Analog peripheral perfusion monitor using NIR PPG and lock-in without microcontrollers
DOI:
https://doi.org/10.70208/3007.8245.v6.n1.373Keywords:
photoplethysmography, perfusion index, synchronous demodulation, lock-in, biomedical instrumentationAbstract
Peripheral perfusion is a relevant indicator of vasomotor tone and hemodynamic status, particularly in triage, hypothermia, and early shock. Conventional photoplethysmography (PPG) can be degraded by ambient light and motion artifacts, compromising derived metrics such as the perfusion index (PI). This paper presents the design of a fully analog peripheral perfusion monitor powered by DC rails (+12 V and +5 V) using near-infrared PPG (940 nm) with optical modulation (3–10 kHz) and lock-in synchronous demodulation implemented via analog switching and baseband filtering. PI is defined as AC/DC×100% of the plethysmographic waveform, and its estimation is proposed using the AC envelope (pulsatile component) and the DC level (non-pulsatile component), providing 0–5 V analog outputs and a TTL pulse output for heart-rate counting. The work details functional blocks, initial component values, transimpedance stability considerations, and safety-oriented guidelines informed by IEC 60601-1. A reproducible validation protocol (occlusion, cold exposure, and post-exercise) is also proposed to quantify PI trends and compare them against a commercial pulse oximeter as an operational reference.
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