How do I measure maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP)?

How to Measure Maximal Inspiratory Pressure (MIP) and Maximal Expiratory Pressure (MEP)

Use a flanged mouthpiece connected to a pressure transducer with a small leak (2-mm internal diameter, 20-30 mm length), have the seated patient perform maximal inspiratory effort (Mueller maneuver) at residual volume for MIP and maximal expiratory effort (Valsalva maneuver) at total lung capacity for MEP, sustaining each effort for at least 1.5 seconds, and record the maximum value from three maneuvers that vary by less than 20%. 1

Equipment Setup

• Mouthpiece: Use a flanged mouthpiece attached to a short, rigid tube with a three-way tap or valve system that allows normal breathing followed by maximal maneuvers 1

• Small leak requirement: The system must incorporate a leak approximately 2-mm internal diameter and 20-30 mm in length to prevent glottic closure during MIP measurement and reduce buccal muscle use during MEP measurement 1

• Pressure measurement device: Use a digital pressure transducer connected to a recording system rather than aneroid manometers (difficult to read accurately) or mercury manometers (safety concerns) 1

• Calibration: Regularly calibrate pressure transducers against a fluid manometer with baseline pressure equal to atmospheric pressure 1

Patient Positioning and Preparation

• Position: Seat the patient; noseclips are not required 1

• Instruction: Provide careful instruction and strong encouragement, as this is an unfamiliar maneuver requiring maximal effort 1

• Coaching points: Coach patients to prevent air leaks around the mouthpiece and to support their cheeks during expiratory efforts by pinching their lips around the mouthpiece 1

Measurement Technique

For MIP (Maximal Inspiratory Pressure):

• Perform the Mueller maneuver at or near residual volume (RV) 1
• Have the patient exhale completely, then make a maximal inspiratory effort against the occluded airway 1
• The small leak prevents glottic closure during this maneuver 1

For MEP (Maximal Expiratory Pressure):

• Perform the Valsalva maneuver at or near total lung capacity (TLC) 1
• Have the patient inhale completely, then make a maximal expiratory effort against the occluded airway 1
• The small leak reduces buccal muscle contribution 1

Pressure Sustaining and Recording:

• Each pressure must be maintained for at least 1.5 seconds 1
• Record the maximum pressure sustained for 1 second rather than peak pressure, as sustained pressure is more reproducible 1
• Perform at least three maneuvers and record the maximum value when three attempts vary by less than 20% 1

Reference Values

Normal values vary by sex and mouthpiece type (flanged vs. tube) 1:

For flanged mouthpiece (preferred method):

• Males: MIP approximately 10.3-13.7 kPa (105-140 cmH₂O); MEP approximately 13.7-15.1 kPa (140-154 cmH₂O) 1
• Females: MIP approximately 6.9-9.1 kPa (70-93 cmH₂O); MEP approximately 8.7-9.2 kPa (89-94 cmH₂O) 1

More recent regression equations for adults up to age 70 years provide age-adjusted values 2:

• Male MIP = 120 - (0.41 × age) cmH₂O; Female MIP = 108 - (0.61 × age) cmH₂O 2
• Male MEP = 174 - (0.83 × age) cmH₂O; Female MEP = 131 - (0.86 × age) cmH₂O 2

Common Pitfalls and How to Avoid Them

• Inadequate seal: Patients must pinch lips tightly around the mouthpiece to prevent pressure losses, particularly at high pressures 1

• Submaximal effort: Even low variability between attempts does not guarantee maximal effort was achieved; strong encouragement from an experienced operator is essential 1

• Incorrect lung volume: MIP must be performed at RV and MEP at TLC, as lung volume significantly affects measured pressures 1

• Glottic closure: Without the small leak, patients may close their glottis during MIP measurement, yielding falsely low values 1

• Buccal muscle use: The small leak also prevents patients from generating pressure primarily with cheek muscles during MEP 1

Alternative Methods

For patients unable to maintain a proper seal around the mouthpiece, balloon catheter systems can be passed into the esophagus or stomach to measure pressures directly 1. A unidirectional expiratory valve method using a face mask has been validated and produces values approximately 14% higher than standard method 3.

Hand-held pressure meters are now available for bedside use, making the technique easily portable 1. Digital manometers (both clinical and validated non-clinical devices) show excellent agreement and are preferred over analog devices 4.