Background: Intrathecal administration of morphine produces intense analges
ia, but it depresses respiration, an effect that can be life-threatening. W
hether intrathecal morphine affects the ventilatory response to hypoxia, ho
wever, is not known.
Methods: We randomly assigned 30 men to receive one of three study treatmen
ts in a double-blind fashion: intravenous morphine (0.14 mg per kilogram of
body weight) with intrathecal placebo; intrathecal morphine (0.3 mg) with
intravenous placebo; or intravenous and intrathecal placebo. The selected d
oses of intravenous and intrathecal morphine produce similar degrees of ana
lgesia. The ventilatory response to hypercapnia, the subsequent response to
acute hypoxia during hypercapnic breathing (targeted end-tidal partial pre
ssures of expired oxygen and carbon dioxide, 45 mm Hg), and the plasma leve
ls of morphine and morphine metabolites were measured at base line (before
drug administration) and 1, 2, 4, 6, 8, 10, and 12 hours after drug adminis
tration.
Results: At base line, the mean (+/-SD) values for the ventilatory response
to hypoxia (calculated as the difference between the minute ventilation du
ring the second full minute of hypoxia and the fifth minute of hypercapnic
ventilation) were similar in the three groups: 38.3+/-23.2 liters per minut
e in the placebo group, 33.5+/-16.4 liters per minute in the intravenous-mo
rphine group, and 30.2+/-11.6 liters per minute in the intrathecal-morphine
group (P=0.61). The overall ventilatory response to hypoxia (the area unde
r the curve) was significantly lower after either intravenous morphine (20.
2+/-10.8 liters per minute) or intrathecal morphine (14.5+/-6.4 liters per
minute) than after placebo (36.8+/-19.2 liters per minute) (P=0.003). Twelv
e hours after treatment, the ventilatory response to hypoxia in the intrath
ecal-morphine group (19.9+/-8.9 liters per minute), but not in the intraven
ous-morphine group (30.5+/-15.8 liters per minute), remained significantly
depressed as compared with the response in the placebo group (40.9+/-19.0 l
iters per minute) (P=0.02 for intrathecal morphine vs. placebo). Plasma con
centrations of morphine and morphine metabolites either were not detectable
after intrathecal morphine or were much lower after intrathecal morphine t
han after intravenous morphine.
Conclusions: Depression of the ventilatory response to hypoxia after the ad
ministration of intrathecal morphine is similar in magnitude to, but longer
-lasting than, that after the administration of an equianalgesic dose of in
travenous morphine.