There seems to be a wide range of practice in relation to the optimum
oxygen setting before, and at the start of,
cardiopulmonary bypass. Even manufacturers of blood
oxygenators vary in their suggestions for this phase of
extracorporeal circulation. Most of these suggestions are based on peak performance, Association for the Advancement of Medical Instrumentation (AAMI) standards, experience, and legal considerations. Therefore, suggested gas:blood flow ratios will vary from no gas flow at the start of bypass, to a ratio setting of 1:1. On the other hand, suggested inspired
oxygen concentrations will generally vary between 0.80 to 1.0 at the start of
cardiopulmonary bypass. In regard to perfusate temperatures before going on bypass, there are no clearly defined standards other than those of clinical preference. The manufacturer of the
oxygenator used in this study clearly states in the operating instructions that gas flow should be proportional to blood flow at the start of bypass, and gas flow should be turned off when there is no fluid flow through the
oxygenator. The presence of hyperoxic perfusates and wide patient/perfusate temperature gradients at the start of bypass has been suspected in the appearance of gaseous microemboli during this critical period. Hyperoxemia during the bypass period is also implicated in the introduction of
oxygen free radicals and
nitric oxide into the hypoxic myocardium during
cardioplegia delivery. Presented here are the results of a randomized clinical study involving 39 adult patients undergoing
cardiopulmonary bypass for the surgical treatment of
coronary artery disease. All patients were randomly selected into five groupings. The first group had 1 L of gas flow through the perfusate before bypass, and bypass was then started with an FIO2 of 0.80. The second two groups had no gas flow through the perfusate prior to bypass and a starting FIO2 of 0.21. Groups 4 and 5 had 1 L of gas flowing through the perfusate and a starting FIO2 of 0.21. Results indicate that gas flow through
Normosol R/
Albumin perfusates will prevent the
acidosis that is found in this
solution when the system is previously flushed with
carbon dioxide. Also, suggested high FIO2 settings will produce hyperoxic perfusates at the start of
cardiopulmonary bypass. However, the use of an FIO2 of 0.21 at the start of bypass will produce normoxemic conditions that are both safe and reliable for the conduct of initiating
cardiopulmonary bypass.