Amyotrophic lateral sclerosis (ALS) is a progressive neurological condition that affects motor neurons. Motor neurons are nerve cells that control the movement of voluntary muscles, including respiratory muscles.
Respiratory failure is the most common cause of death in ALS patients. Lung function tests are used to assess the risk of respiratory failure and the need for assisted ventilation.
Lung function tests for ALS
There are several devices and measurement parameters that are used to evaluate respiratory function in ALS patients. They include:
Spirometer
A spirometer measures airflow and determines how much air is exhaled and the rate (speed) at which it is exhaled. The primary measurement parameter dobtained with a spirometer is the vital capacity. After a full inspiration, the volume of either relaxed (slow vital capacity or SVC) or forced full exhalation (forced vital capacity or FVC) is measured. FVC is the most frequently used respiratory function test in ALS patients.
The expiration volume can also be measured over time. The forced expiratory volume 1 (FEV1) measures the amount of air a person can exhale within the first second of forced expiration. A similar measure is the forced expiratory flow (FEF), which measures the exhalation speed during the middle portion of a forced expiration. FEF is also called maximum mid-expiratory flow rate (MMEFR).
A spirometer can also measure the peak cough flow (PCF). After a maximal expiration, the patient coughs as forcefully as possible and the exhalation volume over time is measured.
Respiratory pressure meter
A respiratory pressure meter measures respiratory muscle strength.
The maximum inspiratory pressure (MIP) measures the strength of the muscles that are used during inspiration. During the test, the patient first exhales and then inhales with as much effort as possible.
Contrary to MIP, the maximum expiratory pressure (MEP) measures the strength of the muscles that are used during exhalation; inhaling is followed by forced expiration.
Another method to measure inspiratory muscle strength is the sniff nasal inspiratory pressure (SNIP) measurement. After expiration, the patient performs a maximal sniff while one nostril is blocked, and the nasal pressure in the blocked nostril is measured.
Peak flow meter
A peak flow meter measures the maximum, or peak, expiratory flow (PEF) in liters per minute. Similarly, it can also assess the peak cough flow (PCF).
Predictive value of lung function tests in ALS
A retrospective study analyzed case records from 2008 to 2015 to assess which kinds of lung function tests best predict the appropriate timing for the initiation of non-invasive ventilation (NIV).
ALS patients with an FVC of less than 70 percent, symptoms of hypoventilation at night, signs of increased breathing activity, or daytime hypercapnia (high levels of CO2 in the blood) were referred to a home ventilation service (HVS). The HVS performed the lung function tests. Data from patients who had undergone at least two respiratory function tests were included in the study. Tests included FVC, PCF, SNIP, MIP, and MEP.
For 87 out of 110 patients, the physician at the HVS decided that NIV was necessary. All patients had previously undergone different lung function tests, and all five function tests showed a descending trend before the need for NIV. SNIP measurements showed the greatest decline within the three months before the need for NIV, and the PCF test showed the biggest difference between patients who needed NIV and patients who did not need the intervention yet. The authors of the study concluded that the PCF and SNIP measurements were suitable indicators for timely initiation of NIV.
Additional information
An ongoing randomized controlled clinical trial (NCT03214224) at the Milton S. Hershey Medical Center in Pennsylvania aims to assess whether remote pulmonary function tests (rPFTs) could enable timelier detection of respiratory insufficiency. The tests include FVC and MIP measurements.
Patients are randomized into two groups. The first group receives a standard respiratory assessment for pulmonary function every three months. The second group receives, in addition to the standard assessment, rPFTs in the interim months. Remote respiratory assessment is performed at home with the help of a respiratory therapist over a telemanagement portal.
The study is assessing the effects of rPFT on respiratory outcomes, quality of life, and survival.
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