Patent Description:
In hemodialysis, blood is taken from a patient through an intake needle (or catheter) which draws blood from an artery located in a specific accepted access location (arm, thigh, subclavian, etc.). The drawn blood is pumped through extracorporeal tubing via a peristaltic pump, and then through a dialyzer which removes unwanted toxins such as blood urea, nitrogen, potassium, and excess water from the blood. As the blood passes through the dialyzer, it travels in straw-like tubes which serve as semi-permeable membrane passageways for the uncleaned blood. Fresh dialysate liquid, which is a solution of chemicals and water, flows through the dialyzer in the direction opposite the blood flow. As the dialysate flows through the dialyzer, it surrounds the straw-like membranes in the dialyzer. The fresh dialysate collects excess impurities passing through the straw-like tubes by diffusion, and also collects excess water through an ultrafiltration process due to a pressure drop across the membranes. The used dialysate exits the dialyzer with the excess fluids and toxins via an output tube, thus cleansing the blood flowing through the dialyzer. The dialyzed blood then flows out of the dialyzer via tubing and a needle (or catheter) back into the patient. Sometimes, a heparin drip or pump is provided along the extracorporeal blood flow loop in order to prevent clotting during the hemodialysis process. Several liters of excess fluid can be removed during a typical multi-hour treatment session. , a chronic patient will normally undergo hemodialysis treatment in a dialysis center three times per week, either on Monday-Wednesday-Friday schedule or a Tuesday-Thursday-Saturday schedule.

Hemodialysis has an acute impact on the fluid balance of the body due in part to the rapid change in circulating blood volume. When the fluid removal rate is more rapid than the plasma refilling rate of the body, intravascular blood volume decreases. The resulting imbalance has been linked to complications such as hypotension, loss of consciousness, headaches, vomiting, dizziness and cramps experienced by the patient, both during and after dialysis treatments. Continuous quantitative measurement of parameters relating to the circulating blood volume (in real-time) during hemodialysis reduces the chance of dialysisinduced hypotension, and otherwise optimizes dialysis therapy regimes by controlling fluid balance and aiding in achieving the appropriate dry weight for the patient. <CIT> and <CIT> are referred to as prior art. <CIT> describes a method of detecting gas bubbles in a living body, comprising: transmitting at least one original electromagnetic signal to a body portion; detecting a signal modulated by a flow of blood in said body portion; and analyzing a perturbation in said signal to determine at least one of an existence and a property of a bubble in said blood flow.

One embodiment of the disclosure provides a method according to claim <NUM>.

Another embodiment of the disclosure provides a system according to claim <NUM>.

Yet another embodiment of the disclosure provides a non-transitory computer-readable medium having processor-executable instructions according to claim <NUM>. Optional features are mentioned in the dependent claims <NUM>-<NUM>,<NUM>-<NUM> and <NUM>-<NUM>.

In a renal dialysis treatment session, occlusion of an arterial blood access line due to blood flow at high flow rates (<NUM>-<NUM>/min) may result in very low negative pressures (< <NUM> mmHg) which would result in degassing of the blood. As such, air bubble detection is important for indicating whether an arterial occlusion has occurred. One way of optically detecting air bubbles is with a single light intensity wavelength. The optical air bubble detection on the arterial line can thus be used for indicating an arterial occlusion event and alarming a user. While optical air bubble detection is desirable, and can be achieved with a single light intensity wavelength, it is affected by several factors affecting the single light wavelength absorbance.

Single light wavelength absorbance is affected by the color of the medium or blood, which varies significantly based on the blood hematocrit concentration and oxygenation level. Single light wavelength is also affected by sensor to sensor variability across multiple sensors. A sensor includes a single wavelength source or emitter and a single wavelength detector. Sensor to sensor variability may be reduced through a self-normalization step where sensor measurements are normalized relative to a baseline of its measurement. For example, an empty tubing may be used with a sensor to characterize the sensor's profile while adjusting light input to the sensor. This self-normalization or sensor calibration can be performed on each source-detector combination to ensure signal integrity and sensor response at wavelength of interest.

Also during dialysis, the disposable tubing that contains blood and through which the optical detection is taking place undergoes discoloration and loses some of its transparency over time, thus affecting measurements relying on single light wavelength absorbance. Other factors affecting single light wavelength absorbance include transient effects during dialysis. For example, when a saline solution dose is introduced and mixes with blood, the blood becomes diluted the color of the medium contained in the tubing lightens. Furthermore, when the saline delivery ends, and blood returns to the tubing mixing with saline currently in the tubing, the color of the medium contained in the tubing darkens.

Saline delivery is typically clinically introduced during dialysis treatment in order to deliver medications to the patient, or in the case of a patient suffering from hypovolemia, saline is introduced to allow for rapid patient recovery. Such variability in factors affecting the absolute intensity of light absorption makes the detection of air based purely on intensity level of light of a single wavelength challenging and difficult to adapt with the various factors. Using a single wavelength, an optical sensor typically interprets detection of blood when the single wavelength light is mostly absorbed (or equivalently reduced transmission of light), thus resulting in a low-range sensor output. Using a single wavelength, the presence of air in the tubing produces scattering, which also reduces transmission, but not as much as blood, and thus resulting in a mid-range sensor output. Using a single wavelength, the presence of clear water or saline in the tubing results in focused transmission with little absorbance of the single wavelength light, thus resulting in a high-range sensor output. A single wavelength optical sensor may then be used to obtain "typical" levels of sensor outputs for blood, air, and saline in order to interpret contents of the tubing during dialysis.

Embodiments of the disclosure provide an improved methodology for detection of a transient air bubble by removing one or more effects of common mode factors affecting the absorbance of the light, such as, medium color, tubing color, angle of illumination, or temperature of the optical detector. The methodology involves using a normalized ratio (R) of the absorbance intensity of two light wavelengths through the medium. By using the normalized ratio R, common factors affecting the absolute scale of light intensity are eliminated. Only factors affecting the wavelengths differently result in a change to the value and sensitivity of the normalized ratio R. When taking a ratio, their effects cancel out.

<FIG> is a perspective view of a typical patient undergoing hemodialysis treatment with a non-invasive, optical blood monitor monitoring the patient's blood in real-time as it passes through extracorporeal tubing in the hemodialysis system. The environment illustrated in <FIG> is usable with exemplary embodiments of the present disclosure. Further, it will be appreciated that the environment shown in <FIG> is merely exemplary, and that the principles discussed herein with respect to exemplary embodiments of the present disclosure may be implemented in other environments as well.

<FIG> illustrates a patient <NUM> undergoing hemodialysis treatment using a conventional hemodialysis system <NUM>, as well as a non-invasive, optical blood monitor <NUM>. A typical hemodialysis clinic will have several hemodialysis systems <NUM> for treating patients on a Monday-Wednesday-Friday schedule or a Tuesday-Thursday-Saturday schedule. While the invention is not limited to the number of hemodialysis systems located at a clinic, or the specific type of hemodialysis system, the general operation of the hemodialysis system <NUM> is helpful for understanding the environment in which the invention is intended to operate.

An input needle or catheter <NUM> is inserted into an access site of the patient <NUM>, such as in the arm, and is connected to extracorporeal tubing <NUM> that leads to a peristaltic pump <NUM> and then to a dialyzer or blood filter <NUM>. The dialyzer <NUM> removes toxins and excess fluid from the patient's blood. The dialyzed blood is returned from the dialyzer <NUM> through extracorporeal tubing <NUM> and return needle or catheter <NUM>. In some parts of the world, the extracorporeal blood flow may additionally receive a heparin drip to prevent clotting. The excess fluids and toxins are removed by clean dialysate liquid which is supplied to the dialyzer <NUM> via tube <NUM> and removed for disposal via tube <NUM>. A typical hemodialysis treatment session takes about <NUM> to <NUM> hours in the United States.

In the exemplary environment depicted in <FIG>, the optical blood monitor <NUM> includes a blood chamber <NUM>, an optical blood sensor assembly <NUM>, and a controller <NUM>. The blood chamber <NUM> is preferably located in line with the extracorporeal tubing <NUM> upstream of the dialyzer <NUM>. Blood from the peristaltic pump <NUM> flows through the tubing <NUM> into the blood chamber <NUM>. The preferred sensor assembly <NUM> includes LED photo emitters that emit light at optical wavelengths. The blood chamber <NUM> includes lenses so that the emitters and detectors of the sensor assembly <NUM> can view the blood flowing through the blood chamber <NUM>, and determine whether a transient air bubble is detected in the blood.

In another exemplary environment, the optical blood monitor includes an optical blood sensor assembly and a controller and does not include a blood chamber. The optical blood sensor assembly clasps onto the extracorporeal tubing upstream of the dialyzer. The blood sensor assembly includes LED photo emitters that emit light in optical wavelengths including red light, infrared light, ultraviolet light, blue light, green light, and/or any wavelength in the optical spectrum. The emitted light travels from the LED photo emitters through the extracorporeal tubing to light detectors (e.g. photodiodes) also included in the optical blood sensor assembly.

<FIG> illustrates an exemplary system <NUM> that may be used to measure blood constituents using an optical blood sensor assembly according to some embodiments of the disclosure. An LED current driver <NUM> produces multiple currents to drive an LED array <NUM>. The LED array <NUM> contains LED1 to LEDN. Each LED in the LED array <NUM> operates at a different wavelength, and the LED current driver <NUM> is configured to provide power to turn ON or turn OFF each LED in the LED array <NUM>.

<FIG> shows that processor <NUM> controls the LED current driver <NUM>, so in some embodiments, the processor <NUM> may determine which LEDs should be turned ON at a certain time during measurement. Light from the LED array <NUM> is incident on a blood container <NUM>, passes through a blood flow path <NUM>, and is detected at a photosensor <NUM> (or "photodetector") which may be a photodiode. The blood container <NUM> may be, for example, a blood chamber <NUM> located in line with an extracorporeal tubing <NUM>, or may be, for example, the extracorporeal tubing <NUM> itself. The photosensor <NUM> may be one or more photosensors sensitive to wavelengths emitted by the LED array <NUM>. The photosensor <NUM> collects and integrates detected LED light and generates a current. The current is then amplified and filtered by the receiver <NUM>. The receiver <NUM> may also convert the amplified and filtered current from an analog signal to a digital signal, providing the digital signal to the processor <NUM> for further analysis.

The processor <NUM> is designed to interface with other electronic components allowing input and output communication of signals to and from the optical blood sensor assembly. For example, apart from the blood container <NUM> including the blood path <NUM>, every other component identified in system <NUM> may be part of a optical blood sensor assembly, for example, an optical blood sensor assembly <NUM> as depicted in <FIG>. Using the embodiment in <FIG>, the optical blood sensor assembly <NUM> may communicate with a controller <NUM> for displaying system outputs and/or receiving command signals or calibration signals.

In some embodiments, the processor <NUM> performs calculations and analyses based on the digital signal received from the receiver <NUM> and determines whether air bubbles are present in the blood flow path <NUM>. In other embodiments, the processor <NUM> may provide the digital signal received from the receiver <NUM> to an external computing device using the input/output communication channels so that the external computing device, for example, controller <NUM>, determines whether air bubbles are present in the blood flow path <NUM>. Processor <NUM> may be a microprocessor, a microcontroller, a field programmable gate array (FPGA), a complex programmable logic device (CPLD), an application specific integrated circuit (ASIC), etc..

<FIG> illustrates another exemplary system <NUM> that may be used to measure blood constituents using an optical blood sensor assembly according to some embodiments of the disclosure. The system <NUM> includes a processor <NUM>, LED current driver <NUM>, a multi-wavelength LED <NUM>, a blood flow path <NUM>, a tubing <NUM>, a wideband photosensor <NUM>, and a receiver/signal processor <NUM>. The multi-wavelength LED <NUM> may generate more than one wavelength, and the wideband photosensor <NUM> is sensitive to the more than one wavelength generated by the multi-wavelength LED <NUM>. The receiver/signal processor <NUM> is configured to obtain a current from the photosensor <NUM> and separate the magnitude of the current contributions of the different wavelengths sensed by the wideband photosensor <NUM>. The receiver/signal processor <NUM> may separate the current contributions from the different wavelengths using digital and/or analog signal processing. The receiver/signal processor <NUM> then provides the separated current contributions to the processor <NUM>.

The exemplary systems depicted in <FIG> and <FIG> may be used for detection of air bubbles by continuous monitoring of a blood flow path. The Beer-Lambert law defines the concentration ci of an absorbent in solution to be determined by the intensity of light transmitted through the solution, knowing the path length dλ, the intensity of the incident light I<NUM>,λ, and the extinction coefficient εi,λ at a particular wavelength λ. In generalized from, the Beer-Lambert law is expressed as Eq. (<NUM>): <MAT> where <MAT>, and where µa,λ is the bulk absorption coefficient that represents the probability of absorption per unit length, assuming photon scattering in the solution is negligible.

The detected absorbance of a first light wavelength's intensity, called IA, is divided by the detected absorbance of a second light wavelength's intensity, called IB, thus producing a ratio of R=IA/IB, which represents the normalized ratio R of Beer-Lambert's light intensity at wavelength A over the Beer-Lambert light intensity at wavelength B. Note that IA and IB represent detected absorbance parameters. Detected absorbance is defined as light intensity absorbed by absorbents in the medium or solution as defined in the Beer-Lambert law. The detected absorbance is obtained through a source-detector measurement system by subtracting the light intensity received at the detector from the light intensity provided at the source. For example, in <FIG>, LED <NUM> in LED array <NUM> emits <NUM> wavelength light with intensity Iemit,<NUM>, and the light travels through the blood flow path <NUM> and is incident on one photosensor in photosensor <NUM>. The processor <NUM> determines the light intensity to be Idetect,<NUM>. The detected absorbance I<NUM> is determined as Iemit,<NUM> - Idetect,<NUM>.

The ratio R=IA/IB normalizes all common components affecting the plurality of light wavelengths' absorbance intensity. The ratio R will therefore be limited in its sensitivity to the actual factors distinctly affecting the light wavelengths individually as opposed to common factors affecting all wavelengths' intensities. Such normalization methods are effective in limiting the sensitivity of the detection ratios to the transient factors, such as, air bubble presence.

During dialysis, the color of the medium in the tubing, for example, the extracorporeal tubing <NUM>, presents itself as a persistent value to the absorbance variable that is slowly varying. For example, the color of the medium when changing from blood to saline and back to blood will take on a gradual, slowly changing transient characteristic with an intermediary that is a mixture of both saline and blood. When air bubbles are present, however, a rapid variation occurs in the measured absorbance of the medium. Therefore, in some embodiments, a variance of the measured absorbance in the blood flow path <NUM> may be used to detect the presence of air bubbles. Embodiments of the disclosure thus provide an ability to avoid false alarms due to gradual changes in color of the medium (blood or saline or mixture thereof) caused by slow-varying transitions in the respective concentrations of blood and saline. Blood tubing color changes are also eliminated since these color changes are also slowly varying compared to air bubble presence. As such, the system is more robust and reliable for air bubble detection for arterial occlusion indication.

Due to the transient nature of the composition of the medium, a sampling rate may be defined for the measurement system of <FIG>, <FIG> and <FIG>. For example, the optical sensor assembly <NUM> may make measurements with a sampling rate varying from <NUM> to <NUM>. The sampling rate should be fast enough to realize fast passing transient bubble events under flowing fluid conditions.

In an exemplary embodiment, the optical sensor assembly <NUM> includes an LED that emits light in a red light wavelength, an LED that emits light in an infrared wavelength, a photosensor sensitive to red light wavelengths, and a photosensor sensitive to infrared wavelengths. Since the photosensors are sensitive to specific wavelength regimes, the LED-photosensor pairs for the red light wavelength and the infrared wavelength are operated at the same time to obtain measurements. That is, both LEDs are ON at the same time, and both photosensors are receptive at the same time to obtain simultaneous measurements at the red light wavelength and the infrared wavelength. A normalized ratio is then determined based on the measurements.

<FIG> illustrates an exemplary process <NUM> of detecting an air bubble according to some embodiments of the disclosure. The process <NUM> may be performed, for example, using the optical sensor assembly <NUM> in the environment illustrated in <FIG>. In the setup of <FIG>, light sources in the optical sensor assembly <NUM> are turned ON. At step <NUM>, the light intensities emanating from light emitters are determined. Using the illustration in <FIG>, light emitters or light sources are the LED array <NUM>. Light intensity of an LED in the LED array <NUM> is directly proportional to electrical current flowing through the LED. The processor <NUM> controls the LED current driver <NUM>, which provides electrical current to the LED array <NUM>, thereby controlling the light intensity of LEDs in the LED array <NUM>. As such, at step <NUM>, the processor <NUM> knows the current provided to an LED by the LED current driver <NUM> (which corresponds to the output intensity for the LED). For example, if two LEDs are in the LED array <NUM>, with a first LED operating in the red light wavelength and the second LED operating in the infrared wavelength, the processor <NUM> determines the current provided to the first LED (iemit,A) and the current provided to the second LED (iemit,B).

At step <NUM>, the light intensities detected at the photosensors are determined. A photosensor may generate a current proportional to an intensity of light incident upon the photosensor. For example, in accordance with <FIG>, two photosensors are provided: a first photosensor (sensitive to light in the red light wavelength) and a second photosensor (sensitive to light in the infrared wavelength). After the light emanating from each of the first LED and the second LED travels through the blood flow path <NUM>, the intensity of the light in each wavelength is changed. The light received at the first photosensor and the second photosensor causes a current proportional to the light intensity to be generated at each respective photosensor. Note that the first photosensor is sensitive to red light wavelength and will generate current proportional to only red light wavelength received, and the second photosensor is sensitive to infrared wavelength and will generate current proportional to infrared wavelength received. The receiver <NUM> then processes each current generated by each photosensor. The receiver <NUM> may have multiple channels to receive multiple currents from multiple photosensors detecting light at different wavelengths. The processor <NUM> receives from the receiver <NUM> a value for the current detected at the first photosensor (idetect,A) and a value for the current detected at the second photosensor (idetect,B).

At step <NUM>, one or more normalized ratios are determined from light intensities absorbed in the blood flow path <NUM>. That is, the normalized ratios are determined using the emanating light intensities obtained at step <NUM> and the received light intensities measured at step <NUM>. For example, in accordance with <FIG>, the processor <NUM> determines a normalized ratio R using iemit,A, iemit,B, idetect,A, and idetect,B, Determining the normalized ratio R involves first subtracting idetect,A from iemit,A and subtracting idetect,B from iemit,B to obtain current differences iA and iB, respectively. The current differences iA and iB are proportional the light intensities of red light wavelength and infrared wavelength, respectively, that are absorbed in the blood flow path <NUM>. The normalized ratio R then equals iA/iB.

At step <NUM>, the processor <NUM> or a measurement system, for example, controller <NUM> determines whether an air bubble is detected. Step <NUM> involves comparing historical values of the normalized ratio R to determine a change in the normalized ratio R. For example, steps <NUM>, <NUM>, and <NUM> were performed at a previous time, and a normalized ratio Rprev was stored in a non-transitory computer readable medium accessible by the measurement system. The measurement system then determines the change in the normalized ratios R and Rprev, normalized ratios at the current time period and the previous time period, respectively. If the change from a previous time period to a next time period meets or exceeds a predetermined threshold, the measurement system determines that an air bubble is detected. The system may then take an appropriate action at step <NUM> in response to detecting an air bubble, such as generating an alarm or stopping the pumping operation. The alarm may include, for example, an auditory alert generated at the controller <NUM>, a visual alert such as a message or graphic displayed at the controller <NUM> or some other measurement system, or other forms of auditory, visual, data logging, and/or haptic notifications. Additionally or alternatively, the measurement system may automatically stop a pump, for example, the peristaltic pump <NUM> associated with pumping blood from the patient <NUM> through the dialyzer <NUM>.

If the change from the previous time period to the next time period is below the predetermined threshold, the measurement system continues monitoring the normalized ratios at step <NUM> by performing steps <NUM>, <NUM>, and <NUM> to obtain a new normalized ratio for comparison at step <NUM>.

The detection of an air bubble at step <NUM> may indicate a degassing of blood-i.e., that the blood is under extreme negative pressure. Negative pressure signifies that there is an occlusion or collapse in the blood flow path, and continuing to pump under these conditions generates more negative pressure and may cause further complications. Thus, it is advantageous in certain situations to stop the pump when an air bubble is detected.

For example, if an arterial wall collapse causes an air bubble, stopping the pump gives a caregiver time for the artery walls to expand before resuming operation at a slower flowrate. In some cases, the caregiver may gradually increase or ramp up the flowrate from the slower flowrate to a higher flowrate. Different patients come with different artery compliances, and attempting to pull blood from a collapsed artery will not result in blood being pulled. Thus, embodiments of the disclosure provide a system that detects when an artery wall collapses and alerts caregivers so they may reduce the flowrate of the peristaltic pump <NUM> to match a patient's artery compliance.

In another example, if a needle being lodged against the arterial wall causes an air bubble, stopping the pump gives a caregiver the chance to adjust the needle and restart the pulling of blood from the patient. The needle being lodged against the arterial wall blocks the blood flow path, thus creating negative pressure that may result in the creation of air bubbles.

In yet another example, if a kink in tubing causes an air bubble, stopping the pump gives a caregiver the chance to adjust the tubing and restart the pulling of blood from the patient.

Process <NUM> involves monitoring the rate of change of the normalized ratio R over time. This may be accomplished using a predefined sampling rate, for example, a sampling rate between <NUM> and <NUM>. Thus, a normalized ratio R is determined at specific times, and the receiver <NUM> can sample the current generated at the photosensors at the specified sampling rate. In this configuration, there is no need to turn the LED array <NUM> ON and OFF during measurements.

In another embodiment, more than two wavelengths may be used. For example, if a third wavelength is introduced, then the system may obtain further ratios across the three wavelengths, including, R<NUM>=IA/IB, R<NUM>=IA/IC, R<NUM>=IB/IC. When more than two ratios are used, medium absorbance (e.g., absorbance of blood) will be a function of the multiple ratios. For example, in an embodiment with three ratios R<NUM>, R<NUM>, and R<NUM>, the medium absorbance may be written as f(R<NUM>, R<NUM>, R<NUM>). Linear regression methods may be used to define the characteristic of the function f.

In the case where three wavelengths are used, absorbances IA, IB, IC can provide three ratios IA/IB, IA/IC or IB/IC. The process for detecting an air bubble using three wavelengths is similar to that for two wavelengths, except that more permutations of comparisons are possible. Potential advantages to having more than two wavelengths include: (<NUM>) different ratio results may reveal additional information, or (<NUM>) one ratio's results may be used to confirm another ratio's results. Also, using more than two wavelengths and obtaining more than one ratio provides the ability to adjudicate confusing or mixed results. For example, in the case where three wavelengths are used, if historical values of R<NUM> show an air bubble is present while historical values of R<NUM> and R<NUM> show that no air bubble is present, then the measurement system may not generate an alarm since two ratios out of three indicate no air bubble. Thus, the use of more than two wavelengths may yield more robust true positives.

<FIG> illustrates example waveforms showing absorbance characteristics and a ratio of the absorbances. Absorbance detected by an infrared sensor is shown compared to absorbance detected by a red wavelength sensor, and the ratio of the absorbances is obtained by dividing the absorbance detected by the infrared sensor by that detected by the red wavelength sensor. The absorbance characteristics plotted in <FIG> are sample measurements that may be obtained while monitoring a tubing with different transients due to a change in the medium contained in the tubing. As shown in <FIG>, as the medium changes, the absorbance characteristics change accordingly, thus the red wavelength sensor and the infrared sensor have a similar shape with different amplitudes. The differing amplitudes suggest that one wavelength is absorbed more by the medium and tubing properties than the other wavelength.

The effect of common mode is removed when looking at the ratio of the absorbances. The ratio of the absorbances is mostly flat and non-varying compared to each absorbance characteristic used to determine the ratio. Effects of the color of the medium within the tubing or slowly varying color of the tubing are effectively removed when looking at the ratio. Thus, the ratio does not vary as much when compared to each of the absorbance characteristics.

The rate of change of the ratio can be used to detect fast transients as shown in <FIG>. Fast transients are indicative of the presence of an air bubble. By taking the ratio of absorbances, slowly varying effects are muted, but fast transients will be captured and will be easily distinguished. Thus, as can be seen in <FIG>, the fast transients corresponding to the present of an air bubble can clearly be seen when looking at the ratio of the absorbances over time.

Claim 1:
A method for detecting air bubbles in a blood flow path (<NUM>, <NUM>, <NUM>, <NUM>) using at least one light emitter (<NUM>, <NUM>) to generate different wavelengths and at least one photosensor (<NUM>, <NUM>), the method comprising:
setting initial intensities of light at a first wavelength emanating from the at least one light emitter (<NUM>, <NUM>) and of light at a second wavelength emanating from the at least one light emitter (<NUM>, <NUM>);
determining intensities of the light at the first wavelength and of the light at the second wavelength received at the at least one photosensor (<NUM>, <NUM>), wherein the light at the first wavelength and the light at the second wavelength received at the at least one photosensor (<NUM>, <NUM>) traverses the blood flow path (<NUM>, <NUM>, <NUM>, <NUM>);
determining a normalized ratio between an absorbance for the light at the first wavelength and an absorbance for the light at the second wavelength using the initial intensities of the light at the first wavelength and of the light at the second wavelength emanating from the at least one light emitter (<NUM>, <NUM>) and the intensities of the light at the first wavelength and of the light at the second wavelength received at the at least one photosensor (<NUM>, <NUM>), wherein the absorbance for the light at the first wavelength is obtained by subtracting the intensity of the light at the first wavelength at the at least one photosensor (<NUM>, <NUM>) from the initial intensity of the light at the first wavelength emanating from the at least one light emitter (<NUM>, <NUM>), and wherein the absorbance for the light at the second wavelength is obtained by subtracting the intensity of the light at the second wavelength at the at least one photosensor (<NUM>, <NUM>) from the initial intensity of the light at the second wavelength emanating from the at least one light emitter (<NUM>, <NUM>); and
determining whether an air bubble is present in the blood flow path (<NUM>, <NUM>, <NUM>, <NUM>) based on the determined normalized ratio by determining a change in the normalized ratio and a normalized ratio at a previous time period, wherein the air bubble is present if the change meets or exceeds a predetermined threshold.