I am a Pulmonary and Critical Care Physician with a love for teaching people from all walks of life through my instagram page @pulmcritdoc. I enjoy brunching, working out, pumpkin spice lattes and everything basic in between, all while nerding out over resuscitation, airway, and anything to do with lungs.
The Pre-brief
In volume control ventilation, changes in resistance and compliance cause changes in the measured peak and plateau airway pressures. Once we understand these measurements we can recognize patterns to diagnose changes in the patient/ventilator system.
Definitions
Peak pressure: This is the pressure that is generated by the ventilator to overcome BOTH airway resistance AND alveolar resistance. The goal of the pressure is to obtain the set inspiratory flow and the tidal volume goal that is set by the provider.
As noted in our last post about peak pressures, the formula for peak pressure with a square waveform is:
Peak inspiratory pressure = (Resistance x Flow) + (Elastance of respiratory system x Tidal volume) + PEEP
Hence, if any of these variables increase for reasons that we will discuss below, the peak pressure will go up. Keep in mind that you’ll never need to calculate this or use a ventilator maneuver to obtain this number. This number is seen on the ventilator.
Plateau pressure: This is the pressure that is essentially left over in the lung after the tidal volume has been delivered.
Plateau pressure = Tidal Volume/Compliance
At zero flow
In order to measure this, an inspiratory hold has to be done on the ventilator. This is where the ventilator pauses for several seconds after the tidal volume has been delivered, which eliminates flow, and thus, airway resistance.
This number is a reflection of the compliance of the lung. If the lung is very compliant as in patients with COPD, the plateau pressure is lower. If the lung is not compliant, as in patients with ARDS or fibrotic lung disease, the plateau pressure will be high.
Elevations in the Peak and Plateau Pressures:
High Peak/Normal Plateau: issue with elevated resistance
There is resistance in the ETT or distal bronchus, which decreases the diameter and thus requires the ventilator to increase pressure to achieve the same desired tidal breath. Below are common issues with isolated high peak pressures and solutions to these issues.
- Kink in the circuit: examine ventilator tubing
- Fluid accumulation: clear the fluid from the circuit
- Biting the ETT: increase sedation or insert bite block
- A small ETT with biofilm forming: consider changing out the tube
- High flow rate or tidal volume: adjust ventilator settings
- Ventilatory asynchrony: increase sedation
- Laryngospasm or bronchospasm: consider steroids, epinephrine
- Mucous plugging: clear out ETT
- Foreign body: removal
High Peak and high plateau pressures: issue with compliance
For the reasons stated below, there may be an issue with low compliance of the lungs, which forces the ventilator to have an increase the pressure.
- Pneumonia: if this has been gradually worsening, consider a ventilator associated pneumonia. Obtain chest x-ray, respiratory cultures, and start broad spectrum antibiotics
- Pulmonary edema: Consider diuresing and adjusting PEEP
- Auto PEEP: this is seen in breath stacking where the patient does not have a full exhalation. It is common in COPD and reactive airway diseases. Adjust the PEEP, tidal volumes, or inspiratory:expiratory time ratio. Consider sedation as well. If very severe, this can be immediately corrected by removing the patient from the ventilator (and manually ventilating), while allowing the air to leave the circuit.
- Right main stem intubation: retract endotracheal tube
- Pneumothorax: This should be considered with sudden onset hypoxia and hypotension. Evaluate with ultrasound and/or chest xray. Resolution is attained with decompression and chest tube placement.
- Atelectasis, pulmonary fibrosis, and even abdominal issues like abdominal compartment syndrome can increase the plateau pressures.
For more on pressures, compliance, and resistance, check out the recent post by Aman Thind HERE.
The Debrief
- Peak pressure: This is the pressure that is generated by the ventilator to overcome BOTH airway resistance AND alveolar resistance.
- Plateau pressure: This is the pressure that is essentially left over in the lung after the tidal volume has been delivered.
- High Peak pressures and normal plateau indicate an issue with elevated resistance
- High Peak and high plateau pressures indicate an issue with compliance
References
- Al-Rawas N, Banner MJ, Euliano NR, Tams CG, Brown J, Martin AD, Gabrielli A. Expiratory time constant for determinations of plateau pressure, respiratory system compliance, and total resistance. Crit Care 2013;17(1):R23.
- Amato MBP, Meade MO, Slutsky AS, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015 Feb 19;372(8):747-755.
im an anesthesia resident and this was really helpful in breaking down these concepts. Thank you!
Thank you for your post however there’s one concept that’s bogging me that I hope you may help me with. If we model the lungs to a balloon, why is the peak pressure not equal to plateau pressure?
Let me give the question more context for why I’m having trouble understanding that peak pressure is greater than plateau pressure.
A quick search yielded that the plateau pressure is lower because the pressure equilibrates and so it becomes lower than peak pressure but that makes no sense. Another explanation/analogy I’ve read was, “it takes high pressure to inflate the balloon, but once inflated, a lot less effort to keep it inflated.” This also doesn’t make any sense because pressures don’t work that way. If the volumes are the same the pressures are the same.
Here’s my rational. If the lungs were a balloon and I inflated that balloon with a set volume of air and then tied the balloon, the pressures reached just before finishing inflation and after tying it are equal because the volumes haven’t changed. Similarly, I’m thinking in the lungs at the end of inhalation (i.e., peak pressure) and after the breath hold (i.e., plateau pressure) the volume of air doesn’t change, hence the two pressures should be equal, unless gas exchange is occurring and there’s a net volume of gas entering the blood stream resulting in lower pressure after the breath hold, however I’m not sure if that can account for the difference between the two pressures.
I came across this concept on a uworld 2ck question. The given explanation on the Q bank wasn’t clear. I came across your description via a random google search of keywords. And this write-up made everything so clear now. THANK YOU!