13.1 Respiratory Muscle Strength: MIP, MEP, SNIP, and Cough Peak Flow

13.1 Respiratory Muscle Strength: MIP, MEP, SNIP, and Cough Peak Flow

The Detailed Content Outline names respiratory muscle strength (for example, MIP, MEP, cough peak flow) as procedure item 17 in Domain II and as data item 14 in Domain III. That places it under all six task verbs — select, perform, evaluate validity, calculate, evaluate reliability, and evaluate clinical implications. It is a small test with an outsized role in neuromuscular disease, where it detects weakness months before vital capacity falls below the lower limit of normal.

---

What the Measurements Represent

The starting lung volumes are not arbitrary. Inspiratory muscles are at their most favorable length–tension relationship at residual volume; expiratory muscles are at theirs at total lung capacity. Starting from the wrong volume systematically underestimates strength.

---

Equipment and the Critical Leak

The measuring system is an occluded mouthpiece assembly connected to an aneroid manometer or a pressure transducer capable of recording at least $\pm$ 200 cmH$_2$O, with a rigid, non-collapsible circuit.

The single most testable equipment detail is the deliberate leak: the mouthpiece assembly must contain a small orifice, conventionally about 2 mm in diameter and 20–30 mm long. Its purpose is not comfort. Without it, a patient can:

• generate a large negative mouth pressure by expanding the cheeks and using buccal muscles rather than the diaphragm, and
• generate a large positive pressure by closing the glottis and compressing air in the mouth.

The leak is large enough to dissipate pressure created by the cheeks and mouth, but far too small to relieve pressure generated by the thorax over the 1–2 second measurement window. A flanged or scuba-type mouthpiece is preferred over a tube mouthpiece because it distributes lip pressure and prevents perioral leak in weak patients; a face mask is used when facial weakness prevents a seal.

---

Technique

MIP:

1. Seat the patient upright with a nose clip in place.
2. Coach a full, unforced exhalation to residual volume.
3. Occlude the mouthpiece and instruct a maximal inspiratory effort — "pull in as hard as you can and keep pulling" — sustained for at least 1.5 seconds.
4. Record the maximum pressure sustained over 1 second (the plateau), not the transient spike. A very brief spike is usually a mouth artifact.

MEP:

1. Coach a full inspiration to total lung capacity.
2. Occlude and instruct a maximal expiratory effort — "blow as hard as you can and keep blowing" — sustained for at least 1.5 seconds.
3. Support the cheeks with the hands or an assistant's hands to eliminate buccal contribution.

Number of efforts and repeatability: perform at least three acceptable maneuvers, with 30–60 seconds of rest between efforts. The two largest values should agree within about 10% or 10 cmH$_2$O, and the highest acceptable value is reported. Because both tests are highly effort- and learning-dependent, values often improve across the first several efforts; a rising series means coaching is still working and more efforts are warranted, up to the point of fatigue.

---

Reference Values and Thresholds

Reference equations vary widely between published sets, so laboratories should state which set they use. Commonly cited adult lower limits:

• MIP: more negative than about $-80$ cmH$_2$O in men and $-70$ cmH$_2$O in women.
• MEP: greater than about $+100$ cmH$_2$O in men and $+80$ cmH$_2$O in women.

Interpretive anchors:

• A normal MIP effectively excludes clinically significant inspiratory muscle weakness. Its negative predictive value is its greatest strength.
• A low MIP is far less specific, because submaximal effort, poor comprehension, and mouthpiece leak all lower it.
• MIP less negative than $-30$ cmH$_2$O is associated with hypercapnic respiratory failure risk and nocturnal hypoventilation.
• Reduced MEP predicts an ineffective cough, retained secretions, and a rising RV/TLC ratio, because weak expiratory muscles cannot compress the thorax toward true residual volume.

---

SNIP and the Supine Vital Capacity

Sniff nasal inspiratory pressure (SNIP) measures pressure in an occluded nostril during a short, sharp maximal sniff from FRC. It requires no mouthpiece, so it avoids perioral leak entirely and is often easier for patients with facial or bulbar weakness. It underestimates in nasal obstruction. SNIP and MIP are complementary: if either is normal, significant inspiratory weakness is unlikely.

Supine vital capacity is the cheapest and most specific bedside test of diaphragm function. Lying supine displaces abdominal contents against the diaphragm; a healthy diaphragm compensates, a weak one does not.

• Normal: VC falls less than 10% from upright to supine.
• Fall of 10–20%: suggests diaphragm weakness.
• Fall greater than 25–30%: strongly suggests bilateral diaphragm paralysis.

This is the same postural maneuver the content outline lists as spirometry item 2b (upright/supine), and it is the reason a technologist should never quietly perform a "supine because the patient was tired" spirometry without documenting position.

---

Cough Peak Flow

Cough peak flow (peak cough expiratory flow) is measured with a peak flow meter or pneumotachometer, through a mouthpiece or face mask, after a maximal inspiration to TLC followed by a maximal voluntary cough. Report the best of three efforts.

Cough peak flow integrates three things — an adequate inspiration, adequate expiratory muscle force, and normal glottic closure and release — so a low value in a patient with a preserved MEP should prompt evaluation of bulbar function.

---

Evaluating Validity: Falsely Strong and Falsely Weak

Document the reference set used, the number of efforts, whether a plateau was achieved, the interface (mouthpiece versus mask), and patient position. Without those, a serial comparison across visits means nothing.