Asthma and O2 therapy
- somewhat controversial
- bottom line: give O2 if in resp distress (incr WOB or incr resp rate), dc
when improved
- O2 is a mild bronchodilator
- if in respiratory distress give O2, patient almost always benefits
- different opinions: give O2 when sat< 95, or when sat <90
- not necessary to try to get a sat of 100, can dc O2 when Resp rate improves
The child described in the vignette is presenting with a significant
exacerbation of his reactive airway disease. Data obtained from sampling the
capillary blood gas demonstrate that the patient still is ventilating well. This
information is gleaned from the finding of a low Pco2,
which would be expected in a person who has an increased respiratory rate,
and the compensatory elevated pH that documents a
respiratory alkalosis. The pulse oximetry reading of 94% demonstrates
reasonable oxygenation. However, the capillary blood gas
is a poor method of determining oxygen saturation because it is obtained after
the blood already has begun to go through the tissues.
The use of supplemental oxygen in the
patient who has status
asthmaticus is somewhat
controversial. Initially, it was thought that oxygen should not be
administered to these patients because the oxygen
might suppress their hypoxic drive, which could result in respiratory arrest.
However, it has been demonstrated that this does not occur in asthmatic
children, and it has also been shown that oxygen is a mild bronchodilator.
Thus, patients who have low pulse oximetry readings should receive supplemental
oxygen, although the exact oximetry reading at which it should be administered
has not been determined. Some
recommend oxygen supplementation for patients whose pulse
oximetry
readings are below 95%; others do not initiate therapy until the readings reach
90%. The patient who has an increased respiratory rate and work of breathing
almost always benefits from supplemental oxygen. The National
Heart,Lung,
Blood Institute guidelines for asthma recommend oxygen for anyone who has
significant hypoxia and patients who have forced expiratory volume in 1 second
or peak expiratory flow rates less than 50% of predicted. Obtaining
pulmonary function studies is difficult in children
during an asthma exacerbation. Because oxygen has virtually no adverse effects,
its use is recommended in affected children, such as the boy described in the
vignette.
Oxygen therapy should be administered
until the patient's respiratory rate improves. There is no reason to
maintain an oxygen saturation of 100%, and it is inappropriate to wait
until the oxygen saturation falls below 92% in a patient who is showing signs of
respiratory distress. It is not appropriate to wait until existing
respiratory distress worsens before beginning oxygen administration.
Routine measurement of arterial blood gases is not recommended because such
evaluations are associated with complications and are quite painful, and changes
in oxygen saturation can be monitored via pulse oximetry.
The illustrated oxygen saturation curve demonstrates that
Pao2 can vary greatly, depending on the
patient's pH. A patient who is experiencing a moderately severe
exacerbation of asthma would have an elevated pH, which would shift the curve up
and to the left, giving a falsely elevated value. In contrast, a severe
exacerbation with an acidotic pH would result in a shift down and to the right,
obscuring a lower Pao2.
References:
AAAAI Initiatives: Update on pediatric asthma: promoting best practice.
Available at:
http://www.aaaai.org/professio
default.stm Lemanske RE Jr, Green CG. Asthma in infancy and childhood. In:
Middleton
E Jr, Reed CE, Ellis EF, Adkinson NF Jr, Yunginger JW, Busse WW, eds.
Allergy: Principles and Practice. 5th ed. St Louis, Mo: Mosby-Year Book,
Inc; 1998:877-900
National Asthma Education Program, Expert Panel Report II. Guidelines
for the Diagnosis and Management of Asthma. Bethesda, Md: US Department
of Health and Human Services, Public Health Service, National Institutes
of Health; 1997:No. 97-4051