The ICU Gotchas: Peak pressure limits

[Read the last Gotcha: OverPEEPing]

The patient is a 46-year-old morbidly obese male with congestive heart failure (EF of 40%) and obesity hypoventilation syndrome, admitted for hypoxemic respiratory failure. He is intubated and placed on volume control with a volume of 450 ml, PEEP of 10 cm H2O, and FiO2 of 60%. On these settings, his SpO2 is 94% and his PCO2 is adequate.

A short time later, you are called to the bedside to find him tachypneic and desaturating to the 80s. His ventilator is repeatedly sounding high pressure alarms, and looking closer, you note that his expired tidal volumes are less than 100 ml.

You disconnect the ventilator and begin to bag him with 100% oxygen. He immediately settles out and his oxygenation normalizes. As you bag, you wonder what you’re going to change before you place him back on the vent.

This one is short and sweet, a true “gotcha,” and surprisingly high yield in terms of frequency to badness. You’ll see the phenomenon rather often, you won’t recognize the problem unless you’re looking for it, and if you don’t get a clue, patients can decompensate and crash.

Breaking under pressure

The problem lies in the way most ventilators handle their high pressure alarms. See, beneath all the settings we typically think about—rate, PEEP, etc—lies a host of other configurations usually only managed by respiratory therapists. Chief among these are the alarms. Alarms are essential to safe mechanical ventilation, because they announce when bad things are happening that require your attention (such as a disconnected circuit or gas failure). But every so often, they’ll create their own problems.

One of the more commonly-sounded klaxons is the peak pressure alarm. This occurs whenever the peak airway pressure exceeds a certain number. The cause for the majority of these alarms is either innocuous, such as the patient coughing, or easily remedied, such as secretions that need to be suctioned. Still—you need them.

In some patients, of course, the peak pressure may always be high. An obvious example might be a patient with a very small endotracheal tube or bronchoconstriction (causing a high peak, but normal plateau). Another case might be the patient with obesity, a distended abdomen, circumferential burns, or other reasons for extrinsic compression on the lungs. These patients will have a high peak and high plateau, and can’t be ventilated without fairly high airway pressures… pressures that just might set off your alarms.

Here’s the heart of the Gotcha: when a breath reaches the peak pressure threshold (or within some range of it), most ventilators won’t just sound an alert, they’ll terminate the breath. In other words, the set number is not merely an alarm, but a limit.

You can imagine the problem. It’s nice to keep airway pressures low when you can. But one patient might have a normal plateau pressure with a high peak, and another patient might have high plateaus due to unresolvable factors like obesity. In both cases, if the pressure limit—often defaulted to around 40 cm H2O—isn’t sufficient to deliver the tidal volume, they won’t get a full breath. They might not get much air at all. As a direct result of your vent’s alarm settings, they’ll just sit there and suffocate.

Nota bene: This is primarily a phenomenon of volume modes, such as volume control (i.e. assist control with a volume limit). In pressure control, you’re the one setting the driving pressure. Since you should be following the resulting tidal volumes as you do, in a patient with poor compliance, it should be immediately obvious that their expired volumes are low, and a higher pressure is needed. On the other hand, PRVC, which is essentially a pressure mode that self-titrates the inspiratory pressure, is most certainly subject to this Gotcha.

The cure

The solution is simple: you need to increase the pressure limit.

Ingrain this pattern recognition into your head, because it’s virtually pathognomonic. When you walk into the room and notice:

  1. Low expired tidal volumes, with
  2. A peak pressure consistently pegged at the set limit (e.g. 40 cm H2O)

Your first reaction should be to increase the high pressure alarm. Feedback will be prompt: if you bump the limit from 40 to 50, and the patient’s peak pressure immediately jumps to 50 and the tidal volumes increase, you’ve discovered the problem. Keep increasing until you’ve reached an acceptable tidal volume.

Now, this is only the first step. You’ll still need to ask the question of why the patient’s compliance is so poor. Perhaps the answer is because of tight airways, and so you don’t care (i.e. because the plateau pressure is normal). Perhaps the answer is because of obesity, and so you don’t care (i.e. because you suspect the transpulmonary pressure is normal). Or perhaps there’s been an acute change due to a pneumothorax that needs to be decompressed. The point isn’t that high pressures don’t matter, that alarms are meaningless, and that you should ignore them. The point is: whatever the problem, not ventilating the patient rarely improves it.

Keep your antennae out for this tricky little scenario, and you’ll be able to quickly respond to a situation that will leave others baffled. Due to the distress and tachypnea that often results, your colleagues may be tempted to say the underlying the problem is ventilator dyssynchrony, and the answer is more sedation or paralysis. But you’ll know better.

Go forth, and ventilate—don’t suffocate.