Comparison of the effects of sub-hypnotic concentrations of propofol and halothane on the acute ventilatory response to hypoxia |
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Authors: | Nagyova B; Dorrington K L; Gill E W; Robbins P A |
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Institution: | University Laboratory of Physiology, Parks Road, Oxford OX1 3PT; University Department of Pharmacology, Mansfield Road, Oxford OX1 3QT |
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Abstract: | To compare the effects of sub-anaesthetic concentrations of propofol and
halothane on the respiratory control system, we have studied the acute
ventilatory response to isocapnic hypoxia (AHVR) in 12 adults with and
without three different concentrations of propofol and halothane. Target
doses for propofol were 0, 0.05, 0.1 and 0.2 of the effective plasma
concentration (EC50 = 8.1 micrograms ml-1). Target doses for halothane were
0, 0.05, 0.1 and 0.2 minimum alveolar concentration (MAC = 0.77%). The
doses achieved experimentally were 0.01, 0.06, 0.13 and 0.26 of the EC50
for propofol and 0, 0.05, 0.11 and 0.20 MAC for halothane. During the
experiment subjects breathed via a mouthpiece from an end-tidal forcing
system. End-tidal PO2 (PE'O2) was held at 13.3 kPa for 5 min, and then at
6.7 kPa for 5 min. End- tidal PCO2 (PE'CO2) was held constant at 0.13-0.27
kPa greater than the subject's natural level throughout. The mean values
for AHVR with propofol were: 12.8 (SEM 2.4) litre min-1 (0.01 EC50), 10.0
(1.9) litre min-1 (0.06 EC50), 9.8 (2.3) litre min-1 (0.13 EC50) and 4.9
(1.2) litre min-1 (0.26 EC50). The values for AHVR with halothane were:
11.9 (2.4) litre min-1 (0 MAC), 7.8 (1.6) litre min-1 (0.05 MAC), 5.9 (1.2)
litre min-1 (0.11 MAC) and 3.2 (1.6) litre min-1 (0.2 MAC). The decline in
AHVR with increasing dose for both drugs was statistically significant
(ANOVA, P < 0.001); there was no significant difference between the two
drugs with respect to this decline. Normoxic ventilation with propofol
declined from 13.2 (1.6) litre min-1 (0.01 EC50) to 8.3 (0.9 litre min-1
(0.26 EC50), and with halothane declined from 13.5 (2.0) litre min-1 (0
MAC) to 11.8 (1.6) litre min-1 (0.2 MAC). This was significant for both
drugs (ANOVA, P < 0.001).
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