Alar photoplethysmography: a new methodology for monitoring fluid removal and carotid circulation during hemodialysis.

OBJECTIVE

Hemodialysis (HD) hemoconcen tration measurements may not predict hypotension, and are confounded by impaired compensatory responses to ultrafiltration (UF). We devised noninvasive photople thysmograph (PPG) technology to monitor carotid blood flow at the nasal alar, quantify cardiac and respiratory components, and study the effect of UF and resistance breathing in HD patients and blood donors.

METHODS

The PPG was recorded using a novel alar probe in 40 subjects (20 each group), before and after their procedure, with 3 airway resistances. Raw data were separated into a low frequency component (LFC, based on the effect of respiration on thoracic pressure and blood capacitance) and a rapid pulsatile cardiac component (PCC), yielding 3 measures of carotid flow and vascular tone.

RESULTS

The device produced stable signals amenable to automated processing. The LFC (respiration-induced variation in carotid flow) increased with UF (P as low as 0.03, depending on airway resistance), not changing in blood donors. Two PCC variables (measuring blood vessel distention with each heart beat) decreased (P < or = 0.03) with blood donation, but not UF.

CONCLUSIONS

This new noninvasive PPG method detects altered respiration-associated carotid circulation during UF. With blood donation there is dampening of pulsatile vessel distention, consistent with increased vascular tone. That compensatory mechanism was impaired in HD patients and helps explain their instability with fluid removal.