Pulse Oximetry - developinganaesthesia



PULSE OXIMETRY

Above: “Noah’s sacrifice after the Deluge”, John Martin (1789-1854), oil on canvas.

Right: “An Allegorical Monument to Sir Isaac Newton”, 1727-1729, oil on canvas, Giovanni Battista Pittoni the younger, Fitzwilliam Museum, Cambridge

“…Do not all charms fly

At the mere touch of cold philosophy?

There was an awful rainbow once in heaven:

We know her woof, her texture; she is given

In the dull catalogue of common things.

Philosophy will clip an Angel’s wings,

Conquer all mysteries by rule and line,

Empty the haunted air, and gnomed mine -

Unweave a rainbow…”

John Keats, “Lamia”, 1820

“..and don’t you remember Keats proposing “Confusion to the memory of Newton”, and upon your insisting on an explanation before you drank it, his saying “Because he destroyed the poetry of the rainbow by reducing it to a prism?” Ah my dear old friend you and I shall never see such days again…”

Benjamin Haydon, letter to his friend William Wordsworth, mid Nineteenth Century.

In December 1817, Benjamin Haydon, the English painter and critic hosted a famous dinner. He would in later years refer to it as the “immortal” dinner. He invited the cream of the artistic and literary genius of Georgian London to view his latest allegorical work, “Christ entering Jerusalem”. In it was featured Sir Isaac Newton, as a believer and Voltaire as a sceptic. Among the guests were John Keats, William Wordsworth and Charles Lamb. After much drinking and merriment the conversation turned to Haydon’s painting thereupon which the gathering was somewhat stunned to hear Keats propose a rather derogatory toast to the memory of Sir Isaac Newton. When William Wordsworth demanded an explanation for the apparent slight on one of England’s greatest heroes, Keats explained that Newton had forever destroyed the wonder and beauty of a rainbow by reducing it to a set of physical laws. He recorded this sentiment, years later in his poem “Lamia” in which he accuses Newton of destroying the wonder of nature, by “unweaving the rainbow”. Many, of the early Nineteenth century agreed with him, including Haydon who echoed this sentiment to his friend Wordsworth many years later in a letter recalling the memory of Keats.

It is an argument still familiar to us today, many prefer the idea of a rainbow that is made and sent by God as opposed to a set of mundane “physical laws”. The natural wonder of childhood has been replaced by the reality of a brutal and unthinking universe. The magisterial Richard Dawkins takes a different view. He argues that the incredible insights and scientific discoveries of the Enlightenment that had barely preceded Keats’s own time should only increase our wonder and marvel of the natural world. In the words of the brilliant 20th century physicist Richard Feynman…

“…The beauty that is there for you is also available for me too. But I see a deeper beauty that isn’t so readily available to others. I can see the complicated interactions of the flower. The color of the flower is red. Does the fact that the plant has color mean that it evolved to attract insects? This adds a further question. Can insects see color? Do they have an aesthetic sense? And so on. I don’t see how studying a flower ever detracts from its beauty, it only adds…”

Richard Dawkins passionately argues that we can still maintain our childhood wonder in the natural world even when we understand much of how it works, indeed as we learn more it should only increase this wonder to a greater degree than any child even could think possible. By blending the two aspects of all humanity, the innate wonder of the world we find ourselves so temporarily and miraculously part of, together with a sense of never ending enquiry as to the actual explanation for our existence, great and wonderful strides have been made. In the words of Richard Dawkins, “A Keats and a Newton listening to each other might hear the galaxies sing”

Newton’s childhood wonder of nature combined with his relentless thirst to understand the marvels of the world around him have led to scientific advances that would have been undreamed of by even himself. In the field of 21st century medicine one of the “dull catalogue of common things” that we process is the pulse oximeter. By this device we can monitor the oxygen saturation levels of our patients and though we have “unweaved its rainbow” we should never fail to appreciate the wonder of the fact that we are able to do this.

PULSE OXIMETRY

Introduction

● The oxygen reserves of the body are very limited and life threatening hypoxemia can development rapidly within minutes and with few initial clinical signs to warn of this.

● The availability therefore of reliable pulse oximeters has revolutionized the safe monitoring of patients with unstable cardiorespiratory conditions as well as patients undergoing anaesthesia or sedation.

● Pulse oximetry is an easy to use, non-invasive, continuous real time monitor of a patient’s level, (percentage) of oxygenated haemoglobin.

● It reduces the need for frequent and invasive measurement of arterial PaO2 levels required to monitor unstable or sedated or anesthetised patients.

Physiology of Tissue Oxygenation

● It is important to note that whilst the oxygen saturation is a very important component of tissue oxygenation, it is in fact only one component of a range of factors which determine the overall oxygen delivery to the tissues.

Total Oxygen Delivery

The total oxygen delivery (or “oxygen flux”) to the tissues is determined by the following equation:

O2 flux = CO x O2 content of the blood.

= Cardiac output x (O2 carrying capacity of Hb + dissolved O2 )

= CO (mls /min) x (1.39 Hb gm / 100 mls x SAT % + 0.3 / 100mls at PaO2 of 100 mmHg)

O2 carrying capacity of Hb = 1.39 x 15gm / 100mls blood x 100 / 100, (assuming 100% saturation)

Assuming: 1 gm of Hb binds 1.39 mls of O2 and that the Hb level is 15gm/ 100mls. This comes to 20.8 mls O2 per 10 mls of blood.

The amount of dissolved O2 = Solubility x PaO2 (Henry’s law: the amount of O2 dissolved in blood is ( to its partial pressure)

The solubility of O2 in blood = 0.003 mls / 100 mls / mm Hg of O2

Therefore at 100 mmHg of O2 = 0.3 mls O2 100 mls of blood

Therefore the oxygen content of blood = 20.8 mls / 100 mls blood + 0.3 mls / 100mls blood. This comes to 21.1 mls O2 / 100 mls blood.

Assuming the average adult at rest has a CO of 5 liters / min

X / 5000 = 21.1 / 100

X = approximately 1000 mls of O2 is delivered to the tissues per minute.

Therefore, in summary, adequate tissue oxygenation depends not only on the PaO2, but also on an adequate cardiac output, and hemoglobin level.

See also Tissue Oxygenation Assessment Guidelines.

Physics of Pulse Oximetry

Pulse oximeters function on the basis of the physics of absorption spectroscopy, and 3 principles that stem from this:

● The differences in absorption (and hence transmission) of light of differing wavelengths for OxyHb and DeoxyHb.

● The Beer-Lambert law.

● The assumption that no other Hb forms exist apart from OxyHb and DeoxyHb.

The relationship of wavelength to light absorption and transmission:

Absorbed light

Wavelength (nanometres)

Pulse oximeters have 2 LED (light emitting diode) sources. One is of 660 nm wavelength, (visible red light) whilst the other is of 940nm wavelength, (invisible infra-red light).

The absorption and transmission of light through different media varies according to the exact wavelength of the light that is traversing the media.

At 660nm, DeoxyHb is absorbed much more than OxyHb, (ie OxyHb transmits red light to a much greater degree than DeoxyHb)

At 940nm DeoxyHb is absorbed about the same as OxyHb, (ie, OxyHb and DeoxyHb are transmitted to about the same degree)

Therefore:

660nm Absorbed light (mostly DeoxyHb)

940nm ( Absorbed light (OxyHb same as DeoxyHb)

Therefore:

660nm Transmitted light (mostly OxyHb) OxyHb level

940nm ( Transmitted light (OxyHb same as DeoxyHb) ( OxyHb & DeoxyHb level

And:

OxyHb level 100

OxyHb & DeoxyHb level X 1 = Saturation level of oxygen.

The Beer Lambert law relates the OxyHb level as a function of the amount of transmitted light, (at constant light intensity and Hb). This law therefore allows the microprocessors to convert a transmitted light reading into an OxyHb level.

Measurement of the Arterial Component of the blood oxygen saturation level:

The total amount of light that is transmitted through the tissues will depend on the sum of the light that is transmitted through the pulsatile (or arterial) component and the non-pulsatile (venous and tissue) component.

The detecting microprocessors of the pulse oximeter are able to differentiate the pulsatile component of light that is transmitted from the total non-pulsatile plus pulsatile component of light that is transmitted.

The pulsatile component of transmitted light will therefore provide a reading for the arterial oxygen saturation level.

Oxygen saturation values are calculated in this way by the detecting microprocessors many hundreds of times a second, and the result is then time averaged over approximately 5 seconds.

Indications

1. Pulse oximeters are now considered mandatory for the monitoring of all patients who:

● Have impaired, or are at risk of impaired, cardio-respiratory function.

● Are undergoing anaesthesia or heavy sedation.

2. Respiratory function assessments

● Chronic lung disease

● Sleep anpea studies

Interpretation

Once a reading is given in terms of the patient’s arterial oxygen saturation an assessment can be made of the patient’s oxygenation status (but not necessarily of the adequacy of the total oxygen delivery to the tissues)

By using the oxygen-dissociation curve function an estimate can be made of the patient’s PaO2 level.

The Oxygen Dissociation Curve, showing the factors that can affect it.

Interpretation of SaO2 levels: 2

| | | |

|Hb Saturation |PaO2 (mmHg) |Clinical Correlation |

| | | |

|100% |> 250 |Breathing 40% oxygen. |

| | | |

|97% |95-100 |PaO2 in the young and healthy breathing room air. |

| | | |

|96% |80 |PaO2 in the elderly breathing room air. |

| | | |

| | | |

|93% |70 |Lower limit of PaO2 in the elderly breathing roo air. |

| | | |

|90% |60 |Definition of respiratory failure |

| | | |

| | | |

| | | |

|85% |50 |Cyanosis may be visible below this level. |

| | | |

|75% |40 |Mixed venous PvO2 |

| | | |

|50% |26 |P50 level, a marker of the position of the oxygen-dissociation |

| | |curve. |

| | | |

|32% |20 |Coronary sinus blood. |

Note that a saturation of 90%, represent the beginning of the steep descent of the oxygen-dissociation curve and so within this region, even small decreases in the SaO2 will represent a profound decrease in the PaO2. It is the PaO2 which determines the flow of oxygen down the “oxygen cascade” from alveoli to mitochondria. For this reason a saturation level below 90% represents imminent respiratory failure.

Limitations in Pulse oximeters

It is important to be aware of the normal limitations as well as the pathological and technical factors which may reduce the accuracy of these instruments.

Normal limitations:

● As mentioned above whilst giving a reading for a patient’s oxygenation status, the adequacy of the total oxygen delivery to the tissues is not necessarily determined, as this will further depend on functional haemoglobin levels and the cardiac output.

● The pulse oximeter will not provide any information on the important parameters of PaCO2 and acid-base status.

● A normal SaO2 reading especially in cases of increased inspired oxygen concentrations will not necessarily indicate an adequate ventilatory effort. Hypoventilation may not become apparent till late because of falsely reassuring SaO2 readings.

Pathological factors which may limit the use of pulse oximeters:

1. Anemia:

● This will reduce the oxygen carrying capacity of the blood, but the recorded SaO2 will remain unaltered.

2. Dysfunctional haemoglobins:

● CO, there may be “normal” readings despite the existence of severe tissue hypoxia.

● Methemoglobinemia, this absorbs similarly in the red and infra-red bands and results in pulse oximeter readings of around 85% irrespective of the degree of actual oxygenation.

3. Factors which shift the oxygen-dissociation curve:

● Conditions that shift the oxygen-dissociation curve will make interpretation of results more problematic.

4. Poor perfusion

● Any condition in which there is poor peripheral perfusion (from any cause) will make readings inaccurate and unreliable. The vascular bed being monitored must be pulsatile.

Technical factors which may limit the use of pulse oximeters:

1. Accuracy range:

● Readings are accurate within the range of 70-100% SaO2 and reasonable but variable in the range of 55%-70%. Below 55% they are not accurate. 1

2. Motion artefact.

3. Darkly pigmented skin:

● Patients with darkly pigmented skin, though careful nail bed placement of the probe will usually overcome this.

4. Nail polishes:

● Some dark nail polishes, depending on the exact type may cause signal failure.

Technical Aspects

Application

● Pulse oximetry uses light emitting diodes (LEDs) of two wavelengths (red and infrared) that transmit through a reasonably translucent site with good blood flow.

● Opposite the emitter is a photodetector that receives the light that passes through the tissues. The photodetector converts the light energy into electrical energy to provide a digital electronic reading and a visual waveform.

● Typical adult/pediatric sites are the finger, toe, pinna (top) or lobe of the ear. Infant sites include the foot or palm of the hand and the big toe or thumb.

Features

Advantageous features of pulse oximeters include:

● Ease of use

● Non-invasive

● No calibration required

● Digital real time recordings of the oxygen saturation level.

● Digital real time recordings of the pulse rate.

● Real time recording of the arterial pulse waveform.

● An audible tone for pulse rate with a variable intensity audio capability that correlates with the level of arterial saturation.

● Alarm limits for oxygenation saturation level recordings

References

1. Young I.H, “Oximetry”, Aust Prescr 2003; 26:132-5.

2. Northern Hospital ICU Website: “What’s the use of pulse oximetry?”

Dr J. Hayes.

Dr D. Pescod, Staff Anaesthetist Northern Hospital.

13 June 2007

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