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A thermodilution technique is described for measuring steady-state cardiac output (CO) in conscious rats for periods of 2-3 wk. The method utilizes small implantable aortic thermocouples inserted via the femoral artery at the time of placement of other chronic indwelling catheters. Data are presented to validate the accuracy and reproducibility of the technique by acute in situ comparison with electromagnetic flowmeter measurements. Responses in chronically instrumented rats were tested with hemorrhage and infusions of vasodilator and vasoconstrictor agents administered repeatedly over a period of 9 days. The results show that the system is capable of measuring CO chronically and provides reproducible responses for periods of at least several weeks. We conclude that these methods provide a single, inexpensive, and accurate way to measure steady-state CO in conscious, unrestrained rats.

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