12.2 ECMO Circuit, Lung Rest & Anticoagulation

12.2 ECMO Circuit, Lung Rest & Anticoagulation

ECMO Circuit Mechanics & "Rest" Mechanical Ventilation

Circuit Architecture & Sweep Gas Control

The extracorporeal circuit consists of drainage tubing, a venous reservoir (bladder or console sensor), a blood pump (centrifugal or roller pump), an advanced polymethylpentene (PMP) hollow-fiber membrane oxygenator, an in-line heat exchanger, and arterial reinfusion tubing.

• Oxygenation ($PaO_2$): Regulated by extracorporeal blood flow rate ($mL/kg/min$) and the $FiO_2$ of the sweep gas running through the membrane lung.
• Ventilation ($PaCO_2$): Governed primarily by the sweep gas flow rate ($L/min$) passing through the oxygenator. Increasing sweep gas flow accelerates the concentration gradient across the hollow fibers, dramatically increasing $CO_2$ clearance and lowering patient $PaCO_2$.

The "Lung Rest" Ventilator Strategy

Once extracorporeal support is established, ventilator intensity is generally reduced to limit ventilator-induced lung injury while preserving recruitment appropriate to the underlying disease. Mode, PEEP, driving pressure, rate, and FiO2 are individualized; the table gives an illustrative range rather than a universal prescription.

+-----------------------------------------------------------------------------+
|              ILLUSTRATIVE ECMO LUNG-PROTECTIVE SETTINGS                    |
+-----------------------+-----------------------+-----------------------------+
| Ventilator Parameter  | Typical Rest Range    | Clinical Rationale          |
+-----------------------+-----------------------+-----------------------------+
| Ventilator Mode       | PCV or SIMV-PC        | Pressure-limited delivery   |
+-----------------------+-----------------------+-----------------------------+
| Peak Inspiratory      | 20 to 25 cmH2O        | Low driving pressure;       |
| Pressure (PIP)        |                       | prevents alveolar stretch   |
+-----------------------+-----------------------+-----------------------------+
| Positive End-Expir    | 8 to 12 cmH2O         | Maintains FRC; prevents     |
| Pressure (PEEP)       |                       | cyclic alveolar collapse    |
+-----------------------+-----------------------+-----------------------------+
| Respiratory Rate (RR) | 10 to 15 breaths/min  | Minimizes shear strain;     |
|                       |                       | avoids dynamic trauma       |
+-----------------------+-----------------------+-----------------------------+
| Inspiratory Time (Ti) | 0.8 to 1.0 seconds    | Promotes even distribution  |
+-----------------------+-----------------------+-----------------------------+
| Delivered FiO2        | 0.21 to 0.30 (21-30%) | Avoids hyperoxic absorption |
|                       |                       | atelectasis and free radical|
|                       |                       | oxidative lung damage       |
+-----------------------+-----------------------+-----------------------------+

Critical Clinical Concept: Avoid both injurious ventilator intensity and needless derecruitment. PEEP may be kept high enough to preserve lung volume in some patients, while other disease states or air-leak risks require a different strategy. Follow serial imaging, compliance, hemodynamics, and the ECMO center’s recovery plan.

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Anticoagulation Protocols & Circuit Monitoring

Blood contact with circuit surfaces promotes thrombosis while critical illness and anticoagulation promote bleeding. Systemic anticoagulation is commonly used, but it may be reduced or temporarily held for major bleeding under ECMO-team direction.

Unfractionated Heparin Infusion

• Mechanism: Heparin binds to and potentiates Antithrombin III (ATIII), accelerating the inhibition of thrombin (Factor IIa) and Factor Xa.
• Bolus & Maintenance: Many protocols use a weight-based cannulation bolus followed by infusion, but dose and timing depend on bleeding risk, circuit, age, laboratory response, and center protocol.

Anticoagulation Monitoring Parameters

1. Activated Clotting Time (ACT): Rapid point-of-care whole-blood measure; affected by platelets, fibrinogen, temperature, hemodilution, and coagulation-factor abnormalities. Some protocols use ranges near 180–220 seconds, but the target is not universal.
2. Anti-Factor Xa Assay: Estimates heparin effect more directly; many centers use a range near 0.3–0.7 IU/mL. Hemolysis, hyperbilirubinemia, hyperlipidemia, antithrombin concentration, and assay method can affect interpretation.
3. aPTT and viscoelastic testing: Provide complementary information about coagulation and clot dynamics; no single assay captures both bleeding and circuit-thrombosis risk.
4. Platelets and fibrinogen: Transfusion thresholds are individualized to active bleeding, procedures, age, circuit condition, and local protocol rather than fixed for every patient.
5. Antithrombin: Low activity can contribute to apparent heparin resistance. Confirm the clinical and laboratory pattern before replacement; antithrombin concentrate or plasma decisions belong to the ECMO team.

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Worked Clinical Case: ECMO Candidacy Evaluation & OI Calculation

A $3.5\text{ kg}$ term male infant born at 40 weeks gestation is being managed in the NICU for severe meconium aspiration syndrome and persistent pulmonary hypertension. The patient is intubated on High-Frequency Oscillatory Ventilation (HFOV) with the following settings:

• Mean Airway Pressure (mPAW): $24\text{ cmH}_2\text{O}$
• $FiO_2$: $1.0$ (100%)
• Inhaled Nitric Oxide (iNO): $20\text{ ppm}$

An indwelling post-ductal arterial line blood gas reveals: $\text{pH }7.20$, $PaCO_2\text{ }54\text{ mmHg}$, $PaO_2\text{ }42\text{ mmHg}$, $HCO_3^-\text{ }21\text{ mEq/L}$, and pre-ductal/post-ductal $SpO_2\text{ }82%$. These values have been sustained for 5 hours despite fluid resuscitation and inotropic support. A head ultrasound shows no intracranial hemorrhage.

Step 1: Calculate the Oxygenation Index (OI)

$OI = \frac{\text{mPAW} \times FiO_2 \times 100}{PaO_2} = \frac{24 \times 1.0 \times 100}{42} = \frac{2400}{42} = 57.1$

Step 2: Clinical Assessment

• The calculated OI is $57.1$, which significantly exceeds the threshold of $OI > 40$ sustained for 4 to 6 hours.
• This is a high-risk trajectory requiring urgent ECMO-center consultation despite optimized rescue care.
• Gestational age, weight, cannulation feasibility, neurologic status, bleeding risk, reversibility, and goals of care all enter candidacy review.
• Cranial ultrasound is one component of the neurologic and bleeding-risk assessment.

Step 3: Modality Selection & Rest Settings

• Echocardiogram reveals suprasystemic pulmonary artery pressures with right-to-left ductal shunting, but preserved left ventricular ejection fraction (> 55%).
• Escalation: The sustained OI and refractory hypoxemia justify urgent multidisciplinary ECMO-center evaluation. Preserved ventricular function may favor VV support, but anatomy, cannula availability, hemodynamics, and center expertise determine the final mode.
• Ventilator plan if supported: Reduce injurious settings and titrate recruitment, driving pressure, rate, and FiO2 to the patient’s physiology and the center protocol rather than applying one fixed recipe.

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NPS Exam Traps

Exam Trap 1: Calculating the Oxygenation Index (OI)

On the NPS exam, questions frequently provide ventilator and ABG values and ask for immediate clinical triage. Candidates often invert the formula or forget to multiply by 100. Remember: $OI = (\text{mPAW} \times FiO_2 \times 100) / PaO_2$. A persistent OI above 40 is a traditional high-risk threshold that should trigger urgent ECMO-center consultation and full candidacy assessment. If the question gives $PaO_2$ in $kPa$, you must convert it to $mmHg$ ($1\text{ kPa} = 7.5\text{ mmHg}$) before calculating.

Exam Trap 2: Gestational Age and Birth Weight Cutoffs

Very low gestational age and weight substantially increase cannulation and intracranial-hemorrhage risk and were traditional exclusion thresholds. Treat them as major candidacy constraints, not timeless universal rules: optimize rescue therapy and discuss the current center’s equipment, outcomes, and criteria.

Exam Trap 3: Adjusting Ventilator FiO2 vs. Sweep Gas FiO2 on ECMO

When a patient on ECMO experiences arterial hypoxemia, examinees often incorrectly choose to "increase the ventilator $FiO_2$ to 1.0." The patient's native lungs are diseased and resting. Arterial oxygenation on ECMO depends strongly on extracorporeal blood flow and membrane-lung gas oxygen concentration, but also on native cardiac output and lung function, hemoglobin, oxygenator performance, recirculation or differential oxygenation, and cannula position. Ventilator FiO2 is generally reduced when extracorporeal support is adequate, then individualized to native-lung contribution, differential oxygenation, recruitment, and the center plan.