Case Study:
Optics for Ventilator Monitoring

How are Optics Used in Blood Gas Monitoring?

Carbon dioxide, oxygen, pH, and hemoglobin levels are key parameters in arterial blood gas monitoring. Below are several examples of how those parameters may be photometrically determined.

Carbon Dioxide

Figure 1 shows an optical system for photometric determination of CO₂ from a blood sample. Radiation from a thermic source is reflected from a concave mirror through a slit and onto a second concave mirror. The radiation is reflected from the second concave mirror to a grating, which is optimized for IR wavelengths from 4200-4300nm. The grating rotates, allowing the diffracted wavelengths to vary across this spectrum. The radiation is then transmitted back to the concave mirror, passed through a 3500nm longpass filter, transmitted through the measuring chamber, and then directed to a pyroelectrical detector. The detector, which is synchronized with the rotating grating, emits a signal proportional to the radiation intensity at 4210, 4260, and 4310nm. The amount of absorption from these wavelengths provides the baseline amount of CO₂ in the sample³. Arterial blood should be relatively depleted of carbon dioxide because of pulmonary gas exchange, or external respiration⁴.

Figure 1: Photometric determination of CO₂.

Oxygen

Figure 2 shows an optical system used for the determination of the amount of oxygen in a blood sample. Light from a green diode is transmitted through a 580nm short wave pass (SWP) filter. The radiation is transmitted from the SWP filter to a 580nm dichroic filter that reflects the green diode light onto the focus lens. The green light is then focused on the sample where it interacts with an oxygen-sensitive dye⁵. The excited luminophores emit photons with longer wavelengths than the incident green light. These excitation photons are then transmitted from the sample container through the lens, through the dichroic filter, then through a 665nm longpass color filter, and onto a silicon photodiode. The signal intensity at the detector is used to calculate the oxygen level. Oxygen levels are dependent on movement of air in and out of alveoli, blood flow through pulmonary capillaries, and oxygen carrying protein hemoglobin⁴.

Figure 2: Photometric determination of O₂ then through a SCHOTT 665nm longpass color filter.

pH

Figure 3 shows a cross section of an optical system for photometric determination of pH. A halogen lamp emits radiation that is filtered by a heat absorbing filter, such as SCHOTT KG5 glass. The radiation then interacts with the blood sample and is finally focused by a lens through several bandpass filters onto silicon photodiodes. The photodiodes emit current signals representing the intensity of 458, 589, and 750nm radiation. These radiation intensities allow for the pH value of the sample to be calculated³. Blood pH provides insight into the interaction of chemical buffers present in blood (principally bicarbonate), red blood cells, and the function of three organs: the kidneys, lungs, and brain stem⁴.

Figure 3: Photometric determination of blood pH.

Hemoglobin

Oxygen is poorly soluble in blood, so the body relies on the oxygen carrying protein, hemoglobin, to transport oxygen to tissue cells. An optical system known as a CO-oximeter measures the absorption of light passing through blood from as few as two or three wavelengths of light to several dozens of wavelengths. This is done in order to distinguish oxyhemoglobin and deoxyhemoglobin (formerly called "reduced" hemoglobin) and determine the oxyhemoglobin saturation, or the percentage of oxygenated hemoglobin compared to the total amount of available hemoglobin (Hb). Measuring greater numbers of wavelengths enables the instrument to distinguish between these and carboxyhemoglobin (COHb), methemoglobin (metHb), other hemoglobin moieties, and "background" light-absorbing species.

Figure 4: Photometric determination of hemoglobin.

Figure 4 shows an example of a CO-oximeter. Radiation is transmitted from a halogen lamp through a heat absorbing filter onto the measuring chamber. The light that passes through the sample is focused through a dichroic filter, which then splits the spectrum into either a 600 or 506nm bandpass filter. The resulting intensities are detected by silicon photodies and used for the calculation of the total hemoglobin content (Hbtot) and oxygen saturation. The calculation is based on Lambert Beer's Law and predetermined values of the extinction coefficients for Hb and HbO₂ ³.

Below is another example of a CO-oximeter, where the red blood cells are exposed to ultrasonic vibration which destroys the cell walls releasing the hemoglobin. Hemoglobin in the flow cell is exposed to visible light and the absorbance spectrum (478-672nm) is observed using a grating mirror and photodiode array. From this absorbance spectra, bilirubin concentrations can be computed and used to screen for conditions such as neonatal jaundice⁶.  

Figure 5: Hemoglobin measuring system with an ultrasound source.