139 COMPARISON OF FOREARM OXYGENATION DURING ISCHAEMIA BY NEAR INFRARED SPECTROSCOPY AND PO₂ HISTOGRAPHY

Abstract

Background: There is concern regarding the quality and validity of the Cyt_aa3(cu-oxi)) signal (Cyt) from near infrared spectroscopy (NIRS); including, the increase in brain Cyt seen by us during circulatory arrest on cardiac bypass.

Purpose: To study episodes of arrested blood flow with simultaneous measurements between NIRS and pO₂ histograph to compare rates of change and reproducibility, for Hb, HbO₂, Cyt and tissue pO₂.

Method: NIRS optodes were placed 3cm apart on the forearm of healthy adults. A pO₂ tissue probe was inserted subcutaneously 5cm lateral to the optodes. Four periods of ischaemia (4 minutes each), were generated by sphygmomanometer cuff occlusion.

Results: Twenty-two paired data sets were obtained. Hb and HbO₂ were practically noise free. Cyt signals were very small, but had a distinctive pattern of response. Rates of change during ischaemia: mean HbO₂ −6.82 ± 0.73 umol/min, Hb 6.99 ± 0.83 umol/min, Cyt 0.23 ± 0.12 umol/min, HbSum 0.02 ± 0.82 umol/min, vO₂ −2.57 ± 0.27 umol/1OOg/min, pO₂ −3.32 ± 0.99 mmHg/min.

Discussion: There was a minor shift in NIRS baselines perhaps due to incomplete recovery from ischaemia. Hb, HbO₂ and pO₂ changes were consistent and reproducible; Cyt had a less obvious trend (large signal to noise ratio). Tissue pO₂ fell at a comparable rate to HbO₂.

Conclusion: pO₂ (an absolute measure) relates to HbO₂; the small positive change in Cyt was consistent with our cardiac bypass studies.

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