use mixed venous oxygen to guide blood transfusion in ECMO
- related: ECMO extracorporeal membrane oxygenation
- tags: #literature #icu
There are a number of considerations when evaluating a patient being supported on venovenous ECMO. The goal is to improve oxygen delivery to the tissues. Factors affecting oxygen delivery in patients receiving ECMO include integrity of the circuit, right ventricular function, oxygenation by the lungs, and oxygen-carrying capacity. The oxygen saturation depends on the balance between oxygen delivery and oxygen demand. Mixed venous oxygen saturation reflects the adequacy of oxygen delivery; when oxygen demand begins to exceed oxygen delivery, tissues extract more oxygen from the blood delivered, and mixed venous oxygen saturation drops. In this case, there has been a drop in both arterial and mixed venous oxygen saturation.
The ECMO circuit is running normally without any problems in circuit flow, and attempts at increasing flow have been unsuccessful. The Hagen-Poiseuille law for flow states that flow of fluid (Q) is proportional to the fourth power of the diameter and the pressure gradient across the tubing (P) and inversely proportional to the length and fluid viscosity. Optimizing flow requires considering cannulation configuration, cannula size (resistance), and pump mechanics. In this case, circuit flow is near maximal, given chattering with attempts to increase flow further. Increasing the size of the inflow cannula could be a consideration to increase the circuit’s ability to tolerate more flow and address the hemodynamic needs if the patient’s vascular anatomy allows.
Oxygen delivery capability in patients receiving ECMO is determined by blood flow, hemoglobin concentration, inlet hemoglobin saturation, and membrane lung properties. The decrease in mixed venous oxygen saturation in this case reflects the inability of oxygen delivery to keep up with oxygen demand. Because blood flow appears to be at the limit of the circuit properties, and there is no indication of membrane lung failure, the best option to increase oxygen delivery is increasing the hemoglobin concentration. Changing the ventilator settings will likely be of little benefit.
The role of the ventilator for patients receiving venovenous ECMO is to provide enough support for lung units to remain open while minimizing ventilator-induced lung injury. The extent in which the native lung contributes to gas exchange depends on the integrity of the alveolar units and the capillary membranes. The role of the ECMO circuit is to provide adequate oxygenation and promote lung rest by minimizing shear stress and dynamic strain related to volutrauma (via low tidal volume ventilation), as well as static strain and barotrauma (through the lowest PEEP necessary to reach critical opening pressure of the alveoli). Elevated levels of PEEP also have the potential to decrease venous return and negatively affect overall hemodynamics. Despite these principles, there is a paucity of data to describe the optimal ventilator strategy for patients receiving venovenous ECMO. Increasing sedation in this patient has no role because sedation and analgesia should be titrated to alleviate anxiety and ensure patient comfort, and this patient is already calm. Increasing sedation and providing therapeutic paralysis may reduce oxygen consumption to a small extent but is unlikely to result in a clinically significant improvement in the calm, comfortable patient.12
You are called to evaluate a 35-year-old patient receiving venovenous extracorporeal membrane oxygenation (ECMO) due to ARDS from pneumococcal pneumonia. The cannula is in the right internal jugular vein with a dual-lumen single-site catheter. She is on day 7 of ECMO and has had gradual decrease of her sedation over the previous 24 h. You are called to the bedside because of an Spo2 of 80% on the bedside monitor. The respiratory therapist has increased the FiO2 on the ventilator from 0.5 to 1.0 with no effect on saturation. The current ECMO flow is 3 L/min. An attempt was made to increase flow to 3.5 L/min, and chattering of the circuit occurred. The flow was reduced back to 3 L/min.
On your arrival, she is calm. Her BP is 100/75 mm Hg and heart rate is 110/min. A circuit check confirms there has been no change in the position of the cannula, and the tubing is not kinked or compressed by her body. The pressure control is 10 cm H2O, rate is 10/min, PEEP is 10 cm H2O, and FiO2 is 1.0. Oxygen gas flow to the oxygenator is 100%. Arterial blood gas measurements are pH of 7.28, Pco2 of 45 mm Hg, and Po2 of mm Hg 45. The venous Spo2 is 35%, down from 65% previously.
What is the best next step?
