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 ca rdiac instrumentation or blood sampling. Methods: Observations were made in the hamster (N = 32, 70 mg/kg pentobarbi tal) cremaster using in vivo fluorescence videomicroscopy. Fluorescein isot hiocyanate-conjugated bovine serum albumin (10 mg/ml) was injected as a bol us dose (right jugular) while video recording the light intensity in a 20-m um arteriole (intensified charge-coupled device [CCD] camera at fixed gain) . The intensity signal was analyzed over time (background subtracted) and c alibrated to the dye concentration. The ex vivo calibration was performed u sing a constant optical path length (20 mum) and a range of dye and hematoc rit concentrations. In vivo tube hematocrit was determined using standard m ethods with fluorescently labeled erythrocytes, Thus, quenching of the fluo rescence signal by hemoglobin was corrected for the calibration, and the pl asma space in the arteriole was determined. The steady state dye concentrat ion measured by the light intensity at 2 min 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 pe r 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. C ardiac output was increased twofold when enough intravenous nitroprusside o r nitroglycerine was injected to decrease mean arterial pressure from 90 to 70 mmHg. Cardiac output was elevated during dobutamine infusion (16 mug 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. Conclusion: Fluorescent dye dilution and video microscopy can be used to re peatedly determine cardiac output or blood volume in small animals.