Læknaneminn - 01.10.1996, Page 95

Overdoses and poisonings: physiological responses
gap to exist between the measured oxygen saturation as
determined by spectrophotometric methods such as
co-oxymetry and the calculated oxygen saturation.
The calculated oxygen saturation is derived from the
measured arterial pressure of oxygen using the oxy-
hemoglobin dissociatation curve, assuming that nor-
mal-functioning hemoglobin is present.
If abnormal hemoglobins are present the calculated
value will overestimate the actual percentage of hemo-
globin present that is saturated with oxygen.
Abnormal hemoglobin’s causing this type of problem
include:
• Methemoglobin
• Carboxyhemoglobin
• Sulfhemoglobin
• Cyanomethemoglobin
The co-oxymeter expresses oxygen saturation
percentages by comparing the concenfrations of oxy-
hemoglobin to total hemoglobin which includes any
variants present:
• Total hemoglobin
• Oxyhemoglobin
• Deoxyhemoglobin
• Carboxyhemoglobin
• Methemoglobin
Co-oximetry, therefore, is an important technology
which can assist in the clinical management of patients
suspected of having abnormal hemoglobin present. A
saturation gap should be calculated in such patients.
Clinical contexts which should raise suspicion include:
• Cyanotic patients whose color fails to improve with
oxygen administration (methemoglobin)
• Victims of house fires or smoke inhalation (car-
boxyhemoglobin or methemoglobin)
• Patient being treated for cyanide ingestion (methe-
moglobin and cyanomethemoglobin)
V. SUMMARY
With these various toxicologic fingerprints in mind,
the clinician is able to proceed with life-saving therapy
even when available information about the causative
agent is minimal. All that is required is an understand-
ing of the autonomic nervous system and an awareness
of which drugs or toxins affect it in a given way (e.g.
sympathomimetic). In addition a specific symptom
(such as a seizure) or sign (such as a wide QRS) may al-
ert the clinician to the presenCe of a specific toxin.
REFERENCES
1. Ellenhorn MJ, Barceloux DG. Medical Toxicology: Diagnosis and
Treatment of Human Poisoning. New York, Elsevier, 1988.
2. Goldfrank LR, Flomenbaum NE, Lewin NA, et al. Goldfrank's
Toxicologic Emergencies., 4th ed. East Norwalk, Appleton &
Lange, 1990.
SELF-ASSESSMENT QUESTIONS
0VERD0SES AND P0IS0NING:
PHYSIOLOGICAL RESPONSES
Try your luck in identifying the toxidrome in each
case.
Ctise 1
A 77-year-old woman is admitted to an orthopedic
service for a fracture of the distal femur. On the sec-
ond day post-admission she is to be taken to surgery,
but when the attendants come to take her to the oper-
ating room, she tells them about the fire engines she
sees flying across the room. Also, this normally lucid
woman is tachycardic and agitated and insists that her
daughter (who is in the room) is her mother (who has
been dead for many years). Later on that day she is
observed to have a tonic-clonic seizure. On review of
her records it is determined that she had been taking
Fiorinal and ethchlorvynol chronically for headaches
and to help her sleep for years. Her daughter notes she
has gone from doctor to doctor to obtain these med-
icines. On admission these medicines were discontinu-
ed and the patient written for prn narcotic pain med-
icines.
Case 2
A 7-year-old girl is brought in by her mother who
says she was awakened at l:00am to find her daughter
striking at her with her fists. The child was stark naked
and yet insisted upon questioning that she was trying
to take off her clothes. Her mother also notes that her
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2. tbl. 1996, 49. árg.