Initial Vent settings question

[sozme]

Discussed this case from Case Files Critical Care with my attending today (pasted if you feel like reading, but if tldr: asthma exacerbation, young person).

A 35-year-old woman with persistent severe asthma is being seen in the ED. On a previous admission, she required mechanical ventilation and was transferred to the intensive care unit (ICU) for treatment of an asthmatic exacerbation. For the past week, she has increased her use of a β2 agonist as rescue medication by 6 to 8 times normal, and has nighttime exacerbations every evening. On physical examination she is in acute respiratory distress with nasal flaring and a quiet chest with very distant wheezing. An ABG drawn on 30% oxygen shows a pH of 7.35, PaCO2 42 mm Hg, PaO2 89 mm Hg, and bicarbonate (HCO3) of 23 mEq/L. Peak expiratory flow rates are all below 40% of the patient's predicted range. Her respiratory rate is 30 breaths/minute, heart rate is 110 beats/minute and regular, and blood pressure is 150/78 mm Hg with a pulsus paradoxus of 10 mm Hg.

My question is regarding the appropriate initial settings for this patient. The Owen book recommends:

§ AC/VC mode
§ VT: 8 mL/kg IBW
§ RR: 10-14
§ I:E ratio: 1:3
§ PEEP: Start with 0.

She says the 6 mL/kg IBW is better these days, with higher RR and PEEP 5.

However, The Ventilator Book asserts that lower VT can lead to air trapping/worsening hyperinflation. Also states that applied PEEP will worsen hyperinflation.

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[tartesos]

The craze these days is low tv for everyone for fear of barotrauma. I'm a fan of 6ml/kg to start.
Not all patients are treated equal. There are exceptions to every rule and if you practice long enough you will break them all.
I would start with 6 Mts tv and peep of 5 and watch her( like watch her, stay at the bedside and wait in front of the vent kind of watch )


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[Hernandez]

It depends......and varies patient to patient, and frankly I'm of the school of thought that PEEP is an appropriate treatment for air trapping many times, I've seen it work and yes I'm aware of the controversies around it. I personally find this a good role for volumetric capnography.

To me that's a depends on the patient, their actual rate and many many other factors.


The I:E is far more important than the rr or tv. It doesn't matter what you set those to, the patient wants to do what they want to do, set a TV that's too low and they'll increase their RR, set you're rate too high and its a problem if you're own making, you can play with flow rates to help, on occasion you have to paralyze these people to take full control, use agrees bronchodilators and sometimes things like ketamine, mag, etc.

And the appropriate settings can change hour to hour, re-assess.

One size fits all doesn't work in conditions with variable severity presentations.

 

[cbrons]

A 35-year-old woman with persistent severe asthma is being seen in the ED. On a previous admission, she required mechanical ventilation and was transferred to the intensive care unit (ICU) for treatment of an asthmatic exacerbation. For the past week, she has increased her use of a β2 agonist as rescue medication by 6 to 8 times normal, and has nighttime exacerbations every evening. On physical examination she is in acute respiratory distress with nasal flaring and a quiet chest with very distant wheezing. An ABG drawn on 30% oxygen shows a pH of 7.35, PaCO2 42 mm Hg, PaO2 89 mm Hg, and bicarbonate (HCO3) of 23 mEq/L. Peak expiratory flow rates are all below 40% of the patient's predicted range. Her respiratory rate is 30 breaths/minute, heart rate is 110 beats/minute and regular, and blood pressure is 150/78 mm Hg with a pulsus paradoxus of 10 mm Hg.

Nm

 

[lymphocyte]

applied PEEP will worsen hyperinflation.

I think badly applied PEEP will worsen hyperinflation in some.

PEEP helps by dilating collapsed distal airways, but only if PEEP is less than auto-PEEP. Like creating a waterfall that flows from within the trapped airways outward--you're trying to set up a pressure gradient that facilitates airway opening. But why does some PEEP sometimes help rather than no PEEP? Three reasons:

1. If the ventilator is set to deliver patient-initiated breaths, auto-PEEP is extra impedance that your respiratory muscles have to overcome. Providing a little bit of PEEP bridges that impedance. (This is also how you can conceptualise setting pressure for CPAP in spontenously ventilating asthmatics.)

2. PEEP stents collapsible airways and increases expiratory flow. The lungs are regionally heterogenous and not all airways are trapped in the same way (or even at all).

3. The conventional wisdom is that high tidal volume is bad. But when you have significant airway trapping, tidal volume isn't being evenly distributed across the lungs anyways. You're likely damaging the open (and ventilating) alveolar units even at lower tidal volume. PEEP might help with this. You're trading volutrauma for barotrauma, but Vt is a very misleading measure for potential volutrauma in this situation.

So, instead of AC, which would needlessly risk dynamic hyperinflation, I would go with SIMV with no pressure support and then titrate up the PEEP while watching ventilator cycling pressures. You're hoping to find enough PEEP to reduce work of breathing and distribute tidal volume more evenly across the lungs, but you also don't want to exceed auto-PEEP. PEEP slightly less than auto-PEEP is the sweet spot--enough to bridge impedance and do good stuff for the rest of the lungs--but too much PEEP that it will just exacerbate hyperinflation (as well as cause barotrauma and haemodynamic compromise). So start low, work up, and see what peak dynamic and static cycling pressures do. If they increase in response to PEEP, no beuno. But if they don't, PEEP might be helpful, since auto-PEEP remains the dominant effect (and the waterfall will flow in the right direction). You gotta constantly reassess though.

Marini has a really good paper on this.

Marini JJ. Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years. Am J Respir Crit Care Med. 2011;184(7):756-62.

 

[jdh71]

I think badly applied PEEP will worsen hyperinflation in some.

PEEP helps by dilating collapsed distal airways, but only if PEEP is less than auto-PEEP. Like creating a waterfall that flows from within the trapped airways outward--you're trying to set up a pressure gradient that facilitates airway opening. But why does some PEEP sometimes help rather than no PEEP? Three reasons:

1. If the ventilator is set to deliver patient-initiated breaths, auto-PEEP is extra impedance that your respiratory muscles have to overcome. Providing a little bit of PEEP bridges that impedance. (This is also how you can conceptualise setting pressure for CPAP in spontenously ventilating asthmatics.)

2. PEEP stents collapsible airways and increases expiratory flow. The lungs are regionally heterogenous and not all airways are trapped in the same way (or even at all).

3. The conventional wisdom is that high tidal volume is bad. But when you have significant airway trapping, tidal volume isn't being evenly distributed across the lungs anyways. You're likely damaging the open (and ventilating) alveolar units even at lower tidal volume. PEEP might help with this. You're trading volutrauma for barotrauma, but Vt is a very misleading measure for potential volutrauma in this situation.

So, instead of AC, which would needlessly risk dynamic hyperinflation, I would go with SIMV with no pressure support and then titrate up the PEEP while watching ventilator cycling pressures. You're hoping to find enough PEEP to reduce work of breathing and distribute tidal volume more evenly across the lungs, but you also don't want to exceed auto-PEEP. PEEP slightly less than auto-PEEP is the sweet spot--enough to bridge impedance and do good stuff for the rest of the lungs--but too much PEEP that it will just exacerbate hyperinflation (as well as cause barotrauma and haemodynamic compromise). So start low, work up, and see what peak dynamic and static cycling pressures do. If they increase in response to PEEP, no beuno. But if they don't, PEEP might be helpful, since auto-PEEP remains the dominant effect (and the waterfall will flow in the right direction). You gotta constantly reassess though.

Marini has a really good paper on this.

Marini JJ. Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years. Am J Respir Crit Care Med. 2011;184(7):756-62.

Ha! I'll have to let Jon know someone quoted his paper here. He'll love that.

 

[jdh71]

Discussed this case from Case Files Critical Care with my attending today (pasted if you feel like reading, but if tldr: asthma exacerbation, young person).

A 35-year-old woman with persistent severe asthma is being seen in the ED. On a previous admission, she required mechanical ventilation and was transferred to the intensive care unit (ICU) for treatment of an asthmatic exacerbation. For the past week, she has increased her use of a β2 agonist as rescue medication by 6 to 8 times normal, and has nighttime exacerbations every evening. On physical examination she is in acute respiratory distress with nasal flaring and a quiet chest with very distant wheezing. An ABG drawn on 30% oxygen shows a pH of 7.35, PaCO2 42 mm Hg, PaO2 89 mm Hg, and bicarbonate (HCO3) of 23 mEq/L. Peak expiratory flow rates are all below 40% of the patient's predicted range. Her respiratory rate is 30 breaths/minute, heart rate is 110 beats/minute and regular, and blood pressure is 150/78 mm Hg with a pulsus paradoxus of 10 mm Hg.

My question is regarding the appropriate initial settings for this patient. The Owen book recommends:

§ AC/VC mode
§ VT: 8 mL/kg IBW
§ RR: 10-14
§ I:E ratio: 1:3
§ PEEP: Start with 0.

She says the 6 mL/kg IBW is better these days, with higher RR and PEEP 5.

However, The Ventilator Book asserts that lower VT can lead to air trapping/worsening hyperinflation. Also states that applied PEEP will worsen hyperinflation.

There is no one "right" pat answer in intubated asthma.

Intubation in asthma is usually the beginning of the woes not the end of it. But you do it when you gotta do it. And the real question is are we dealing with a straight forward hypercapnic resp fail asthma exacerbation or is it what I like to call ZOMFGasthma!! (Two exclamation points).

And in my mind it's really a question of are you going to paralyze then or not? If they are spontaneously breathing I match their auto peep with the vent to decrease their work of breathing. If I paralyze them, they get ZERO peep. I agree about TV and RR are kind of driven by the spontaneously breathing patient but as long as pressures aren't too high I let them have big breaths. Most of that pressure is hitting the bronchioles not the alveoli. And yes. Yes. The transplumonary pressure. Keep an eye on your best calculation of if. Esophageal ballon helps dial it a bit more in if you have it available. However, if I have them paralyzed? I start at 8ml/kg and RR to target a MV of around 10. And ride it out.

A better paper than Jon's for the practical application is another UMN Professor's James Leathermsn 2015 review

http://medicinaintensiva.net/wp-con...-severa-y-ventilacion-mecanica-2015-CHEST.pdf

Jim is the best bedside critical care physiologist I ever worked with. Most of the important data in that paper was discovered by him. A lot of the dogma doesn't completely add up.

Bottom line: treat the patient.

 

[lymphocyte]

Ha! I'll have to let Jon know someone quoted his paper here. He'll love that.

Hey thank you. Tell him it's with a lot of gratitude all the way from the middle of nowhere, Australia!!

 

[bashwell]

Hey thank you. Tell him it's with a lot of gratitude all the way from the middle of nowhere, Australia!!

I'm in Australia too. We have some fantastic critical care physicians here as well.

 

[lymphocyte]

I'm in Australia too. We have some fantastic critical care physicians here as well.

It's not like any big RCTs arise from Australia or anything... but we definitely seem to punch above our weight in critical care.

What I meant: I actually did read the Marini paper (and large chunks of his book) from almost literally nowhere. Rural rotation with fly-in fly-out clinics...

 

[jdh71]

Hey thank you. Tell him it's with a lot of gratitude all the way from the middle of nowhere, Australia!!

He started as an engineer. Probably why you like his stuff.