Prosserman Centre for Health Research (Samuel Lunenfeld Research Institute), Mount
Sinai Hospital, 2009
Author: Oded Friedman
Discussion: Lower Body Positive Pressure Experiment
“Upper airway cross-sectional area was determined using acoustic pharyngometry (Eccovision Acoustic pharyngometry; Sleep Group Solutions, Miami, Fl) at end-expiration. Two parameters, oropharyngeal junction area and mean cross-sectional area from
the velum to the glottis, were recorded via the mean of four consecutive measurements. The device was positioned in the mouth (using a modified scuba mouthpiece designed to secure the tongue in place) of subjects while they lay supine with head fixed in the neutral position by resting the head in form-shaping sand bags. The pulse emitter produced five pulses/second and two microphones detected the amplitude and temporal changes of the reflected pulse. Acoustic pharyngometry is highly reproducible and has been validated against computed tomography and
magnetic resonance imaging for assessment of upper airway cross- sectional area in non-snorers and in snorers with and without OSAH in both the supine and upright positions. Given that lung volume is a determinant of upper airway cross-sectional area, changes in end-expiratory lung volume (i.e. functional residual
capacity) were assessed by respiratory inductance
plethysmography calibrated against a spirometer in the DC coupled mode. BP and heart rate were monitored during LBPP application by an automated sphygmomanometer (Dinamap 1846SX NIBP; Critikon, Tampa, FL).
“Following a 30-minute stabilization period in which subjects lay supine, baseline measurements of all variables were made. Subjects were then exposed to increasing LBPP beginning at 10
mmHg and ending at 40 mmHg, at 10 mmHg increments. Each of the 4 applied pressures was maintained for 5 minutes and measurements of all variables were made at the end of the 5- minute period.
“The relationship between the reduction in mean upper airway cross-sectional area and the LBPP-induced reduction in leg fluid volume was fit best using log-transformed values of the reduction in mean upper airway cross-sectional area.
“Leg fluid volume and neck circumference prior to LBPP application were significantly greater in the RH [drug resistive hypertension] group compared to the CH [controlled hypertension] group; on the other hand, the baseline oropharyngeal junction area and mean upper airway cross-sectional area were comparable in
the RH and CH groups. Compared to the CH group, the LBPP- induced reductions in oropharyngeal junction area and mean upper airway cross0sectional area were significantly greater in the RHG group.The LBPP-induced increase in neck circumference was also significantly greater in the RH group.
“Given that the effects of LBPP application on neck circumference and upper airway cross-sectional area were assessed during the wake period, the findings may not be applicable to those occurring during the sleep period. However, it would have been impractical for subjects to sleep uninterrupted while undergoing simultaneous LBPP application and acoustic pharyngometry since they would be unable to sleep or would move if they did fall asleep causing artifactual changes in the acoustic pharyngometry signal. Additionally, the literature would sugest that rostral fluid shift may have and even more pronounced effect on upper airway cross- sectional area during the sleep than wake cycle because of a withdrawal of neural input to the pharyngeal dilator muscles at sleep onset resulting in pharyngeal luminal narrowing with a corresponding increase in pharyngeal resistance and collapsibility. A limitation of acoustic pharyngometry, given that transmitted sound waves travel below the soft palate, is its inability to evaluate
the lumen size at the l3evel of the retropalatal pharynx, the site at which the paharynx collapses in most patients with OSAH. Nonetheless, significan changes in the mean upper airway cross- sectional area and/or oropharyngeal junction have already been documented following LBPP application and diuretic therapy. Furthermore, several studies have shown consistently that the upper airway cross-sectional area from velum to glottis in the
awake state is narrowed in patients with OSAH, both while upright and supine.”