A fiberoptic reflection densitometer with cardiac output calculator

Abstract

A catheter-tip densitometer for indocyanine green is described consisting of a cardiac catheter containing optical fibers, an incandescent light source, a light detection unit and a processing unit. Half of the optical fibers guide the light to the blood at the tip of the catheter, the other half the back-scattered (reflected) light to the detection unit. In the detection unit the light is measured by two silicium barrier layer photocells after it has been split into two beams by a beam splitter. In the measuring channel the light passes an 800 nm filter before reaching the photocell. When fiberoptic catheters with glass fibers are employed, the other channel, used for compensation of non-specific effects such as blood flow variations, contains no filter, thus measuring light in a broad spectral band. It is shown that in this way compensation of flow effects may be about two times better than when a 920 nm filter is used. When using plastic optical fibers a 950 nm filter must be used, because above λ=850 nm plastic fibers transmit only a band around that wavelength (950 nm). At zero dye concentration the densitometer output or ratio of compensating and measuring photocell outputR/R ⁸⁰⁰ is almost insensitive to changes in haemoglobin concentration. When the blood contains dye, however, the influence of haemoglobin concentration is considerable. The desitometer outputR/R ⁸⁰⁰ is linearly related to dye concentration up to 50 mg · l⁻¹, the outputR ⁹²⁰/R ⁸⁰⁰ up to 30 mg · l⁻¹. The outputR/R ⁸⁰⁰ decreases with decreasing oxygen saturation; the slope of the calibration line, however, appears to be unaffected. The processing unit also contains an analog cardiac output calculator consisting of an integrator and a divider. Central dye dilution curves recorded from the pulmonary artery after injection of dye into the right atrium or a caval vein come down to the baseline. At this moment the reading of a digital voltmeter displaying the divider output calibrated in l · min⁻¹ can be held and the reading taken.