DOSE-DEPENDENT EFFECTS OF HALOTHANE ON TBE CARBON-DIOXIDE RESPONSES OF EXPIRATORY AND INSPIRATORY BULBOSPINAL NEURONS AND THE PHRENIC-NERVEACTIVITIES IN DOGS
Eae. Stuth et al., DOSE-DEPENDENT EFFECTS OF HALOTHANE ON TBE CARBON-DIOXIDE RESPONSES OF EXPIRATORY AND INSPIRATORY BULBOSPINAL NEURONS AND THE PHRENIC-NERVEACTIVITIES IN DOGS, Anesthesiology, 81(6), 1994, pp. 1470-1483
Background: Expiratory bulbospinal and inspiratory bulbospinal neurons
in the ventral respiratory group provide drive for thoracoabdominal e
xpiratory and phrenic and thoracic inspiratory motor neurons. Potent i
nhalational agents such as halothane may have differential effects on
inspiratory and expiratory neurons, but detailed studies comparing neu
rons at a homologous level are lacking. Methods: The dose-dependent ef
fects of anesthesia with 1.0-2.5 minimum alveolar concentration haloth
ane on the CO2 responses of single expiratory and inspiratory bulbospi
nal neurons of the ventral respiratory group and on phrenic neural act
ivities were studied in nonpremedicated, anesthetized, paralyzed, vago
tomized dogs. Hyperventilation with O-2 and the addition of CO2-O-2 mi
xtures were used to produce low, medium, and high steady-state levels
of central CO2 drive. Results: Peak neuron discharge frequency decreas
ed progressively with increasing halothane dose at all levels of CO2 d
rive for both types of neurons. The sensitivities of inspiratory and e
xpiratory bulbospinal neuronal activities to halothane were not signif
icantly different from one another, whereas the sensitivity to halotha
ne of the peak phrenic activity was markedly greater than those of the
neurons. Increasing halothane dose caused a downward, predominantly p
arallel shift of the CO2 response curves. Phrenic nerve activity also
showed a decrease in slope of the CO2 response. Conclusions: The activ
ities of respiratory premotor neurons are less depressed by increasing
doses of halothane than is phrenic nerve activity. The greater depres
sion of phrenic activity may result from additional anesthetic actions
on the efferent motor pathways, resulting in decreased descending syn
aptic inputs to phrenic motor neurons.