Lung Volume Modulation of Sternohyoid Muscle Function: An Anesthetised Rat Model.

Lung volume change modifies pharyngeal airway patency by altering breathing-related passive force transmission between lower and upper airways (via tracheal and other connections). We hypothesise that such force transmission may also impact active upper airway dilator muscle function by altering resting muscle length. The aim of this study was to determine the relationship between end expiratory lung volume (EELV) and ability of sternohyoid muscle (SH) contraction to alter pharyngeal airway patency. Eleven supine, anesthetized, spontaneously breathing Wistar rats with sealed snout mask to monitor airflow (̇) and mask pressure (P_M) were positioned in head-out plethysmograph (P_BOX). Graded changes in P_BOX (±8cmH₂O) were applied, tracheal pressure (P_tr), SH length (SHL) and tension (SHT_P) were monitored. Change in EELV (ΔEELV) and upper-airway resistance (R_UA-Passive) (%baseline) were calculated. Electrical stimulation of SH at each P_BOX was performed, and changes in SH tension (SHT_A) and R_UA (ΔR_UA= R_UA-Passive - R_UA-Active) calculated. Data were analysed using mixed-effects models. Increasing EELV increased SHL by 0.062 mm/ml (0.037 - 0.086) [mean(95%CI)], increased SHT_P by 5.42 mN/ml (3.96 - 6.87), and decreased R_UA-Passive by 4%/ml (1.1 - 7.4), while decreasing EELV decreased SHL by 0.31 mm/ml (0.14 - 0.49), SHT_P by 17.8 mN/ml (11.7 - 23.9), and increased R_UA-Passive by 132%/ml (95.6 - 168.3). Increasing EELV decreased SHT_A by 19.7%/ml (10.6 - 28.8) and ΔR_UA by 60.1%/ml (27.1 - 93.1), while decreasing EELV also decreased both SHT_A, by 57.5%/ml (31.4 - 84.2) and ΔR_UA by 139.0%/ml (46.8 - 231.2), (all p<0.05). Maximal SHT_A and ΔR_UA occurred at baseline EELV. We conclude that while shifts in EELV impact passive upper airway function, they also negatively impact active SH pharyngeal dilator function, potentially via generation of a sub-optimal resting SHL.