Pulse oximetry

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SPEAKER DR UMA MANDALMODERATOR DR DEBASHISH
GHOSH

• PULSE OXIMETRY

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INTRODUCTION

• Oximetry is the measurement of the oxygen
  saturation of haemoglobin.
• Pulse oximeters are the electrical devices used
  for in vivo, non invasive continuous measurement
  of Hb oxygen saturation.
• Pulse oximeter readings are denoted as SpO2.
• Sometimes it is called the fifth vital sign.
• pulse oximetry has been an integral component of
  intraoperative anaesthetic management since the
  first set of anaesthetic monitoring standards was
  introduced in 1986

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• It was adopted as a minimum monitoring standard
  by the ASA and has subsequently been defined as a
  minimum standard for intraoperative monitoring by
  the world federation of societies of
  anaesthesiologists(WFSA)
• It is also a part of the WHO safe surgery
  checklist

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HISTORY

• The concept of pulse oximetry is not new.
• In 1935 Carl Matthes built the first device to
  continuously measure oxygen saturation in vivo by
  transilluminating tissue.
• (Limitations , difficult to calibrate and
  absolute value could not be obtained)
  
• In 1940 J.R. Squire devised a technique of
  calibration by compressing tissue to eliminate
  blood ? later incorporated in the first
  generation of pulse oximeters used in ot.
• In the early 1940s, Glen Millikan coined the
  term oximeter" to describe a lightweight
  earpiece to detect the oxygen saturation of
  hemoglobin, for use in aviation research to
  investigate high altitude hypoxic problems.

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• In 1972 Takuo Aayogi working on a dye dilution
  cardiac output monitor using a ear densitometer,
  discovered that the absorbency ratios of the
  pulsations at different wavelenght varied with
  the oxygen saturation .
• The subsequent development of light emitting
  diodes (LEDs), photo detectors and
  microprocessors further refined the technique,
  and pulse oximeters were widely introduced into
  clinical practice.

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PHYSIOLOGIC FUNDAMENTALS

• The primary roles of the cardiorespiratory system
  are the uptake and delivery of O2 to the body.
• O2 delivery (Do2) is quantified as the product of
  arterial O2 content (CAO2) and cardiac output .
  CAO2 (in milliliters of O2 per 100 mL of blood
  hemoglobinHb, mL/100 mL) is calculated as
• CaO2 (1.34 SaO2 Hb) 0.0031
  PaO2
• Four species of Hb are found in adult blood
  oxygenated Hb (O2Hb), deoxygenated Hb (deO2Hb),
  carboxyhemoglobin (COHb), and methemoglobin
  (MetHb).
• Under normal circumstances, the concentrations of
  COHb and MetHb are small (1 to 3 and less than
  1, respectively).
• Functional O2 saturation (SaO2) refers to the
  amount of O2Hb as a fraction of the total amount
  of O2Hb and deO2Hb and is expressed as

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• Functional SaO2 O2Hb /O2Hb deO2Hb 100
• The O2Hb fraction or fractional saturation is
  defined as the amount of O2Hb as a fraction of
  the total amount of Hb12
• Fractional SaO2 O2Hb /O2Hb deO2Hb COHb
  MetHb 100
• Efficient o2 transport relies on the ability
  of hb to reversibly load and unload o2.
• The relationship b/w Pao2 and Sao2 is seen in
  oxy hb dissociation curve.

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PHYSICS

• Pulse oximetry depends on spectral analysis for
  measurement of oxygen saturation i.e. the
  detection and quantification of components in
  solution by their unique light absorption
  characteristics.
• The pulse oximeter combines the two technologies
  of
• spectrophotometry (which measures
  hemoglobin oxygen saturation) and
• optical plethysmography (which measures
  pulsatile changes in arterial blood volume
  at the sensor site.

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• When light passes through matter, it is
  transmitted, absorbed, or reflected.
• The relative absorption or reflection of light
  at different wavelengths is used in several
  monitoring devices to estimate the concentrations
  of dissolved substances, for example, carbon
  dioxide (CO2) in respiratory gas and Hb in
  plasma. This type of measurement is called
  spectrophotometry.
• It is based on the Beer-Lambert law of
  absorption

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• BEER LAMBERT LAW
• states that if a known intensity of light
  illuminates a chamber of known dimensions, then
  the concentration of a dissolved substance can be
  determined if the incident and transmitted light
  intensity is measured
• Itrans Iine-Dce
• where Itrans is the intensity of transmitted
  light, Iin is the intensity of the incident
  light, e is base of the natural logarithm, D is
  the distance the light is transmitted through the
  solution, C is the concentration of the solute,
  and e is the extinction coefficient of the
  solute( a constant for a given solute at a
  specified wavelength)
• Solved for C,
• C (1/da) ln
  Ii/It
• The unknown concentration C is thus inversely
  proportional to the light path length d and
  directly proportional to the log of the ratio of
  incident to transmitted light intensity.

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• So using the principal of beer-lambert law , the
  conc. Of a given solute in a solvent is
  determined by the amount of light that is
  absorbed by the solute at a specific wavelength
  is detected by transmitting the light of specific
  wavwlength across the solution and measuring the
  intensity on the otherside.
• The extinction cofficient of each solute vary
  with the wavelength of light, as seen with
  different forms of haemoglobins.
• peak absorption of reduced Hb at 660nm(red
  light) and oxygeneted Hb at 940nm( infrared
  light)

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• OPTICAL PLETHOSMOGRAPHY
• Because the absorption of light is proportional
  to the amount of blood between the transmitter
  and photodetector, changes in the blood volume
  are reflected in the pulse oximeter trace.
  Pulsatile expansion of the arteriolar bed
  increases the light path length , thereby
  increasing absorbenCY.
• The signal displayed by the pulse oximeter is
  proportional to light absorption between the nail
  and the anterior face of the finger.
• During systole, the amount of hemoglobinpresent
  in the fingertip is increased and, consequently,
  light absorption is decreased. An inverse
  phenomenon is observed during diastole.
  Therefore, the POP waveform depends on the
  arterial pulse.

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PRINCIPLES AND TECHNOLOGY

• Pulse oximetry takes advantage of the pulsatility
  of arterial blood flow to provide an estimate of
  SaO2 by differentiating light absorption by
  arterial blood from light absorption by other
  component.
• light absorption by tissue can be divided into a
  pulsatile component, historically referred to as
  AC, and a nonpulsatile component, referred to as
  DC. In a standard pulse oximeter, the ratio (R)
  of AC and DC light absorption at two different
  wavelengths is calculated.
• Typically used wavelengths of light are 660 and
  940 nm. At 660 nm, light absorption is greater by
  deO2Hb than by O2Hb. At 940 nm, light absorption
  is greater by O2Hb than by deO2Hb.

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• R AC660/DC660
• AC940/DC940
• where AC660, AC940, DC660, and DC940 are the AC
  and DC components for the 660- and 940-nm
  wavelengths.
• This ratio (R) is then empirically related to O2
  saturation based on a calibration curve internal
  to each pulse oximeter

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TECHNOLOGY
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• The pulse oximeter needs two different
  wavelengths to perform measurements. These
  wavelengths are generated using two Light Emitter
  Diodes (LEDs), a Red LED (660 nm,) and an Infra
  Red LED (940 nm).
• Samples cannot be taken at the same time because
  there is only one photodetector for two signals,
  therefore signals must be multiplexed by a
  multiplexer that allows selecting the wavelength
  to be sampled.
• The output generated by the photodetector is a
  current that represents the light absorption.
  This current needs to be converted into a voltage
  in order to be properly filtered and treated.
  Conversion is performed using a current to
  voltage converter where it is filtered,
  amplified, and converted into voltage.
• The signal is now multiplexed to its respective
  filter and amplification stage, depending on
  whether it is Red or Ired LED. At this stage, the
  signal is treated and most of the noise is
  removed. The signal is also amplified in order to
  be detected
• easily by the MCU ADC. The filtered signal is
  then sent to an ADC channel on the MCU.

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• One sample of the Red filtered signal, Red
  baseline, IRed filtered signal, and IRed baseline
  are taken every 1 ms. Samples are captured using
  the embedded 16-bit ADCs and filtered using a 0.5
  Hz to 150 Hz FIR (Finite Impulse Response)
  software filter on the Kinetis K53 MCU for high
  frequency and DC component removal, taking
  advantage of the MAC (Multiply and Accumulate)
  DSP instruction.
• Samples are stored on software buffer and
  averaged. A peak detection algorithm is used to
  determinate the AC component of the signal that
  is generated by the pulsatile arterial blood
  absorption.
• This is the part of the signal which is used for
  SpO2 and beats per minute (bpm) calculation. The
  samples taken and the calculated data (SpO2 and
  bpm) are sent to a GUI on a computer.

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OXIMETER STANDARDS

1. There must be means to limit the duration of
   continuous operation at temperature above 41deg
   C.
2. The accuracy must be stated over the range of 70
   TO 100 Spo2. if the manufacturer claims accuracy
   below 65, it must be stated over the additional
   range.
3. If the manufacturer claims accuracy during motion
   this aand the test methods used to establish it
   must be disclosed in the instruction for use.
4. If the manufacturer claims accuracy during
   conditions of low perfusion , this and the test
   methods used to establish it must be disclosed in
   the instruction for use.
5. There must be an indication when the Spo2 or
   pulse rate data is not current.
6. If the pulse oximetr is provided with any
   physiologic alarm, it must be provided with an
   alarm system that monitors for equipment faults,
   and there must be an alarm for low Spo2 that is
   not less than 85 Spo2 in the manufacturer
   configured alarm preset. An alarm for high Spo2
   is optional.

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• 7. An indication of signal inadequacy must be
  provided if the Spo2 or pulse rate value
  displayed is potentially incorrect.
• 8. If a variable pitch auditory signal is
  provided to indicate the pulse signal , the pitch
  change shall follow the Spo2 readings( i.e., as
  the Spo2 readings lowers, the pitch shall also be
  lowered)

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• TYPES OF PULSE OXIMETRY
• 1. TRANSMISSION PULSE OXIMETRY
• The most common type of pulse
  oximeter .
• the light beam is transmitted
  through a vascular bed and is detected on the
  opposite site.
• 2. REFLECTNCE PULSE OXIMETRY
• Relies on light that is reflected (
  backscattered ) to determine oxygen saturation .
• the probe has both an LED and
  photodetector in the same side.

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• EQUIPMENT
• 1. PROBE
• that part which comes in contact with the
  patient.
• Contains one or more LED that emits light at
  specific wave length and a photodetector .
• Several types of probe
• Reusable either clip on or attached by using
  adhesive or velcro.
• Self adhesive
  probes are less susceptible to motion artifact
  and less likely to come off if the pt moves but
  usually not well shielded from ambient light
• Disposable usually attached by adhesive,
  may be easier to use but not economical.

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• 2.CABLE
• The probe is connected to the oximeter by an
  electrical cable.
• Cables from different manufacturers are not
  interchangeable.
• 3.CONSOLE
• Many different consoles are available .
• Most oximeter that are used in the operating
  room are part of a physiological monitors.
• Console may be free standing unit,
• Small handheld , battery operated pulse oximeter
  are often used especially during transport.

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SITES

• 1. FINGER
• the probe is most commonly attached over
  fingertip.
• Failure rate is less and accuracy is better than
  on the ear lobe
• The arm oppsite from that on which the BP cuff is
  applied or in which an arterial catheter has been
  inserted should be used.
• The performance is unaffected by an arterial
  cannula, present on the same arm., bt poor
  function may occur with intravenous infusion d/t
  local hypothermia and vesoconstriction.
  
• Motion artifact is less when probe is placed on
  larger finger.
• Should not be placed on index finger when pt is
  on recovery- corneal abrasion my occur.

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• Finger is relatively sensitive to sympathetic
  system vesoconstriction poor circulation
  finger block, digital pulp space infiltration or
  a vasodilator may improve performance.(
  vigorously rubbing the fingertip may temporarily
  improve circulation).
• Dark finger nailpolish or synthetic finger nail
  the probe should be oriented so that it transmit
  light from one side of finger to the other.
  Acrylic nail do not affect.
• Detection of desaturation and resaturation is
  slower than a centrally plaed probe
• Response time may be quicker when placed on thumb.

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• 2.TOE
• alternate site when the finger is not
  available or the signal from the finger is
  unsatisfactory.
• Detection of desaturation or resaturation will
  not be as rapid as with a centrally placed probe.
  The delay time is 1 to 2 mins.
• Toe may provide more reliable signal in pts. Who
  have an epidural block
• ?
• increased in pulse amplitude
• ?
• sign of successful block

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• 3. NOSE
• usually a convenient location
• Nasal bridge , the wings of the nostril and nasal
  septum have been used.
• Responds more rapidly to change in saturation
  than probes placed on extrimities.
• Accuracy is controversial
• It has been recommended under condition such as
  hypothermia , hypotension and infusion of
  vesoconstrictive drugs.
• If the pt is placed in Trendelenberg position ,
  venous congestion may occur around the nose
  show artificially low saturation.

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• 4. EAR
• Can be particularly useful when there is finger
  motion
• Ear probe may be held in position by a plastic
  semicircular device hung around the ear or
  stabilising devices i.e. headbands or the ear
  loops may be useful.
• Earlobe should be massaged for 30 to 45 sec with
  alcohol or vasodilator or EMLA cream can be
  applied for 30 min prior to probe application to
  increase the perfusion.
• Response time faster.
• Relatively immune to vasoconstritive effects of
  the sympathetic system.
• Amplitude of ear plethysmography will respondes
  to changes in pulse pressure.
• Give errneous readings than finger probe in --
• TR
• a steep head down
  position

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• 5. TONGUE
• Probe can be made by placing a malleable
  alluminium strip behind the probe to allow it to
  bend around the tongue.
• A disposable probe wrapped around the tip of the
  tongue in the saggital plane may be used.
• Reflactance pulse oxymetry has been used in the
  superior surface of the tongue( mouth should be
  closed)
• Glossal pulse oximetry has been shown to be
  accurate
• This site is useful in pts. Who have burn over a
  large percentage of their body surface.
• Desaturation and resaturation detected quicker
• More resistant to signal interference from
  electrosurgical unit.
• Difficult to maintain in place during emergency
  situation
• tongue quivering may mimic tachycardia
• Venous congestion from a head down position and
  excess oral secretions poor performance
• Easily dislodged
• It should be placed after tracheal intubation or
  insertion of supraglottic airway.

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• 6. CHEEK
• A probe with a metal strip backing can be used to
  hold a disposable probe around the cheek or lips.
• A clip on probe with a cover over the part of
  the probe on the buccal surface can also be
  used.
• This method of use is not recommended by the
  manufacturer.
• Buccal pulse oximetry is more accurate than
  finger pulse oxymetry.
• Detect increases and decreases in saturation
  more quickly than finger and toe probes.
• Effective during hypothermia , decreased cardiac
  output, increased systemic vascular resistance
  and other low pulse pressure states.
• This site is also useful in pts. with burn, in
  neonate.
• Difficult placements, poor acceptance by awake
  patients, and artifacts during airway maneuvers.

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• 7. ESOPHAGUS
• This probe uses reflectance oximetry.
• The esophagus a core organ , is better perfused
  than the extrimities during states of poor
  peripheral purfusion and may therefore provide a
  more consistent , reliable source for pulse
  oxymetry with haemodynamic instability.
• Reflets changes in saturation more quickly.
• This site may be useful in pt with extensive burn

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• 8. FOREHEAD
• A flat reflactance pulse oximeter sensor can be
  used on the forehead.
• It should be placed just above the eyebrow so
  that it is centerd slightly lateral to the iris.
• The sensor site should be cleaned with alcohol
  befoe applying the sensor to help secure the
  adhesive.
• Pressure on the probe from a headband or
  pressure dressing may improve the signal.
• This site is usually accesible
• The forehead is less affected by
  vesoconstriction from cold or poor purfusion
  than the ear or finger.
• It should not be used if the patient is in the
  Trendelenberg position.

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• OTHERS
• Pharyngeal pulse oximetry by using a pulse
  oximeter attached to a laryngeal mask may be
  useful in patients with poor peripheral
  perfusion.
• Flexible probes may work through the palm, foot,
  penis , ankle , loer calf , or even the arm in
  infants.
• Pulse oximetry may be used to monitor fetal
  oxygenation during labor by attaching a
  reflactance pulse oximetry probe to the
  presenting part.

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ADVANTAGES

• ACCURACY
• pulse oximetry is accurate, and
  accuracy does not change with time.
• numerous studies have shown that the
  difference b/w sturation determined by puse
  oximetry and arterial blood gas analysis is
  clinically insignificant above an Spo2 of 70.
• most manufacturer claim that errors are
  less than 3 at saturation above 70.
• changes in accuracy are negligible over
  temperatures encountered in clinical practice.

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• 2. INDEPENDENCE FROM GASES AND VAPORS
• 3. FAST RESPONSE TIME
• 4. NON INVASIVE
• which allows it to be used as a routine monitor.
  
• Readily accepted by awake pt ,
• bleeding , arterial insufficiency, embolization,
  and infection sometimes seen after arterial
  puncture are avoided.
• 5. SEPARATE RESPIRATORY AND CIRCULATORY
  VARIABLES
• perfusion is indicated by the pulse signal
  strength and oxygenation by saturation.

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• 7. CONVENIENCE
• The probe is simple and fast to apply .
• site preparation is minimal.
• Arterialization of skin is not usually required,
  except in case ear lobe .
• No calibration or changing of electrolyte or
  membrane is required.
• A variety of different probes are available for
  different site applications.
• 8. FAST START TIME
• There is minimal delay in obtaining the oxygen
  saturation .
• Readout typically begins within a few beats after
  application of the probe. ( in case trancutaneous
  monitoring , requires prolonged warm up time)

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• 9. TONE MODULATION
• Changes in pulse tone with varying saturation
  allow the user to be continuously updated on Sp02
  without taking his or her eyes off the pt.
• It allows a much quicker recognition of hypoxic
  episodes than does a fixed tone.
• Most anesthesia providers can detect the
  direction ( not the magnitude) of a change in
  saturation by listening to the change in pitch of
  a pulse oximeter tone.
• 10.USER- FRIENDLINESS
• 11. LIGHT WEIGHT AND COMPACTNESS
• The console can be made lightweight and compact
  this facilitates use during transport.
• Hand held pulse oximeter s are avilable.
• Oxygen saturation monitoring is available in
  nearly all physiological monitors.

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• 12. PROBE VARIETY
• The wide variety of probe configurations confer
  broad clinical applicability to all types of
  patients , including preterm infant.
• 13. NO HEATING REQUIRED
• 14. BATTERY OPERATED
• most stand alone units and those incorporated
  into transport monitors can be operated on
  batteries
• 15. ECONOMY

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LIMITATIONS AND
DISADVANTAGES
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• 1. Dyshemoglobinemias
• The accuracy of pulse oximetry is excellent
  when the oxygen saturation is between the range
  of 70 to 100, provided the only hemoglobin
  species present in the blood are reduced
  hemoglobin and oxygenated hemoglobin.
• COHb and MetHb absorb light at one or both of
  the wavelengths used by the pulse oximeter.
  Accordingly, the presence of these Hb species
  produces errors in SpO2.
• The absorption of light at 660 nm by COHb is
  similar to that of O2Hb. At 940 nm, COHb absorbs
  virtually no light. Thus, in a patient with
  carbon monoxide poisoning, the SpO2 is falsely
  elevated.
• For every 1 of circulating carboxyhemoglobin,
  the pulse oximeter over reads by 1.

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• MetHb absorbs a significant amount of light at
  both 660 and 940 nm. As a result, in its
  presence, the ratio of light absorption R
  approaches unity.
• An R value of 1 represents the presence of equal
  concentrations of O2Hb and deO2Hb and corresponds
  to an SpO2 of 85. In a patient with
  methemoglobinemia, the SpO2 is 80 to 85
  irrespective of the SaO2.
• When the presence of either of these
  dyshemoglobins is suspected, pulse oximetry
  should be supplemented by in vitro
  multiwavelength co-oximetry.

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• sickle cell disease,as well as in the presence of
  fetal Hb - pulse oximetry readings are accurate.
• A relatively uncommon cause of reduced SpO2
  readings is the presence of congenital variants
  of Hb. Some variants, such as Hb Bassett, Hb
  Rothschild, and Hb Canabiere, have a reduced
  affinity for O2, and changes in SpO2
  appropriately reflect changes in SaO2.
• Other variants, such as Hb Lansing, Hb Bonn, and
  Hb Hammersmith, have altered absorption spectra
  that result in low SpO2 readings in the setting
  of normal SaO2.
• Heinz Body hemolytic anemia causes the pulse
  oximeter to read low.

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• 2.POOR FUNCTION WITH POOR PURFUSION
• Adequate arterial pulsations are required to
  distinguish the light absorbed by arterial blood
  from that absorbed by venous blood and tissue and
  readings may be unreliable or unavailable if
  there is loss or diminution of the peripheral
  pulse.
• Significantly erroneous reductions in SpO2
  readings may be observed for SBP lt80 mm Hg.
• proximal blood pressure cuff inflation,leaning on
  an extremity, improper positioning, hypotension,
  hypothermia, cardiopulmonary bypass, low cardiac
  output, hypovolaemia, peripheral vascular disease
  or infusion of vasoactive drugs, Valsalva
  maneuver (such as is seen in laboring patients),
  Cold extremities-----give a message such as Low
  Quality Signal or Inadequate Signal.

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• Methods to improve the signal include application
  of vasodilating cream, digital nerve blocks,
  administration of intra-arterial vasodilators, or
  placing a glove filled with warm water in the
  patient's hand. Warming cool extremities may
  increase the pulse amplitude, provided the
  cardiac output is not depressed.
• Pulse oximeters with signal extraction technology
  may perform better during low perfusion states.

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• DIFFICULTY IN DETECTING HIGH OXYGEN PARTIAL
  PRESSURE
• At high saturations, small changes in
  saturation are associated with relatively large
  changes in PaO2. Thus the pulse oximeter has a
  limited ability to distinguish high but safe
  levels of arterial oxygen from excess
  oxygenation, which may be harmful, as in
  premature newborns, or patients with severe COPD
  who need the hypoxic drive.
• DELAYED DETECTION OF HYPOXIC EVENTS
• Delay in response is related to sensor location.
  Desaturation is detected earlier when the sensor
  is placed more centrally. Lag time will be
  increased with poor perfusion and a decrease in
  blood flow to the site monitored.
• Performance of a neural block may cause the lag
  time to decrease while venous obstruction,
  peripheral vasoconstriction, hypothermia and
  motion artifacts delay detection of hypoxaemia.

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• ERRATIC PERFOMANCE WITH IRREGULAR RHYTHMS
• Irregular heart rhythms can cause erratic
  performance.
• During aortic balloon pulsation, the augmentation
  of diastolic pressure exceeds that of systolic
  pressure. This leads to a double or triple-peaked
  arterial pressure waveform that confuses the
  pulse oximeter so that it may not provide a
  reading.
• Pulse oximetry is notoriously unreliable in the
  presence of rapid atrial fibrillation.
• Pulsatile veins cause under reading as the
  oximeter cannot differentiate between pulsatile
  arteries and veins. Tricuspid regurgitation and
  neonates with hyperdynamic circulation may have
  inaccurate readings.

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• ELECTRICAL INTERFERENCE
• Electrical interference from an electrosurgical
  unit can cause the oximeter to give an incorrect
  pulse count (usually by counting extra beats) or
  to falsely register decrease in oxygen
  saturation.
• This problem may be increased in patients with
  weak pulse signals.
• The effect is transient and limited to the
  duration of the cauterization.
• Manufacturers have made significant progress in
  reducing their instruments' sensitivity to
  electrical interference and some monitors display
  a notice when significant interference is
  present.
• Steps to minimize electrical interference include
  ? locating the
  electrosurgery grounding plate as close to and
  the oximeter sensor as far from, the surgical
  field as possible
• ? routing the cable from the
  sensor to the oximeter away from the
  electrosurgery apparatus

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• ? keeping the pulse oximeter sensor and
  console as far as possible from the surgical site
  and the electrosurgery grounding plate and
  table
• ? raising the high pulse rate alarm
• ? operating the unit in a rapid response
  mode.
• ? The electrosurgical apparatus and pulse
  oximeter should not be plugged in to the same
  power source

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• OPTICAL INTERFERENCE
• No significant effect on SpO2 accuracy with
  exposure to five types of light sources
  quartz-halogen, incandescent, fluorescent, infant
  bilirubin lamp, and infrared
• Different pulse oximeters show variable
  susceptibility to such interference.
• One clue that optical interference is occuring in
  consistency b/w the pulse rate on the pulse
  oximeter and that on the monitors.
• Misplacement of the probe can allow for
  direct detection of LED light by the
  photodetector. This optical shunt gives rise to
  an SpO2 reading of 85.
• There are number of ways to minimize the
  effects------
• selection of a correct probe,
• photodetector is across from LEDs,
• shielding the probe from the light
  and other nearby probes (covering with
  opaque material ,i.g towel gauge pieces,
  alluminium foil.

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• The photodiodes used in the sensor to detect
  light cannot differentiate one wavelength of
  light from another.
• Therefore the detector does not know whether the
  light received originates from the red (660 nm)
  light-emitting diode (LED), the infrared (940 nm)
  LED, or the room lights.
• This problem is solved in most pulse oximeters by
  alternating the red and infrared LED sources.
• In this way, the oximeter attempts to eliminate
  light interference even in a quickly changing
  background of room light.
• Some sources of fluctuating light can cause
  problems, despite this clever design.

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• MOTION ARTIFACTS
• Motion of the sensor relative to the skin
  can cause an artifact that the pulse oximeter is
  unable to differentiate from normal arterial
  pulsations.
• Motion may produce a prolongation in the
  detection time for hypoxaemia without giving a
  warning.
• Evoked potential monitors and nerve stimulators
  can produce motion artifacts if the pulse
  oximeter sensor is in the same extrimity.
• Neonates and children with their tiny digits and
  poor contact with probes are most susceptible to
  motion artifact.
• Motion artifacts can usually be recognized by
  false or erratic pulse rate displays or distorted
  plethysmographic. waveforms
• Artifacts caused by motion can be decreased by
  careful sensor positioning on a different
  extremity from that being stimulated.
• Ear, cheek and nose probes may be more useful
  than finger probes in restless patients, and
  flexible probes that are taped in place, or
  probes lined with soft material are less
  susceptible to motion artifacts than clip-on
  probes

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• Patient motion, causing venous pulsations with a
  high AC-to-DC signal ratio.
• Engineers have tried several approaches to this
  problem, beginning with simply increasing the
  signal averaging time.
• If the device averages its measurements over a
  longer period, then the effect of an intermittent
  artifact is usually less.
• slows the response time to an acute change in
  Sao2
• In a new signal-processing algorithm developed by
  Masimo, Inc., the oximeter actually computes a
  venous noise reference signal, which is common to
  both light wavelengths.
• The noise reference is then subtracted from the
  total signal, and a true arterial signal is left.
  This is one of five parallel engine algorithms
  that Masimo SET (Signal Extraction Technology)
  simultaneously uses to find the most reliable
  SpO2 value for the current signal conditions.
• Some manufacturers use the R-wave of the
  patient's ECG to synchronize the optical
  measurement

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• NAIL POLISH AND COVERINGS
• Although all colors of nail polish can reduce the
  SpO2 reading, black, purple, and dark blue have
  the greatest effect. However, the error is
  generally within 2.
• Depending on the brand of pulse oximeter used,
  artificial acrylic nails may impair SpO2
  readings, although generally not to a clinically
  significant extent.
• Under conditions of normal SaO2, skin
  pigmentation has no effect on SpO2 estimates.
  However, increased skin pigmentation is
  associated with SpO2 values that overestimate
  SaO2 by as much as 8 for SaO2 less than 80.

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• The administration of intravenous dyes can result
  in inaccurate SpO2 readings.
• Methylene blue leads to a transient, marked
  decrease in SpO2 down to 65.
• Indigo carmine and indocyanine green also
  artificially decrease SpO2 measurements, although
  to a lesser extent than methylene blue.
• Isosulfan blue can produce a prolonged reduction
  with larger doses.

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• With continued clinical use, the performance of
  the LEDs in the probe may be degraded, leading to
  inaccuracy of the SpO2 value outside of the range
  specified by the manufacturer.
• These inaccuracies are expected to be more
  pronounced at lower saturations (i.e., lt90).
• Loss of accuracy at low values
• Measurement of SpO2 is less accurate at low
  values, and 70 saturation is generally taken as
  the lowest accurate reading.

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• Hyperemia
• If a limb is hyperemic, the flow of capillary
  and venous blood becomes pulsatile.
• In this situation the absorption of light from
  these sources will be included in the saturation
  computations with resulting decrease in accuracy
  of the oxygen saturation measured by pulse
  oximetry.
• A pulse oximeter placed near the site of blood
  transfusion may show transient decreases in
  oxygen saturation with rapid infusion of the
  blood.
• Failure to detect hypoventilation
• Hypoventilation and hypercarbia may occur without
  a decrease in hemoglobin oxygen saturation,
  especially if the patient is receiving
  supplemental oxygen.
• Pulse oximetry should not be relied upon to
  assess the adequacy of ventilation or to detect
  disconnections or oesophageal intubations.

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• Failure to detect impaired circulation
• A pulse oximeter signal and a normal reading do
  not necessarily imply adequacy of tissue
  perfusion.
• Some pulse oximeters show a pulse despite
  inadequate tissue perfusion or even when no pulse
  is present, as ambient light may produce a false
  signal.
• Discrepancies in readings from different
  monitors
• on the same patient at the same time is not
  uncommon.
• One reason for this is differences in methods of
  calibration and the variation in the time it
  takes various monitors to detect desaturation.
• Interference with other monitors
• Electromagnetic interference from pulse oxymeter
  power supply may cause artifacts and false
  readings.

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PATIENT COMPLICATION

• CORNEAL ABRASION
• Commonly occurs during recovery from GA, with
  pulse oximeter in the index finger, when pt rub
  their eyes.
• So finger other than index finger most
  appropriate site during recovery.
• PRESSURE AND ISCHEMIC INJURIES
• Persistent numbness to ischemic injury d/t
  prolonged probe application , compromised
  perfusion of the extremity and tight application
  of probe.
• So frequent examination of the site and moving
  the probe to different site.

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• BURN
• Burn can result from incompatibility b/w the
  probe from one manufacturer with pulse oximeter
  of another , use of a damaged probe .
• A pulse oximeter probe may provide an
  alternate pathway for electrosurgical
  currents.
• Frequent inspection of the probe site and site
  rotation are recommended.
• In case of finger or toe , the light source
  should be placed on the nail ..a glove can be
  used to protect it.
• Burns that are associated with pulse oximetry
  during MRI as a result of induced skin current
  beneath looped cable act as antennae.

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APPLICATIONS OF
PULSE OXIMETRY
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• 1 . MONITORING OF OXYGENATION
• ANESTHETIZING AREAS
• As a safe, non-invasive monitor of the
  cardiorespiratory status of high-dependency
  patients - during general anesthesia,and pt
  undergoing regional and monitored anesthesia
  care.
• During thoracic anaesthesia, and particularly
  one-lung anaesthesia, to determine whether
  oxygenation via the remaining lung is adequate or
  whether increased concentrations of oxygen must
  be given.
• Pulse oximetry may help to detect inadvertent
  bronchial intubation and may be useful to
  confirm correct tracheal tube placement when a
  funtional carbondioxide monitor is not available.
  This method is not reliable .

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• POST ANESTHESIA CARE UNIT
• The recovery room is another location
  where desaturation is common. Routine oxygen
  administration to recovering pts may not be
  necessary when pts are monitored with pulse
  oximeter.
• TRANSPORT
• Unrecognised oxygen desaturation may occur
  while the pt is being transported b/w the
  operating room and PACU . Included on most
  transport monitors and portable one is available.

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• OTHER INTRA HOSPITAL AREAS
• Pt frequently experience hypoxic episodes in the
  post operative period after leaving the PACU.
  Pulse oximetry can detect this episodes and help
  in deciding when oxygen therapy should be
  discontinued.
• Useful for monitoring patients in the ICU. May be
  helpful during weaning from artificial
  ventilation.
• Has been used during CPR. Because of artifacts
  and lag times , it is more useful during primary
  respiratory arrest than cardiac arrest.
• Emergency department.
• Reflectance pulse oximeters can be useful for
  assessing fetal status during labor and delivery
  by applying a forehead probe.
• Useful in identifying which pt with tonic-clonic
  seizures are at increased risk of hypoxic
  cerebral brain damage.

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• OUT OF HOSPITAL USE
• During the transport of patients,
  especially when noise is an issue, for
  example in aircraft, helicopters or ambulances.
  The audible tone and alarms may not be heard,
  but if a waveform can be seen, together with an
  acceptable oxygen saturation, this gives a global
  indication of a patients cardiorespiratory
  status.
• CONTROLLING OXYGEN ADMINISTRATION
• To limit oxygen toxicity in premature
  neonates supplemental oxygen can be tapered to
  maintain an oxygen saturation of 90 - thus
  avoiding the damage to the lungs and retinas of
  neonates.
• MONITORING PERIPHERAL OXYGENATION
• To assess the viability of limbs after
  plastic and orthopaedic surgery and, for example,
  following vascular grafting, or where there is
  soft tissue swelling. As a pulse oximeter
  requires a pulsatile signal under the sensor, it
  can detect whether a limb getting a blood supply.

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• DETERMINING SYSTOLIC BLOOD PRESSURE
• A pulse oximeter can be used to determine the
  SBP. More accurate in pediatric pt. than
  determined by automatic noninvasive blood
  pressure monitor.
• Has been used for patients with pulseless
  diseases of extrimities to monitor saturation and
  SBP.
• LOCATING ARTERIES
• when the axiilary artery cannot be palpated
  , it may be located by placing a pulse oximeter
  on a finger on that side and pressing in the
  axiila until the pulse wave disappears. So also
  in case femoral and dorsalis pedis arteries.

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• OTHER USES
• may be useful in high frequency jet
  ventilation, determinig the effectiveness of
  therapeutic bronchoscopy, can be combined with
  measurement of mixed venous hbo2 saturation to
  estimate oxygen consumption.
• Pulse oximetry for the detection of congenital
  heart diseases in neonates
• Pulse oximetry has also been proposed
  as a newborn-screening test for the detection of
  critical congenital heart disease (CCHD), defined
  as CHD requiring surgery or catheter intervention
  in the first year of life.
• The effectiveness of pulse-oximetry screening
  has been demonstrated in multiple international
  clinical trials .
• In a study in the UK,36 more than 20,000 neonates
  were examined in the right hand and either foot.
  Saturation of ,lt95 in either limb or a
  difference of gt2 between the limb readings was
  taken as abnormal. In this study, pulse oximetry
  had a sensitivity of 75 for critical cases and a
  specificity of 99.16

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• 2. ANALYSIS OF PLETHYSMOGRAPH WAVE FORM

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HEART RHYTHM

• the pulse oximeter waveform can be a useful
  tool for detecting and diagnosing cardiac
  arrhythmias
• To be used to maximum benefit, the pulse oximeter
  waveform is used in conjunction with the
  electrocardiogram.
• This can greatly help in correctly interpreting
  artifacts due to patient movement or electrical
  cautery.

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PULSE AMPLITITUDE

• One of the more useful plethysmographic features
  is the waveform amplitude.
• the amplitude of the plethysmograph signal is
  directly proportional to the vascular
  distensibility.
• If the vascular compliance is low, for example
  during episodes of increased sympathetic tone,
  the pulse oximeter waveform amplitude is also
  low.
• With vasodilatation, the pulse oximeter waveform
  amplitude is increased.
• An intriguing potential use of the plethysmograph
  may be as a indicator of MAC-BAR , the dose of
  anesthetic required to block adrenergic response
  in 50 of individuals who have a surgical skin
  incision

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• DICROTIC NOTCH POSITION
• It has been speculated that the vertical
  position of the dicrotic notch , as detected with
  the pulse oximeter , can be used as an indicator
  of vasomotor tone.
• It appears that the dicrotic notch tends to
  descend towards the baseline during increasing
  vasodilation and climbs to the apex of the
  pulse wave form with vasoconstriction.

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RESPIRATORY VARIABILITY ANALYSIS

• With ventilation (spontaneous and positive
  pressure) there is fluctuation of both the
  baseline (D/C) and pulsatile (A/C) components of
  the plethysmographic waveform.

• Devices based on respiratory cycleinduced
  variation in the pulse plethysmogram have been
  developed as a less invasive measure of
  prediction of intravascular fluid
  responsiveness.
• Measures such as photoplethysmography variation
  (?POP) or the plethysmography variability index
  (PVI) appear to be useful.
• The respiratory variations in the POP waveform
  amplitude (POP) are closely related to the
  respiratory variations in the arterial pulse
  pressure (PP) and are sensitive to changes in
  ventricular preload.
• Respiratory variation in POP waveform seems to be
  a noninvasive and widely available index of fluid
  responsiveness in mechanically ventilated
  patients during general anesthesia.

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PLETH VARIABILITY INDEX

• PVI is an automatic measure of the dynamic change
  in perfusion index (PI) that occurs during a
  complete respiratory
• cycle.
• Pulse oximetry uses red and infrared light. A
  constant amount of light (DC) from the pulse
  oximeter is absorbed by skin, other tissues, and
  non-pulsatile blood, whereas a variable amount
  (AC) is absorbed by the pulsating arterial
  inflow.
• For PI calculation, the infrared pulsatile
  signal is indexed against the non-pulsatile
  infrared signal and expressed as a percentage
  PI(AC/DC)100 reflecting the amplitude of the
  pulse oximeter waveform.
• PVI calculation measures changes in PI over a
  time interval sufficient to include one or more
  complete respiratory cycles as PVI(PImaxPImin)/
  PImax100 and is displayed

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• PVI has been shown to help clinicians to predict
  fluid responsiveness in mechanically ventilated
  patients under general anesthesia, defined as a
  significant increase in cardiac output after
  fluid administration.
• gtgt PVI gt14 prior to volume expansion is
  predictive that a patient will respond to fluid
  administration (81 sensitivity)
• gtgt PVI lt14 prior to volume expansion is
  predictive that a patient will not respond to
  fluid administration(100 specificity)

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• PULSE CONTOUR CARDIAC OUTPUT MONITORING
• pulse contour cardiac output, determine stroke
  volume from computerized analysis of the area
  under the arterial pressure waveform recorded
  from an arterial catheter or even a noninvasive
  finger blood pressure waveform(plethysmograph
  wave form)
• noninvasive, continuous, beat-to-beat cardiac
  output monitoring.
• A reasonably welldefined arterial pressure
  waveform with a discernible dicrotic notch is
  required for accurate identification of systole
  and diastole, a condition that might not exist
  under severe tachycardia or dysrhythmia, or other
  very low output state.

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• Pulse co-oximetry
• Multiwavelength pulse oximeters have been
  developed that use additional wavelengths of
  light to allow for the continuous noninvasive
  measurement of total Hb concentration (SpHb),as
  well as concentrations of MetHb and COHb
• More recently, the first multiwavelength pulse
  oximeter, the Rainbow Rad-57 Pulse CO-oximeter,
  has been introduced.
• This device uses eight light wavelengths rather
  than the usual two, making it capable of
  measuring both COHb and MetHb, in addition to the
  conventional SpO2 value and pulse rate.
• It also measure perfusion index(PI), and pleth
  variability index(PVI).
• The first human volunteer study of this
  instrument demonstrated that it can measure COHb
  with an uncertainty of 2, and MetHb with an
  uncertainty of 0.5.

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