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The effects of sevoflurane, isoflurane, and halothane on the pattern of phrenic nerve activity in response to acute hypoxia in rats

Background and Goal of Study: (required) Anaesthetics have influence on chemical control of respiration either on central and/or peripheral level, modifying normal response to hypoxia. Phrenic nerve activity (PNA) is typically used as an equivalent of the central control of breathing. Even low concentrations of volatile anaesthetics are shown to decrease hypoxic ventilatory response (HVR). The aim of our study was to compare effect of halothane, isoflurane, and sevoflurane at equal MAC values of 1.75, 2.0 and 2.5 on the PNA in rats during acute normocapnic hypoxia induced by 10% oxygen concentration in inspired nitrogen. Materials and Methods: (required) Following the approval of the University of Split, School of Medicine Medical Ethics Committee, Sprague-Dawley males, laboratory rats, BM of 300-350 g were used. Three experimental groups of seven animals each, were formed regarding volatile anaesthetics used (halothane - H, isoflurane I and sevoflurane S). After tracheotomy was performed the animals were mechanically ventilated. Femoral veins and arteries were cannulated. All animals were vagotomized bilaterally. Phrenic nerve was dissected and recorded. Rhythm and pattern related changes of phrenic nerve were analysed during normocapnic hypoxia for three different volatile anaesthetics under three levels of anaesthesia. Results and Discussion: (required) Halothane anaesthesia at 1.75 MAC markedly depressed PNA and abolished the ventilatory response to hypoxia. On the contrary, at the same level of isoflurane anaesthesia, hypoxia evoked marked increase of PNA (p=0.008). At 2.0 MAC and 2.5 MAC halotane and isoflurane abolished the HVR monitored by phrenic nerve activity. Hypoxia at sevoflurane anaesthesia at 1.75 MAC and at 2.0 MAC evoked increase in amplitude (p=0.002 and p=0.01, respectively) and rhythm related variables of PNA. At 2.5 MAC sevoflurane did not persistently affect phrenic nerve activity in response to hypoxia ; increased PNA in 5 animals (p=0.029), and evoked no change in 2 animals. Conclusion(s): (required) These results indicate there are both qualitative and quantitative differences in effects of halothane, isoflurane, and sevoflurane on the activity of phrenic nerve in response to acute normocapnic hypoxia in rats.

Hypoxia; Ventilation; hypoxic response; Anaesthetics volatile

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