Pulse Oximetry

Pulseoximetry

Pulseoximetry Is it aNoninvasiveMethod for monitoring the health of a personoxygen saturation. Peripheral oxygen saturation (SpO2The accuracy of readings of arterial oxygen saturation (Sa) is usually within 2% (within 4% in the worst 5% cases).O2()arterial blood gasAnalysis.[1]However, the two methods are well correlated enough to make pulse oximetry a valuable method for measuring oxygen saturation.ClinicalUse.

Abstract

The universal use of pulse oximetry in critical care monitoring is to monitor patients. This article updates the 1999 Critical care review of pulse oximetry. This article provides a summary of multiwavelength pulseoximeters that have been recently developed and their ability to detect dyshemoglobins. Critically, the impact of new signal processing techniques and reflectance tech on improving pulse oximeter performance in low perfusion and motion artifact conditions is examined. Also, data on the effects of pulse oximetry upon patient outcomes are presented.

A flexible organic reflectance oximeter array | PNAS

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Introduction

The monitoring of oxygenation in critical care is done using pulse oximetry. Pulse oximeters can be used to alert clinicians of hypoxemia. This may allow for quicker treatment and avoidance of more serious complications. In this review, I update the principles of pulse oximetry from my article in 1999 and discuss recent technological advances that have been developed to enhance the accuracy and clinical applications of this monitoring technique [1]. We will also review the available studies that evaluate the effects of pulse oximetry upon patient outcomes.

Pulse Oximetry: Mechanism, History, Use and Sources of error

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Principles of pulseoximetry

The technique of pulse oximetry has been previously described [1]. Pulse oximetry uses spectrophotometric methods to measure oxygen saturation. This is done by lighting the skin and measuring light absorption of oxygenated blood (oxyhemoglobin/#CR1] [1,2] (Fig. 1). To establish the pulse oximeter’s measurement of arterial saturation (SpO2), the ratio of absorbance at these wavelengths has to be calculated and calibrated against direct measurements. Waveforms are available on most pulseoximeters and can be used to help clinicians distinguish an artifact from the real signal (Fig. 2).

Pulse oximetry: Understanding its basic principles facilitates appreciation  of its limitations - Respiratory Medicine

Fig. 1

Four hemoglobin species are represented by their transmitted light absorbance spectra: Oxyhemoglobin (reduced hemoglobin), Caroxyhemoglobin (caroxyhemoglobin), and Methemoglobin (methemoglobin).

Fig. 2

Pulsatile signals that are common on a pulseoximeter. (Top panel: Normal signal showing a sharp waveform and a clear dicrotic notch. (Second Panel) A typical sine wave is seen in pulsatile signals during low perfusion. Third panel: Pulsatile signal with superimposed noise artifact, giving it a jagged appearance. (Bottom Panel) Pulsatile signal with motion artifact showing an irregular waveform. Reprinted with permission from BioMed Central Ltd [1]

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Accuracy

Critically ill patients with SaO2 levels of 90 % or more have a mean difference between SpO2 & SaO2 (that’s, bias), measured using a CO-oximeter as a reference standard. The standard deviation between the measurements (that’s, precision) is below 3 % [3-5]. However, the accuracy and bias of pulse oximetry readings are affected when SaO2 levels fall below 90 %. Pulse oximetry can only accurately reflect changes in SaO2 in one point, but it is not reliable in predicting them, especially in ICU patients [5,8] (Fig. 3).

Fig. 3

In critically ill patients, changes in oxygen saturation (SpO2) measured by pulseoximetry (SpO2) are compared to arterial oxygen saturation (SaO2) measured using a COoximeter (CO-oximeter). The actual changes in SaO 2 were consistently underestimated by the pulse oximeter. Reprinted with permission from BioMed Central Ltd [8]

Conventional pulse oximeters employ transmission sensors, in which the light emitter is on one side and the detector on the other. These sensors are suitable for use on the finger, toe, or earlobe; when tested under conditions of low perfusion, finger probes performed better than other probes [9]. Recently, pulse oximeter probes that use reflectance technology have been developed for placement on the forehead [10]. Reflectance sensors have emitter and detector components that are adjacent so that oxygen saturation can be estimated using back-scattered rather than transmitted light. The precision and bias between SpO2 (and SaO2) in critically ill patients with low perfusion were lower for the forehead probe than the finger probe [11-12]. The superiority of forehead reflectance probes over conventional digital probes, however, was not observed in patients with acute respiratory distress syndrome (ARDS) during a positive end-expiratory pressure (PEEP) recruitment maneuver [13].

The response time of traditional oximeter probes is variable; ear probes respond faster to changes in O2 saturation than finger or finger probes [14-15]. A recent study compared the response time of the conventional finger probe with the reflectance forehead probe in patients undergoing general anesthesia [16] (Fig. 4). It took 94 seconds to detect a decrease of SpO to 90% after apnea had been instilled (desaturation reaction time), and 100 seconds to detect the change in the finger probe. The time it took for mask ventilation to detect an increase of SpO 2 to 100% (resaturation response) was 23.2 seconds for forehead probes and 28.9 seconds with finger probes. Investigators believed that the faster response time for the forehead probe with reflectance was due to its location rather than the technology. The forehead probe measures O2 saturation in the supraorbital arterial, which has high blood flow and is less susceptible to vasoconstriction [17].

Fig. 4

The oxygen saturation was measured using pulse oximetry (SpO2) with transmittance finger probes (diamonds) and reflectance forehead probs (squares), during apnea, mask ventilation with 100% O2. At every point, the reflectance probe responded faster than the transmission probe. * P There was a difference of 0.05 between these two groups. Reprinted with permission from Wiley [16]

Pulse oximeter accuracy and precision at five different sensor locations in  infants and children with cyanotic heart disease – topic of research paper  in Medical engineering. Download scholarly article PDF and read

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Limitations

The limitations of oximeters can lead to incorrect readings [15] (Table 1). The sigmoid curve of the oxyhemoglobin oxidation curve may cause hypoxemia to be missed in patients with high arterial oxygen tension (PaO2) [1,18].

Table 1.

Limitations in pulse oximetry

Shape of oxygen dissociation curve
Dyshemoglobins
– Carboxyhemoglobin
Methemoglobin
Dyes
Low perfusion
Skin pigmentation
Anemia
Nail polish
Motion artifact
The technique is not well-known.

Conventional pulse oximeters are able to distinguish two substances: reduced hemoglobin or oxyhemoglobin. It assumes that dyshemoglobins, such as methemoglobins (MetHb), and carboxyhemoglobins (COHb), are absent (Fig. 1). Research has shown that SpO2 readings can be affected by elevated levels of COHb or MetHb [1,19]. Accordingly, multiwavelength oximeters that are capable of estimating blood levels of COHb and MetHb have recently been designed [20]. In healthy volunteers, the accuracy of a multiwavelength oximeter (Masimo Rainbow-SET Rad-57 Pulse CO-oximeter; Masimo Corporation, Irvine, CA, USA) in measuring dyshemoglobins was evaluated by inducing carboxyhemoglobinemia (levels range from 0 % to 15 %) and methemoglobinemia (levels range from 0 % to 12 %) [20]. The difference in COHb levels between the standard method and the pulse COoximeter was -1.22 percent. The corresponding precision was 2.19 percent. Bias +/- precision of MetHB was measured using the pulse COoximeter and MetHb was measured using the laboratory COoximeter. The accuracy of pulse CO-oximeters in measuring COHb levels was also assessed during hypoxia [21]. The accuracy of the pulse COoximeter in measuring COHb was measured in 12 healthy volunteers (bias of 0.7% and precision of 4.0%). However, COHb levels were not measured when the SaO2 fell below 85%. Patients with carbon monoxide poisoning were evaluated at the emergency department. The bias between the pulse CO-oximetric measurement COHb and the laboratory CO-oximetric measurement COHb was less that 3 percent [22,23]. However, the limits of agreement between measurements were high (-11.6 % – 14.14 %) [23]. This led some authors to conclude these new pulse CO2-oximeters should not be interchangeable with standard laboratory COHb measurements [22-24].

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There have been instances of inaccurate readings using pulse oximetry with intravenous dyes that are used for diagnostic purposes, low perfusion (that is, low cardiac output and vasoconstriction and hypothermia), patients with sickle cells anemia, pigmented subjects, and patients with low levels of hemoglobin [1,6,25,26]. The two wavelengths used by pulse oximeters to measure SpO2 (660 and 940nm) can be produced from different ambient light sources. This could lead to false SpO2 readings. Fluck and colleagues [27] conducted a controlled randomized trial on healthy subjects to determine the accuracy of pulseoximetry in the presence ambient light. SpO2 measurements were taken in a darkroom with five different light sources, including fluorescent, infrared and incandescent. The difference in SpO 2 between the control (complete darkness) and any five light sources was less that 5%. Nail polish can interfere with pulse oximetry readings [28]. Hinkelbein and his colleagues [29] discovered that the difference in SpO2 and SPO2 was greatest in black (+1.6% +– 3.0%), purple (+1.2% +– 2.6%) and dark blue (+1.1% +– 3.5%) nail polish. The limits of agreement ranged between 6 % for unpainted fingernails to 14.4 percent for dark blue (Fig. 5). The error caused by nail polish cannot be corrected by rotating the probe 90 degrees.

Fig. Figure 5.

Bias in O2 saturation pulse-oximetry (SpO2), and arterial O2 saturation [SaO2] of different nail polish colors in critically ill people. The thick horizontal lines indicate mean bias. The whiskers indicate maximum and minimum bias. The bottom and top boxes represent the first and second quartiles. **P 0.01 if compared to arterial oxygen saturation. Reprinted with permission from Elsevier Inc. [29]

Motion artifact can be a significant cause of false alarms and error [30-33]. Many signal processing techniques were introduced to pulse oximeters during the 1990s to reduce motion artifact [34–38]. One such technique is Masimo signal extraction technology (SET(tm)) [39]. During motion and hypoxia, the Masimo SET oximeter performed better than the Agilent Viridia 24C (Agilent Technologies, Santa Clara, CA, USA), the Datex-Ohmeda 3740 (Datex-Ohmeda, Madison, WI, USA), and the Nellcor N-395 (Covidien Corporation, Dublin, Ireland) oximeters [34].

It is still a relatively new field of knowledge for clinicians about pulse oximetry. Interviews with 551 nurses in critical care revealed that 37 percent of them didn’t know that oximeters are more susceptible to inaccurate readings during patient motion. 15 % were unaware that poor signal quality could lead to inaccurate readings. 30 % believed that SpO2 readings can be used instead of arterial blood gases when caring for ICU patients [40].

Moderate sedation monitoring

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Clinical applications

The early detection of hypoxemia can be provided by pulse oximetry [41,42]. The largest randomized study, which involved more than 20,000 patients, found that hypoxemia rates (SpO2 less than 90%) in patients with pulse oximetry were 7.9% and 0.4% in those without one [43]. Anesthesiologists reported that at least 17 percent of patients had received oximetry. Ehrenfeld and colleagues [44] used 95,407 electronically recorded pulse-oximetry measurements of patients who had non-cardiac surgery at two hospital. They found that 6.8% of patients experienced a hypoxemic episode (SpO2 less than 90), and 3.5% had a severe hypoxemic episode (SpO2 not more than 85%) lasting longer than 2 minutes. Hypoxemic events occurred mostly during the induction or emergent phase of anesthesia; these time periods are consistent with the clinical view that anesthesia-transitional states are high-risk periods for hypoxemia [45]. Continuous monitoring of SpO2 showed that all patients who had undergone gastric bypass surgery experienced episodic hypoxemia. This was a condition in which the SpO2 level was less than 90 % for 30 seconds or more. For each patient, desaturation lasted as long as 21 +- 15 minutes [46].

The reliability of pulse oximetry in titrating fractional inspired oxygen concentrations (FIO2) in patients who require mechanical ventilation has been demonstrated. A SpO2 level of 92 % seems reasonable to ensure adequate oxygenation for Caucasian patients [6]. The SpO2 and FIO2 ratios (S/F) were used to determine if the ratio of PaO2 or FIO2 can be used to predict the ratio of PaO2/FIO2 (P/F). Data from 1,074 patients who had suffered an acute lung injury (or ARDS) were compared in two large clinical trials [47]. A S/F ratio 235 predicted a 200 P/F ratio (oxygenation criteria for ARDS), with a sensitivity 0.85 and a specificity 0.85. An S/F of 310 predicted a P/F of 300 (oxygenation criteria for acute lung injury), with a sensitivity 0.91 and a specificity 0.56. The S/F rate was found to be a reliable proxy of the P/F (correlation coefficient (r), 0.46) in patients undergoing surgery. This is especially true for patients who require PEEP levels greater than 9 cm H2O (r=0.68) and patients with a lower P/F (r=0.61) [48]. In the ICU, the S/F ratio can also be a surrogate measure for the P/F ratio when calculating the sequential organ failure assessment score, which measures the severity of organ dysfunction in critically ill patients [49].

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PCD Pulse Oximetry Integration with Clinical Applications - IHE Wiki

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Cost-effectiveness

Studies have shown that pulse oximetry can reduce the amount of blood gas samples taken in the ICU and emergency department [50-51]. However, the lack of incorporating explicit guidelines for the appropriate use of pulse oximetry may lessen the cost-effectiveness of pulse oximetry in the ICU [1].

Design of a finger base-type pulse oximeter: Review of Scientific  Instruments: Vol 87, No 1

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Influence on the outcome

To date, the largest randomized controlled trial that has evaluated the impact of pulse oximetry on outcome was the study by Moller and colleagues [43] in 20,802 surgical patients. Although myocardial ischemia occurred less frequently in the oximetry than the control group, the numbers of post-operative complications and hospital deaths were similar in the two groups [43].

In a more recent randomized study in 1,219 post-operative patients, Ochroch and colleagues [52] assessed the impact of pulse oximetry on the rate of transfer to the ICU from a post-surgical care floor. Patients were randomly assigned to receive pulse oximetry monitoring upon admission to the study floor. The oximeter was used to monitor patients continuously (n =589) or intermittently (n=630). This was based on clinical needs, as determined by either a nurse (oximeter group), or a physician (control). Patients transferred to the ICU in both the oximeter and control groups had similar percentages (6.7% versus 8.5%). In the oximeter, there was a lower rate of ICU transfers due to pulmonary complications. The estimated cost of ICU transfer for patients who needed it was lower in the oximeter than in the control group ($18.713), despite the fact that the former is older and has more comorbidities. According to the authors, the reduction in ICU pulmonary transfers may be due to earlier diagnosis and treatment for post-operative pulmonary complications.

Pulse oximetry may not have a demonstrable effect on clinical trial outcomes. This could be due to its signal-to-noise ratio [41-53]. Because the outcome under evaluation (readmission to the ICU, myocardial infarction, or death) is rare, a huge number of patients are needed to show a reduction in these events [41]. For example, to show a decrease in complications in the Moller and co-workers’ study, a 23-fold increase would have to be achieved in enrollment [41,53].

Although randomized trials have failed to show that pulse oximetry routine monitoring improves patient outcomes, it has not stopped anesthesiologists using pulse oximeters [53-54]. When surveyed, 94 % of the anesthesiologists in the study by Moller and colleagues [43] considered the pulse oximeters to be helpful in guiding clinical management. The idea that pulse oximetry could help prevent irreversible injury by keeping oxygen levels within the physiological limits was believed to be a way of avoiding irreversible injury. It is this perspective that has made pulse oximetry a crucial part of standard of care despite the absence of proven efficacy [41].

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Conclusions

The ICU uses pulse oximetry to monitor the respiratory status of patients. Modern advances in signal analysis technology and reflectance technology have made pulse oximeters more effective under conditions of motion artifact or low perfusion. Multiwavelength oximeters could prove useful in diagnosing dyshemoglobinemia. Despite the insufficient data, pulse oximetry monitoring is still a vital component of standard care for critically ill patients.

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Abbreviations

ARDSAcute respiratory distress syndrome
COHbcarboxyhemoglobin
FIO2Fractional inspired oxygen concentration
ICUintensive care unit
MetHbMethemoglobin
PaO 2arterial oxygen tension
PEEPPositive end-expiratory pressure
P/FRatio of PaO2-to FIO2
rcorrelation coefficient
SaO 2arterial oxygen saturation
SETSignal extraction technology
S/FRatio SpO2-to FIO2
SpO 2Pulse oximetry measures oxygen saturation

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Footnotes

See related review by Jubran, http://ccforum.com/content/3/2/R11

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Competing interests

The author declares that there are no competing interests.

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