Frequently Asked Questions
We have compiled some of the most frequently asked questions from the discussion forums and posted answers here; we will continue to add to these as new frequently asked questions arise.
How should you choose the ventilator mode (volume control or pressure control)? What are the pros and cons of each ventilator mode (volume control or pressure control)?
Much of the selection between volume control and pressure control will depend upon local preferences. Studies have shown that there are no differences in outcomes between the two. The benefits of volume control include providing a dedicated tidal volume, and clinicians can ensure that they are targeting a set mls/kg of PBW. The benefit to pressure control is similar – clinicians can ensure that the pressures are within a lung-protective range. Also, there is some though that pressure control is more comfortable to patients, as the variation in tidal volume and flow can feel more physiologic than volume control. However, selecting one and becoming very comfortable with the intricacies of that mode is beneficial, especially if one is learning mechanical ventilation. For the purposes of managing ARDS in COVID-19, we recommend using volume control, selecting a low tidal volume, and monitoring pressures.
What is the difference between dynamic and static compliance?
When we discuss “compliance” in mechanical ventilation, we are discussing static compliance. Static compliance is the difference in volume (tidal volume) divided by the pressure between the plateau pressure (Pplat) and the PEEP. This can be recalled as the Pplat is a static number – it is determined during an inspiratory hold maneuver, which stops all flow. This is the compliance of the lungs, as resistance is not part of the equation, given the lack of air flow.
Dynamic compliance, however, is the difference in volume (tidal volume) divided by the pressure between the peak inspiratory pressure (PIP) and the PEEP. This value does include air flow. Because of the variation in resistance, the dynamic compliance is clinically less useful.
What is derecruitment?
Derecruitment is large-scale atelectasis, or collapsing of the alveoli in a large section of the lung – such as an entire section or the posterior portions. We refer to reopening of the alveoli as “recruitment” (think – they have been “recruited” to participate in gas exchange) and therefore, their closing is “derecruitment.”
How do you perform an inspiratory hold or an expiratory hold, and why you might want to?
Inspiratory and expiratory holds are performed by pressing buttons on the ventilator. They may be labeled as “inspiratory (or expiratory) pause” or “inspiratory hold.” We check an inspiratory hold to evaluate the plateau pressure and an expiratory hold to evaluate for auto-PEEP, also known as intrinsic PEEP.