Book contents
- Frontmatter
- Contents
- List of Abbreviations
- Acknowledgements
- Abdominal Trauma: Investigations
- Accessing the Thorax
- Acid-Base
- Acute Renal Failure (see also table in ‘Low urine output’)
- Acute Respiratory Distress Syndrome (ARDS)
- Agitation and Sedation
- Airway Management
- Analgesia
- Aortic Dissection
- Atelectasis
- Blood Pressure Monitoring
- Blood Products
- Blood Transfusion
- Brainstem Death and Organ Donation
- Bronchiectasis
- Burns
- Calcium Balance
- Cardiac Assessment
- Cardiogenic Shock
- Central Line Insertion
- Chronic Renal Failure
- Coagulation Defects
- Disseminated Intravascular Coagulation (DIC)
- ECG I – Basic Concepts
- ECG II – Rate and Rhythm Disturbances
- Endotracheal Intubation
- Enteral Nutrition
- Extubation and Weaning
- Fat Embolism Syndrome
- Flail Chest
- Fluid Therapy
- Haemorrhagic Shock
- Head Injury I – Physiology
- Head Injury II – Pathophysiology
- Head Injury III – Principles of Management
- Inotropes and Circulatory Support
- ITU Admission Criteria
- Jugular Venous Pulse (JVP)
- Lactic Acidosis
- Low Urine Output State
- Magnesium Balance
- Mechanical Ventilatory Support
- Metabolic Acidosis (see also ‘Acid-base’ and and ‘Lactic acidosis’)
- Metabolic Alkalosis
- Nutrition: Basic Concepts (see also parenteral nutrition & TPN)
- Oxygen: Basic Physiology
- Oxygen Therapy
- Parenteral Nutrition (TPN)
- Pneumonia
- Pneumothorax
- Potassium Balance
- Pulmonary Artery Catheter (see also ‘Central line insertion’)
- Pulmonary Thromboembolism
- Pulse Oximetry
- Renal Replacement Therapy
- Respiratory Assessment
- Respiratory Failure (see also ‘Oxygen therapy’)
- Rhabdomyolysis
- Septic Shock and Multi-Organ Failure
- Sodium and Water Balance
- Spinal Injury
- Systemic Response to Trauma
- Tracheostomy
- Transfer of the Critically Ill
- Tube Thoracostomy (Chest Drain)
Pulse Oximetry
- Frontmatter
- Contents
- List of Abbreviations
- Acknowledgements
- Abdominal Trauma: Investigations
- Accessing the Thorax
- Acid-Base
- Acute Renal Failure (see also table in ‘Low urine output’)
- Acute Respiratory Distress Syndrome (ARDS)
- Agitation and Sedation
- Airway Management
- Analgesia
- Aortic Dissection
- Atelectasis
- Blood Pressure Monitoring
- Blood Products
- Blood Transfusion
- Brainstem Death and Organ Donation
- Bronchiectasis
- Burns
- Calcium Balance
- Cardiac Assessment
- Cardiogenic Shock
- Central Line Insertion
- Chronic Renal Failure
- Coagulation Defects
- Disseminated Intravascular Coagulation (DIC)
- ECG I – Basic Concepts
- ECG II – Rate and Rhythm Disturbances
- Endotracheal Intubation
- Enteral Nutrition
- Extubation and Weaning
- Fat Embolism Syndrome
- Flail Chest
- Fluid Therapy
- Haemorrhagic Shock
- Head Injury I – Physiology
- Head Injury II – Pathophysiology
- Head Injury III – Principles of Management
- Inotropes and Circulatory Support
- ITU Admission Criteria
- Jugular Venous Pulse (JVP)
- Lactic Acidosis
- Low Urine Output State
- Magnesium Balance
- Mechanical Ventilatory Support
- Metabolic Acidosis (see also ‘Acid-base’ and and ‘Lactic acidosis’)
- Metabolic Alkalosis
- Nutrition: Basic Concepts (see also parenteral nutrition & TPN)
- Oxygen: Basic Physiology
- Oxygen Therapy
- Parenteral Nutrition (TPN)
- Pneumonia
- Pneumothorax
- Potassium Balance
- Pulmonary Artery Catheter (see also ‘Central line insertion’)
- Pulmonary Thromboembolism
- Pulse Oximetry
- Renal Replacement Therapy
- Respiratory Assessment
- Respiratory Failure (see also ‘Oxygen therapy’)
- Rhabdomyolysis
- Septic Shock and Multi-Organ Failure
- Sodium and Water Balance
- Spinal Injury
- Systemic Response to Trauma
- Tracheostomy
- Transfer of the Critically Ill
- Tube Thoracostomy (Chest Drain)
Summary
What is pulse oximetry, and what does it measure?
Pulse oximetry is a non-invasive and continuous method of assessing arterial oxygen saturation (SaO2) and pulse rate.
Note that it is not a measure of the total oxygen content of the blood nor the PaO2. It does not assess ventilation, which requires a measure of the PaCO2.
By which principle does pulse oximetry work?
Pulse oximetry works on the principles of spectrophotometry. It contains a probe emitting light at the red (660 nm) and infra-red (940 nm) wavelengths, and a photodetector.
It relies on the differing amount of light absorbed by the saturated and unsaturated Hb molecules. The percentage oxygen saturation of the blood is calculated from the ratio of these two forms of the molecule.
What are its disadvantages and sources of error?
Problems encountered include
Diminished accuracy below a saturation of ∼70%
There is a delay of ∼20 s between actual and displayed values, limiting its use in the emergency setting
Poor peripheral perfusion leads to a poor signal quality
So does ambient light pollution
Abnormal pigments affect the results. External pigments include nail varnish. Internal pigments include bilirubin of the jaundiced patient, methaemoglobin, and carboxyhaemoglobin. Jaundice underestimates the true SaO2 and carbon monoxide poisoning overestimates the true SaO2
Abnormal pulsations, such as cardiac arrhythmia or venous pulsations of right heart valve defects may interfere with the signal
Note, however, that there is no interference from polycythemia or fetal haemoglobin
What is methaemoglobin?
This is a haemoglobin molecule that contains iron in the ferric (Fe3+) state within its haem portion, as opposed to the normal ferrous (Fe2+) state.
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- Surgical Critical Care Vivas , pp. 192 - 193Publisher: Cambridge University PressPrint publication year: 2002