Assessing and interpreting arterial blood gases and acid-base balance
BMJ 1998; 317 doi: https://doi.org/10.1136/bmj.317.7167.1213 (Published 31 October 1998) Cite this as: BMJ 1998;317:1213
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I was interested to read that bilirubin can lead to a falsely low
oxygen saturation by pulse oximetry. However, no reference or
quantitative information on this problem was provided. Newborns commonly
develop high bilirubin levels. I wonder if Dr. Williams could provide an
estimate of the degree to which the oxygen saturation measured by pulse
oximetry might be affected by jaundice at the levels commonly seen in
newborns. A reference that provides the basis for such an estimate would
also be much appreciated.
Sincerely,
Thomas B. Newman, MD, MPH
Departments of Epidemiology and Biostatistics
University of California
San Francisco, CA 94143-0560
USA
Competing interests: No competing interests
Dear Sir,
In his recent article on arterial blood gas analysis, Dr Williams repeats
the commonly read advice to perform a modified Allen's test prior to
attempting radial artery puncture [1].
It is my impression that this advice is never carried out in practice and
a brief survey of my anaesthetist colleagues confirmed that none of six
specialist registrars and eight consultants (with a combined experience of
several thousand radial artery punctures) used the test routinely.
In fact Allen's test has a poor sensitivity and specificity for
complications following radial artery cannulation. In a series of 1699
patients undergoing arterial cannulation for coronary artery surgery, of
411 patients who had an Allen's test, 16 were abnormal. None of these 16
had complications from radial arterial cannulation [2]. There is a further
report of serious complications in 2 of 982 patients who had a normal
Allen's test prior to radial arterial cannulation [3].
The available evidence does not support the routine use of Allen's test
prior to radial artery puncture. Nevertheless, because of the rare
incidence of serious complications common sense suggests that all patients
should have regular clinical observation of their hand and finger blood
supply following arterial puncture or cannulation.
Yours sincerely,
Adrian Steele BA MRCP FRCA
Specialist registrar in Anaesthesia, St Helier Hospital, Wrythe Lane
Carshalton, Surrey
1.Williams AJ. Assessing and interpreting arterial blood gases and
acid-base balance. BMJ 1998;317:1213-16 (31 Oct)
2. Slogoff S, Keats AS, Arlund C. On the safety of radial artery
cannulation. Anaesthesiology 1983;59:42-47
3. Mandel MA, Dauchot PJ. Radial artery cannulation in 1000 patients:
precautions and complications. Journal of Hand Surgery 1977;2:482-85
Competing interests: No competing interests
Sir, in response to his excellent review [1] I feel that Adrian
Williams has not emphasised the importance of looking at the bicarbonate
level in patients presenting with respiratory failure. With depressing
frequency intensive care doctors are presented with patients rendered
unnecessarily hypoxic by the casualty or medical teams because any patient
with a raised arterial carbon dioxide level is immediately starved of
oxygen in case they stop breathing. If they looked at the bicarbonate
level and it is normal, this virtually proves that the respiratory failure
is of acute onset, metabolic compensation having not had time to occur,
and it is safe to give a high inspired oxygen level. If the bicarbonate
level is abnormally raised, this is suspicious of a patient having long
term carbon dioxide retention and then a more cautious approach to oxygen
therapy is justified.
1. Williams AJ.ABC of oxygen: Assessing and interpreting arterial
blood gases and acid-base balance. BMJ 1998; 317: 1213-1216
Competing interests: No competing interests
Pulse oximetry: Readings not affected by serum bilirubin.
DearSir,
Dr Williams (BMJ 1998;317:1213-1216) is right to point out that pulse
oximeters, whilst a valuble monitoring tool, have certain limitations. He
does however state erroneously that the readings they give are affected by
serum bilirubin. An examination of the absorption spectrum of bilirubin
demonstrates no absorption at 660 nm and 940 nm, the wavelengths at which
pulse oximeters operate, and thus no interference is possible(1). The
confusion arises due to the fact that co-oximeters, which are used to
measure blood pO2 in vitro operate over a wide range of wavelengths in
order to measure the concentrations of abnormal haemoglobins (eg
methaemoglobin and carboxyhaemoglobin). The operating wavelengths of these
machines includes the visible spectrum, over which bilirubin obviously
does absorb.
Yours sincerely
Dr Michael Coupe
(1) Ward's Anaesthetic Equipment; J.T. Moyle, A. Davey: 4th edition,
1998: WB Saunders and Company Limited.
Competing interests: No competing interests