Augmented activity of suprahyoid muscles during hypothermia in sevoflurane-anesthetized mice

Halogenated volatile anesthetics not only cause profound respiratory depression but also exert a facilitatory influence on the upper airway dilator (UAD) muscles in small rodents. A high concentration of sevoflurane inhalation induces gasping respiration characterized by augmented breaths with mandibular movements, to which elevated activity of suprahyoid muscles (SHMs) contributes significantly. Although similar gasping-like breathing has been observed during hypothermia in sevoflurane-anesthetized spontaneously breathing mice, the effect of sevoflurane during hypothermia on SHMs' activity remains elusive. We investigated the synergistic effects of sevoflurane, pentobarbital, and hypothermia on ventilation and SHMs' activity in spontaneously breathing mice. The twenty-one tracheally intubated mice were divided into three groups, i.e., the sevoflurane (N = 7), the pentobarbital (N = 7), and the pentobarbital-sevoflurane (N = 7) groups. Progressive hypothermia was produced by cooling mice in each group from 37 to 36℃ to 25-24℃ while measuring body temperature, breathing patterns, and the SHMs' activity through subcutaneous electromyography (EMG_SH). The pentobarbital group showed minimal change in tidal volume (V_T) and respiratory-related SHMs' activity during cooling. In contrast, in the sevoflurane and pentobarbital-sevoflurane groups, the EMG_SH, which behaves like the UAD muscle, was augmented with increased V_T during hypothermia. Notably, the pentobarbital-sevoflurane group showed significantly larger EMG_SH values at body temperatures of 34-33 and 31-30℃, indicating a more pronounced effect. Our study confirms the significant role of sevoflurane in inducing increased V_T and augmented SHMs' activity during hypothermia. Furthermore, adding pentobarbital to sevoflurane anesthesia during hypothermia led to a further increase in augmented EMG_SH, highlighting the synergistic effects of these factors.