Daily spontaneous breathing trials (SBT) for patients receiving mechanical ventilation are considered standard-of-care across intensive care units (ICUs); however, when we discuss weaning strategies, there’s a bit more variability. Many intensivists opt for putting the patient on “minimals”: 5 cm H2O of inspiratory pressure support (PS) on top of 5 cm H2O end-expiratory pressure, otherwise known as “5 over 5.” The patient is then monitored for 15-30 minutes looking for signs of distress/fatigue, rapid shallow breathing index, negative inspiratory force, arterial blood gases, etc. It’s thought that the inspiratory pressure helps to overcome the resistance to airflow created by the endotracheal tube (ETT). And it makes sense… if I’m essentially breathing through a straw (especially smaller ETTs), I’d like some help.
The work of breathing (WOB) through an ETT depends on many factors including the inspiratory effort generated by the patient, the ETT’s diameter/length, but most importantly, an airflow rate that varies tremendously from breath-to-breath (and even within the same breath). Having a fixed PS, therefore, might actually be providing too little support with the onset of inspiration (high flow) and too much after most of the tidal volume has been delivered (low flow).
ATC is an interesting option. It continuously calculates the difference in pressure across the proximal and distal ends of the ETT (ΔP). This ΔP becomes the inspiratory PS and will dynamically change as ΔP changes. Basic settings for ATC include the type of tube (ETT vs tracheostomy tube), inner diameter, and the percentage of compensation (ie, 80%).
Although some data suggests that the resistance created by the ETT doesn’t significantly differ from intrinsic resistance created by the native upper conducting airways, I think it’s still awesome to be familiar with weaning options like “minimals”, T-pieces, and ATC! 🙂
Drop me a comment below if you have experience with ATC or questions!