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RESEARCH IN PRACTICE
Pulse oximetry in children Ian P Sinha, Sarah J Mayell, Clare Halfhide
Respiratory Unit, Alder Hey Children’s Hospital, Liverpool, UK Correspondence to Dr Ian Sinha, Respiratory Unit, Alder Hey Children’s Hospital, Liverpool L12 2AP, UK; [email protected] Received 23 October 2013 Accepted 18 November 2013 Published Online First 4 December 2013
To cite: Sinha IP, Mayell SJ, Halfhide C. Arch Dis Child Educ Pract Ed 2014;99: 117–118.
ABSTRACT Pulse oximetry is routinely used in hospitals in high-income settings, but its theoretical basis is often poorly understood.1 This paper summarises the physiological background, technological basis and limitations of pulse oximetry.
THE CARRIAGE OF OXYGEN IN BLOOD: WHAT DO SATURATIONS SIGNIFY? When oxygen (O2) molecules cross the alveolar–capillary membrane, the majority enter erythrocytes and bind with haemoglobin (Hb) to form oxyhaemoglobin.2 Some remain unbound and dissolve in plasma. The amount that does so is the partial pressure of oxygen in arterial blood (PaO2). The total oxygen content of arterial blood (CaO2) includes the amount bound in erythrocytes and the amount dissolved in plasma. The term ‘saturation’ describes the percentage of available Hb saturated with oxygen (SaO2 when measured by blood gas analysis or SpO2 when measured by oximetry). PaO2 and SaO2, which can only be measured by analysing arterial blood samples, exist in dynamic equilibrium: if PaO2 rises (eg, in pulmonary capillaries), O2 binds with Hb; if PaO2 falls (eg, tissue capillaries), O2 dissociates and dissolves in plasma. The relationship is depicted in the oxygen dissociation curve (figure 1). Pulse oximetry provides a continuous, non-invasive indication of O2 saturation. Although important for evaluating children, it is only a component of the respiratory assessment. Normal SpO2 may indicate that CaO2 is adequate, but an anaemic child may be hypoxaemic (low CaO2) despite having normal saturations. A child with adequate Hb and low SpO2 may be hypoxaemic or may have ‘shunting’ of deoxygenated blood into the systemic circulation (CaO2 may be normal). Furthermore, SpO2 in isolation is not a sensitive or specific measure of hypoventilation.
Sinha IP, et al. Arch Dis Child Educ Pract Ed 2014;99:117–118. doi:10.1136/archdischild-2013-305526
HOW PULSE OXIMETERS WORK The first device for measuring oxygen saturations was developed in Germany in 1935 by Dr Carl Matthes.3 Despite huge subsequent technological advances, modern pulse oximeters still use the original method of near-infrared photospectroscopy,2 using the principle that Hb absorbs different wavelengths of light depending on the degree of oxygenation (deoxygenated Hb absorbs more red than infrared light; oxygenated Hb absorbs more infrared than red light—this also accounts for the visual difference). The probe emits red and infrared beams of light across a fingertip, toe or earlobe. The photodetector measures the wavelengths passing through the tissue, which are determined by proportion of oxygenated and deoxygenated Hb. SpO2 is derived from these measurements, which are made several times per second. For decades, oximeters were hindered by the constant absorption of light by nail, skin, subcutaneous tissue and venous blood. In the 1970s, it was discovered that when the volume of arterial blood at the measuring site increases momentarily with each heartbeat, more light is absorbed. This gives a plethysmographic waveform signal for light absorption, with the trough representing the ‘constant absorbers’ and the peak reflecting arterial pulsation. Most modern oximeters now incorporate technology to identify between pulsatile arterial blood and other substances. FACTORS AFFECTING THE ACCURACY OF PULSE OXIMETRY Advances over the last decade have focused on improving saturation monitoring accuracy in two challenging situations.4 Motion artefact
During motion, false desaturation can occur. This happens because deoxygenated blood in the low-pressure venous 117
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Research in practice
Figure 1 Oxygen dissociation curve. The curve depicts the relationship between partial pressure of oxygen (PaO2) and oxygen saturation. A haemoglobin (Hb) molecule contains four haem components, each of which can bind with one O2 molecule. Each time this happens, Hb changes shape, which facilitates binding of the next O2 molecule. The curve is sigmoidal because the affinity of Hb for O2 increases as successive molecules bind (the steep portion of the curve) until all available Hb is saturated with O2 (the plateau). The curve shifts, depending on the desired properties of Hb in different situations. Left shift (arrow a) increases affinity (O2 binds to Hb more easily). This happens when blood passes through the lungs to enable a more efficient uptake of oxygen in erythrocytes. Right shift (arrow b) increases dissociation of O2 from Hb. This happens in metabolising tissues, particularly during exercise, states of cardiovascular compromise or physiological stress.
system easily deforms and produces artefactual ‘signal noise’. New generation saturation monitors incorporate ‘read-through’ or ‘motion-tolerant’ technology, based on sophisticated analytical techniques, that improves pulse oximetry performance.
CONCLUSION Pulse oximetry enables reasonable estimation of oxygen saturations in most children. It is important to remember, however, that SpO2 provides only an indirect, proxy evaluation of whether total oxygen carriage in the blood is satisfactory. New technologies integrate sophisticated algorithms that improve measurement during motion and in low-perfusion states. Whichever machines are used, healthcare professionals should be trained to interpret pulse oximetry appropriately and in context of a child’s clinical status. Contributors IS had the idea for the article, after discussion with the editorial team. IS, SM and CH all performed literature searches. IS wrote the first draft of the article. CH and SM were both involved in amending subsequent drafts. IS is the guarantor for the final version of the article. Competing interests None. Provenance and peer review Commissioned; internally peer reviewed.
REFERENCES
Inadequate perfusion
If peripheral perfusion is reduced, arterial pulsation is more difficult to detect. This makes it difficult to separate arterial light absorption from background signal noise, causing erroneously high or low readings. Certain new generation oximeters incorporate algorithms to better distinguish true and false signals in these situations. Other factors
Saturation readings may be falsely low if smooth transition of the light beams across the artery is impaired. Causes include increased light absorption
118
(by pigmentation, nail polish or intravascular dyes), increased venous pulsation (eg, tricuspid regurgitation) or reduced passage of light beams through arterial pulsations ( probe positioning tangential to the artery, or arrhythmias). Low-saturation readings may also be caused by high-ambient infrared light.5 Saturations may be falsely high because of time lag (usually a delay of several seconds between a desaturation and its detection) or if Hb is bound to something other than O2 that affects light wavelength absorption (carboxyhaemoglobinaemia or methaemoglobinaemia). SpO2 is inaccurate below saturations of 80%. Machine calibration was conducted in healthy volunteers over a range of induced desaturations down to, but not below, 80%. Lower saturations are derived by extrapolation rather than direct measurement.
1 Fouzas S, Politis P, Skylogianni E, et al. Knowledge on pulse oximetry among pediatric health care professionals: a multicenter survey. Pediatrics 2010;126:e657–62. 2 Ochs M, O’Brodovich . The structural and physiologic basis of respiratory disease. In: Wilmott RW, Boat TF, Bush A, et al., eds. Kendig & Chernick’s Disorders of the Respiratory Tract in Children (8th edition). Philadelphia: Saunders, 2012:35–74. 3 Fouzas S, Priftis KN, Anthracopoulos MB. Pulse oximetry in pediatric practice. Pediatrics 2011;128:740–52. 4 Kamat V. Pulse oximetry. Ind J Anaesth 2002;46:261–8. 5 Fluck RR, Schroeder C, Frani G, et al. Does ambient light affect the accuracy of pulse oximetry. Resp Care 2003;48:667–80.
Sinha IP, et al. Arch Dis Child Educ Pract Ed 2014;99:117–118. doi:10.1136/archdischild-2013-305526
Downloaded from http://ep.bmj.com/ on June 13, 2015 - Published by group.bmj.com
Pulse oximetry in children Ian P Sinha, Sarah J Mayell and Clare Halfhide Arch Dis Child Educ Pract Ed 2014 99: 117-118 originally published online December 4, 2013
doi: 10.1136/archdischild-2013-305526 Updated information and services can be found at: http://ep.bmj.com/content/99/3/117
These include:
References Email alerting service
This article cites 4 articles, 2 of which you can access for free at: http://ep.bmj.com/content/99/3/117#BIBL Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
RESEARCH IN PRACTICE
Pulse oximetry in children Ian P Sinha, Sarah J Mayell, Clare Halfhide
Respiratory Unit, Alder Hey Children’s Hospital, Liverpool, UK Correspondence to Dr Ian Sinha, Respiratory Unit, Alder Hey Children’s Hospital, Liverpool L12 2AP, UK; [email protected] Received 23 October 2013 Accepted 18 November 2013 Published Online First 4 December 2013
To cite: Sinha IP, Mayell SJ, Halfhide C. Arch Dis Child Educ Pract Ed 2014;99: 117–118.
ABSTRACT Pulse oximetry is routinely used in hospitals in high-income settings, but its theoretical basis is often poorly understood.1 This paper summarises the physiological background, technological basis and limitations of pulse oximetry.
THE CARRIAGE OF OXYGEN IN BLOOD: WHAT DO SATURATIONS SIGNIFY? When oxygen (O2) molecules cross the alveolar–capillary membrane, the majority enter erythrocytes and bind with haemoglobin (Hb) to form oxyhaemoglobin.2 Some remain unbound and dissolve in plasma. The amount that does so is the partial pressure of oxygen in arterial blood (PaO2). The total oxygen content of arterial blood (CaO2) includes the amount bound in erythrocytes and the amount dissolved in plasma. The term ‘saturation’ describes the percentage of available Hb saturated with oxygen (SaO2 when measured by blood gas analysis or SpO2 when measured by oximetry). PaO2 and SaO2, which can only be measured by analysing arterial blood samples, exist in dynamic equilibrium: if PaO2 rises (eg, in pulmonary capillaries), O2 binds with Hb; if PaO2 falls (eg, tissue capillaries), O2 dissociates and dissolves in plasma. The relationship is depicted in the oxygen dissociation curve (figure 1). Pulse oximetry provides a continuous, non-invasive indication of O2 saturation. Although important for evaluating children, it is only a component of the respiratory assessment. Normal SpO2 may indicate that CaO2 is adequate, but an anaemic child may be hypoxaemic (low CaO2) despite having normal saturations. A child with adequate Hb and low SpO2 may be hypoxaemic or may have ‘shunting’ of deoxygenated blood into the systemic circulation (CaO2 may be normal). Furthermore, SpO2 in isolation is not a sensitive or specific measure of hypoventilation.
Sinha IP, et al. Arch Dis Child Educ Pract Ed 2014;99:117–118. doi:10.1136/archdischild-2013-305526
HOW PULSE OXIMETERS WORK The first device for measuring oxygen saturations was developed in Germany in 1935 by Dr Carl Matthes.3 Despite huge subsequent technological advances, modern pulse oximeters still use the original method of near-infrared photospectroscopy,2 using the principle that Hb absorbs different wavelengths of light depending on the degree of oxygenation (deoxygenated Hb absorbs more red than infrared light; oxygenated Hb absorbs more infrared than red light—this also accounts for the visual difference). The probe emits red and infrared beams of light across a fingertip, toe or earlobe. The photodetector measures the wavelengths passing through the tissue, which are determined by proportion of oxygenated and deoxygenated Hb. SpO2 is derived from these measurements, which are made several times per second. For decades, oximeters were hindered by the constant absorption of light by nail, skin, subcutaneous tissue and venous blood. In the 1970s, it was discovered that when the volume of arterial blood at the measuring site increases momentarily with each heartbeat, more light is absorbed. This gives a plethysmographic waveform signal for light absorption, with the trough representing the ‘constant absorbers’ and the peak reflecting arterial pulsation. Most modern oximeters now incorporate technology to identify between pulsatile arterial blood and other substances. FACTORS AFFECTING THE ACCURACY OF PULSE OXIMETRY Advances over the last decade have focused on improving saturation monitoring accuracy in two challenging situations.4 Motion artefact
During motion, false desaturation can occur. This happens because deoxygenated blood in the low-pressure venous 117
Downloaded from http://ep.bmj.com/ on June 13, 2015 - Published by group.bmj.com
Research in practice
Figure 1 Oxygen dissociation curve. The curve depicts the relationship between partial pressure of oxygen (PaO2) and oxygen saturation. A haemoglobin (Hb) molecule contains four haem components, each of which can bind with one O2 molecule. Each time this happens, Hb changes shape, which facilitates binding of the next O2 molecule. The curve is sigmoidal because the affinity of Hb for O2 increases as successive molecules bind (the steep portion of the curve) until all available Hb is saturated with O2 (the plateau). The curve shifts, depending on the desired properties of Hb in different situations. Left shift (arrow a) increases affinity (O2 binds to Hb more easily). This happens when blood passes through the lungs to enable a more efficient uptake of oxygen in erythrocytes. Right shift (arrow b) increases dissociation of O2 from Hb. This happens in metabolising tissues, particularly during exercise, states of cardiovascular compromise or physiological stress.
system easily deforms and produces artefactual ‘signal noise’. New generation saturation monitors incorporate ‘read-through’ or ‘motion-tolerant’ technology, based on sophisticated analytical techniques, that improves pulse oximetry performance.
CONCLUSION Pulse oximetry enables reasonable estimation of oxygen saturations in most children. It is important to remember, however, that SpO2 provides only an indirect, proxy evaluation of whether total oxygen carriage in the blood is satisfactory. New technologies integrate sophisticated algorithms that improve measurement during motion and in low-perfusion states. Whichever machines are used, healthcare professionals should be trained to interpret pulse oximetry appropriately and in context of a child’s clinical status. Contributors IS had the idea for the article, after discussion with the editorial team. IS, SM and CH all performed literature searches. IS wrote the first draft of the article. CH and SM were both involved in amending subsequent drafts. IS is the guarantor for the final version of the article. Competing interests None. Provenance and peer review Commissioned; internally peer reviewed.
REFERENCES
Inadequate perfusion
If peripheral perfusion is reduced, arterial pulsation is more difficult to detect. This makes it difficult to separate arterial light absorption from background signal noise, causing erroneously high or low readings. Certain new generation oximeters incorporate algorithms to better distinguish true and false signals in these situations. Other factors
Saturation readings may be falsely low if smooth transition of the light beams across the artery is impaired. Causes include increased light absorption
118
(by pigmentation, nail polish or intravascular dyes), increased venous pulsation (eg, tricuspid regurgitation) or reduced passage of light beams through arterial pulsations ( probe positioning tangential to the artery, or arrhythmias). Low-saturation readings may also be caused by high-ambient infrared light.5 Saturations may be falsely high because of time lag (usually a delay of several seconds between a desaturation and its detection) or if Hb is bound to something other than O2 that affects light wavelength absorption (carboxyhaemoglobinaemia or methaemoglobinaemia). SpO2 is inaccurate below saturations of 80%. Machine calibration was conducted in healthy volunteers over a range of induced desaturations down to, but not below, 80%. Lower saturations are derived by extrapolation rather than direct measurement.
1 Fouzas S, Politis P, Skylogianni E, et al. Knowledge on pulse oximetry among pediatric health care professionals: a multicenter survey. Pediatrics 2010;126:e657–62. 2 Ochs M, O’Brodovich . The structural and physiologic basis of respiratory disease. In: Wilmott RW, Boat TF, Bush A, et al., eds. Kendig & Chernick’s Disorders of the Respiratory Tract in Children (8th edition). Philadelphia: Saunders, 2012:35–74. 3 Fouzas S, Priftis KN, Anthracopoulos MB. Pulse oximetry in pediatric practice. Pediatrics 2011;128:740–52. 4 Kamat V. Pulse oximetry. Ind J Anaesth 2002;46:261–8. 5 Fluck RR, Schroeder C, Frani G, et al. Does ambient light affect the accuracy of pulse oximetry. Resp Care 2003;48:667–80.
Sinha IP, et al. Arch Dis Child Educ Pract Ed 2014;99:117–118. doi:10.1136/archdischild-2013-305526
Downloaded from http://ep.bmj.com/ on June 13, 2015 - Published by group.bmj.com
Pulse oximetry in children Ian P Sinha, Sarah J Mayell and Clare Halfhide Arch Dis Child Educ Pract Ed 2014 99: 117-118 originally published online December 4, 2013
doi: 10.1136/archdischild-2013-305526 Updated information and services can be found at: http://ep.bmj.com/content/99/3/117
These include:
References Email alerting service
This article cites 4 articles, 2 of which you can access for free at: http://ep.bmj.com/content/99/3/117#BIBL Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
