When is mask ventilation necessary and appropriate?

Mask ventilation may be necessary if ALS has led to respiratory failure. There are essentially two reasons for initiating mask ventilation:

The onset of symptoms (particularly labored breathing) or evidence of respiratory failure based on specific measurements (vital capacity). In the majority of patients, both reasons for mask ventilation are present (symptoms and abnormal test results).

However, ventilator support may also be necessary if symptoms are present even though the measured values are still within the normal range (e.g., when breathing is labored during physical exertion, even though vital capacity is at the lower limit of the normal range and blood gas analysis shows no abnormalities). Conversely, ventilatory therapy may be medically appropriate if vital capacity measurements indicate reduced respiratory function, even in the absence of symptoms (no subjective sensation of respiratory distress).

Common symptoms that may indicate respiratory failure include shortness of breath during physical exertion (exertional dyspnea) or when lying down (orthopnea), as well as shortness of breath at rest (resting dyspnea). Other symptoms of respiratory dysfunction may include daytime sleepiness, fatigue, loss of appetite, and low mood. However, these symptoms can also occur independently of respiratory dysfunction. Doctors familiar with the symptoms of ALS are able to interpret these symptoms in the context of respiratory failure.

When evaluating symptoms, the findings of the neurological examination (particularly regarding trunk instability and weakness of the respiratory muscles) as well as various measurement values must be taken into account. Measurement parameters for assessing respiratory function include vital capacity, cough volume, and blood gas analysis (BGA), as well as nocturnal monitoring of oxygen and carbon dioxide levels in the blood (capnometry). BGA and capnometry are usually performed in specialized ventilatory care centers and during inpatient diagnostic evaluation. For the initial assessment of the need for ventilatory therapy, vital capacity is of crucial importance (in addition to the symptoms).

A decrease in vital capacity to below 70% is an established criterion for initiating mask ventilation. In blood gas analysis, a carbon dioxide concentration of 45 mmHg is a criterion for hypoventilation. In nocturnal capnometry (continuous measurement of carbon dioxide concentration through the skin), the corresponding threshold is 50 mmHg. Overall, the decision to initiate mask ventilation depends on various factors that must be weighed against one another and discussed in a doctor-patient dialogue.