A new window into hemodialysis: How optical sensors could make treatment safer

(Press-News.org) For the millions of people living with end‑stage kidney disease, hemodialysis is more than a medical procedure, it is a thrice‑weekly lifeline that keeps the body’s chemistry in balance. Yet even with decades of clinical experience and numerous technological advances, one stubborn challenge persists: determining how much fluid to remove during treatment without tipping a patient into dangerous instability. Too little fluid removal leaves patients overloaded, too much can trigger sudden drops in blood pressure, cramping, nausea, or even early termination of the session. These events are not rare, in fact, they affect nearly half of all dialysis patients.

A new pilot study from Boston University and Boston Medical Center suggests a promising way to address this longstanding problem. By using a custom optical device that illuminates the skin and underlying muscle with near‑infrared light, the researchers captured real‑time changes in tissue water content and other physiological signals during hemodialysis. As reported in Biophotonics Discovery, their research results point to a future in which clinicians may be able to anticipate adverse events earlier, and intervene before a patient becomes unstable.

The hidden physics of dialysis complications

Current monitoring tools provide only a partial picture of what happens during fluid removal. For example, devices like Crit‑Line track how the blood’s hematocrit changes as water is removed from circulation. But this leaves out the largest reservoir of fluid in the body: the extravascular compartments, where more than 60% of total body water resides. Those spaces exchange water with the bloodstream at varying rates depending on the patient’s vascular refill capacity, comorbidities, and the intensity of ultrafiltration.

When this delicate balance breaks down, when fluid leaves the bloodstream faster than it can be replenished, patients become vulnerable to hypotension and other complications. Because existing tools cannot reliably detect this mismatch in real time, clinicians often rely on patient symptoms or heuristic rules to adjust treatment.

The Boston team saw an opportunity: instead of listening only to the bloodstream, why not measure what is happening directly in the tissue itself?

A hybrid optical system designed for the clinic

The researchers created a compact device that combines two forms of near‑infrared spectroscopy: frequency‑domain (FD) and broadband continuous‑wave (CW), to gather complementary information. The FD light component measures absolute absorption and scattering as specific wavelengths, and when combined with the broadband CW measurements, the amounts of water, lipids, and hemoglobin can be quantified in tissue.

Together, these measurements provide a high‑resolution portrait of the optical properties of tissue. Each minute during dialysis, the system captured:

absorption changes linked to hemoglobin and water scattering changes linked to tissue composition and hydration derived quantities such as oxygen saturation and water‑to‑lipid ratio The probe itself, held against the calf muscle with medical tape, remained unobtrusive throughout treatment. Patients continued their dialysis as usual while the device recorded continuous optical data.

Inside the study: who was measured and how

Twenty‑seven adult inpatients receiving fluid‑removal dialysis participated. Their medical histories reflected a typical inpatient dialysis population, including high rates of hypertension, diabetes, and cardiovascular disease. Researchers logged any signs of trouble including cramping, dizziness, vomiting, headache, shortness of breath, or hypotension, and tagged these events in the data stream.

The central question: Could tissue‑level optical changes distinguish patients who experienced complications from those who did not?

The Signal That Stood Out: Tissue Water Ratio

Among all optical markers measured, one metric proved especially telling: the water ratio, defined as water content divided by the combined water‑plus‑lipid content of the tissue.

Across the cohort:

Patients who remained stable during treatment tended to show a gradual decrease in water ratio, consistent with effective ultrafiltration and normal tissue‑to‑blood water exchange. Patients who experienced adverse events, however, showed little decrease, or even slight increases, in this ratio. This difference was statistically significant.

The implication is compelling: when the body struggles to shift water from tissues into the bloodstream fast enough to keep up with fluid removal, the water ratio may flatten or rise, signaling an emerging mismatch that precedes symptoms.

Another clue: how tissue scattering changes with hydration

The researchers also found that reduced scattering amplitude, a parameter reflecting how light interacts with tissue structure, differed between groups. Patients who remained stable tended to show distinct patterns in scattering compared to those who became unstable. This lines up with earlier work showing that tissue scattering decreases as hydration increases.

Together, water ratio and scattering measures formed a signature that could discriminate between the two groups. A multifeature model using three optical parameters classified subjects with strong accuracy, better than any single marker alone.

How did it compare to current clinical tools?

Crit‑Line, the widely used optical hematocrit monitor, did not distinguish between stable and unstable patients in this cohort in a statistically significance manner. Likewise, systolic blood pressure only showed a difference because most complications were hypotension‑related, and by the time blood pressure drops, an adverse event is already underway.

The optical system, by contrast, revealed subtle physiological divergence earlier in treatment, sometimes within the first quarter of the session. This points to possible future use as an early warning tool, rather than a reactive one.

Why this matters for the future of dialysis

Dialysis patients frequently suffer from fluid‑management‑related complications. Yet clinics have few objective, real‑time tools for assessing the physiological mechanisms that actually drive those complications.

This study is small, but it opens a meaningful path forward:

Noninvasive monitoring of interstitial water could allow clinicians to adjust ultrafiltration rates before instability develops. Real‑time tissue measurements may provide a more direct link to symptoms than blood‑based metrics alone. Improved accuracy in determining dry weight, a notoriously difficult but essential clinical benchmark, may reduce long‑term cardiovascular stress. The authors emphasize limitations: the cohort was small and medically complex, the tissue model was simplified, and optical methods require further validation. But they also highlight the broad potential of the technology. Beyond dialysis, tools that measure tissue hydration could help manage edema in heart failure, monitor weight‑loss interventions, or support athletes tracking hydration status.

A glimpse of a more responsive dialysis experience

Fluid removal during hemodialysis will always be a balancing act. But with the ability to monitor tissue water in real time, clinicians may soon have a clearer view of the underlying physiology that shapes each patient’s response. This optical approach doesn’t just add another number to the chart, it opens a new window into how the body manages fluid, second by second.

For patients whose treatments can suddenly shift from routine to dangerous, that insight could make all the difference. 

For details, see the original Gold Open Access article by D. Suciu et al., “Frequency-domain broadband near-infrared spectroscopy for noninvasive monitoring of fluid volume status during hemodialysis," Biophoton. Discovery 3(1) 015003 (2026), doi: 10.1117/1.BIOS.3.1.01500

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