The static respiratory pressures, Maximal Inspiratory Pressure (MIP or PIMAX) and Maximal Expiratory Pressure (MEP or PEMAX) are a way to non-invasively assess respiratory muscle strength. Respiratory muscle weakness is present in a number of conditions, most notably neuromuscular diseases and disorders, but also malnutrition, cardiovascular disease, polymyositis, sarcoid and COPD. Strictly speaking, the maximal inspiratory and expiratory pressures are not generated solely by the respiratory muscles but also by the elastic recoil. The elastic recoil of the lung at TLC contributes up to 40 cm H2O towards MEP and the elastic recoil of the chest wall at RV contributes up to 30 cm H2O towards MIP. Even so, an individual cannot reach TLC or RV without the use of their respiratory muscles so the measurements are still valid regardless of how the pressures are generated.
I have mixed feeling about MIPs and MEPs but this is mostly because many patients perform these tests poorly, making it hard to interpret results. Normal results can rule out respiratory muscle weakness but reduced results are not necessarily diagnostic. Nevertheless, they are still valuable tests and it is important for them to be performed correctly.
MIP is measured at RV and MEP is measured at TLC. The ATS/ERS statement on respiratory muscle testing indicates that each effort should last at least 1.5 seconds and that at least three measurements within 20% of the highest value should be obtained. A maximum number of attempts has not been specified but most research studies limited this to 5 or 6.
The actual maneuver depends somewhat on the equipment configuration. Respiratory pressures were originally measured using a pressure gauge and most early systems consisted of just a mouthpiece and a gauge (or gauges).
To use this type of system the patient either exhales to RV or inhales to TLC, places their lips around the mouthpiece and then forcefully inhales or exhales. Because of the limited amount of time available for lip placement a round plastic or cardboard mouthpiece is usually used.
Currently, most MIP-MEP mouthpiece manifolds have a side hole that the patient can breathe through prior to performing a MIP or MEP maneuver.
This allows the use of a flanged mouthpiece which usually takes longer to insert and seat properly inside a patient’s mouth. With these types of manifolds the patient is placed on the mouthpiece first and allowed to breath normally until they are instructed to exhale to RV or inhale to TLC. The technician then blocks the hole and the patient then forcefully inhales or exhales. After the measurement the technician unblocks the hole and allows the patient to return to normal breathing.
Regardless of what type of a mouthpiece system is used a small leak must be built into it. This is intended to prevent glottal closure and the use of cheek muscles. The ATS/ERS statement specifies a hole with a 2 mm internal diameter that is 20-30 mm long (the length gives the leak a controlled resistance).
Although a side-hole manifold can be used with a pressure gauge the ATS/ERS standard makes the use of an electronic pressure transducer more or less mandatory. There are a couple reasons for this. First, reading pressure off a gauge during the relatively short time a patient inhaling or exhaling can be difficult. Although there are recording gauges (a secondary needle that is moved by the primary needle and remains at the highest pressure until reset) what is most likely to be reported is the peak pressure and that is not necessarily what is supposed to be recorded. Instead of the peak pressure (which could be a spike lasting only a short time), the ATS/ERS standards state that the maximum pressure should be averaged over a 1 second period and this is not possible with a gauge. Averaging requires an electronic pressure transducer and a computer with software designed to analyze the pressure signal and perform the averaging.
I have been unable to find any study that looked at the difference between peak and averaged MIP and MEP measurements. Since the point of the MIP and MEP tests are to measure respiratory muscle strength averaging is probably the best way to reject transient pressure spikes. The 1 second averaging period is relatively arbitrary and some studies have used different averaging periods, but since the averaging of MIP and MEP waveforms has not been studied in detail its not clear if there is an optimum averaging period.
More importantly though, the ATS/ERS statement only specifies that a 1-second average should be computed, not how it should be computed. This leaves it to individual researchers and manufacturers to decide how to implement an averaging algorithm. Presumably the peak MIP or MEP is used to select the proper segment of data for an averaging algorithm, but its not clear how much data before or after the peak pressure should be averaged. When I review the MIP and MEP results from our test systems it is unclear what part of the pressure waveform is being averaged (or should be averaged for that matter).
It also depends on how an algorithm is written whether a short effort (<1 second; which is admittedly a suboptimal effort but may also be the best the patient can do) will be underestimated or not. This may or may not make a significant difference in the reported MIP or MEP but it should be addressed the next time the ATS/ERS updates their standards.
Interestingly, several studies have shown that patients tend to get higher MIP and MEP pressures with a simple round mouthpiece than with a flanged mouthpiece. The reasons for this are unclear but the ATS/ERS statement nevertheless recommends a flanged mouthpiece which is because “These mouthpieces are also easier for patients to use, especially those with neuromuscular disease.”
So once you’ve gotten good quality MIP and MEP measurements, what’s normal?
The predicted values for both MIP and MEP decrease with age and are larger for males than for females. Interestingly, height and ethnicity are not significant factors in most or all MIP and MEP reference equations. Unlike most other pulmonary function tests weight in one form or another (i.e. BSA or BMI) is a significant factor in most MIP and MEP reference equations. Not surprisingly, increased weight increases the predicted MEP in most instances. Somewhat surprisingly however, increasing weight also usually increases the predicted MIP. The reason for this relationship was not generally discussed except for one researcher who attributed it to an increased diaphragmatic strength.
As with many pulmonary function tests the normal values for MIP and MEP are spread over a relatively large range, and as usual selecting a single set of reference equations is problematic. The studies with the largest populations are limited to older individuals (45-85) and the studies with the widest age ranges were performed on relatively small populations. Because it’s particularly difficult to make a nuanced interpretation of MIP and MEP (basically all you can say is normal or abnormal) in this particular instance I’d recommend looking at the lower limit of normal instead.
The ATS/ERS standard states that a “PIMAX of -80 cm H2O usually excludes clinically important inspiratory muscle weakness”. This statement isn’t referenced to any study however, and is not necessarily supported by research data. Evans et al performed an extensive analysis of the LLN for MIP and MEP and developed several reference equations which seem pretty reasonable.
|MIP LLN:||62 – (0.15 x age)||62 – (0.50 x age)|
|MEP LLN:||117 – (0.83 x age)||95 – (0.57 x age)|
MIP and MEP are useful tests for assessing respiratory muscle strength that can be performed with relatively simple equipment. They are particularly useful for monitoring patients with known neuromuscular diseases and disorders. They should also be performed when patients with otherwise normal pulmonary function tests continue to complain of dyspnea and exercise intolerance.
MIP and MEP are very effort-dependent test and more than one study has indicated that a properly trained technician can make a significant difference in a patient’s test results. Patients need to be encouraged properly and particularly since these tests are frequently ordered for patients with neuromuscular diseases their lips and mouth needs to be checked during the test for a proper seal. For those patients that are unable to maintain a tight seal it may be possible to substitute a mask. Mask and mouthpiece MIPs and MEPs have been studied in normal subjects and although mask pressures were less than those obtained by mouthpiece the difference was not statistically significant.
MIP and MEP testing requires only relatively simple and inexpensive equipment. Many lab testing systems come equipped to perform MIP and MEP testing without the need to order it as an additional option. There is no CPT4 code for MIP and MEP testing however, (we use 94799, unlisted service) and some insurance companies will not pay for testing. I’ve read that some PFT labs don’t perform MIPs and MEPs for this reason but there are several tests we routinely perform (supine spirometry, for example) that we aren’t fully reimbursed for. I think this is just part of taking care of our patients, but the decision to offer or not to offer specific tests is sometimes made by hospital administration, not the lab.
|Male:||MIP Reference Equations:|
|[A]||143 – (0.55 x age)|
|[B]||2.71828183^(4.02 – (0.004 x age) + (0.47 x BSA))|
|[C]||161 – (0.99 x age)|
|[D]||149 – age + (0.1 x wt_lb)|
|[G]||126-(1.028 x age) + (0.343 x wt_kg)|
|[H]||10.2 x (0.158 x BMI) – (0.051 x age) + 8.22)|
|[I]||155.3 – (0.80 x age)|
|[J]||9.8-(0.31 x age)+(1.47 x wt_lb) – (0.0026 x (wt_lb^2)) – (0.0059 x age x wt_lb)|
|[K]||142 – (1.03 x age)|
|Female:||MIP Reference Equations:|
|[A]||104 – (0.51 x age)|
|[B]||2.71828183^(3.76 – (0.004 x age) + (0.47 x BSA))|
|[D]||118 – (0.9 x age) + (0.1 x wt_lb)|
|[G]||171 – (0694 x age) + (0.861 x wt_kg) – (0.743 x ht_cm)|
|[H]||10.2 x (8.55 – (0.24 x age))|
|[I]||110.4 – (0.49 x age)|
|[J]||-388+(1.77 x age)-(0.014 x (age^2))+(wt_lb x 0.41)-(0.0041 x age x wt_lb)+(4.69 x ht_cm)-(0.014 x (ht_cm^2))|
|[K]||(0.71 x ht_cm) – 43|
|Male:||MEP Reference Equations:|
|[A]||268 – (1.03 x age)|
|[B]||2.71828183^(4.48 – (0.0004 x age) + (0.25 x BSA))|
|[C]||215 – (0.43 x age)|
|[D]||278 – (2.27 x age) + (0.28 x wt_lb)|
|[I]||165.3 – (0.81 x age)|
|[K]||180 – (0.91 x age)|
|Female:||MEP Reference Equations:|
|[A]||170 – (0.53 x age)|
|[B]||2.71828183^(4.3 – (0.0004 x age) -(0.003 x age) + (0.25 x BSA))|
|[D]||179 – (1.68 x age) + (0.36 x wt_lb)|
|[I]||115.6 – (0.61 x age)|
|[K]||3.5 + (0.55 x ht_cm)|
MIP/MEP Study Populations:
|Source:||#male||age range||#female||Age Range:||Ethnicity:|
|[E] MEP||329||65-85||427||65-85||96% Caucasian, 4% Black|
|[E] MIP||2259||65-85||2942||65-85||96% Caucasian, 4% Black|
|[F]||n/a||n/a||Synthesized from other studies|
|[J]||1886||45-84||1963||45-84||White, Black, Asian, Hispanic|
MIP/MEP Study Methods:
|[E] MEP||1994||Gauge||Rubber, flanged||0.5 sec averaged|
|[E] MIP||1994||Gauge||Plastic, round||0.5 sec averaged|
|[G]||1998||Transducer||Cardboard, round||2 sec Plateau|
|[H]||2000||Transducer||Rubber, round||2 sec Plateau|
|[I]||1999||Guage||Plastic, flanged||1 sec|
|[J]||2009||Gauge||Cardboard, round||Visually averaged, rounded to nearest 5 cmH2O|
|[K]||1984||Gauge||Plastic, flanged||1 sec, Peak|
ATS/ERS Statement on respiratory muscle testing. Am J Respir Crit Care Med 2002; 166: 518-624.
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