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Methods to test airway responsiveness to inhaled
agonists in children were originally developed for use in
adults with omron nebulizers, and agonist dosage regimens
do not adequately correct for the size of the child. Because
small children receive a higher dose relative to their body
size than respiratory system do large children, the
age-related decline in airway asthma nebulizer
responsiveness reported in many recent studies might reflect
failure to adequately size-correct test respiratory therapy
dosages rather than a genuine devilbiss
physiological event. Until the administered doses of inhaled
agonists can be satisfactorily size-corrected, tests of
airway responsiveness in children should be regarded as
qualitative rather than albuterol nebulizer quantitative.
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respiratory
When treatment diffuser
accessory cost no
Portable Nebulizer
kit care and portable respiratory system
free
salbutamol nebulizer
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Techniques to assess airway responsiveness to inhaled histamine and
methacholine are important epidemiological tools designed for use in
adults,(n1-n3) but despite a lack of adequate validation in children, most
textbooks of paediatric respiratory medicine describe their use with
little or no modification. These techniques have been used in many
epidemiological studies of respiratory disorders in childhood.(n4-n19)
Interpretation of the results of these studies is dependent on the
validity of the methods, which in turn rests on assumptions that reaction
to either the nebuliser solution concentration or the dose of agonist
administered reflects airway responsiveness. Apparent age-related changes
in responsiveness, and new evidence on aerosol delivery to the lower
airways, indicate that these assumptions should be re-examined.
Method and assumptions portable nebulizer machine respiratory system
Two basic methods are used to test airway responsiveness to inhaled airway
agonists. In one, a jet nebuliser delivers doubling concentrations of
agonist, usually histamine or methacholine; respiratory therapist
the aerosol is inhaled during tidal breathing or full-vital-capacity
Manoeuvres until a predetermined fall in respiratory function, maximum
concentration of agonist, or number of inspirations is reached. In the
other, a dosimeter is used to deliver agonist, again with doubling
concentrations or an increasing number of inspirations, until a
predetermined response or dose is reached.(n2,n3) With either technique,
the final concentration or dose of agonist is based on its likelihood to
produce a response in most asthmatic adults ("bronchial
hyperresponsiveness") but not in most non-asthmatic adults.(n1,n2) These
same concentrations or doses of agonist are also used to decide whether or
not children have "bronchial hyperresponsiveness".
In children, use of the jet nebuliser/tidal breathing technique(n1) to
assess airway responsiveness is based on the assumption that the
concentration of agonist in the nebuliser solution indicates the level of
airway responsiveness, and that this responsiveness can be compared
between children and adults and between children of different
ages.(n9,n11,n13) With the dosimetric technique(n2) (or indeed any
technique designed to deliver a dosage regimen(n3,n4)), the same agonist
dosage schedule is used to test responses between individuals of different
size,(n4-n6,n8,n10) on the assumption that the dose regimen does not need
to be corrected for size.
Validity of assumptions respiratory therapy
Adults, though they do come in different shapes and heights, show less
striking variations in size than do children. For the jet nebuliser
solution agonist concentration to be a valid guide to airway
responsiveness in children, the actual dose of agonist inspired for a
given nebuliser solution concentration should be roughly proportional to
the size of the child. But measurement of inspiratory flow shows that, for
a given nebuliser solution concentration of agonist and a typical
, all children over the age of about 12 months will inspire the
same dose because after that age inspiratory flow exceeds flow
and the entire nebuliser output is inhaled during inspiration.(n20) Only
small infants below this age inspire at a lower flow than that of the
output, and only then will dose be affected by subject size.
respiratory therapist
Thus, with current techniques, all children over the age of about I year
receive much the same dose of agonist for a given nebuliser solution
concentration despite the striking difference in size between a child of 1
and, say, 10 years of age. The figure shows the multiples of the adult
dose received by children when doses are corrected for weight,(n21)
thoracic gas volume,(n22) and the surface area of the air/tissue portable
jet nebulizer interface.(n23) The discrepancy of dose delivery between
different ages is striking: even at 10 years of age, children receive
double the adult dose after size-correction by any of those variables.
Clearly, small children are more likely than large children to react to a
given nebuliser solution concentration of agonist, and the assumption that
the nebuliser solution concentration of agonist can be used to quantify
responsiveness in children for comparisons between different-sized
individuals is invalid. Some form of size-correction of dose is required
but the similarity of the three plots, although intriguing, is not proof
that it is valid to size-correct airway responsiveness data in this way.
The dosimetric technique uses the same adult-derived dosage schedule(n2)
for all children;(n7,n8,n10,n14,n16) according to the above reasoning the
assumption that size-correction of dose is not required also appears to be
invalid.
Age and airway responsiveness hand held portable nebulizer.
Does airway responsiveness change with age? If not, there would be no
change in response to an appropriately size-corrected dose with increasing
age and an inverse relation between airway response to a set dose and age.
In symptom-free infants and children, an inverse relation between response
to a set dose and age has been reported(n4,n6,n8,n10,n13) but many
researchers have interpreted this finding to mean that airway
responsiveness decreases with age. Closer analysis of size-correction data
indicates that this conclusion may owe more to artifact than to
physiology. In one study,(n4) a 5-year-old and an 18-year-old child were
said to respond to a methacholine area under the curve (AUC) of
approximately 2670 and 3875 breath units, respectively, but
weight-correction of these values (from standard means of 18.4 kg for age
5 and 59.7 kg for age 18 years(n21) would give values of 145.1 and 64.9
breath units/kg, respectively--reversing the age-related changes.
Similarly when I and colleagues compared airway responsiveness to the
concentration of histamine in the inhaled gas rather than in the nebuliser
solution,(n12) infants aged 1 month responded to a much lower nebuliser
solution concentration of histamine than did older children (median age 10
years). However, when the inspired gas flow and nebuliser gas flow were
taken into account to calculate the inspired histamine
concentration,(n12,n20) there was no difference in responsiveness between
the two age-groups. I argue that the apparent age-related decline in
airway responsiveness in children is an artifact and is evidence for a
need to size-correct agonist doses rather than for decreased
responsiveness with increasing age in normal children. respiratory system
If the need to size-correct agonist dose is accepted, the idea that airway
responsiveness in asthmatic children does not change or falls with age may
also need to be reviewed. Gerritsen et al(n11) reported decreased
responsiveness with age in children with asthma, but if the geometric
means of the provocative concentration to produce a 10% fall in FEV1
(PC10) for children (4.05 mg/ml) and adults (8.46 mg/ml) are
size-corrected by predicted weight for age,(n21) the trend is reversed to
0.137 mg/ml per kg and 0.128 mg/ml per kg, respectively.
Standardisation of standard tests
Can methods to assess airway response to inhaled agonists be reliably
standardised for children of different sizes? We know surprisingly little
about aerosol delivery and drug deposition in children, and have little
assurance that a given agonist dose can reproducibly be delivered to or
deposited in the airway of any particular child, and less still that this
aim can be achieved in different children. Delivery systems for children
need further evaluation and drug deposition within airways needs further
study, for example with radioisotopelabelled aerosol.(n24)
Even with reliable delivery and deposition of agonist, how should the dose
be corrected for the size of the child? Little is known about the basic
mechanisms involved in an adult's response to an inhaled agonist, and
there is no rational way to choose which variable (eg, height, weight,
thoracic gas volume, airway surface area) to use for size-correction. Use
of a standardised inhaled concentration would involve fewer assumptions
but the data obtained should still be interpreted with caution. Minute
ventilation and therefore aerosol delivered may be proportionately higher
in infants than in older children(n12) but infants tend to breathe through
their noses, and lung deposition after nasal inhalation may be only about
33-50% of that after oral inhalation.(n24) Similarly, inspiratory pattern
changes during childhood; in adults, different inspiratory patterns
greatly influence deposition and airway responsiveness.(n25,n26) And even
after appropriate standardisation, does a given percentage change in an
aspect of lung function have the same physiological implications at
different ages?
Discussion
Some of the variability between different-sized individuals might be
reduced by comparison with the quantity of agonist inspired or by
size-correction during standard response studies--but until more reliable
data are obtained, the results of such studies respiratory therapist
should be treated as qualitative rather than quantitative.
Despite the drawbacks of current methods to assess airway responsiveness
in children of different ages and sizes, short-term within-subject changes
in responsiveness should be valid where individuals are used as their own
controls, and current techniques should allow comparisons between
individuals of the same or a similar size. Portable battery operated
nebulizer Cohort studies of children within very limited age ranges allow
intra-group analyses although airway responsiveness cannot readily be
compared with other studies. However, long-term longitudinal studies of
airway responsiveness in such cohorts, in which values for the group are
arranged in centiles or ranked at specific ages, might also provide useful
information about shifts in an individual's centile or ranked values at
different ages.
GRAPH: Size-corrected doses of agonist inspired (expressed as multiples of
adult dose) versus age.
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