SESSION TITLE: Postoperative ICU Issues Posters
SESSION TYPE: Original Investigation Poster
PRESENTED ON: Wednesday, October 29, 2014 at 01:30 PM - 02:30 PM
PURPOSE: Evaluation of the feasibility of a new method to measure arterial oxygenation (SaO2) noninvasively in patients in whom pulse oximetry fails: those with continuous flow left ventricular assist devices (LVADs).
METHODS: Five normal subjects were studied while they breathed room air, or supplemental oxygen or nitrogen by nasal cannula, monitored by standard pulse oximetry (SpO2). Three pulseless LVAD patients with normal methemoglobin levels were studied (1 on 2 occasions) at clinically-determined levels of oxygenation. A fingertip was transilluminated alternately by LEDs emitting 660nm and 905nm. Transmitted light (L) was monitored and digitized at 16 bit resolution at 27.5 Hz, before, during and after ~5 second manual occlusions of radial and ulnar arteries. For the 1 to 2 seconds after release of an occlusion, arterial blood attenuance (the sum of absorbance and scatter) was calculated for each wavelength at each time point as A=log(Lmax/L), where Lmax is the light level (at the relevant wavelength) just before release of occlusion. A at 660nm was plotted against A at 905nm, yielding a straight line, the slope of which represents the average red/infrared arterial blood attenuance ratio (R/IR). Studies were done in triplicate at each oxygenation level. For normal subjects, median SpO2s were plotted against median R/IRs, yielding a preliminary calibration curve, which was used to derive predicted SaO2 for comparison with measured SaO2 in the LVAD patients.
RESULTS: Reproducible measurements were obtained in all subjects. Normal subject data showed close correlation of SpO2 with R/IR, [SpO2=111.45-(26.73 x R/IR), R2=0.975]. For LVAD patients, predicted SaO2 overestimated measured SaO2 by an average of a clinically insignificant 2.0 percentage points (maximum 2.6).
CONCLUSIONS: Preliminary results in a small number of patients demonstrate that a comparison of changes in attenuance of light at two wavelengths, transmitted through a fingertip, brought about by release of momentary arterial occlusion, can be used to estimate arterial saturation with acceptable accuracy, without reliance on pulsatile flow.
CLINICAL IMPLICATIONS: A noninvasive oximeter that does not rely on pulsatile flow would be a valuable advance in assessing oxygenation in patients with LVADs, for whom the only current option is arterial puncture, which is painful, carries a risk of arterial injury, and only provides a snapshot evaluation of oxygenation. Whether the same results can be achieved in peripheral vascular disease remains to be determined.
DISCLOSURE: The following authors have nothing to disclose: Pragya Gupta, Anthony Carlese, Daniel Goldstein, Thomas Aldrich
The technique described in the abstract is a new application of a well-established technique, pulse oximetry. It is not approved for clinical use, although it has been reviewed and approved for research by Montefiore's Institutional Review Board for the Protection of Human Subjects. The technique described was designed by the authors, and is not sponsored by any third party.