Upper Airway Muscles

(See Electromyography in Section 8 of this Statement.)

Rationale. Assessment of upper airway muscle activity and airway collapsibility is useful for assessing infants and children with increased upper airway resistance.

Measurements. Surface and fine wire EMGs have been used in research settings to evaluate the activity of upper airway muscles including the intrinsic laryngeal muscles (8), alae nasi (105), and the genioglossus (105). The function of upper airway muscles is normally assessed clinically by visualization of the size of the airway using fiberoptic endoscopy (106), flu-oroscopy (107), or, more recently, fast magnetic resonance imaging techniques (108). Another useful indirect measurement, especially when assessing patients for obstructive sleep apnea, is to monitor airflow at the mouth/nose for evidence of reduced flow (109). Upper airway collapsibility can be assessed either by monitoring pharyngeal pressure during an airway occlusion or by applying positive and negative pressures at the airway opening and looking for evidence of airflow limitation during spontaneous breaths (24). The critical pressure (Pcrit) of the upper airway is defined from such measurements as the pressure at which flow equals zero as extrapolated from a maximum flow versus applied nasal pressure curve. Closing pressure of the upper airway has been measured in postmortem infants (110) and in normal sleeping infants (23).

Advantages. Measurement of upper airway muscle activity provides valuable information when assessing infants and children with suspected upper airway dysfunction. Visualization of the upper airway is a standard clinical procedure.

Disadvantages. Electromyographic measurements are not feasible for clinical assessment because of the technical difficulties, limited number of muscles that are accessible to surface measurements, and the inability to compare levels of EMG activity between patients. At the present time, measures of col-lapsibility are time-consuming and require special expertise.

Normal values. No normal values exist for the EMG of the upper airway muscles. in general, abductor muscles are active during inspiration with quiet breathing and some adductors are active during expiration. Measurements of Pcrit in a small number of children yielded values of —19.5 cm H2O for children with primary snoring and +1.0 cm H2O in children with obstructive sleep apnea (24). Closing pressures in postmortem infants averaged 0.8 cm H2O (110) whereas those in sleeping infants averaged —3.8 cm H2O (23). The more positive value in the postmortem measurements probably reflects the lack of upper airway muscle activation.

TABLE 4. AXIAL DIAPHRAGM DISPLACEMENT: NORMAL ULTRASONOGRAPHIC VALUES DURING TIDAL BREATHING

Reference

Axial Diaphragm

i Displacement (mm)

Anterior

Middle

Posterior Mean

Infancy

97

2.6

3.7

4.6 3.6

4

7.4

12.1 9.7

Adulthood

98

— 15.2

Clinical application. Measurements of upper airway function are useful in assessing children with suspected obstructive sleep apnea due to abnormalities of either structure or upper airway muscle activation including those with craniofacial abnormalities, hypertrophied lymphoid tissue, and neuromuscular disease. They can also be useful in cases of vocal cord dysfunction.

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