CPT code 99190 denotes the hourly assembly and operation of a mechanical pump–oxygenator or heat exchanger for extracorporeal circulation and oxygenation of blood, a critical perfusion service used during cardiac surgeries requiring cardiopulmonary bypass. Nationally, this code captures a specialized intraoperative activity that directly supports complex cardiac procedures and resource-intensive perioperative care. Key payers referenced in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise explanation of what the code represents, how it fits into the clinical workflow for cardiac surgery, and the typical site of service. The publication outlines payer coverage considerations, common modifiers associated with the service (listed elsewhere), and operational benchmarks where available. It also summarizes coding context relevant to hospital billing and perfusion service documentation and highlights areas where policy updates or payer-specific rules may affect claim processing. Data not available in the input is identified as such in relevant sections.