Respiratory Muscle Assessment

BY
Riham Hazem Raafat
Lecturer Of Pulmonary Medicine, Ain Shams University

OBJECTIVES:
When should respiratory muscle function be assessed?
Clinical signs of respiratory muscle weakness
Pathological conditions in which respiratory muscle weakness can
be suspected
Principles of assessment of respiratory muscles
Respiratory muscle endurance
Assessment Of Respiratory Muscle Function In The ICU

INTRODUCTION
 Respiratory muscles generate the pressure differences driving ventilation
 In advanced stages, respiratory muscle weakness leads to respiratory
pump failure.
 Inspiratory muscle weakness may partially explain dyspnea and exercise
intolerance. In addition, reduced respiratory muscle force has been shown
to be an important predictive factor for poor survival in chronic lung
diseases
 In advanced stages the functional consequence of respiratory muscle
weakness may require mechanical ventilation.
 Expiratory muscle weakness leads to problems with speech, and mucus
retention due to impaired cough efficacy.

Muscles Of Respiration:
1. Diaphragm
2. Accessory Muscles:
I. Intercostals (Internal and External)
II. Scalenes
III. Sternomastoid
IV. Pectoralis Major
I. Abdominal muscles:
II. Rectus Abdominis
III. Transverse Abdominis
IV. External And Internal Obliques

Clinical indications for Respiratory Muscles Strength
Assessment:
1. Assessment of neuromuscular disorders (e.g : ALS, MG, MS)
2. Evaluations of patients with impaired cough and retained
secretions.
3. Evaluation of reduced muscle strength (as in emphysema and
chest wall deformities)
4. Unexplained dyspnea .
5. Respiratory muscle weakness may compound other diseases:
malnutrition, steroid, drugs, thyroid disorders, heart failure etc.

Clinical signs of respiratory muscle weakness (Clinical
Assessment):
1. Dyspnoea during normal activities
2. Orthopnoea
3. Unexplained reduction in vital capacity
4. Short sentences during speech
5. Paradoxical movement of the abdominal or thoracic wall
6. Problems with cough (and recurrent infections)
7. Generalized muscle weakness
8. CO2 retention while awake or during sleep, specifically in the
absence of severe airflow obstruction

Principles of assessment of respiratory muscles
In clinical practice respiratory muscle force is indirectly measured through
the pressure generated during inspiration or expiration
Respiratory muscle force is generally expressed as kilopascal (kPa) or cm
water pressure
The pressure is generated by all the muscles under investigation
(inspiratory or expiratory), and is hence not muscle specific nor cause
specific.

Respiratory Muscle Testing
1. Tests of Overall Respiratory Function
2. Tests of Respiratory Muscle Strength
3. Tests of Respiratory Muscle Endurance
4. Imaging Respiratory Muscle Function
5. Assessment of Respiratory Muscle Function in the
Intensive Care Unit

I. Tests Of Overall Respiratory Function

1- Spirometry & Lung Volumes:
 The most frequently noted abnormality of lung
volumes in patients with respiratory muscle weakness
is a reduction in vital capacity (SVC & FVC).
 Also TLC, RV and FRC might decrease
 The VC is limited by weakness of both the inspiratory
muscles, and expiratory muscles, inhibiting full
inflation and expiration.
 In most normal subjects, VC in the supine position is
5–10% less than when upright and a fall of 30% or
more is generally associated with severe
diaphragmatic weakness.

2- Resistance:
• Airway resistance is normal
in uncomplicated respiratory
muscle weakness .

3- Maximum Voluntary Ventilation
• The maximum voluntary ventilation was formerly
recommended as a more specific test for muscle weakness
than volume measurements but, in practice, the proportionate
reduction is usually similar to that of VC .

4- Arterial Blood Gases
Arterial blood gases assess the major functional consequence
of respiratory muscle weakness.
In chronic muscle weakness Pao2 and the alveolar–arterial
O2 difference are usually normal.
In acute muscle weakness, Pao2 may be more markedly
reduced, but the picture may be complicated by atelectasis or
respiratory infection .

With mild weakness, Paco2 is usually less than normal ,
implying alveolar hyperventilation.
In the absence of primary pulmonary disease, daytime
hypercapnia is unlikely unless respiratory muscle strength is
reduced to 40% of predicted and VC is reduced to 50% of
predicted.
In patients with Duchenne muscular dystrophy, hypercapnia has
been shown to predict shorter survival .

II- Tests Of Respiratory Muscle Strength

Volitional tests of respiratory include:
a- Mouth Pressures (Maximal Static Inspiratory and Expiratory
Pressure).
b- Sniff Test
c- SNIP Test (Sniff pressure at nose measured )
d- Cough Tests

a- Mouth Pressures (Maximal Static Inspiratory and
Expiratory Pressure).
 Measurement of the maximum static inspiratory
pressure that a subject can generate at the mouth
(PI max) or the maximum static expiratory
pressure (PE max) is a simple way to gauge
inspiratory and expiratory muscle strength.
 MIP = 142 - (1.03 x Age) cmH2O
 PImax of -80 cm H2O usually excludes clinically
important inspiratory muscle weakness.
 A normal PEmax with a low PImax suggests
isolated diaphragmatic weakness.

b- Sniff Test
A sniff is a short, sharp voluntary inspiratory maneuver
performed through one or both unoccluded nostrils.
It involves contraction of the diaphragm and other
inspiratory muscles.
• Esophageal (Pes), gastric (Pg), and transdiaphragmatic (Pdi)
pressures measured during maximum voluntary sniffs in a
normal subject and in a patient with severe diaphragm
weakness.
 Pdi = Pga - Pes
• The normal subject reproducibly generates a Pdi of 100 cm
H2O, whereas the weak patient can generate only 15 cmH2O

C- Sniff nasal inspiratory pressure (SNIP)
Simple bedside test
Normal valve established
(men > 70 cm H2O. women
> 60 cm H2O)

D- Cough Tests
The main expiratory muscles and the abdominal muscles are
used in cough.
• PE max and cough esophageal pressures comparable
• The mean maximal Pga, co was 230 cm H2O for men and 166
cm H2O for women.

III- Electro-physiologic Techniques for the
Assessment of Respiratory Muscle Function

The EMG can be recorded
with surface electrodes (for
diaphragm, intercostal, scalene,
abdominal, and accessory
muscles) or an esophageal
electrode (for the crural
diaphragm).

VI- Imaging Respiratory Muscle Function

Imaging Respiratory Muscle Function
1. Transmission Radiography
2. Ultrasound
3. Volumetric Imaging
4. Nuclear Medicine

3- VOLUMETRIC IMAGING
Volumetric computed tomography
scans and magnetic resonance
imaging can determine the
configuration of the thoracic cavity.
Both methods can be used to
determine the detailed shape of the
diaphragm , the rib cage, and ribs in
normal and emphysematous
subjects.
Volumetric imaging has limited
research application.

4- NUCLEAR MEDICINE
Both single-photon emission computed tomography (SPECT) and
positron emission tomography (PET) scans provide volumetric
information, which can be used to identify the surface of
ventilated or perfused lungs.
Because the outer surface of the lung is apposed to the rib cage
and diaphragm, the configuration of the diaphragm and rib cage
can also be determined by these methods.
Both techniques offer poor spatial and temporal resolution and
there are no published data in which they have been used to
analyze respiratory muscle function.

V- Assessment Of Respiratory Muscle Function In The
ICU

Assessment Of Respiratory Muscle Function In The ICU
1. Breathing Pattern
2. Lung Volumes
3. Pressure Measurements
4. Prediction of Weaning

1.Breathing Pattern
Abnormalities of the pattern of breathing are common in ICU patients,
especially in those with respiratory muscle dysfunction.
Tachypnea is a sensitive marker of deteriorating clinical status, but is
not specific.
Paradoxical motion of the rib cage and abdomen occurs with elevated
respiratory load, but is not diagnostic of respiratory muscle fatigue.
2.Lung Volumes
Few studies have been conducted in critically ill patients examining the
usefulness of lung volume measurements.

3. Pressure Measurements
1. Measurements of maximum inspiratory pressure have poor
reproducibility in critically ill patients and are of limited use for decision
making in the ICU.
2. Airway occlusion pressure (P0.1) is easy to measure in patients
receiving assisted ventilation, and high values signal increased respiratory
motor output.

Respiratory Muscle Assessment

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