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EBMG

Pulse Oximetry

Essentials

  • Hypoxaemia is common, difficult to detect and deleterious.
  • Pulse oximetry is an easy-to-use and effective method for detecting hypoxaemia when the device recognizes a good pulse wave.
  • Pulse oximetry describes blood oxygenation. It does not describe, for example, the adequacy of ventilation (see Respiratory Failure) or circulation, and is therefore not sufficient on its own to monitor tissue oxygenation in a severely ill patient.

Operation principles

  • The measurement of blood oxygen saturation is based on the fact that two different wavelengths of light are absorbed unequally in reduced haemoglobin and oxyhaemoglobin.
  • Only net absorption during a pulse wave is measured. This minimizes the influence of tissues and venous or capillary blood on the result.
  • The device is usually calibrated at 75-99% SaO2 values with an error marginal of approx. 2%.
  • Response time to saturation changes is shorter with earlobe sensors than with fingertip sensors.

Restrictions

  • Pulse oximetry does not give information on acid-base balance (see Blood Gas Analysis and Acid-Base Balance).
  • Pulse oximetry does not detect hypoventilation (increased partial pressure of carbon dioxide) in a patient breathing air with increased concentration of oxygen. In a patient breathing normal air significant hypoventilation usually reduces oxygen saturation. Pulse oximetry does not thus replace clinical monitoring of respiration e.g. after anaesthesia.
  • In a critically ill patient pulse oximetry is unfortunately often unreliable because of peripheral vasoconstriction (the device does not recognize the pulse wave).

Clinical use

  • Detection of hypoxaemia associated with for example
    • anaesthesia
      • More reliable than ECG e.g. during diathermy
      • Indicates whether arrhythmias have an effect on haemodynamics.
    • in an acutely ill person
    • sleep apnoea syndrome
  • Controlling oxygen therapy

Interpretation

  • A decrease in oxygen saturation to below 90% (a shift to the steeply sloping part of the oxyhaemoglobin dissociation curve) is an indication of a significant reduction in oxygen partial pressure. Higher saturation values do not mirror reliably the partial pressure of oxygen.
  • Circulation at the point of measurement has to be sufficient for the measurement to be reliable.

Interventions in hypoxaemia

  • See table T1.

Actions to be taken in hypoxaemia

Oxyhaemoglobin saturation (%)Actions
90-95Determine oxygen saturation regularly, particularly at night. If the result is unexpected, rule out sources of error. Find out the reason for the hypoxia.
80-90As above + administer oxygen until saturation exceeds 90%. Increase FiO2 or consider CPAP therapy if target cannot be achieved.
<80As above + start continuous monitoring of oxygen saturation. Consider assisted ventilation.
In every case, act according to the alarm levels agreed upon at your workplace (MET [Medical Emergency Team] and EWS [Early Warning Signs] criteria).

Sources of error

Decreased peripheral circulation

  • The most important source of error
  • Causes
    • Cold weather or low body temperature
    • Hypotension, vasoconstriction (pain, medications)
  • Peripheral circulation can be improved by
    • warming
    • massage
    • local vasodilating therapy (e.g. a small amount of nitroglycerin ointment)
    • removing tight clothing or blood pressure torniquette.
  • Error caused by movement: tremor, waving of the hands and vibration of the ambulance

Venous pulsation

  • Heart failure
  • Tricuspidal insufficiency

Dyshaemoglobinaemias

  • Carboxyhaemoglobin (carbon monoxide poisoning
    • A misleadingly high saturation is recorded (the instrument interprets carboxyhaemoglobin as oxyhaemoglobin)
  • Methaemoglobinaemia
    • The reading is about 85% irrespective of true oxygen saturation.

Problems with illumination

  • The probe is placed incorrectly
    • Light must not leak into the probe
  • Xenon, LED and infrared lights
  • Bright daylight, fluorescence-inducing light

Obstacles to absorption

  • Non-natural pigment, such as nail polish (blue the most), tattoos and discoloration of skin caused by smoking or work interferes with the normal operation of the device.
  • Natural pigment of the skin does not usually interfere with the operation of the device. In very dark-skinned people, the device may give incorrectly high readings in hypoxemia.

Advantages compared with blood gas analysis

  • Continuous noninvasive monitoring
  • Easy-to-use and reliable instrument
  • Fewer sources of error
  • Pain and nervousness during artery sampling cause hyperventilation, which increases oxygen saturation values. Blood gas analysis may lead to an overestimation of oxygenation.

    References

    • Jouffroy R, Jost D, Prunet B. Prehospital pulse oximetry: a red flag for early detection of silent hypoxemia in COVID-19 patients. Crit Care 2020;24(1):313. [PubMed]
    • Luks AM, Swenson ER. Pulse Oximetry for Monitoring Patients with COVID-19 at Home. Potential Pitfalls and Practical Guidance. Ann Am Thorac Soc 2020;17(9):1040-1046. [PubMed]
    • Sjoding MW, Dickson RP, Iwashyna TJ, et al. Racial Bias in Pulse Oximetry Measurement. N Engl J Med 2020;383(25):2477-2478. [PubMed]
    • Pretto JJ, Roebuck T, Beckert L, et al. Clinical use of pulse oximetry: official guidelines from the Thoracic Society of Australia and New Zealand. Respirology 2014;19(1):38-46. [PubMed]
    • Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med 2013;107(6):789-99. [PubMed]
    • Lee WW, Mayberry K, Crapo R, et al. The accuracy of pulse oximetry in the emergency department. Am J Emerg Med 2000;18(4):427-31. [PubMed]

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