Dr. Farkas’ post below is a fast read but is chocked full of useful information. As usual for this sort of post, this post is simply a way for me to quickly access medical information that I want to have easily available.
The post that follows is only excerpts from Dr. Farkas’ post so be sure to review his post.
Five pearls for the dyspneic patient with Guillain-Barre Syndrome or Myasthenia Gravis
February 22, 2015 by Josh Farkas:
As always, see the post for the full picture.
Pearl #1: Do not intubate a patient solely because of poor PFTs
Pearl #2: Don’t check the MIP [Maximum Inspiratory Pressure] or MEP [Maximum Expiratory Pressure]
FVC [Forced Vital Capacity] is arguably the best single test of ventilatory capability, since it integrates inspiratory and expiratory muscle strength as well as pulmonary compliance. It is also the most reproducible test over time. Therefore it should come as no surprise that nearly all studies have focused exclusively on the FVC in predicting respiratory failure, completely ignoring the MIP and MEP (e.g., Sunderrajan 1985, Chevrolet 1991, Sharshar 2003, Durand 2006, Kanikannan 2014).
Pearl #3: Don’t assume respiratory failure is due to respiratory muscle weakness
Patients who have been labeled with GBS or MG are susceptible to anchoring bias: there is a tendency to assume that any respiratory problem encountered must be due to their neuromuscular weakness. Once we were told that a patient transferred to Genius General Hospital with MG and respiratory failure required urgent intubation. Indeed, the patient arrived quite dyspneic and hypoxemic. Bedside ultrasonography showed a large right-sided pleural effusion, and further evaluation revealed that the patient had congestive heart failure with severe volume overload. Therapeutic thoracentesis and heart failure management caused immediate improvement, avoiding the need for intubation. Although the patient may have known respiratory muscle weakness, don’t forget to look for other problems as well. When in doubt, unholster the triple-barreled shotgun [Approaching undifferentiated cardiopulmonary failure: Which tests are most useful? February 1, 2015 by Josh Farkas]: POCUS, EKG, and CXR.
Pearl #4: Consider early pre-emptive respiratory support with BiPAP or high-flow nasal cannula.
The pulmonary outcome of a patient with MG or GBS will often depend on the balance between the respiratory muscle strength and the work of breathing. If the scale is tipping slightly in the wrong direction, the patient will gradually fatigue and eventually fail. For patients who are hanging in the balance, even a small reduction in the work of breathing could be critical. However, in order for this to succeed respiratory support must be initiated early, well in advance of respiratory exhaustion.
Pearl #5: Try not to chase dysautonomia in GBS. However, be prepared to handle it in the peri-intubation period.
Patients with GBS may have dysautonomia with hemodynamic lability. One risk involved in this situation is that if the “highs” are over-treated, this may exacerbate the “lows.” That is, if hypertension or tachycardia is treated (for example, with a beta-blocker), then the patient could subsequently have an episode of severe hypotension and bradycardia. It is often best to avoid treating these fluctuations if possible. If treatment is needed, a very short-acting agent may be safest so that it can be discontinued rapidly if needed. Any factors aggravating hemodynamic swings (e.g. untreated pain, underlying hypovolemia) should be corrected.
Dysautonomia is a particular concern in the peri-intubation period, as it may combine with hemodynamic fluctuations following intubation, amplifying the risk of hypotension. These patients are often volume depleted due to poor oral intake, so it is sensible to assess volume status prior to intubation (e.g. with ultrasonography) and resuscitate to a euvolemic state. Peri-intubation bradycardia is mediated by the parasympathetic nervous system, so atropine is a logical first-line treatment for this and should be close at hand.
- The only bedside pulmonary function test which is useful is the forced vital capacity (FVC).
- Patients with a FVC < 20 ml/kg are at risk for respiratory failure and should receive ICU-level monitoring.
- Intubation is typically required when the FVC falls below 10-15 ml/kg. However, the decision to intubate is aclinical decision based primarily on ability to protect the airway, work of breathing, vital signs, overall appearance, and trajectory.
- For patients who are dyspneic but don’t require intubation, consider trialing BiPAP or high-flow nasal cannula to see if this may improve their comfort and reduce the work of breathing.
- Patients with GBS may have dysautonomia with wide fluctuations in blood pressure. Avoid treating hypertension if possible, as this may exacerbate subsequent episodes of hypotension.