Sniff Tests

Scientific basis. A sniff is a short, sharp voluntary inspiratory maneuver performed through one or both unoccluded nostrils. It involves contraction of the diaphragm and other inspiratory muscles. To be useful as a test of respiratory muscle strength, sniffs need to be maximal, which is relatively easy for most willing subjects, but may require some practice.

The sniff was described in 1927 as a radiological test of diaphragm paralysis because, in normal subjects, it was associated with crisp diaphragm descent during inspiration (57, 58). Esau and coworkers (59) suggested that a short, sharp sniff would approximate the diaphragm contraction elicited by a brief stimulation of the phrenic nerves (59, 60). Miller and coworkers (61) showed that normal subjects generated greater Pdi during maximal sniffs than during maximal static inspira-tory efforts, perhaps because the maneuver achieves rapid, fully coordinated recruitment of the inspiratory muscles (62). The detailed respiratory mechanics of this dynamic maneuver have been little studied, but numerous studies using the sniff in normal subjects and patients have found it to be a robust measure. The nose appears to act as a Starling resistor, so that nasal flow is low and largely independent of driving pressure, Pes (63). Pdi measured during a sniff (Pdi,sn,max) reflects diaphragm strength and Pes reflects the integrated pressure of the inspiratory muscles on the lungs (Figure 6).

More recently it has been suggested that pressures measured in the mouth, nasopharynx, or one nostril give a clinically useful approximation to esophageal pressure during sniffs (64, 65). Because these measurements do not require the passage of esophageal or gastric balloons, they are easier for operator and subject. However, pressure transmission may be impaired, particularly when there is significant disease of the lungs (66).

Methodology. For measurement of maximal sniff pressures, patients are encouraged to make maximum efforts. Sniffs can be achieved only when one or both nostrils are un-occluded, to allow the passage of air. An occluded sniff may be called a "gasp," and is more difficult for subjects to perform reproducibly. subjects should be instructed to sit or stand comfortably, and to make sniffs using maximal effort starting

Figure 5. (A) A typical pressure tracing from a subject performing a maximum expiratory maneuver (PEmax). A peak pressure is seen and the 1-second average is determined by calculating the shaded area. (B) A pressure tracing from a subject performing a maximum inspiratory maneuver (PImax).

from relaxed end expiration. Detailed instruction on how to perform the maneuver is not necessary, and may be counterproductive. However, subjects should be exhorted to make maximal efforts, with a rest between sniffs. Most subjects achieve a plateau of pressure values within 5-10 attempts.

Transdiaphragmatic pressure during sniff. Esophageal and gastric balloons are passed by the usual technique. Transdiaphragmatic pressure during sniff (Pdi,sn) is reasonably reproducible within normal subjects, although there is wide variability between subjects (61) (Table 3). The values tend to be as large, or larger, than Pdi during maximum static inspiratory efforts (61).

Esophageal pressure during sniff. The methodology is as for Pdi,sn but with the passage of an esophageal balloon alone.

Nasal sniff pressure. Pressure is measured by wedging a catheter in one nostril. Various techniques for wedging are available including foam, rubber bungs, and dental impression molding. The subject sniffs through the contralateral unobstructed nostril. The pressure in the obstructed nostril reflects the pressure in the nasopharynx, which is a reasonable indication of alveolar pressure. This in turn approximates esoph-ageal pressure, particularly if the lungs are normal with a mean Pnas/Pes ratio of 0.92 (64, 66). In COPD, nasal sniff pressure (Pnas,sn) tends to underestimate esophageal pressure during sniff (Pes,sn) but can complement Pimax in excluding weakness clinically (67).

Mouth and nasopharyngeal pressures can also be measured, and also reflect alveolar pressure, but are less easy for the subject than nasal pressure and have no significant advantages (65) (Figure 7).

The sniff is a dynamic maneuver and so a pressure measurement system is required with a frequency response of > 10 Hz. This can be achieved by a standard balloon catheter system with a suitable pressure transducer. Use of catheter-mounted transducers has also been described (18).

Advantages. The sniff is easily performed by most subjects and patients and requires little practice. It is relatively reproducible and has a smaller range of normal values than mouth pressures (61, 62). It is a useful voluntary test for evaluating diaphragm strength in the clinical setting (55), giving equal or greater pressures than maximal static efforts (61, 68).

It is possible to achieve greater transdiaphragmatic pressures by certain maneuvers, such as the modified Mueller maneuver (see previous passages), in highly trained and well-motivated subjects. This may be important in physiological studies but is not usually clinically relevant.

Sniff nasal pressure is technically simple.

Disadvantages. The pressures measured during a sniff may be less than maximal static values because of shortening of the inspiratory muscles (pressure-velocity relationship) (69). The average volume change during a sniff is approximately 500 ml with some gas rarefaction, which may be somewhat more than the volume by which gas expands during a static maneuver against a closed airway (63).

Sniffs are difficult or impossible if there is upper airway distortion, and particularly if the nose is completely obstructed. It may be difficult to pass the balloons if there is severe bulbar weakness, but this would not preclude measurement of Pnas.

Sniffs are voluntary maneuvers and, therefore, poorly motivated subjects may perform submaximal efforts. These can often, but not always, be detected as variability tends to be greater than for maximal maneuvers.

The sniff generates rapid pressure changes, so measurement requires a catheter system and transducer with a higher frequency response (see previous passages) than for static maneuvers.

Normal Diaphragm weakness

Normal Diaphragm weakness

Figure 6. Esophageal (Poes), gastric (Pg), and transdiaphragmatic (Pdi) pressures measured during maximum voluntary sniffs in a normal subject and in a patient with severe diaphragm weakness. The normal subject reproducibly generates a Pdi of 120 cm H2O (11.8 kPa), whereas the weak patient can generate only 15 cm H2O (1.5 kPa).

Figure 6. Esophageal (Poes), gastric (Pg), and transdiaphragmatic (Pdi) pressures measured during maximum voluntary sniffs in a normal subject and in a patient with severe diaphragm weakness. The normal subject reproducibly generates a Pdi of 120 cm H2O (11.8 kPa), whereas the weak patient can generate only 15 cm H2O (1.5 kPa).

Applications. Maximal sniffs have been widely used and validated as reproducible and reliable tests of diaphragm or global inspiratory muscle function. This test can, therefore, be used in research studies, although care must be taken with re-producibility, as in any voluntary maneuver.

The sniff maneuver is a useful part of the clinical evaluation of respiratory muscle strength and correlates well with response to therapy (Figure 6) (68). The normal values are shown in Table 3 (61). There is a wide normal range, reflecting the wide range of normal muscle strength in different individuals. In clinical practice Pdi,sn,max values greater than 100 cm H2O in males and 80 cm H2O in females are unlikely to be associated with clinically significant diaphragm weakness (55). Values of maximal sniff Pes or Pnas numerically greater than —70 cm H2O (males) or —60 cm H2O (females) are also unlikely to be associated with significant inspiratory muscle weakness. However, these reflect the integrated pressure of all the inspiratory muscles and it is possible that there could be a degree of weakness of one or more of these muscles that would not be detected at this level.

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  • aulis
    What is a normal sniff kpa?
    1 year ago

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