Respiratory management in patients with neuromuscular disorders (NMD)

Changes in pulmonary function of NMD patients

  • Restrictive pattern in static and functional testing
  • Vital capacity (VC), forced vital capacity (FVC), total lung capacity (TLC) and functional residual capacity (FRC) are low
  • Low VC is caused by:
    1. Reduced inspiratory muscle strength
    2. Reduced static lung compliance because of increased alveolar surface tension by breathing at low lung volumes.
  • Residual volume (RV), FEV1/FVC ratio and diffusion capacity are normal

Ensure good technique when measuring dynamic parameters

  • Good lip closure around measuring equipment
  • No tongue propulsion

Definitions

FVC = Forced Vital Capacity

  • Largest volume of air that the patient can exhale after a maximal inspiration
  • Assesses the general ventilatory ability. Depends on both inspiratory and expiratory muscle strength as well as chest wall and lung compliance
  • Measured by having the patient exhale maximally after a maximal inhalation and measuring the exhaled volume.

NIF = Negative Inspiratory Force

(also referred to as MIP - Maximal Inspiratory Pressure)

  • It is the greatest negative pressure the patient can generate
  • Usually reported as absolute value (eg. by ”>60” are meant values like -61, -62 etc)
  • Assesses the strength of the inspiratory muscles
  • Measured by having the patient exert maximal inhalation effort against a measurement gauge

MEP = Maximal Expiratory Pressure

  • It is the greatest positive pressure the patient can generate
  • Assesses expiratory muscle strength
  • Indirect assessment of the ability to cough and clear sectetions
  • Measured by having the patient exert maximal exhalation effort against a measurement gauge

Normal values

  • FVC 50-70 ml/kg
  • NIF >60 cm H2O
  • MEP >100 cm H2O

Consider ICU admission or noninvasive ventilation

  • FVC 25-30 ml/kg
  • NIF 30-40cm H2O
  • MEP <50 cm H2O

Consider intubation and mechanical ventilation

  • FVC 10-15 ml/kg
  • NIF <20 cm H2O
  • MEP <40 cm H2O

Consider weaning off ventilator

  • FVC >15 ml/kg
  • NIF >25 cm H2O
  • MEP >40 cm H2O

Factors to consider when deciding to intubate

  • Increasing generalized muscle weakness
  • Bulbar signs (dysarthria, dysphagia, dysphonia)
  • Dyspnea at rest
  • Tachypnoea, shallow breathing
  • Weak cough, difficulty mobilizing secretions
  • Aspiration, cough after swallowing
  • Orthopnea
  • Use of accessory inspiratory muscles
  • Interrupted speech
  • Inability to count to 10 on a single breath is roughly equivalent to FVC <1L
  • Decreased level of consciousness
  • VC <1L or 50% decrease in VC within a day
  • NIF <30 cm H2O
  • MEP <40 cm H2O
  • Hypoxia or hypercapnia are late findings in NMD patients with respiratory insufficiency. Normal arterial blood gases should not deter from intubating an unstable patient