Clinical Case Scenario
A 59-year-old man who has a history of renal transplant 3 years ago on immunosuppression suddenly developed respiratory distress, he was diagnosed with COVID-19 ARDS with a PaO2/FiO2 ratio of less than 70 for the past one day.VV-ECMO was initiated for the ARDS, he developed AKI one day post-cannulation, and his Hb was 7 g/dl. The parameters on ECMO machine and his chest X-ray are shown below:
Wrong Answer: A. Fluid bolus with 500 mL normal saline.
Right Answer: B. Transfuse one unit of PRBC
Explanation The monitor above shows flow fluctuations, which suggests the chattering of the ECMO. Preload affects the performance of contemporary centrifugal pumps used in ECMO. According to the Hagen-Poiseuille equation, blood flows through the drainage cannula. Excessive negative pressure can cause the compliant walls of the vena cava to collapse when the pump speed is adjusted too high in relation to the available venous return. This can result in chattering and intermittent clogging of drainage ports. Due to non-laminar, irregular blood flow, chatter is characterized by repetitive, jerky motions of the ECMO tubing. These movements are frequently seen visually and verified by variations in the pressure waveform and flow fluctuations as shown in the figure above. Low venous capacitance and excessive negative intake pressures are the main causes of drainage insufficiency and chatter because they interfere with regular venous return and jeopardize efficient circuit operation. To fix the Chatter, preload has to be increased, by administering a fluid bolus or by transfusing blood, as the Hemoglobin of the patient is low, therefore transfusing one unit of PRBC is the best management. Increasing the flow shall lead to more flow fluctuations and chattering, so this is not the best option. Recirculation is not an option as the positions of drainage and return cannulas are more than 10 cm as shown in Chest X-ray.
Clinical pearl: Chattering Chatter is defined as the ECMO tubing's rhythmic pulsations brought on by irregular non laminar blood flow. The primary causes of drainage insufficiency are either too much negative pressure or inadequate venous capacitance. Risk factors of Chattering 1) Hypovolemia: Venous return to the drainage cannula is decreased when blood volume is decreased. 2) Vasodilation: Blood vessel widening raises vascular capacity and may result in insufficient venous return. Vasodilation, a component of the systemic inflammatory response syndrome (SIRS) reaction when blood is exposed to the extracorporeal circuit, may be the cause of "Postcannulation Syndrome." 3) Valsalva Manoeuvre: This technique temporarily reduces venous return by raising intrathoracic pressure. 4) Inflow Obstruction: Blood flow into the drainage cannula is impeded by obstructions or kinks in the venous channel. 5) Small Drainage Cannula: Using a cannula with a small diameter restricts blood flow by increasing resistance. 6) Inferior Vena Cava (IVC) Compression: The IVC is compressed by elevated IAP, which lowers its capacitance and prevents venous return from the lower body. 7) Redistribution of Blood Volume: Hemodynamics may be impacted by an increase in IAP that causes blood to move from the abdomen to the thoracic compartment. 8) Impaired Drainage to Superior Vena Cava (SVC) and Right Atrium: Conditions like pneumothorax, cardiac tamponade, and the Valsalva manoeuvre elevate intrathoracic pressure, hindering venous return to the SVC and right atrium. 9) Increased Right Ventricular Afterload: Elevated intrathoracic pressure raises pulmonary vascular resistance, increasing the workload on the right ventricle. 10) High Pump Speed Relative to Inflow Resistance and Blood Volume: Operating the pump at speeds too high for the available blood volume and inflow resistance can create excessive negative pressure, leading to vessel collapse or haemolysis. 11) Insufficient Venous Return: When venous return is compromised, the negative pressure generated by the pump can exacerbate drainage issues Management of drainage cannula chatter A systematic approach has to be followed. First, reduce pump speed to achieve stable blood flow, ensuring the patient is hemodynamically compensated. Confirm adequate sedation to avoid agitation or coughing. Inspect the ECMO circuit thoroughly for obstructions, occult bleeding, or cannula malposition, especially the femoral drainage cannula. Evaluate for increased intra-abdominal or intrathoracic pressures such as pneumothorax or tamponade. If chatter persists, perform a volume challenge using Trendelenburg positioning. If effective, administer fluids in small aliquots. Persistent chatter despite these steps may indicate the need for an additional drainage cannula. Using a larger cannula (≥23 Fr) can ensure adequate flow at lower pressures. Titrate ECMO flow appropriately based on the patient’s native lung function recovery.
Wrong Answer: C. Increase Flow
Wrong Answer: D. Recirculation
