adding inspiratory hold to improve double triggering
- related: basic understanding of ventilators
- tags: #literature #boards
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This patient has severe ARDS from COVID-19, requiring a very low tidal volume to ensure lung protection. His breathing pattern on the ventilator waveforms shows that he double triggers with many breaths. Despite the fact that the tidal volume on assist control is set at 350 mL, his tidal volumes are much higher with each double-triggered breath. Adding an inspiratory hold time is the measure most likely to decrease this phenomenon (choice B is correct).
Results from the seminal ARMA trial in 2000 showed that low tidal volume ventilation (6 mL/kg ideal body weight, along with a plateau airway pressure <30 cm H2O) in ARDS, compared with a high tidal volume (12 mL/kg ideal body weight), was associated with a 25% relative improvement in survival. This trial did not address the approach to double triggering, during which patients may receive much larger tidal volumes than what is set on the ventilator. However, this covert delivery of large tidal volumes has become increasingly apparent.
With greater awareness of the problem of covert large tidal volumes associated with double triggering, several strategies have emerged to combat this problem. Adding a 1-s inspiratory pause closes the ventilator circuit at the very time when a patient attempts to initiate a second (double-triggered) breath. If the patient attempts to take a second breath, the effort is not rewarded because the inspiratory valve on the ventilator is closed for a full second after the programmed tidal volume is delivered. With this maneuver, the actual programmed low tidal volume is delivered reliably, and double triggering is essentially eliminated. This appears to be the most effective way to eliminate double triggering and maintain lung-protective ventilation.
This patient's Richmond Agitation-Sedation Scale score was already −2. Increasing the level of sedation and/or analgesia is generally ineffective (choice A is incorrect). Although the patient may appear less agitated, such an intervention typically leads to the transition to a patient who is comatose and still double triggering.
Changing the mode of ventilation from assist control to pressure support almost always eliminates all double triggering. However, the majority of patients with ARDS with double triggering have an exceedingly high respiratory drive. Therefore, the elimination of double triggering with pressure-support ventilation comes at a cost of high tidal volumes. Such high tidal volumes exceed the goal of a low tidal volume lung-protective strategy (choice C is incorrect).
As mentioned, the majority of patients with ARDS with double triggering have an exceedingly high respiratory drive. Accordingly, raising the inspiratory trigger threshold to −5 cm H2O is unlikely to eliminate double triggering (choice D is incorrect).1