The effect of increased genioglossus activity and end-expiratory lung volume on pharyngeal collapse

Abstract

Increasing either genioglossus muscle activity (GG) or end-expiratory lung volume (EELV) improves airway patency but not sufficiently for adequate treatment of obstructive sleep apnea (OSA) in most patients. The mechanisms by which these variables alter airway collapsibility likely differ, with increased GG causing airway dilation, whereas increased EELV may stiffen the airway walls through caudal traction. We sought to determine whether the airway stabilizing effect of GG activation is enhanced when EELV is increased. To investigate this aim, 15 continuous positive airway pressure (CPAP)-treated OSA patients were instrumented with an epiglottic catheter, intramuscular GG-EMG electrodes, magnetometers, and a nasal mask/pneumotachograph. Subjects slept supine in a sealed, head-out plastic chamber in which the extra-thoracic pressure could be lowered (to raise EELV) while on nasal CPAP with a variable deadspace to allow CO(2) stimulation (and GG activation). The pharyngeal critical closing pressure (P(CRIT)) was measured by sudden reduction of CPAP for three to five breaths each minute during non-rapid eye movement (NREM) sleep in 4 conditions: a) baseline, b) 500 ml increased EELV, c) 50% increased GG, and d) conditions b and c combined. P(CRIT) was found to be reduced from 2.2 + or - 0.7 cmH(2)O at baseline to -1.0 + or - 0.5 with increased EELV, 0.6 + or - 0.7 with increased GG and -1.6 + or - 0.7 when both variables were raised (P < 0.001). The slope of the P(CRIT) curves remained unchanged in all conditions (P = 0.05). However, the CPAP level at which flow limitation developed was lower in both increased EELV conditions (P = 0.001). These findings indicate that while both increased GG and EELV improve airway collapsibility, the combination of both variables has little additional effect over increasing EELV alone.

Fig. 1.

Experimental setup. Patient lay with his/her body within and head out of the sealed plastic shell in the bedroom. He/she wore a nasal mask/pneumotachograph that was connected to continuous positive airway pressure (CPAP) tubing without an expiratory leak valve but with a fresh air input bleed line in place. To maintain eucapnia, high flow rates of medical air were introduced into the CPAP tubing flushing expiratory gas out of the tubing. However, when hypercapnia was required, the fresh air flow rate could be reduced such that the subjects partially rebreathed their own expired air. Thus the degree of hypercapnia could be maintained throughout changes in CPAP pressure. The subjects also wore magnetometers placed on the chest and abdomen such that changes in EELV, which were induced by applying negative pressure to the sealed chamber, could be documented.

Fig. 2.

Raw data in all 4 conditions in 1 subject. Airflow, tidal volume (V_T), mask pressure (P_MASK), epiglottic pressure (P_EPI), carbon dioxide (CO₂) level, pressure inside the plastic shell (P_LUNG), abdominal diameter (AB), rib cage diameter (RC), and both the raw (EMG) and rectified/moving time averaged (MTA) genioglossal (GG) EMG signals are shown in 1 subject in all 4 conditions: baseline (BL), 50% increased genioglossus activity (Inc GG), 500 ml increased end-expiratory lung volume (Inc EELV), and both 500 ml increased end-expiratory lung volume and 50% increased genioglossal activity (Inc GG+Inc EELV). In each condition a pressure drop to ∼6 cmH₂O was performed with varying degrees of flow limitation resulting.

Fig. 3.

An example of pharyngeal critical closing pressure (P_CRIT) curves in the 4 conditions in 1 subject. The CPAP level (P_MASK) and peak inspiratory flow rate (PIF) are shown for all flow-limited breaths during P_CRIT measurement (3rd to 5th breaths after CPAP drop only) in the 4 experimental conditions: BL, Inc GG, Inc EELV, and Inc GG+Inc EELV. Linear regression lines, extrapolated to zero flow are shown in each condition.

Fig. 4.

Individual and group mean P_CRIT in each subject across the 4 conditions: BL, Inc GG, Inc EELV, and Inc GG+Inc EELV. Group mean values (±SE) are shown in the large black circles. *Significantly lower than BL; #significantly lower than Inc GG.