably prefer MI-E to suctioning for comfort
and effectiveness and they find it less tiring. Deep
suctioning, whether via airway tube or via the upper
airway, can essentially be discontinued for most patients.
The use of MI-E has permitted us to consistently extubate
people with neuromuscular disease following general
anesthesia despite their lack of any ability to breathe
on their own, and to manage them with noninvasive IPPV.
It has also permitted us to avoid intubation or to quickly
extubate people in acute ventilatory failure and with
profuse airway secretions due to intercurrent chest
infections. MI-E in a protocol with manually assisted
coughing, oximetry feedback, and home use of noninvasive
IPPV was shown to effectively decrease hospitalizations
and respiratory complications and mortality for people
with neuromuscular diseases. It may not be effective
if the user can not cooperate sufficiently to keep the
airway open, if there is a fixed upper airway obstruction,
or if upper airway dilator muscles can not maintain
sufficient patency to allow for PCF to exceed 160 L/m.
This is most often seen in advanced amyotrophic lateral
sclerosis with severe throat muscle weakness. MI-E has
been demonstrated to be extremely safe even when used
many times at pressures of 40 to 60 cm H2O. It is rarely
effective for people of any age at pressures less than
35 cm H2O.
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