An Outstanding Introduction To ECMO: #79: ECMO Essentials with PedsCrit – It’s All Trains and Potatoes From The Cribsiders

Post transcription from 17:00 to 19:00.

Today I reviewed, embedded, and excerpted the outstanding show notes from The Cribsiders Podcast, #79: ECMO Essentials with PedsCrit – It’s All Trains and Potatoes.* March 1, 2023 | By Sam Masur.

ECMO stands for Extracorporeal Membrane Oxygenation.

*Hodges Z, Fauman K, Shanklin A, Masur S, Chiu C, Berk J. “#79: ECMO Essentials with PedsCrit – It’s All Trains and Potatoes”. The Cribsiders Pediatric Podcast. https:/www.thecribsiders.com/ March 1, 2023.

All that follows is from the above resource.

Summary

Do the terms “sweep” and “chatter” set you on edge?Uncertain how to interpret the countless monitors and labs for your patients on ECMO? Join us in a collaboration with the PedsCrit Podcast, as our guest, Dr. Karen Fauman, Pediatric Intensivist, Pediatric ECMO Medical Director, Director of Pediatric Palliative Care, Program Director of the PICU Fellowship at Comer Children’s Hospital, and ECMO Enthusiast, breaks down the basics of ECMO care! Listen as she outlines everything you need to know, from oxygen delivery to circuit liberation.

ECMO Pearls

1. ECMO [Extracorporeal Membrane Oxygenation] is not a treatment in and of itself but a bridge to recovery or definitive treatment. The goal is to rest the lungs and/or heart as they recover, bridge to transplant, or to stabilize while further workup may be ongoing.
2. VV ECMO is for isolated respiratory failure, while VA ECMO is for cardiac support, both cardiac and respiratory support, or respiratory support for infants (due to size limits) or other patients with vascular limitations preventing VV support.
3. Important ECMO circuit parameters and labs to trend include flow, sweep, RPMs, pressure monitors, patient ABG, and pre and post-membrane gases.
4. On VV ECMO we tolerate oxygen saturations lower than you’d typically feel comfortable with – goal saturations should be in the 80s. Try not to reflexively increase the FiO2 on the ventilator!
5. Watch out for a reversal of the SvO2 and arterial oxygenation saturation for a patient on VV ECMO – it is likely a sign of re-circulation and means you are oxygenating the circuit and not the patient.

ECMO Notes

What is ECMO and When Do We Use It?

ECMO, or extra-corporeal membrane oxygenation, is a form of life support in which venous blood is removed from a patient, gets oxygenated, CO2 is removed, and then is returned to the patient. The oxygenated blood is either returned to the venous system (VV ECMO), or arterial system (VA ECMO). It is used for refractory respiratory or hemodynamic failure, or both. Some indications include myocarditis, post-operative congenital heart disease, septic shock, toxic ingestions, congenital diaphragmatic hernia, pneumonia, pulmonary hypertension, and ARDS. 

2 Main Types of ECMO:

VA ECMO:

• Indicated for patients requiring cardiac support, cardiac and respiratory support, or isolated respiratory support for babies too small for VV cannulas/others for whom VV cannulation isn’t possible. Cannulation is most often via percutaneous approach or cut-down in the neck or groin. Common cannulation strategies include internal jugular vein/carotid artery, femoral artery/femoral vein, or central canulation (right atrium and aorta) through a median sternotomy most commonly in post-operative cardiac patients.

VV ECMO:

• Indicated for patients with isolated respiratory failure. Cannulation is most often by single site dual lumen cannula, which can remove deoxygenated blood from the SVC and IVC and returns it through a second lumen with flow directed towards the tricuspid valve. If two site cannulation strategy is chosen, blood removal through femoral vein and return through internal jugular vein is most common.
• With VV ECMO there is a risk that the oxygenated blood that is being returned can get pulled back into the circuit due to the proximity of the drainage and return cannula or lumens, in which case there could be decreased oxygen delivery to the patient. This is called “recirculation.”

Recalling The Basics of Oxygen Delivery

Understanding ECMO requires an understanding of oxygen delivery, which is dependent on cardiac output and the oxygen content of the blood, or:

Oxygen delivery = (SV x HR) x (Hgb x SaO2 x 1.34) + (PaO2 x 0.003)

This equation can be difficult to conceptualize, so Dr. Fauman teaches using a metaphor for oxygen delivery:

Imagine that that there is a kingdom made up of a series of villages (the body and its organs). These villages are all connected by a train system (the cardiovascular system). The villagers (cells) only eat potatoes (hemoglobin bound oxygen, or O2), which must be delivered to them. There is a main train station (the heart) that sends out a train (blood) with many train cars (hemoglobin), and when the kingdom is enjoying good times these train cars are filled to the brim with potatoes (oxygen-bound-hemoglobin). In order to keep the kingdom happy, the train must return to the station with a surplus of potatoes, about 75-80% of the original cargo (aka central venous O2 saturation). If the surplus begins dropping then the main station has to figure out how to get more potatoes delivered.

If you have a patient who – despite loading up the train cars with extra potatoes (increasing oxygen saturation), increasing its speed (vasoactives) and adding more train cars (hemoglobin), has a dwindling surplus upon arrival to back to the train station (SvO2 ~50-60% and lower) they are in shock and may need ECMO.

The ECMO Circuit

There are a variety of variations of the ECMO circuit, but they all consist of:

• The venous, or drainage cannula (from patient to circuit)
• The oxygenator or “membrane lung” (which allows for gas exchange)
• The pump (pressurizes the system allowing for forward flow)
• The “arterialized,” or return cannula (from circuit to patient)
• ECMO specialists! They monitor all circuit parameters (and keep an eye on patient vitals) constantly, are a wealth of knowledge for residents and fellows, and are a critical part of ECMO care.

ECMO Parameters and Pressure Monitors

• Flow (L/min)– The rate that blood moves though the pump. “Chugging” or “chatter” means that the circuit is trying to get more flow but is unable to achieve this (patient needs more volume or goal flow should be lower)  
• Sweep (L/min) – Controls how much CO2 the circuit removes.  As the sweep rises, CO2 deceases; you “sweep” away the CO2, so to speak. 
• RPM (revolutions per minute) – The circuit pump speed. If set too high, the circuit will spin without providing increased flow and will create increased hemolysis. 
• Pump inflow and outflow monitors – Inflow tells you about preload: does the circuit have enough volume to maintain what it is doing? If it’s too negative (not enough preload) it can lead to cavitation and air entrainment. Outflow tells you about the pump afterload. If it’s too high it may indicate any number of things including volume, a potential clot in the return cannula, that a cannula is too small, or that systemic vascular resistance is high.
• Pre and Post Oxygenation Membrane – As the delta pressure between these monitors rises, it can indicate more clot burden within the membrane. This can lead to circuit associated DIC and ongoing hemolysis. Worsening clot burden in the oxygenator will eventually require replacement.

Initiation, Management, and Liberation from ECMO

ECMO Candidacy

While the criteria for ECMO utility may be hotly debated, Dr. Fauman outlines some generally agreed upon criteria for “good candidates,” meaning they are likely to be liberated from ECMO and do well. These include: patients with a reversible condition, no other underlying medical conditions, organ failure limited to cardiac and respiratory system, an intact immune system, and an expected course of less than a few weeks. There are some limitations based on size (tends not to be available if infants are less than 1.8 kg or less than 34 weeks post-conceptual age due to risk of IVH).

ECMO Initiation

Cardiac Failure or Hemodynamic Collapse

ECMO is appropriate when the patient is unable to be supported from a hemodynamic standpoint – when despite multiple vasopressors they are unable to maintain the body’s metabolic demands1.

Respiratory Failure

Historically the oxygenation index (OI) has been used to determine severity of respiratory failure and potential need for ECMO. Typically, VV ECMO is considered as the OI approaches 40 despite optimal ventilator management; this value is generally used to determine initiation in infants and small children.

Considerations for ECMO Management 

While caring for a patient on ECMO, Dr. Fauman notes that there are three of blood gases that are of particular importance.

• Patient’s arterial blood gas (ABG) – it is a reflection of patient oxygen delivery.
• Pre-membrane (deoxygenated, or venous) gas – will give you the “central venous” oxygen saturation (SvO2).  ECMO goal is generally above 60% or 20-30% below arterial saturation (SpO2). As the SvO2 decreases below this target it suggests inadequate ECMO support/oxygen delivery.
• Post-membrane (oxygenated, or arterialized) gas – the PaO2 measures efficacy of the membrane lung. An effective oxygenator requires a low FiO2 of inflow gas to provide an adequate PaO2.  As the clot burden in the oxygenator worsens it will become less efficient and require a higher FiO2 to maintain the same level of oxygenation.

Other considerations include:

• Anticoagulation (see PedsCrit “ECMO Anticoagulation” episode for more information)
• Sedation (though as time passes, the goal is mobilization – some units have patients riding tricycles!)
• Infection Risk – Patients have an increased risk of infection and it is also harder to determine when a patient is infected, since many markers we rely on are harder to trend or interpret (ex. fever, leukocytosis, blood pressure changes).
• Ventilator Settings – Rest settings for patients with respiratory failure (ex. PEEP of 10, inspiratory pressure of 10-15, rate of 10-15, FiO2 of 40% or less). If no respiratory failure, the goal is to maintain functional residual capacity at a level that will keep alveoli recruited but doesn’t increase RV afterload.

Patience, more Patience, and Liberation. 

Dr. Fauman notes that one of the more challenging parts of caring for these patients is waiting. It is difficult, but crucial, to wait (and wait and wait) to allow the lungs and heart to rest.

So, when do you know it’s time to move toward liberation?

VA ECMO – Watching for Cardiac Recovery

Cardiac recovery can be tracked by a few markers. Things to watch for include increasing pulsatility of the arterial waveform (ex. pulse pressure increasing from 10 to 30), improved cardiac squeeze on echo as flows are weaned, normalization of lactate, and tolerating movement or physical therapy without drops in SvO2. When these signs of recovery are present it is reasonable to trial weaning the flow (not too low or the circuit will clot). If the patient tolerates this without significant changes in hemodynamics or lactate, then perform a clamp trial. Have pressors and calcium on hand, wean the flow off and clamp the circuit (typically with the patient sedated) for up to an hour. Then decide if they are ready for decannulation based on the amount of vasoactive support needed, in addition to other markers (ex., lactate and SvO2) during this trial.

VV ECMO – Watching for Respiratory Recovery

Patients cannulated for respiratory failure will typically have chest x-rays (CXR) that have completely whited out lung fields. The first sign of recovery on CXR is the appearance of air bronchograms. Dr. Fauman likens these growing branches to A Tree Grows in Brooklyn2, signifying little branches of hope. As these air bronchograms grow, it is reasonable to gently trial recruitment strategies (manual bagging, gently increasing vent settings). It is only appropriate to continue if there are signs of improved compliance, sometimes over days to weeks. In order to liberate, respiratory recovery should be represented by evidence of a tidal volume (4-6cc/kg) on rest or extubatable settings, decreasing sweep, tolerating physical therapy, and low ventilator FiO2 (expert opinion of no more than 50%). No clamping trial is needed, but the sweep may be left off for up to and even longer than 24hrs.

Outcomes

Many patients do very well after ECMO. Patients who tend to do less well are those who do not fit the above listed criteria. For example, oncology patients who don’t have a functioning immune system, have active disease, or were cannulated in the setting of an infection that is unlikely to clear tend to have worse outcomes.

Links

1. Oxygen Index Calculator
2. ELSO Guidelines for Adult and Pediatric Extracorporeal Membrane Oxygenation Circuits.
3. Smith, Betty. 1943. A Tree Grows in Brooklyn. Harper & Brothers.