CPX, MVV & Respiratory Muscle Strength

Objectives - Understand the indications, contraindications, and results of a CPX; perform an MVV test; and compare and contrast MIP and MEP.

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Cardiopulmonary Stress Testing

Definition

Cardiopulmonary stress testing is also called:

• CPX
• CPEX
• CPET

It is designed to assess or measure:

• Ventilation
• Gas exchange
• Cardiovascular function during exercise

Indications

CPX may be used for:

• Evaluation of exercise intolerance
• Unexplained dyspnea
• Evaluation of patients with cardiovascular disease
• Evaluation of patients with respiratory disease
• Specific clinical applications

Contraindications

Contraindications include:

• Limiting neurologic or neuromuscular disorders
• Limiting orthopedic disorders
• PaO2 less than 40 mmHg on room air
• PaCO2 greater than 70 mmHg

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CPX Equipment & Preparation

Equipment

Exercise equipment may include a treadmill or cycle ergometer. There is no significant clinical difference between them; selection is dictated by the patient's condition.

Additional equipment includes:

• PFT machine
• Gas analyzers
• Douglas bag collection, mixing chamber, or breath-to-breath analysis
• 12-lead EKG
• Blood pressure cuff
• Pulse oximetry
• Arterial line, optional

Gas analyzers measure exhaled:

• O2
• CO2
• N2

Pre-Procedure

Before CPX testing, obtain or perform:

• PFTs, including FVC and MVV
• DLCO if indicated
• Arterial blood gas if hypoxemia is suspected
• Chest X-ray
• 12-lead EKG
• Cardiology consult if the patient has a history of CAD
• Signed consent

Patient preparation includes:

• Wear comfortable clothing and tennis shoes
• No cigarette smoking for 8 hours before testing
• No exercise the day of the test
• Continue medications as ordered

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CPX Procedure

Exercise Protocols

Indications for Termination

Terminate the test for:

• Chest pain
• Signs of respiratory failure, including cyanosis, lightheadedness, or mental confusion
• Abnormal ECG reading
• Muscle cramping
• Hypertension greater than 250 mmHg systolic and greater than 120 mmHg diastolic

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CPX Values & Interpretation

Measurements Obtained

CPX may measure:

• Ventilation
• Tidal volume, minute ventilation, and frequency
• Oxygenation by pulse oximeter
• ABG, if detailed analysis of pulmonary limitations to exercise is needed
• VO2, or oxygen consumption
• VCO2, or carbon dioxide production
• ECG
• Blood pressure

VO2 is the volume of oxygen used by the tissues each minute.

Maximal vs Sub-Maximal Studies

The practitioner first assesses the degree of effort and determines whether the study is maximal or sub-maximal.

A maximal study is identified when one criterion is met:

• Heart rate greater than 85-90% of predicted
• SaO2 less than 80%

A sub-maximal study occurs when the patient is physically unable to reach maximal effort. A criterion that can be met despite the patient's impairments is set.

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Maximum Voluntary Ventilation

Definition

Maximum voluntary ventilation, or MVV, evaluates the maximum volume a patient can breathe over a specified period of time, usually 12 seconds.

MVV is measured in L/min BTPS using the same equipment as spirometry.

Indications

MVV may be used:

• In conditions where ventilatory capacity may be impaired by mechanisms different from those affecting FEV1
• For evaluation of some occupationally related disorders
• To set a ventilatory limit during exercise

Contraindications & Hazards

There are no contraindications specific to the MVV maneuver. The same contraindications to spirometry apply.

Hazards include:

• Syncope or dizziness, commonly due to hyperventilation
• Paroxysmal coughing
• Nosocomial infections
• Bronchospasm
• Desaturation due to interruption of O2

Procedure

Coach the patient to breathe as deeply and rapidly as possible for 12-15 seconds.

During testing:

• Enthusiastically coach the patient throughout the test
• Volume should be greater than tidal volume but less than vital capacity
• Rate should be between 90-110 breaths/min
• Reproducible results should be within 20%

Values & Interpretation

MVV values vary as much as 30% from the mean, so only large reductions are significant.

MVV is a test of the entire respiratory system and is influenced by:

• Status of the respiratory muscles
• Compliance of the lung-thorax system
• Resistance offered by the airways and tissues
• Condition of the ventilatory control mechanism

MVV is decreased with moderate to severe obstructive disease and may be normal with restrictive disease.

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Lung Mechanics: MIP and MEP

Definition

MIP and MEP are tests of respiratory muscle strength.

They may also be called:

• Negative inspiratory force, or NIF
• PImax
• PEmax

MIP reflects the strength of the diaphragm and other inspiratory muscles.

MEP reflects the strength of the abdominal and other expiratory muscles.

Indications

MIP and MEP testing may be used when:

• Respiratory muscle weakness is suspected, such as a patient with dyspnea, weak cough, and known neuromuscular disease
• Vital capacity is low for an unknown reason and respiratory muscle weakness is in the differential diagnosis
• Known respiratory muscle weakness needs to be followed for improvement, stability, or worsening

Contraindications & Equipment

There are no contraindications listed for MIP/MEP.

Equipment may include:

• Manual pressure gauge
• Pulmonary function machine capable of measuring MIP and MEP
• Mouthpiece and nose clips

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MIP and MEP Procedures

MIP Procedure

For maximal inspiratory pressure:

1. Instruct the patient to seal lips firmly around the mouthpiece.
2. Have the patient exhale slowly and completely.
3. Coach the patient to "pull in hard."
4. Maintain inspiratory effort for 1-3 seconds.
5. Record the largest negative pressure sustained for at least 1 second, not a transient spike.
6. Allow a brief rest and repeat the maneuver three times.

Report the maximum value of three maneuvers that varied by less than 20%.

MEP Procedure

For maximal expiratory pressure:

1. Instruct the patient to inhale completely.
2. Coach the patient to "push" or "blow" as hard as possible.
3. Maintain expiratory pressure for 1-3 seconds.
4. Record the largest positive pressure sustained for at least 1 second, not a transient spike.
5. Allow a brief rest and repeat the maneuver three times.

Report the maximum value of three maneuvers that varied by less than 20%.

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MIP and MEP Values

MIP and MEP are rarely used alone. They are considered with other measures, such as VC and FVC.

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MIP and MEP Interpretation

Reduced MIP and/or MEP suggests respiratory muscle weakness, which can have many causes.

Neuromuscular Diseases

Neuromuscular diseases include:

• Amyotrophic lateral sclerosis, or ALS
• Myasthenia gravis
• Polymyositis
• Guillain-Barre syndrome

Systemic Conditions

Systemic conditions can affect skeletal muscle strength, including:

• Heart failure
• Malnutrition
• Idiopathic myopathy

COPD can reduce MIP and MEP unrelated to respiratory muscle weakness because of lung hyperinflation.

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Review

High-yield points:

• CPX assesses ventilation, gas exchange, and cardiovascular function during exercise.
• CPX can evaluate exercise intolerance, unexplained dyspnea, cardiovascular disease, and respiratory disease.
• CPX termination criteria include chest pain, respiratory failure signs, abnormal ECG, muscle cramping, and severe hypertension.
• MVV evaluates the maximum volume a patient can breathe over a short period and may help set a ventilatory limit during exercise.
• MIP reflects inspiratory muscle strength; MEP reflects expiratory muscle strength.
• MIP/MEP results should be interpreted with other measures such as VC and FVC.