British Journal of Anaesthesia, 2000, Vol. 85, No. 3 456-470
© 2000 The Board of Management and Trustees of the British Journal of Anaesthesia
Within-breath arterial PO2 oscillations in an experimental model of acute respiratory distress syndrome
1Nuffield Department of Anaesthetics, University of Oxford, Radcliffe Infirmary, Oxford OX2 6HE, UK. 2Service Anesthésie Réanimation, Hopital de la Croix Rousse, 103 Grande-Rue de la Croix, Rousse, F-69317 Lyon Cedex 04, France*Corresponding author
Tidal ventilation causes within-breath
oscillations in alveolar oxygen concentration, with an amplitude which
depends on the prevailing ventilator settings. These alveolar oxygen
oscillations are transmitted to arterial oxygen tension,
PaO2, but with an amplitude which
now depends upon the magnitude of venous admixture or true shunt,
Q·S/Q·T. We
investigated the effect of positive end-expiratory pressure (PEEP) on
the amplitude of the PaO2
oscillations, using an atelectasis model of shunt. Blood
PaO2 was measured on-line
with an intravascular PaO2
sensor, which had a 2–4 s response time (10–90%). The
magnitude of the time-varying
PaO2 oscillation was titrated
against applied PEEP while tidal volume, respiratory rate and inspired
oxygen concentration were kept constant. The amplitude of the
PaO2 oscillation,
PaO2, and the mean
PaO2 value varied with the level
of PEEP applied. At zero PEEP, both the amplitude and the mean were at
their lowest values. As PEEP was increased to 1.5 kPa, both
PaO2 and the mean
PaO2 increased to a maximum.
Thereafter, the mean PaO2
increased but PaO2
decreased. Clear oscillations of
PaO2 were seen even at the lowest
mean PaO2, 9.5 kPa.
Conventional respiratory models of venous admixture predict that these
PaO2 oscillations will be reduced
by the steep part of the oxyhaemoglobin dissociation curve if a constant
pulmonary shunt exists throughout the whole respiratory cycle. The facts
that the PaO2 oscillations
occurred at all mean PaO2 values
and that their amplitude increased with increasing PEEP suggest that
Q·S/Q·T, in the
atelectasis model, varies between end-expiration and
end-inspiration, having a much lower value during inspiration than
during expiration.
Br J Anaesth 2000; 85: 456–9
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