Mechanical ventilation IV: Pressure Support

Here we discuss spontaneous modes of ventilation.

Read the other entries from this series here: (1), (2), (3), (5), (6)

If we completely eliminate a set respiratory rate—in other words, eliminate time-triggered breaths—we arrive at a mode called…

Pressure Support (PSV)

Pressure support is essentially pressure control without any set rate. What does that mean?

We still set an FiO2 and a PEEP. However, no respiratory rate is set, and therefore no timer runs. Time-triggered breaths do not exist. Only patient-triggered breaths (as usual, using either a flow or pressure trigger) occur.

Pressure support ventilation. There is no uniformity to the rate or morphology of each breath, because the triggering, volume, flow, and duration are all patient-determined. The vent only provides added pressure when the patient chooses to breathe.

Just like pressure control, however, we can deliver positive pressure once a breath has been triggered. But unlike control modes, the patient not only decides when the breath begins, they decide when it ends. Rather than being time cycled, pressure control is flow cycled. The ventilator watches the breath in action, and once the inspiratory flow drops below a certain fraction of the initial flow—in other words, once the breath starts to slow down—it decides the patient must be done breathing, and cuts off the support. The patient can then exhale.

The amount of pressure support can be anything from zero to, say, 20 or 30. However, while peak pressures of 20 are common for PC, they would be unusual for PSV, because the latter is usually used as a weaning mode. We use PSV when patients are getting better, waking up, and breathing more on their own. If they need 20 of support, they’re probably not ready for this mode at all.

(The issue, of course, is that PSV mandates no set rate. So the patient must be awake and aroused enough to have a respiratory drive, because if they don’t trigger any breaths… they won’t receive any breaths. PSV can assist them in reaching a physiological tidal volume when they do trigger, but they first need to try.)

Typical levels of support range from 5 to 10, perhaps as high as 12 or 15. A point of note is that this pressure, just like in PCV, is pressure added on top of any set PEEP. In other words, with a PEEP of 5 and a pressure support of 5 (often denoted as 5/5), the total airway pressure is 10. This is by arbitrary convention, and is rather confusing as it differs from the convention for non-invasive ventilation: BiPap settings are independent inspiratory and expiratory pressures and are not additive (an IPAP of 5 and EPAP of 5 means the inspiratory pressure is just 5).

PSV is the most pleasant mode for patients strong enough to generate their own breaths and awake enough to have opinions, and many patients who are “bucking” more controlled modes will be far more comfortable in PSV—and are probably proving that they’re awake enough to try. Spontaneous breathing trials, in which we test a patient’s readiness for extubation, are often performed using PSV with minimal pressures. It is typically believed that a small amount of support is actually needed to negate the resistance of the endotracheal tube itself, as “breathing through a straw” without any pressure support is harder than breathing while extubated. How much support is needed for a fair fight varies by provider practice as well as the size of the endotracheal tube, but most of us fall somewhere in the range of 0/5 through 10/5.

As a pedantic matter, some vents and some providers tend to call this mode “CPAP.” This is a misnomer, as (1) that nomenclature is usually reserved for non-invasive ventilation, and (2) pure CPAP would have a constant level of pressure rather than alternating pressures (which would be BiPAP); true “CPAP” would need to be PSV at 0/5. But I digress.

Next time, we’ll get a little wacky and consider some “hybrid” modes of control.

Read Part V here

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