6 Respiratory Support in Acute Neuromuscular Respiratory Failure

A 21-year-old woman presented to our emergency department complaining of low back pain for the last two days, tingling in her legs since the previous afternoon, and difficulty urinating since the previous evening. Over the preceding hours she had noticed progressive weakness in her legs. On initial evaluation she had weakness in both legs, which was mild proximally and moderate distally. Her deep tendon reflexes were absent in both legs and decreased in both arms. Sensory examination was unremarkable. She had no signs of oropharyngeal weakness or ventilatory problems. Arterial blood gases revealed neither hypoxia nor hypercapnia. Chest film was normal. She was admitted to our intensive care unit for close monitoring.

Early the following morning we notice she is much weaker. She can hardly activate her iliopsoas muscles and cannot move her legs. Her arms can barely stay up against gravity. She is now completely arreflexic. Restless and tachypneic, she reports difficulty breathing and can only speak a few words at the time because of her breathlessness.

What do you do now?

Intubation is imminent, but what causes the patient’s shortness of breath?

An acute neuromuscular disorder should be suspected in any patient with acute respiratory failure who presents with mixed hypoxia and hypercapnea or predominant hypercapnea with signs of oropharyngeal and appendicular muscle weakness. The most likely causes are Guillain-Barré syndrome or myasthenia gravis. Myopathy or a previously undiagnosed motor neuron disease are less frequent but possible considerations. Patients with neuromuscular respiratory failure have a characteristic presentation. They are dyspneic, tachypneic, and tachycardic. Restlessness is a very common feature, inability to speak in full sentences (staccato speech) and diaphoresis (typically seen as sweat on the forehead) denote their great difficulty to breathe. Patients with oropharyngeal weakness will have a weak cough, nasal voice, and problems handling oral secretions. Recruitment of accessory muscles can be visible on inspection, but it is best noted by palpating the sternocleidomastoid muscles. Yet, the hallmark of neuromuscular respiratory failure is the presence of paradoxical breathing pattern, an inward rather than the normal outward movement of the abdominal wall with each inspiration.

These clinical manifestations are due to failure of the breathing mechanics eventually leading to insufficient ventilation. In a nutshell it goes as follows. Failure of the diaphragm (large component) and intercostal muscles (small component) to lift the ribcage can only be partly compensated by other muscles attached to the ribcage (accessory muscles). The abdominal muscles only assist with coughing and expiration. Poor lung expansion leads to reduced air flow and alveolar collapse. Atelectasis causes hypoxemia and, eventually, hypoventilation results in hypercapnia. Aspiration due to coexisting oropharyngeal weakness may worsen gas exchange even more. The inadequate physiologic compensatory response consists of increasing the respiratory frequency, while the tidal volumes remain small. So, when physicians enter the room they may see a patient visibly struggling to breathe, sitting up in bed and maintaining only marginal pulse oximeter values (oxygen saturations in the low 90s) despite increasing oxygen requirements. Hypercapnia occurs later in acute cases, but may be seen early in patients with exacerbations of chronic neuromuscular disorders.

Bedside spirometry to gauge forced vital capacity, and maximal inspiratory and expiratory pressures, arterial blood gases, and a chest X-ray should complement physical examination in the initial evaluation of these patients. Be sure to coach patients carefully before spirometry testing and check if they can satisfactorily seal the mouth piece of the spirometer with their lips before moving forward with the test. When the results are much poorer than expected—based on the physical exam and the blood gases—poor technique, insufficient mouth sealing, or suboptimal effort are the most frequent explanations.

When after this initial assessment your diagnosis is indeed neuromuscular respiratory failure, the next steps are deciding whether the patient needs mechanical ventilation and what is the most likely cause of the weakness. Both of these priorities are closely related. The urgency of action and the type of mechanical ventilation to be chosen will depend on the neuromuscular disorder being treated. Being able to establish the neuromuscular diagnosis is not only crucial to select optimal treatment, but also carries major prognostic implications. Patients with acute neuromuscular respiratory failure of unclear cause after extensive evaluations—a situation we encounter in more than 10% of all cases admitted to the ICU with acute neuromuscular respiratory failure—rarely recover well despite aggressive respiratory treatment.

Guillain-Barré syndrome (GBS) and myasthenic crisis are the most common causes of acute neuromuscular respiratory failure. Although these two immunological disorders are similar in some aspects, the ideal respiratory management differs substantially between the two. Patients with GBS can get worse very fast and when they do, they often have manifestations of dysautonomia, such as rapid swings in blood pressure and cardiac arrhythmias. Also, once they have developed ventilatory impairment their course toward full-blown respiratory failure is unstoppable. Those patients should be intubated without delay before they reach their nadir because they may develop sudden respiratory arrest and emergency intubation can trigger severe cardiocirculatory complications. Spirometry results can be confidently used to guide the timing of elective intubation in GBS (Table 6.1). Other clinical pointers are shown in Table 6.2.

However the approach in patients worsening from myasthenia gravis can be different. Patients that are approaching a myasthenic crisis may benefit from early noninvasive ventilatory support with bilevel positive airway pressure (BiPAP). Contrary to GBS and other acute or chronic nerve disorders respiratory muscles of patients with myasthenic gravis develop progressive but still reversible fatigability before they fail. When aided by BiPAP, these muscles can sustain adequate ventilation longer, thus allowing time for immunomodulatory therapy to become effective. If started timely, BiPAP can avert intubation and prevent pulmonary complications (atelectasis and pneumonia). However, one should not wait to start BiPAP. Once patients become hypercapneic—an indication that the ventilatory muscles have already failed—noninvasive ventilation is very likely to be unsuccessful. Although bedside spirometry results are often used in myasthenic crisis, they are less reliable indicators of the need of mechanical ventilation than in GBS.

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