Situation: A 34-year-old woman with generalized myasthenia gravis (MG) is admitted to the medical ward with fever and a productive cough. She weighs 60 kg. She takes pyridostigmine 60 mg by mouth every 6 hours and prednisone 10 mg once daily. She has no drug allergies. Serial bedside values on the day of admission are shown. Per unit protocol, the intensive care team is called when forced vital capacity (FVC) falls below 20 mL/kg. 08:00 — FVC 1,800 mL; oxygen saturation (SpO2) 98%; respiratory rate 18/min 12:00 — FVC 1,450 mL; SpO2 97%; respiratory rate 22/min 16:00 — FVC 1,100 mL; SpO2 97%; respiratory rate 26/min Which action is BEST at 16:00?

At 16:00 her FVC is 1,100 mL ÷ 60 kg = 18.3 mL/kg, below the 20 mL/kg protocol threshold, and her respiratory rate is climbing. In neuromuscular respiratory weakness, oxygen saturation and blood gases change late, so a normal SpO2 does not mean she is safe. Calling now allows airway support to be planned before a crisis.

At 16:00, the forced vital capacity (FVC) is 1,100 mL. Divided by her weight of 60 kg, this equals 18.3 mL/kg, which is already below the unit’s threshold of 20 mL/kg. In addition, the respiratory rate has risen from 18/min at 08:00 to 26/min at 16:00, indicating increasing work of breathing. The correct action is to call the intensive care team now, before the patient progresses to frank respiratory failure.

In generalized myasthenia gravis (MG), respiratory muscle weakness—especially diaphragmatic involvement—is the principal risk factor for myasthenic crisis and respiratory failure ^[1]. The diaphragm is the dominant muscle of inspiration, and when it fatigues, tidal volume falls and accessory muscles are recruited. This is why serial FVC measurements are used: they provide an objective, reproducible estimate of neuromuscular respiratory reserve. A declining FVC trend (1,800 → 1,450 → 1,100 mL) reflects progressive diaphragmatic weakness, not simply a transient fluctuation.

Watch out! Oxygen saturation remains 97% at 16:00, but this is misleading. In neuromuscular respiratory failure, hypoxemia and hypercapnia are late findings. The patient can maintain gas exchange by increasing respiratory rate and using accessory muscles until the diaphragm is nearly exhausted. By the time SpO2 drops, the patient may already be in impending respiratory arrest. Therefore, a normal SpO2 does not rule out the need for escalation of care.

The fever and productive cough suggest a respiratory infection, which is a common trigger for myasthenic crisis. Infection increases metabolic demand and inflammatory mediators, further compromising neuromuscular transmission at the already vulnerable postsynaptic acetylcholine receptors. Prednisone 10 mg daily provides immunosuppression but does not acutely reverse weakness. Pyridostigmine 60 mg every 6 hours improves synaptic acetylcholine availability, but giving it early is not the priority here. Cholinesterase inhibitors do not rapidly reverse an established downward FVC trend when respiratory muscles are already fatiguing, and excessive dosing can precipitate cholinergic crisis with increased secretions and bronchospasm.

The single count breath test (SCBT) has been studied as a bedside adjunct for respiratory evaluation in MG, particularly when formal spirometry is unavailable or facial weakness limits testing . However, in this scenario, serial FVC measurements are already available and clearly show deterioration. The SCBT would not add decision-making value at 16:00 because the FVC has already crossed the protocol threshold.

Diaphragmatic performance during acute ventilatory failure in MG is characterized by a high tension-time integral (TTdi), meaning the diaphragm works at a high fraction of its maximum capacity for each breath . As weakness progresses, the diaphragm approaches its endurance limit, and the patient cannot sustain ventilation. This pathophysiologic mechanism explains why the respiratory rate climbs as the FVC falls: the patient is attempting to compensate for reduced tidal volume by breathing faster, but this strategy increases diaphragmatic energy demand and accelerates fatigue.

Time	FVC (mL)	FVC (mL/kg)	Respiratory rate	SpO2	Interpretation
08:00	1,800	30.0	18/min	98%	Above threshold; baseline monitoring
12:00	1,450	24.2	22/min	97%	Declining; increased work of breathing
16:00	1,100	18.3	26/min	97%	Below 20 mL/kg threshold; call ICU now

Waiting until 20:00 to recheck the FVC is unsafe because the trend is already downward and the threshold has been crossed. Continuing monitoring based on SpO2 alone is equally dangerous, as oxygenation is preserved until late in neuromuscular respiratory failure. Giving pyridostigmine early addresses a possible pharmacologic need but does not secure the airway or provide ventilatory support. Key point! In MG, the decision to escalate care is driven by objective respiratory mechanics—FVC and respiratory rate—not by oxygen saturation. The intensive care team should be called when FVC falls below 20 mL/kg (or 1 L in many protocols), because intubation is far safer when performed electively than during an emergency crisis.