Optimal Modes of Ventilation for Weaning
Is it better to use the assisted/control or synchronous intermittent mandatory ventilation (SIMV) with pressure support mode of ventilation in patients who fail weaning?

Ahmed Fathy, MD

There has been much written as to the best way to wean patients from mechanical ventilation. Two large trials have shown that 24% to 29% of patients fail in their first attempt at weaning. The "difficult-to-wean" patients fail generally because their underlying illness has not resolved sufficiently to permit liberation from mechanical ventilation. The other major barriers to weaning include malnutrition, excessive secretions, presence of autoPEEP, impaired muscle function secondary to hypokalemia, hypophosphatemia, or hypomagnesemia, or respiratory muscle fatigue.

Dysynchrony between the patient and the ventilator can also increase the work of breathing for the patient and can lead to muscle fatigue and weaning failure. Optimizing patient-ventilator synchrony minimizes the individual's work of breathing and allows the patient to receive as much support from the ventilator as possible. It should be noted that respiratory muscles do not completely rest even when a patient is on mechanical ventilation. The inspiratory muscles continue to contract during assisted breaths in IMV, AC, and PS modes. The flow rate, trigger sensitivity, and method of triggering should be assessed as to the adequacy of meeting the patient's ventilatory demand. If the flow setting is too low or the trigger setting not sensitive enough, there will be an increase in the patient's overall work of breathing by making the triggering of inspiration more difficult. Using a flow-triggering mode of ventilation will reduce inspiratory effort 30% to 40% more than pressure triggering during mechanical ventilation.

Several trials have compared the efficacy of weaning methods. One trial found that the length of weaning was shorter with PS than with IMV or spontaneous breathing trial (SBT). In contrast, another trial demonstrated that a spontaneous breathing trial performed once daily resulted in extubation 3 times more quickly than IMV and 2 times as quickly as PS. The studies differ significantly in the criteria used to determine the suitability for extubation. In the first study, the patients weaned by IMV had to tolerate a rate of 4 for 24 hours prior to extubation. In the second study, once patients tolerated IMV of 5 for 2 hours, they were extubated. Similarly for SBTs, in the first study, the physicians could request 3 SBTs for 2 hours each prior to authorizing extubation attempt. In the second trial, if patients tolerated SBT for 2 hours, they were extubated.

General recommendations for the difficult-to-wean patient include having patience until the underlying disease process resolves sufficiently; optimizing nutritional and metabolic parameters; maximizing patient-ventilator synchrony to reduce the work of breathing; and providing adequate muscle rest. Once the patient is deemed ready for weaning, daily SBT will likely be the most efficacious method of weaning. The goal of the SBT should be 2 hours without clinical distress.