Neuromuscular Respiratory Failure

Questions

• 1.

  What are the vital signs?

• 2.

  Is the patient in respiratory distress or complaining of shortness of breath?

  Patients with acute neuromuscular respiratory failure might not appear in obvious respiratory distress caused by respiratory muscle weakness. Even a subjective complaint of shortness of breath should be evaluated diligently and followed up with bedside pulmonary function testing (PFT). Rapid, shallow breathing and paradoxical breathing could be danger signs of impending ventilatory failure. Patients with acute weakness who are in obvious respiratory distress should be intubated immediately.

• 3.

  Over what time period has the weakness developed? Any preceding illnesses, vaccinations, or travel history?

  Fluctuating weakness with diurnal variation that has been present for weeks or months is characteristic of myasthenia gravis. Progressive ascending paralysis over hours to days is suggestive of GBS. A history of diarrhea 1 to 2 weeks prior to the onset of weakness has been classically described for GBS.

• 4.

  Has the patient had difficulty swallowing or change in voice?

  Dysphagia (coughing or choking after swallowing), dysarthria, and drooling are symptoms of bulbar muscle weakness. Affected patients are at risk for aspiration and should undergo formal swallow assessments ( Table 16.1 ).

  Table 16.1

  Respiratory Muscle Groups and Clinical Symptoms and Signs in Patients With Acute Neuromuscular Dysfunction

  Muscle Group	Clinical Symptoms and Signs
  Inspiratory	Dyspnea, tachypnea, hypoxia
  Inspiratory muscles	Paradoxical breathing, neck flexion/neck extension weakness
  Bulbar	Dysphagia, dysarthria, weak mastication, facial weakness, nasal speech, lingual weakness
  Expiratory muscles	Ineffective cough
  All	Hypoventilation, hypoxemia, hypercarbia
  Accessory muscles	Unlike patients with other causes of respiratory failure, accessory muscles will be weak so flaring of nostrils, retractions of chest wall, etc. may not be that obvious

Orders

• 1.

  Administer oxygen.

  Oxygen can be administered via nasal cannula, face mask, high-flow nasal cannula, 100% nonrebreather mask, or noninvasive positive pressure ventilation (NIPPV). Reassess oxygen requirements on arrival at the bedside. Patients with suspected or diagnosed myasthenia gravis could benefit from NIPPV such as bilevel positive airway pressure (BiPAP), but for patients with GBS, NIPPV is not recommended because they are likely to worsen and require intubation for a few days or weeks prior to any improvement in neuromuscular respiratory failure.

• 2.

  Perform a focused clinical exam. Look for any signs of respiratory distress, hypotension, any arrhythmias.

  On your systemic exam look for any signs of infection, auscultate the lungs for any signs of aspiration pneumonia/pneumonitis and skin for any rashes. On your neurologic exam, focus on eliciting muscle strength, cranial nerve deficits, and any sensory signs. Check a single breath count (ask the patient to take a deep breath and count as far as possible starting from one in a single breath), and check neck flexion and extension strength. The strength of the neck flexors and extensors that are innervated by nerve roots C3-C5 are a good surrogate for diaphragmatic strength.

• 3.

  Perform bedside pulmonary function tests.

  • Vital capacity (VC) is the maximal exhaled volume after full inspiration and is normally 60 mL/kg (approximately 4 L in a 70-kg person). Patients generally require intubation when VC falls below 40 mL/kg.

  • Peak inspiratory pressure or negative inspiratory force (NIF) (normally greater than −50 cm H ₂ O) measures the force of inhalation generated by contraction of the diaphragm and is an index of the ability to maintain lung expansion and avoid atelectasis.

  • Peak expiratory pressure (PEF) (normally greater than 60 cm H ₂ O) correlates with the strength of cough and the ability to clear secretions from the airway.

    A good guide for determining which patients are at risk of impending respiratory failure is NIF < −20 cm H ₂ O, PEF < 30 cm H ₂ O, and/or VC < 40 cc/kg. However, for patients with facial diplegia, these bedside PFTs may not be accurate because of the lack of a good seal. Trends in these numbers are more helpful than isolated values alone ( Box 16.1 ).

    Box 16.1

    MEF, Maximum expiratory flow; NIF, negative inspiratory force.

    Signs of Impending Respiratory Failure

    Clinical

    • 1.

      Single Breath count < 13 (out of 20)

    • 2.

      Neck flexion/extension weakness

    • 3.

      Use of accessory muscles of respiration

    • 4.

      Hypoxemia (late sign)

    • 5.

      Acute hypercarbia (late sign)

    Pulmonary Function Testing

    • 1.

      NIF < −20 cm H ₂ O

    • 2.

      MEF < 30 cm H ₂ O

    • 3.

      Forced vital capacity < 40 cc/kg

• 4.

  Measure arterial blood gas levels on room air.

  Patients with neuromuscular respiratory failure will develop hypercarbia and hypoxia only late in the course of their respiratory failure. Worsening trends in PFTs, single breath count, neck flexion/extension, and subjective dyspnea will help identify patients with impending respiratory failure before the development of hypercarbia or hypoxia. Hypercarbia (P co ₂ greater than 45 mm Hg) results from alveolar hypoventilation. Hypoxia (P o ₂ less than 60 mm Hg) is indicative of impaired ventilation-perfusion matching and in this setting is usually related to atelectasis or pneumonia.

• 5.

  Obtain a chest radiograph and, if available, assess lungs with point-of-care ultrasonography (POCUS) if possible.

• 6.

  The patient should eat nothing by mouth (NPO).

• 7.

  Insert two peripheral intravenous (IV) lines.

  IV access should be established in anticipation of acute deterioration.

Quick-Look Test

Does the patient look well (comfortable), sick (uncomfortable), or critical (about to die)?

Agitation, diaphoresis, difficulty finishing sentences, accessory respiratory muscle contraction, and rapid or labored breathing may signal impending respiratory failure.

Airway and Vital Signs

Is the upper airway clear?

Weakness of the tongue and oropharyngeal muscles can lead to upper airway obstruction, which increases resistance to airflow and the work of breathing. Stridor is indicative of potentially life-threatening upper airway obstruction.

Weakness of the laryngeal and glottic muscles can lead to impaired swallowing and aspiration of secretions. A wet, gurgled voice and pooled oropharyngeal secretions are the best clinical signs of significant dysphagia.

What is the respiratory rate?

Check for paradoxical respirations ( Fig. 16.1 ) (inward movement of the abdomen on inspiration), which is indicative of diaphragmatic paralysis.

What is the heart rate and blood pressure (BP)?

Sinus tachycardia, hypertension, or BP lability can result from dysautonomia in GBS.

What is the temperature?

Fever or hypothermia can occur because of an obvious or occult source of infection.

Selective History

To make the diagnosis, it is important to establish the time course and distribution of weakness ( Table 16.2 ). Key questions to keep in mind when obtaining a history in patients with generalized weakness include the following: (1) Is a spinal cord lesion a possibility? (2) Is the process purely motor or is sensation involved? (3) Is the patient at risk for sudden respiratory failure?

• 1.

  When did the weakness develop?

  Early symptoms of weakness can be subtle. Ask about difficulty arising from a chair, climbing stairs, lifting packages, combing or brushing hair, or turning keys or doorknobs.

• 2.

  Was there an antecedent illness?

  Approximately 70% of cases of GBS are triggered by an antecedent viral illness or Campylobacter jejuni gastroenteritis. Respiratory crisis in myasthenia gravis is triggered by infection in approximately 40% of patients.

• 3.

  Does the weakness fluctuate?

  Fluctuating weakness (on an hourly basis) is almost pathognomonic for myasthenia gravis.

• 4.

  Has there been any blurred or double vision?

  Blurred vision occurs with botulism.

• 5.

  Has there been any neck or back pain?

  Neck pain should raise suspicion for a cervical cord lesion. Low backache occurs frequently with GBS.

• 6.

  Has there been any numbness or tingling?

  Distal paresthesias are common in GBS.

• 7.

  Have there been any muscle aches, cramps, or tenderness?

  Aching and tenderness generally occur with myopathy. Cramps can occur with motor neuron disease or with severe electrolyte derangements.

• 8.

  Has there been any exposure to toxins or insecticides?

  Organophosphate is the most common toxin that can lead to respiratory failure.

Selective Physical Examination

Once the patient has been stabilized, the goal is to identify signs of airway or respiratory compromise and to search for clues to the diagnosis.