Technique for using video microscopy and indicator dilution for repeated measurements of cardiac output in small animals

Background: The authors developed an indicator dilution technique for small animals to repeatedly determine cardiac output and blood volume without cardiac instrumentation or blood sampling. Methods: Observations were made in the hamster (N = 32, 70 mg/kg pentobarbital) cremaster using in vivo fluorescence videomicroscopy. Fluorescein isothiocyanate-conjugated bovine serum albumin (10 mg/ml) was injected as a bolus dose (right jugular) while video recording the light intensity in a 20-μm arteriole (intensified charge-coupled device [CCD] camera at fixed gain). The intensity signal was analyzed over time (background subtracted) and calibrated to the dye concentration. The ex vivo calibration was performed using a constant optical path length (20 μm) and a range of dye and hematocrit concentrations. In vivo tube hematocrit was determined using standard methods with fluorescently labeled erythrocytes. Thus, quenching of the fluorescence signal by hemoglobin was corrected for the calibration, and the plasma space in the arteriole was determined. The steady state dye concentration measured by the light intensity at 2 rain was not different from the dye concentration found by direct spectrophotometric analysis of the plasma. Results: Cardiac index was calculated as milliliters of blood per minute per kilogram body weight. The calculated cardiac index was 359 ± 18 ml · min^-1 · kg^-1, which is not different from the reported values for hamsters. Cardiac output was increased twofold when enough intravenous nitroprusside or nitroglycerine was injected to decrease mean arterial pressure from 90 to 70 mmHg. Cardiac output was elevated during dobutamine infusion (16 μg · kg^-1 · min^-1) and decreased during esmolol infusion (50, 75 · kg^-1 · min^-1). Blood volume determined from the steady state dye concentrations was 6.2 ± 0.5 ml/100 g body weight, within the normal range for hamsters. Conclusions: Fluorescent dye dilution and video microscopy can be used to repeatedly determine cardiac output or blood volume in small animals.