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LETTER TO EDITOR
Year : 2015 | Volume
: 9
| Issue : 2 | Page : 229-230
Low oxygen saturation: Really a hypoxia?
Monish S Raut, Arun Maheshwari
Department of Cardiac Anesthesia, Dharam Vira Heart Center, Sir Ganga Ram Hospital, New Delhi, India
Correspondence Address:
Dr. Monish S Raut
Department of Cardiac Anesthesia, Dharam Vira Heart Center, Sir Ganga Ram Hospital, Old Rajinder Nagar, New Delhi - 110 060
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/1658-354X.152901
| Date of Web Publication | 10-Mar-2015 |
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How to cite this article:
Raut MS, Maheshwari A. Low oxygen saturation: Really a hypoxia?. Saudi J Anaesth 2015;9:229-30 |
Sir,
A 50-year-old male patient with severe mitral stenosis was scheduled for mitral valve replacement. Before anesthesia induction, pulse oximetry probe was applied on left index finger and right radial artery was cannulated for blood pressure monitoring. Patient's saturation was 99% on pulse oximetry at room air. Surgery was done after going on cardiopulmonary bypass (CPB). During CPB, saturation was not displayed on pulse oximetry due to nonpulsatile perfusion flow. Saturation monitoring was done by in line arterial blood gas analysis, which was consistently above 95%. As the weaning from bypass was started, pulsatile arterial waveform appeared with pressure of 153/50 mmHg. However, pulse oximetry showed saturation of 65% with good plethysmographic waveform correlating with heart rate [Figure 1]. Another pulse oximetry probe was attached to ear lobule, which showed oxygen saturation (SpO 2 ) of 99%. Arterial blood gas analysis revealed saturation of 98.9% with PaO 2 of 178 mmHg on FiO 2 of 0.5. Pulse oximetry probe on the finger was examined, it was not misplaced, but dusky discoloration was observed on the left hand below forearm probably because of prolonged compression by leaning over by surgeon while operating. As the compressive effect was removed, SpO 2 on the same finger probe showed saturation of 98% in a short period of time. | Figure 1: Monitor display showing low saturation with good plethysmographic waveform
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Pulse oximeter system consists of a peripheral probe together with a microprocessor unit displaying a plethysmographic waveform, the SpO 2 and the pulse rate. The probe is placed on the fingertip, earlobe or nose. Probe has two LEDs emitting red spectrum (660 nm) and the infrared spectrum (940 nm). Photodetector on the other side of the probe senses the light passed through the tissue. Oxygenation of hemoglobin influences the amount of light absorption at each frequency. [1] Pulse oximeter calculates the ratio of pulsatile to nonpulsatile absorbance and derive the SpO 2 . Adequate arterial pulsations are essential to distinguish the light absorbed by arterial blood from that absorbed by venous blood. Inaccurate reading may be displayed in the presence of poor peripheral pulsations, low cardiac output, hypovolemia, peripheral vascular disease, improper positioning, hypotension, hypothermia, CPB, low cardiac output. [1]
In the present case, patient's left forearm was compressed for a prolonged period due to leaning over by surgeon while operating. This led to venous congestion of hand and fingers. Arterial pulsation in hand may be attenuated, but still be present. Pulse oximetry probe placed on the finger detected more deoxygenated hemoglobin due to venous congestion in the finger and displayed low saturation [Figure 2]. However, arterial pulsation was still carried distally so giving good plethysmographic waveform. Before jumping on the diagnosis of hypoxia just on the basis of low SpO 2 on pulse oximetry, first assess the proper plethysmographic waveform and then check SpO 2 at other site also. | Figure 2: Schematic diagram illustrating the mechanism of low oxygen saturation in the present case
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| References | |  |
| 1. | Severinghaus JW, Spellman MJ Jr. Pulse oximeter failure thresholds in hypotension and vasoconstriction. Anesthesiology 1990;73:532-7.  |
[Figure 1], [Figure 2]
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