Monitoring of Ventilation On and Off Cardiopulmonary Bypass

Abstract

To the Editor: We read the cases described by Thrush et al. [1] with interest. We are in complete agreement with their observations and concerns and agree that the gas flowmeter alone is an inadequate continuous monitor of gas flow to the oxygenator. Venous return tubing oximetry is an available, inexpensive technology that verifies the global adequacy of oxygen delivery. However, we disagree that monitoring exhaust gas for oxygen, carbon dioxide, and anesthetic concentration is technically difficult. If a sidestream sampling gas analyzer is used during conventional ventilation prior to cardiopulmonary bypass, all that is required is to move the gas sampling line from the breathing circuit elbow site (the elbow port can be occluded with a Luer cap or a syringe) to a piece of tubing attached to the oxygenator scavenging port. When this is done, it allows the anesthesiologist to continuously verify that 1) mean expired oxygen concentration is appropriately enriched (i.e., relatively high during normothermia) and that it is lowered when the patient is hypothermic, 2) mean expired CO2 partial pressure is both present (i.e., there is gas flow) and of a reasonable value, and 3) when a potent inhaled anesthetic is used, it is both present when desired and adequately eliminated when not desired just prior to separation from cardiopulmonary bypass [2]. Furthermore, monitoring the oxygenator's mean partial pressure of the inhaled anesthetic also assists in timing the administration of supplemental intravenous drugs for analgesia and amnesia. Obviously, alarm variables can be set for whatever values the user chooses. Some have expressed concern that room air may be entrained via the sampling port. Although possible, this has not been our experience, and one is reassured that entrainment is not an issue by noting that the measured exhaust O2 concentration is within 5% of that set on the O (2) blender. We encourage others to try this inexpensive monitoring adjunct during the care and monitoring of patients undergoing cardiopulmonary bypass. It will make it much more like one standard for monitoring ventilation during cases in which cardiopulmonary bypass is implemented. Emilio B. Lobato, MD Nikolaus Gravenstein, MD Glenn B. Paige, MD Department of Anesthesiology; University of Florida College of Medicine; Gainesville, FL 32610-0254