Thursday, September 29, 2011

Waveform of the Week: “Golden Moment”

Image 1: Screen of the FLOWi anesthesia delivery system, showing the "Golden Moment" highlighted with blue circle.

It has been over a year since writing a waveform of the week post; however I have incorporated ventilator waveform pictures in recent posts. The waveform for this week is termed the “Golden Moment”

This posting was spurred by a colleague of mine when asking me “hey Scott why is my pressure dropping below PEEP during breath termination on my Servoi ventilator”?

Is this the patient, or the ventilator, does it affect breath delivery?

These are all good questions.

First, what is it?

Servo users term this as the “Golden Moment”.  During breath termination the pressure in the exhalation valve is briefly dropped below the set PEEP, to minimize resistance to the flow imposed by the pressure within the exhalation valve. Hence, this decreases expiratory resistance.

This is also common in other ventilators with “Automatic Tube Compensation” [1].

Note-This action is not the patient trying to trigger a breath.

Does it affect breath delivery?

It only affects breath delivery during the termination phase, reducing expiratory resistance, which should reduce the expiratory work of breathing.

Skeptics are concerned with the pressure dropping below PEEP and the potential for lung de-recruitment. However, this is no concern the drop is so brief that there is not enough time for de-recruitment, as evidence from the expiratory flow waveform (blue arrow, image 1) [2].

Foot Notes

[1]. ATC models: not all ATC models are equal, some provide only inspiratory compensation, and some provide both inspiratory & expiratory compensation. Expiratory compensation is similar to the “Golden Moment”.

[2]. When evaluating image 1, one can identify based on the expiratory flow waveform that there is significant expiratory lung volume during the pressure drop. This negates the theory of lung de-recruitment.