Written by: Shawn Luo, MD (NUEM ‘22) Edited by: Sam Stark, MD, MA (NUEM ‘20)
Expert Commentary by: Ruben Mylvaganam, MD
The ventilator: we’ve all learned about it - the lectures, the bed-side demonstrations on those mind-numbingly long ICU rounds. But we were also told, repeatedly, “Don’t touch it!” Unless you are an attending, fellow, or respiratory therapist (RT) of course. So for a lot of us, the ventilator is a black box, mythical and intimidating.
In this blog, I hope to demystify ventilators a little so when duty calls, you can set initial settings and make some basic adjustments.
Physiology
1. How Mechanical Ventilation affects Oxygenation: PEEP & FiO2
You can reference this nice ARDSnet table for FiO2/PEEP combinations.
FiO2 – its effect is immediate
PEEP – takes up to an hour to show full effect
Therefore, when weaning, wean FiO2 before weaning PEEP so that if the patient desaturates, you have room to go up on FiO2.
2. How Mechanical Ventilation affects Ventilation: Tidal Volume, Respiratory Rate, Inspiratory Pressure or Inspiratory Time
This should be titrated in response to the patient's CO2 levels. Patients in respiratory failure from profound metabolic acidosis will need you to set higher minute ventilation to attempt respiratory compensation.
3. Peak Pressure and Plateau Pressure
Peak pressure is the summation of both airway resistance (dynamic compliance) and plateau pressure (static compliance). Most modern ventilators will automatically report peak pressures without any special maneuvers required. When thinking about airway resistance, think of when you blow air through a straw – the narrower the tubing the higher the resistance and thus a lot of pressure is needed to generate that flow. To measure airway resistance, have the RT set the flow rate to 60 LPM, adjust the flow pattern to a square wave form, and ask them to perform an inspiratory hold.
Plateau pressure is related to lung compliance (higher plateau pressure = less compliant lung). It is the pressure “felt” by the alveoli, and keeping it less than 30 cm H2O helps to prevent barotrauma. It’s only measured after the air stops moving (via an inspiratory hold maneuver – ask RT how to do this on your ventilator) so that dynamic airway resistance is not a factor.
4. Breath-stacking / Auto-PEEP
This occurs when the patient does not have enough time to finish exhalation before the next breath is delivered. This results in progressive hyperinflation of the lung, high peak pressures, and eventually hemodynamic collapse if not identified and intervened upon. It is most common in obstructive airway diseases such as asthma and COPD. Be vigilant for the flow diagram below on the ventilator to detect it early.
Modes
Volume vs Pressure – WHAT TYPE of breath is targeted
Volume mode means the vent will deliver a set tidal volume of air and results in whatever pressure (i.e. stiffer lungs result in higher pressure)
Pressure mode in turn means the vent will deliver at a set inspiratory pressure, and results in whatever volume (i.e. stiffer lungs result in lower volume)
A/C (Assist/Control) vs Support – WHEN the breath is delivered
In A/C mode, the machine delivers breath at a pre-set frequency (control), but the patient can also trigger additional breaths (assist) to faster than the set frequency. A quick and dirty trick is that any mode that contains the word “Control” means there will be a minimal respiratory rate set by the clinician.
Support (or Spontaneous) mode, in turn, will only deliver a breath when the patient initiates it. It senses the negative pressure generated by the patient and delivers a breath. If the patient does not breathe, it will not deliver. Usually safety back-up is in place to prevent prolonged apnea.
Volume Control
Delivers set tidal volume at or above a set rate
You set: tidal volume (6-8mL/kg ideal body weight), respiratory rate (16-22 breaths per minute), flow rate (60-80 LPM), and PEEP & FiO2 as needed
Check: Plateau pressure <30 (inspiratory hold maneuver)
This is a good initial setting for most of the patients you just intubated
Pressure Control
Delivers set pressure at or above set rate
You set: inspiratory pressure (5-15 cm H2O), inspiratory time (“I-time”; 0.6-0.8), respiratory rate (16-22), PEEP & FiO2 as needed
Check: to make sure the patient is getting tidal volumes of 6-8 mL/kg
This can be a helpful setting in some patients that do not tolerate volume control. Adjust pressure support to achieve tidal volume of 6-8 mL/kg while ensuring total pressure is less than 30-35 cm H20.
Pressure Support
Delivers set pressure when the patient initiates a breath to help the patient move the air
You set: Pressure support (5-15 cm H2O), PEEP & FiO2 as needed
Check: to make sure the patient is getting tidal volumes of 6-8 mL/kg
Usually a weaning mode to check if the patient is likely to tolerate extubation
*The bottom line is, by adjusting the parameters, you can achieve the same result with different ventilation modes.
My step-wise approach to initiate mechanical ventilation on most patients:
Build initial settings around Volume Control (tidal volume 6-8mL/kg ideal body weight, respiratory rate 16-22, PEEP 5, FiO2 100%)
Tweak according to patient’s clinical scenario – e.g. higher respiratory rate for acidotic patients, higher initial PEEP for hypoxemic respiratory failure, longer expiratory time for asthmatics/COPD patients with auto-PEEP
Start mechanical ventilation, quickly wean FiO2 for a goal SpO2 of 94-98%
Adjust settings further based on clinical response and ABGs
When in doubt, disconnect and bag the patient.
References:
The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342:1301-1308.
Weingart, S. Managing Initial Mechanical Ventilation in the Emergency Department. Annals of Emergency Medicine, Volume 68, Issue 5, November 2016, Pg 614-617
Hyzy, R. Modes of Mechanical Ventilation. In: UpToDate, Parsons P. Finlay G (Ed), UpToDate, Waltham, MA. (Accessed on May 5, 2020.)
Expert Commentary
Thank you for the opportunity to review this very helpful and concise review on the basics of invasive mechanical ventilation. I hope to make this commentary brief, a contrast to our notoriously long ICU rounding habits. I would recommend any reader to view this editorial for a more in depth and nuanced understanding of mechanical ventilation. (1)
As you have described above, one way in which to think about mechanical ventilation is in the context of the most common scenarios in which we implement it, ie: hypoxemia and hypercapnia. Understanding that for hypoxemic patients, our tools to improve physiology are by manipulating the set FiO2 and PEEP to achieve specified targets for oxyhemoglobin saturation or P/F ratios (with regard to ARDS management). It is important to note that a few studies have demonstrated that an FiO2 greater than 50-60% can be toxic and may result in an increase in reactive oxygen species, increased airway damage (tracheobronchitis), and secondary infection from impaired bactericidal action of immune cells. (2,3) For our hypercapnic patients, knowing their prior baseline PCO2 is helpful in determining how to adjust the respiratory rate and tidal volume to appropriately improve their respiratory acidosis.
An important common 3 part methodology to better appreciate modes of mechanical ventilation is understanding the “trigger”, “target”, and “cycle” of each ventilator mode. In the simplest of terms, the “trigger” is what prompts the ventilator to deliver the breath (ie: an assisted breath when the ventilator senses a patient generated decrease in flow/pressure or a control breath when enough time has elapsed as mandated by the set respiratory rate). The “target” is what the ventilator aims to achieve with each breath (in the mode of AC-VC: a targeted flow rate [often ~60 L/min] or in the mode of AC-PC: a targeted inspiratory pressure [often ~15 cwp]). Finally, the “cycle” is a term that describes how the ventilator recognizes when it is time to terminate the breath that is delivered (in the mode of AC-VC: cycling off after the goal TV is reached [~600cc] or in the mode of AC-PC: cycling off after the set inspiratory time has occurred [~ 0.7 seconds]). See table below for a quick summary.
Finally, the best practical way to simplify mechanical ventilation is to request the changes by the respiratory therapist and see the effects. I encourage you to interpret all VBGs and ABGs, approach your respiratory therapist, pulmonary/CCM fellow, and suggest everything from initial ventilator settings, changes to both modes and individual parameter settings, and see the reflection of this work in your subsequent blood gases.
References
1. Walter JM, Corbridge TC, Singer BD. Invasive Mechanical Ventilation. South Med J. 2018 Dec;111(12):746-753. doi: 10.14423/SMJ.0000000000000905. PMID: 30512128; PMCID: PMC6284234.
2. Suttorp N, Simon LM. Decreased bactericidal function and impaired respiratory burst in lung macrophages after sustained in vitro hyperoxia. Am Rev Respir Dis. 1983 Sep;128(3):486-90. doi: 10.1164/arrd.1983.128.3.486. PMID: 6311064.
3. Griffith DE, Garcia JG, James HL, Callahan KS, Iriana S, Holiday D. Hyperoxic exposure in humans. Effects of 50 percent oxygen on alveolar macrophage leukotriene B4 synthesis. Chest. 1992 Feb;101(2):392-7. doi: 10.1378/chest.101.2.392. PMID: 1310457.
Ruben Mylvaganam, MD
Instructor of Medicine
Department of Pulmonology & Critical Care Medicine
Northwestern Memorial Hospital
How To Cite This Post:
[Peer-Reviewed, Web Publication] Luo, S. Stark, S. (2021, Nov 1). Mechanical Ventilation Oversimplified. [NUEM Blog. Expert Commentary by Mylvaganam, R]. Retrieved from http://www.nuemblog.com/blog/mechanical-ventilation-tips
