Cerebrospinal Fluid Drainage in Endovascular Aortic Repair: Comment

To the Editor: We would like to commend Shelton et al. on their insightful and comprehensive narrative review, which provides an excellent overview of cerebrospinal fluid drainage in thoracic endovascular aortic repair.1 The review not only highlights the uncertainty of risk/benefit ratio of cerebrospinal fluid drainage in high-volume aortic and endovascular surgery centers but also draws attention to the inherent complexities of optimizing spinal cord perfusion pressure. We would like to contribute an additional layer of complexity to the discussion. In the review, spinal cord perfusion pressure is defined as the difference between mean arterial pressure and intraspinal pressure—practical equation for rapid estimations. However, the full equation for spinal cord perfusion pressure, as for any organ perfusion pressure, should include both inflow and outflow pressures.2 Because mean arterial pressure is used to approximate inflow pressure, it tends to greatly overestimate the true inflow pressure in ischemic organs, raising concerns about the accuracy of the first part of the equation. Outflow pressure, meanwhile, is influenced by far more complex pathophysiologic mechanisms. The rationale for using intraspinal pressure to estimate spinal cord perfusion pressure is based on the concept of critical closing pressure. When inflow pressure falls below critical closing pressure, the medullary vessels collapse, and perfusion ceases. Three key points about critical closing pressure are worth highlighting: Organ-specific variation: Critical closing pressure likely differs across organs. For example, it is approximately 60 mmHg for resting skeletal muscle and between 15 and 25 mmHg for the heart.3 To our knowledge, the physiologic critical closing pressure of the spinal cord has not yet been established. Modifying factors: Several factors can influence critical closing pressure, including changes in arteriolar tone, the administration of vasoactive drugs, variations in tissue temperature, and external tissue pressure exerted on the vascular system.4 Influence of venous pressure: Under normal conditions, venous pressure does not generally alter perfusion pressure, as explained by the waterfall concept.2 However, if central venous pressure becomes sufficiently elevated, it may limit perfusion pressure. In this scenario, the lower boundary of the waterfall surpasses the upper boundary, halting blood flow.5 Inflow pressure can be compromised during the management of a thoracic endovascular aortic repair patient in the operating room or intensive care unit, as the vascular graft may cover medullary arteries. At the same time, there are many potential reasons for elevated central venous pressure, including right heart dysfunction, excessive fluid administration, positive-pressure mechanical ventilation, or patient positioning (e.g., Trendelenburg). In such situations, accounting for central venous pressure becomes at least as crucial as considering intraspinal pressure. Incorporating central venous pressure and the dynamics of mechanical ventilation into the framework of spinal cord perfusion pressure optimization may yield valuable insights. For instance, the decision to rapidly extubate a patient versus maintaining invasive mechanical ventilation can have a significant impact on spinal cord perfusion pressure and the patient’s overall clinical outcome. In conclusion, we strongly support the call by Shelton et al. for multicenter randomized controlled trials to address existing gaps in our understanding. Future research should include variables such as central venous pressure and mechanical ventilation strategies, aiming to improve our ability to manage and optimize spinal cord perfusion pressure while refining perioperative protocols. Competing Interests Dr. Mongardon received consulting fees from Baxter-France (Guyancourt, France) and travel fees from AOP Health-France (Meudon-La-Forêt, France), and Dr. Guerci received consulting and lecture fees from Edwards Lifesciences-France (Guyancourt, France) and travel fees from LFB SA (Les Ulis, France), all unrelated to this article. Dr. Collange declares no competing interests.