BiPAP in acute respiratory failure due to myasthenic crisis may prevent intubation

We appreciate the interest of Mazia et al. expressed in our article. When using BiPAP, supplemental oxygen is delivered in L/min. In this mode, inspired fraction of oxygen cannot be reliably determined. Mean oxygen requirement in our patients was 4 L/min(range 2 to 10 L/min); most patients maintained excellent saturations with the volumes of oxygen used. Thus, we believe there is no reason to suspect that any significant ventilation/perfusion mismatch was present in our patients. Although higher inspiratory pressures may be considered under certain circumstances, the patients who failed BiPAP in our series were precisely those that received the highest inspiratory pressures.

Hypercapnia marks a severe, if not terminal, respiratory failure in a patient with myasthenia gravis. In our experience, including patients not reported here, supporting a myasthenic patient and established hypercapnia with BiPAP does not seem to avert endotracheal intubation and volume- controlled mechanical ventilation. We are concerned that a perfunctory attempt to control hypercapnia with BIPAP may dangerously delay endotracheal intubation. Rightly so, the Task force of the Medical Scientific Advisory Board of the Myasthenia Gravis Foundation of America avoided defining "myasthenic crisis" and we correctly applied their grading system. We believe it is simple logic that any breathless myasthenic patient is in a dire state-call it crisis or not.

We read with interest the article of Rabinstein et al. [1] reporting the usefulness of BiPAP in myasthenic patients. We value the attempts to use non invasive ventilation in patients with myasthenic crisis and agree with the suggestion that ventilation with BiPAP could prevent intubation, thus avoiding potential complications. However we would like to comment on several aspects of the article.

There is not a general consensus on how to define myasthenic crisis. In a recent effort to standardize this concept it was proposed that myasthenic crisis be defined as weakness from acquired myasthenia gravis that is severe enough to require intubation (2), a situation that corresponds to class V of the MGFA classification (3). However the patients reported in the article by Rabinstein et al. were class IVb.

Fraction of inspired oxygen (FiO2) values had not been included. For this reason it is not possible to determine the presence and severity of respiratory (hypoxemia) or ventilatory (hypercapnia) failure. It seems that the oxygen requirements were elevated considering patients with isolated ventilatory failure without lung injury. The high oxygen requirements used to maintain adequate arterial oxygen saturation (SaO2) suggests ventilation/ perfusion relationship inequalities, lung injury and respiratory failure. Additionally SaO2 was erroneously expressed in mmHg instead in per cent of oxygen of inspired air.

The inspiratory positive airway pressures (IPAP) delivered may not have been high enough, especially when hypoxemic respiratory failure was present .The reported failure to improve the PaCO2 values in the hypercapnic patients may be related to the low inspiratory pressures used.

This paper gives a new therapeutic approach to treat severe compromised myasthenic patients. However it would be important to provide a better definition of the characteristics of the population such as the clinical classification, the degree of respiratory or ventilatory failure and the clinical conditions to decide the BiPAP trial. Finally we propose that a multivariate analysis should be done prior to conclude that BiPAP should not be attempted in patients with overt hypercapnia.

References

1. Rabinstein A., Wijdicks EFM. BiPAP in acute respiratory failure due to myasthenic crisis may prevent intubation. Neurology 2002;59:1647- 1649.

2. Bedlack RS, Sanders D. On the concept of Myasthenic Crisis. J.Clin.Neuromusc.Dis. 2002;440-442.

3. Jaretzki A 3rd, Barohn RJ, Ernstoff RM., et al. Myasthenia Gravis: Recommendations for clinical research standards. Task force of the Medical Scientific Advisory Board of the Myasthenia Gravis Foundation of America. Neurology 2000;55:16-23.