Mouth Pressure and Nostril Pressure

Scientific basis. Pmo is easy to measure and changes may give a reasonable approximation of change in alveolar pressure and thus Pes, providing there is relatively little pressure loss down the airways, or across the lungs. This may be realistic with normal lungs, particularly when changes in lung volume are small, but is unlikely to be fulfilled in patients with severe lung or airway disease. When used in combination with voluntary static and dynamic maneuvers at FRC, Pmo provides a global index of the action of synergistic respiratory muscles. When the diaphragm contracts in isolation against a closed airway, as with phrenic nerve stimulation, Pmo may be a useful reflection of Pdi.

Pnas is also easy to measure (see Volitional Tests of Respiratory Muscle Strength) but has the same caveats as Pmo.

Methodology. Pmo is measured at the side port of a mouthpiece. It should be possible to occlude the mouthpiece at the distal end and a small leak should be incorporated to prevent glottic closure during inspiratory or expiratory maneuvers (26). The type of mouthpiece used can significantly influence the results (27). The issue of the lung volume at which Pmo should be measured during static efforts is addressed in the section on volitional tests (see subsequent section), and the various maneuvers that can be used to obtain useful Pmo data during phrenic nerve stimulation are described in the section on phrenic nerve stimulation (see subsequent section).

Pnas is measured with a polyethylene catheter held in one nostril by a soft, hand-fashioned occluding plug; respiratory maneuvers are performed through the contralateral nostril (23).

A standard mouthpiece for Pmo, or a nasal plug (custom made or commercially available) for Pnas, and one pressure transducer are required. Portable Pmo devices (28) are useful for screening and bedside studies.

Advantages of mouth pressure and nasal sniff pressure. The main advantage of Pmo and Pnas are their simplicity and ease of use, both for the operator and for the subject.

Disadvantages of mouth pressure and nasal sniff pressure. The measurement of Pmo does not allow the investigator to discriminate between weakness of the different respiratory muscles. When Pmo or Pnas is used as a substitute for Pes during dynamic maneuvers (sniff test, phrenic nerve stimulation), glottic closure or airway characteristics may prevent adequate equilibration.

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