Hypoxia refers to a low oxygen content in the blood or a deficiency of
oxygen reaching the tissues of the body. Generally individuals have
oxygen saturations greater than 95% although many physicians do not
initiate treatment unless O2 sats are lower than SaO2 90% (often lower
than 85%) or transcutaneous PO2 less than 50 mm Hg.
Hypercarbia refers to the presence of excess carbon dioxide in the blood.
The normal PaCO2 at rest is generally accepted to be between 35 and 45 mm
Hg.
Hypoventilation defines a condition in which alveolar ventilation is
insufficient to meet an individuals metabolic demands resulting in
increased PaCO2. Chronic aveolar hypoventilation is used interchangably
with the term chronic ventilatory failure.
To help people understand disorders causing aveolar hypoventilation it is
often helpful to group them into two catagories: patients that "Can't
Breathe" (ie because of neuromuscular weakness or excessive work of
breathing) and those that "Won't Breathe" (ie due to inadequate drive from
the ventilatory control centers). Clinically, the presentation of
individuals with chronic hypoventilation can be nonspecific and may be
accentuated by coexistent hypoxia. Hypercapnia, both directly and
indirectly, affects the central nervous system, heart and kidneys.
Consequences of chronic hypoventilation (hypercarbia and/or hypoxia)
include: altered mental state, sleep of excessive depth or duration,
headaches, depression, irritability, sexual dysfunction, learning and
memory difficulties, narcolopsy, pulmonary hypertension, heart failure,
elevated serum bicarb levels, impaired diaphragm function, disability and
potentially death.
Some schools of thought believe that you can not have hypercarbia without
hypoxia but children with CCHS (like my daughter) typically have very
elevated CO2 levels and either normal or only mildly decreased O2 levels.
Although at times the reverse is true, they seem to have significantly
decreased O2 levels and high normal CO2 levels. We (her caretakers) are
starting to recognize which problem causes what type of symptom(s) and
respond accordingly. Likewise, on very rare occasions Kristen will
specifically ask for either oxygen, ventilator or both to improve how she
is feeling. The downside of children with CCHS is their brains do not
recognize the abnormal levels as being abnormal so if she is asking for
some type of intervention she's in real trouble and we have to act very
quickly to keep her conscious.
Hope this brief overview is helpful.
Joanne Kocourek
The University of Chicago
jskocour(AT)midway.uchicago.edu