SNIP when you can’t MIP

The MIP test is used to assess the strength of the inspiratory muscles and is commonly performed in patients with neuromuscular disease. Results however, are often low because the sensation involved in performing a maximal inspiratory effort against an occluded airway is unpleasant and because the MIP requires coordination, cooperation and motivation to be performed correctly. In addition patients with neuromuscular disease frequently lack the muscular strength necessary to grip the mouthpiece and can therefore leak around it. For these reasons although a normal value will rule out significant muscular weakness, a low value can be difficult to interpret.

Sniff Nasal Inspiratory Pressure (SNIP) is an alternative way to measure inspiratory muscle strength that does not require a mouthpiece and uses a fairly natural maneuver. To perform the test one nostril is blocked with a soft probe attached to either a manometer or a transducer. The patient is asked to perform a normal exhalation to FRC and then inhale forcibly (sniff) with their mouth closed. The sniff effort is short since the maximum nasal inspiratory pressure is reached in a half a second or less.

Although soft nasal probes for SNIP measurements are now available from a couple of suppliers, probes can be made from re-purposed equipment such as an eartip intended for auditory evoked potentials or the nasal cushions from sleep apnea equipment. Prior to testing air leakage around the nasal probe should be checked by blocking the contralateral nostril while the patient attempts to breathe in. If there is a leak then either a different sized probe should should be used or the probe size adjusted with something like earplug wax.

There is a learning effect involved in performing the SNIP test. The number of SNIP trials needed to obtain the maximum SNIP value from a given patient has varied, with some investigators using only five while others have shown that up to twenty should be performed. There is a general consensus among most investigators that at least 10 SNIP efforts should be made and that if the maximum SNIP is obtained on the tenth effort then further trials should be made. SNIP efforts should be separated by at least 30 seconds.

In adults SNIP has been shown to be primarily related to gender and age. Height, weight and BMI are not factors and there is no significant difference in SNIP values between supine and upright positions. Normal values for adults are:

Males = 126.8 – (0.42 * age), LLN ~80 cm H2O

Females = 94.9 – (0.22 * age), LLN ~70 cm H2O

For children the results are more complex. SNIP was shown to correlate with height, age and weight with boy but not with girls. The reasons for this are not clear and because of this it may be better to compare results to the overall means and LLNs for children.

Boys (6-12 yrs): mean 99, LLN 63 cm H2O

Boys (13-17 yrs): mean 117, LLN 66 cm H2O

Girls (6-12 yrs): mean 92, LLN 56 cm H2O

Girls (13-16 yrs): mean 97, LLN 54 cm H2O

Investigators have noted that a SNIP below 40 cm H2O is significantly related to nocturnal hypoxemia and that a patient with a SNIP less than 30% of predicted is at risk for hypercapnia.

SNIP has been shown to reflect the sniff esophageal pressure (Pes) in most patients and SNIP is usually higher than MIP. Patients with COPD, however usually show a reduced SNIP and MIP when compared to Pes and this has been attributed to dampening of the pressure waves due to the increased airway time constants in COPD and the shortness of the sniff effort. Somewhat surprisingly this effect seems to be independent of the degree of airway obstruction.

Despite the fact that the SNIP test usually has somewhat higher and more reproducible results than the MIP, some patients are still better able to perform a MIP than a SNIP. Although sniff Pes is considered by some to be the gold standard for assessing respiratory muscle strength it was found to be no more sensitive or accurate than when both MIP and SNIP tests were performed in the same patient. For this reason an initial assessment of respiratory muscle weakness should probably include both SNIP and MIP.

Spirometry continues to be an important means of assessing the status and course of neuromuscular disease. Vital capacity however, can remain more or less normal until there is profound respiratory muscle weakness. SNIP has been found to be a better predictor of mortality in ALS than spirometry or MIP, but this may be associated at least in part with the fact that the SNIP can be performed in patients with advanced stages of the disease while MIP and FVC often cannot. Having said that investigators have reported that ALS patients with a SNIP less than 40 cm H2O was associated with a median survival rate of six months and a SNIP less than 30 cm H2O was associated with a median survival rate of three months.

SNIP nasal probes and instruments with software designed to measure both SNIP and MIP are available from at least a couple of manufacturers but there is no reason that (with the exception of manometers without recording needles) many instruments designed to measure MIP and MEP shouldn’t be able to be re-purposed towards measuring SNIP. Given the limitations of reporting software however, results may need to be clearly labeled or otherwise identified in testing notes.

The SNIP test produces information that is as relevant to the assessment of respiratory muscle strength as the MIP, MEP, FVC and Pes tests. Comparatively however, the SNIP is much easier to perform and much better tolerated by most patients. In addition patients that are unable to perform a MIP or FVC can usually perform a SNIP. This makes the SNIP far more suitable for children and for adults with progressive neuromuscular disease than the other tests. If you’re already MIP-ping, why aren’t you also SNIP-ping?

References:

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