Mechanical Ventilation Oversimplified

Written by: Shawn Luo, MD (NUEM ‘22) Edited by: Sam Stark, MD, MA (NUEM ‘20)
Expert Commentary by: Ruben Mylvaganam, MD

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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:

1. Build initial settings around Volume Control (tidal volume 6-8mL/kg ideal body weight, respiratory rate 16-22, PEEP 5, FiO2 100%)

2. 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

3. Start mechanical ventilation, quickly wean FiO2 for a goal SpO2 of 94-98%

4. Adjust settings further based on clinical response and ABGs

5. 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.)

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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

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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

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ECMO Initiation in the ED

Written by: Kaitlin Ray, MD (NUEM PGY-3) Edited by: Evan Davis, MD (NUEM Alum ‘18) Expert commentary by: Colin McCloskey, MD (NUEM Alum ‘16)

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ECMO Initiation in the Emergency Department

Introduction:

Extracorporeal membrane oxygenation (ECMO) provides prolonged cardiopulmonary support in severe acute respiratory or cardiac failure. As the science behind ECMO continues to grow and with promising data regarding its use in acute hypoxemic respiratory failure, cardiac arrest, and cardiogenic shock, ECMO use in the United States has increased over 400% in the last ten years. This has stimulated an interest in earlier applications of ECMO both in the emergency department (ED) and even in the prehospital setting [1]. Initiation of ECMO in the ED is a relatively new development, with 65% of programs <5 years old and the majority of programs with <3 cases per year. However, this number continues to grow [2].

There two main types of ECMO: venoarterial (VA) and venovenous (VV). While VV ECMO provides respiratory support, only VA ECMO provides both respiratory and hemodynamic support. ECMO drains blood from the native vascular system, then circulates it outside of the body via a mechanical pump where it passes through an oxygenator and heat exchanger. Hemoglobin then becomes saturated with oxygen, CO2 is removed, and blood then reinfuses into the circulation [4]. Venoarterial ECMO is more commonly utilized in the emergency department as eligible ED patients often have concurrent hemodynamic and respiratory collapse.

Emergency medicine physicians have an increasing responsibility to initiate ECMO and/or make the decision to transfer to an ECMO capable facility. Knowing this, it is critical that we are familiar with the types of ECMO available, understand the indications, contraindications, risks, benefits, and logistics of initiating this form of extracorporeal life support.

Indications/Contraindications:

The specific criteria and contraindications to ECMO vary from institution to institution, often making the decision and ability to initiate ECMO challenging. The Extracorporeal Life Support Organization (ELSO) provides specific guidelines for ECMO initiation. These indications include cardiogenic shock as defined as (1) hypotension and low cardiac output with inadequate tissue perfusion despite adequate intravascular volume and (2) persistent shock despite volume, inotropes, pressors, and possibly an intraaortic balloon pump. The ELSO also provides guidelines for ECMO in acute respiratory collapse, as well as contraindications for ECMO including: unrecoverable heart failure and not a candidate for transplant or LVAD, advanced age, chronic multi-system failure, compliance issues, terminal malignancy and prolonged CPR without adequate tissue perfusion. 

As emergency physicians, which patients should we consider as candidates for ECMO? Generally speaking, consider younger and healthier patients who experience an acute but reversible insult leading to rapid cardiopulmonary collapse.  Recall that ECMO should only be considered as a bridge to more definitive therapy. Examples of scenarios that fit this criterion include:

• Massive pulmonary embolism

• Myocardial infarction causing V-Tach/V-Fib or cardiogenic shock

• Acute myocarditis or cardiomyopathy causing cardiogenic shock

• Drowning

• Hypothermia

• Drug overdose causing cardiovascular collapse (such as beta-blocker or Ca-channel blocker)

• Massive smoke inhalation, pulmonary contusion, or pulmonary hemorrhage causing refractory hypoxemia

As mentioned above, generally those with more chronic conditions such as end-stage heart failure, end-stage COPD, or those with chronic multi-organ failure, do not make good ECMO candidates. Other patient populations to consider in an ICU rather than ED setting include those with septic shock and/or ARDS. Note that patients with traumatic injury leading to hemorrhagic decompensation, although acute in nature, typically are not good candidates for ECMO as ECMO does not prevent further blood loss.

Additionally, we must consider what an ECMO-eligible patient clinically looks like. The majority of the patients who present with one of the above conditions will either be responsive to conventional therapies (intubation, fluids, inotropes), or they will be dead on arrival. However it is the rare, in-between patient that should be considered for ECMO. The condition of a good candidate would include things like: 

• Persistent hypotension despite maximum conventional therapy

• Persistent hypoxemia despite maximum ventilator therapy

• Patients brought in in cardiac arrest but achieve periods of unsustained ROSC

• Patients brought in with vitals but arrest in the ED

Unfortunately patients who arrest in the field, are brought to the ED already in cardiac arrest, or who do not achieve ROSC despite 30-45 minutes of well executed ACLS are unlikely to be appropriate ECMO candidates. The critical ECMO population is truly those who are flirting with life vs. death, especially the patients with intermittent periods of ROSC. Key exceptions to this include drowning and/or hypothermic patients. Generally these patients are better candidates for ECMO even if there has not been a recorded pulse, with the caveat that they should have been pulled out of the water or other environment quickly.

Risks/benefits:

ECMO is unique in that it provides full cardiopulmonary support without the physical trauma of chest compressions, thereby decreasing trauma, stress, and number of interruptions. Additionally, it provides a higher flow state than would otherwise be provided by manual compressions [2]. VV ECMO also minimizes barotrauma, volutrauma, and oxidative stress. However ECMO is not without risks and complications. The risk of bleeding is significant in the context of continuous anticoagulation and platelet dysfunction. Thromboembolism may lead to stroke or limb ischemia [4]. Infection may also occur secondary to indwelling lines/tubes [1].

Logistics:

ECMO is a costly intervention that requires a multi-disciplinary approach and an organizational commitment in order to proceed. Consideration must be given to the required equipment, blood bank capabilities, cannulation, and personnel availability. In order for ECMO to be successfully initiated from the ED, coordination between EMS, emergency medicine physicians, the cath lab, nursing staff, neurocritical care, cardiothoracic surgery, and the ICU is required [3]. When cannulating for VV ECMO, one may use a two cannula approach (femoral vein and internal jugular/SVC), or a single dual-lumen cannula (right atrium/IVC via the IJ). VA ECMO typically involves cannulation through the femoral artery and femoral vein [1]. If CPR is ongoing during cannulation attempts, programs may use a modified ACLS algorithm with a continuous epinephrine infusion at 0.7 mg/kg/min and minimization of pulse checks by utilizing continuous TEE monitoring [3]. Aggressive anticoagulation is required with continuous infusion of either unfractionated heparin or direct thrombin inhibitor, and efforts should be made to maintain platelet counts >50K and hemoglobin >12 mg/dL6,7.

Recap:

While initiation of ECMO from the emergency department is still a relatively new endeavor for many certified ECMO centers, the ED is in a unique position to bridge select patients in acute respiratory or cardiac failure to recovery using ECMO.  While institutional criteria for ECMO varies, the ELSO guidelines may be used as a reference to guide decision making in the absence of formal criteria. Generally speaking, pursue ECMO for younger, healthier patients with acute hemodynamic and/or respiratory collapse that is potentially reversible and unresponsive to conventional therapies. Typically patients with massive PE, MI, hypothermia, drowning, acute cardiomyopathy are the best candidates for ED ECMO. Contraindications generally include severe neurologic injury, end stage malignancy, advanced age, and irreversible multi-organ failure. Knowing that the emergency physicians are often the first to receive patients in acute cardiac and respiratory failure, it is critical that we are familiar with the types of ECMO available, understand the indications, contraindications, risks and benefits, and logistics of initiating this form of extracorporeal life support.

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Expert Commentary 

This is a thoughtful and thorough overview of ECMO within the emergency department. I will limit my commentary to VA ECMO for cardiopulmonary failure (ECPR), given the enthusiasm for the topic in the FOAM world and my experience within a ED based ECMO program. Some broad themes I would like to highlight: Evidence, Patient selection and Systems of Care.

Evidence: There is a signal that ECPR is better than conventional CPR. A systematic review and metanalysis found that those with cardiac arrest who received VA ECMO had an association with increased neurologically intact survival, with a number needed to treat of 7 [1]. However, most of the data is retrospective and from single centers, making it subject to a number of confounders, as well as selection bias. Further, those who received ECPR were more likely to receive therapeutic hypothermia and percutaneous catheterization, both interventions known to improve outcome following cardiac arrest.  Another single center experience has been promising, with 9/18 patients surviving to hospital discharge with good neurological outcome [2]. This protocol involved EMS bypassing the ED and taking the patient to the cardiac catheterization lab where they were placed on ECMO and underwent catheterization. Those who had good outcome all had concomitant intervenable coronary artery disease. There are several centers that have similar experiences with published case series [3,4], but it depends thus far on quality patient selection and a viable system of care.

Patient Selection:  All the above trials had strict inclusion and exclusion criteria. Most established protocols include an age cutoff (65-75 depending on center), initial shockable rhythm, and a time from arrest to cannulation between 30-60 minutes. Pertinent exclusions include advanced comorbidities, initial asystole or prolonged downtime. This is done with the intent of cannulating patients with the best chance of surviving their ECMO run; namely young patients with likely coronary artery disease who need ECMO as a bridge to cardiac catheterization. VA ECMO’s other successful ED applications, namely pulmonary embolism [5], drug overdose [6], and acute myocarditis [7] all share the commonality that ECMO provides time for recovery or as a bridge to a viable intervention. A bridge to recovery must exist prior to any cannulation scenario; this cannot be understated.

Systems of care: Cannulation is just one step in a VA ECMO patients hospital course. When conceptualizing a successful ED ECMO program, the institutional commitment should be visualized: A patient requiring 5-7 days in the ICU, formal neuroprognostication and continuous goals of care discussions with family. You rightly include a logistics session in your review, but this system of care is paramount to a successful ECMO program. Prehospital EMS systems must be designed for quick recognition and transport of ECMO candidates.  Emergency physicians need to be trained, and maintain competency in ECMO cannulation; interventional cardiology must be willing to catheterize appropriate patients; ICU consultants must have a standardized protocol for post-arrest care and neurology/ICU must have an institutionally accepted neuroprognostication scheme. In parallel to this, the family discussions regarding prognosis and any transition of care should include social work, case management and palliative care professionals. Cannulation is as exciting a procedure an emergency physician can perform, but without a thoughtful, multidisciplinary system of care, these patients will do poorly.

In closing, VA ECMO in the emergency department is an exciting development to tertiary ED practice. More experience, and more data, will help define the niche of patients and the necessities of post-arrest care that provide these patients with the best outcome.

1. Ouweneel DM, Schotborgh JV, Limpens J, et al. Extracorporeal life support during cardiac arrest and cardiogenic shock: A systematic review and meta-analysis. Intensive Care Med. 2016;42(12):1922-1934.

2. Yannopoulos D, Bartos JA, Martin C, et al. Minnesota resuscitation consortium's advanced perfusion and reperfusion cardiac life support strategy for out-of-hospital refractory ventricular fibrillation. J Am Heart Assoc. 2016;5(6):10.1161/JAHA.116.003732.

3. Stub D, Bernard S, Pellegrino V, et al. Refractory cardiac arrest treated with mechanical CPR, hypothermia, ECMO and early reperfusion (the CHEER trial). Resuscitation. 2015;86:88-94.

4. Bellezzo JM, Shinar Z, Davis DP, et al. Emergency physician-initiated extracorporeal cardiopulmonary resuscitation. Resuscitation. 2012;83(8):966-970.

5. Yusuff H, Zochios V, Vuylsteke A. Extracorporeal membrane oxygenation in acute massive pulmonary embolism: A systematic review. Perfusion. 2015;30(8):611-616.

6. Wang G, Levitan R, Wiegand T, et al. Extracorporeal membrane oxygenation (ecmo) for severe toxicological exposures: Review of the toxicology investigators consortium (toxic). Journal of Medical Toxicology. 2016;12(1):95-99.

7. Nakamura T, Ishida K, Taniguchi Y, et al. Prognosis of patients with fulminant myocarditis managed by peripheral venoarterial extracorporeal membranous oxygenation support: A retrospective single-center study. Journal of intensive care. 2015;3(1):5.

Colin McCloskey, MD
NUEM Alum ‘16, Critical Care Anesthesiology fellow - University of Michigan Medical Center/University of Michigan Health System

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How To Cite This Post

[Peer-Reviewed, Web Publication]  Ray K, Davis E (2018, November 12). ECMO Initiation in the ED  [NUEM Blog. Expert Commentary by McCloskey C]. Retrieved from http://www.nuemblog.com/blog/ECMO

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Resources

1. Mosier, Jarrod M., et al. “Extracorporeal Membrane Oxygenation (ECMO) for Critically Ill Adults in the Emergency Department: History, Current Applications, and Future Directions.” Critical Care, BioMed Central, 17 Dec. 2015, ccforum.biomedcentral.com/articles/10.1186/s13054-015-1155-7.

2. Tonna, Joseph E., et al. “Practice Characteristics of Emergency Department Extracorporeal Cardiopulmonary Resuscitation (ECPR) Programs in the United States: The Current State of the Art of Emergency Department Extracorporeal Membrane Oxygenation (ED ECMO).” Resuscitation, Elsevier Ireland Ltd, 10 Sept. 2016, experts.umich.edu/en/publications/practice-characteristics-of-emergency-department-extracorporeal-c.

3. Tonna, Joseph E., et al. “Development and Implementation of a Comprehensive, Multidisciplinary Emergency Department Extracorporeal Membrane Oxygenation Program.” Annals of Emergency Medicine, Mosby Inc., 1 July 2017, cwru.pure.elsevier.com/en/publications/development-and-implementation-of-a-comprehensive-multidisciplina.

4. Bartlett, Robert. Extracorporeal Membrane Oxygenation (ECMO) in Adults, 16 June 2017, www.uptodate.com/contents/extracorporeal-membrane-oxygenation-ecmo-in-adults.

5. Extracorporeal Life Support Organization - ECMO and ECLS. “Guidelines for Adult Respiratory and Cardiac Failure.” Extracorporeal Life Support Organization - ECMO and ECLS > Resources > Guidelines, ELSO (Extracorporeal Life Support Organization), Dec. 2013, www.elso.org/Resources/Guidelines.aspx.

6. Sklar MC, Sy E, Lequier L, et al. Anticoagulation Practices during Venovenous Extracorporeal Membrane Oxygenation for Respiratory Failure. A Systematic Review. Ann Am Thorac Soc 2016; 13:2242.

7. Spinelli E, Bartlett RH. Anemia and Transfusion in Critical Care: Physiology and Management. J Intensive Care Med 2016; 31:295.