The Takeaway

"Considering the higher stability to hypothermia with a similar accuracy, nasal septum pulse oximetry may be an attractive alternative to finger pulse oximetry."

Study Design

• Prospective, observational, single-center trial
• Primary outcome: levels of measurement agreement between finger SpO₂ and SaO₂ as well as between nasal septum SpO₂ and SaO₂.
• 23 patients, aged 18–70 years, ASA status I or II, scheduled for elective laparoscopic gastrectomy.
• Exclusion criteria: pre-existing peripheral vascular diseases, those receiving cardiovascular drugs, or conditions known to affect pulse oximeter accuracy (e.g., sickle cell disease, methemoglobinemia, carboxyhemoglobinemia).

Physiology Refresh

A pulse oximeter utilizes the principles of spectrophotometry to estimate arterial oxygen saturation (SpO₂). The device emits light at two specific wavelengths - typically 660 nm (red) and 940 nm (infrared) - which pass through a perfused tissue bed, usually a fingertip or earlobe. Oxygenated hemoglobin (HbO₂) and deoxygenated hemoglobin (Hb) absorb these wavelengths differently. HbO₂ absorbs more infrared light and less red light, while Hb absorbs more red light and less infrared.

The photodetector on the opposite side measures the transmitted light at each wavelength. The ratio of absorbance at these wavelengths creates the basis for calculating oxygen saturation. Importantly, the device only analyzes the pulsatile component of the signal (AC), representing arterial blood, while filtering out the constant component (DC) from surrounding tissues, venous blood, and non-pulsatile arterial blood.

The pulse oximeter's microprocessor applies an algorithm to generate the SpO₂ reading we see. Modern pulse oximeters can achieve accuracy within ±2-3% of arterial blood gas measurements when SpO₂ is above 70%.

However, multiple factors can compromise accuracy:

• Poor perfusion states (shock, hypothermia, vasoconstriction) reduce the pulsatile signal, making it difficult for the device to distinguish between arterial and surrounding tissue.
• Motion artifacts can create false pulsatile signals that confuse the device's algorithm
• Abnormal hemoglobin species aren't correctly interpreted. Carboxyhemoglobin absorbs red light similarly to oxyhemoglobin, causing falsely elevated SpO₂ readings in carbon monoxide poisoning. Methemoglobin absorbs both wavelengths equally, potentially driving readings toward 85% regardless of actual saturation.
• Intravascular dyes like methylene blue and indocyanine green can temporarily alter light absorption characteristics.
• Nail polish and artificial nails can affect light transmission.
• Ambient light interference can occur in settings with strong light sources.
• Profound anemia reduces available hemoglobin for measurement, though it doesn't directly affect percentage saturation readings.
• Hyperbilirubinemia may affect readings in severe cases.
• Venous pulsations, as seen in tricuspid regurgitation, can contaminate the arterial signal.
• Pulse oximeters were calibrated using healthy volunteers with saturations rarely below 70%, making readings below this threshold increasingly unreliable.
• Skin pigmentation historically affected accuracy, with some devices showing bias toward overestimating saturation in darker-skinned patients

Abstract

Purpose: Though the finger is generally recommended for pulse oxygen saturation (SpO₂) monitoring site, its reliability may be compromised in conditions of poor peripheral perfusion. Therefore, we compared the performance of nasal septum SpO₂ monitoring with finger SpO₂ monitoring relative to simultaneous arterial oxygen saturation (SaO₂) monitoring in generally anesthetized patients.

Methods: In 23 adult patients, comparisons of SpO₂ measured at the nasal septum and finger with simultaneous SaO₂ were made at four time points during the 90 min study period. A pulse oximetry monitoring failure was defined as a > 10 s continuous failure of in an adequate SpO₂ data acquisition. Core temperature as well as finger-tip and nasal septum temperatures were simultaneously measured at 10 min intervals.

Results: A total of 92 sets of SpO₂ and SaO₂ measurements were obtained in 23 patients. The bias and precision for SpO₂ measured at the nasal septum were - 0.8 ± 1.3 (95% confidence interval: - 1.1 to - 0.6), which was similar to those for SpO₂ measured at the finger (- 0.6 ± 1.4; 95% confidence interval: - 0.9 to - 0.4) (p = 0.154). Finger-tip temperatures were consistently lower than other two temperatures at all time points (p < 0.05), reaching 33.5 ± 2.3 °C at 90 min after induction of anesthesia. While pulse oximetry monitoring failure did not occur for nasal septum probe, two cases of failure occurred for finger probe.

Conclusions: Considering the higher stability to hypothermia with a similar accuracy, nasal septum pulse oximetry may be an attractive alternative to finger pulse oximetry.

Excerpts

Generally, fingers are the recommended sensor sites for pulse oximetry in the operating room. However, the reliability of SpO₂ measured from the fingers may be compromised in conditions of poor peripheral perfusion such as hypotension, hypothermia, and vasoconstriction. In addition, finger SpO₂ monitoring is mechanically interfered with shivering, movement, or surgical maneuvers.

While changes in the nasal septum temperature were not significantly different from those of core temperature, changes in finger-tip temperature were significantly different from those of core and nasal septal temperatures throughout the study period (p < 0.001). Finger-tip temperatures were consistently lower than other two temperatures at all time points (p < 0.05)

The absolute median (range) differences from SaO₂ of pulse oximeters were − 1 (− 4 to 1) for nasal septum probe and 0 (− 5 to 1) for finger probe.

There was no statistically difference between the deviations of SpO₂ measured at both sites relative to SaO₂ (p = 0.154).

This study demonstrated that nasal septum pulse oximeter had a similar degree of accuracy in SpO₂ measurement compared to finger pulse oximeter. And it seemed to provide a better reliability than finger pulse oximeter under the influence of intraoperative hypothermia.

In this study, the nasal septum was chosen as a suitable alternative site for monitoring. The hypothesis underlying this choice was that the nasal septum, being closer to the trunk with no direct exposure to ambient temperature, would remain adequately perfused during low perfusion states.

The nose is a highly vascular structure with multiple anastomoses and redundancy of blood supply. The nasal septum is supplied by the branches of external and internal carotid arteries in which pulsation persists for longer in the presence of intense peripheral vasoconstriction than digital arteries

The digits are under intensive regulation by the autonomic nervous system, and in cases of low surrounding temperature or low cardiac output, their arteries are constricted to reduce heat dissipation or to maintain sufficient blood supply to the critical core organs. Thus, finger or toe skin surface temperatures significantly drop during operation under general anesthesia.

finger-tip temperatures were consistently lower than nasal septum and core temperatures at all time points (p < 0.05)

In contrast with the finger probe, nasal septum probe functioned well continuously in all cases.

commercially available forehead pulse oximetry also appears to be an alternative to finger pulse oximetry. However, due to venous pulsation via valveless veins in the head and face, forehead sensor produces intermittent low saturation readings if not properly pressure-fixed with a headband or similar device. Such a phenomenon is more pronounced during mechanical ventilation or in head-down tilt position

When a transmittance probe is placed at the ear lobe or finger, device-related pressure ulcer has been anecdotally acknowledged. If the pressure exerted by probes exceeds capillary perfusion pressure (12–32 mmHg), pressure ulcer can occur theoretically. However, as pressure ulcers occur from a combination of pressure and time, it takes 1 to 4 h of excessive pressure to cause a pressure ulcer.

Nasal septal monitoring has several other advantages; very easy to apply, less sensitive to motion artifacts, and less interfered with surgical maneuver during most surgeries. In addition, the nasal septum is usually readily accessible to the anesthesiologist, whereas the fingers are often remote or made inaccessible by the surgical drapes or the patient's position (especially in robot-assisted surgery)

Citation

Oh Y, Kim DK, Ryu DK, Choi JW. Evaluation of pulse oximeter at the nasal septum during general anesthesia: comparison with finger oximeter. J Anesth. 2024 Jun;38(3):364-370. doi: 10.1007/s00540-024-03317-5. Epub 2024 Mar 19. PMID: 38502324.

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