• The pulse oximeter noninvasively estimates the percentage of hemoglobin–oxygen saturation in the arterial blood.
• Expressed as SpO₂%.
• Functions as a measure of how much oxygen is bound to hemoglobin, as a percentage of the maximum carrying capacity.
• The two-part probe is applied over a reasonably translucent site with good blood flow, such as the finger or ear lobe.
• Comprised of an emitter that produces light on one end and a photodetector on the other end; the tissue lies in between.

• Delivery of oxygen to the tissues by the blood is dependent on: The total oxygen-carrying capacity of blood, cardiac output, and blood flow to the tissues. Almost all of the oxygen carried in the blood is bound to hemoglobin. Only a small amount is present in the dissolved form due to the low solubility coefficient (0.003 mL of oxygen per dL of blood per mm Hg PaO₂). Conversely, each gram of hemoglobin carries 1.34 mL of oxygen when completely saturated. Note: The percent oxygen saturation of hemoglobin is in equilibrium with and dependent on the PaO₂ of blood.
• Oxyhemoglobin curve: Used to describe the relationship between the PaO₂ and oxygen saturation. Some important numbers to remember include:
  • PaO₂ 760 mm Hg = SpO₂ 100%
  • PaO₂ 100 mm Hg = SpO₂ 97.4% Increasing the PaO₂ > 100 mm Hg will not significantly increase the oxygen-carrying capacity.
  • PaO₂ 70 mm Hg = SpO₂ 93%
  • PaO₂ 28 mm Hg = SpO₂ 50% (known as the P₅₀)
• Sigmoidal shape: The hemoglobin molecule favors oxygen loading in the lungs and unloading to the tissues.
  • The upper part of the curve is almost flat; thus, the PaO₂ in the lungs (alveoli) has to drop <70 mm Hg before a significant change in the hemoglobin saturation of oxygen can occur. This provides a margin of safety.

  • The steep portion between a PaO₂ of 10–40 mm Hg represents how a small change in the PaO₂ will result in larger changes in oxygen saturation. This serves a critical physiologic function in peripheral tissues where the PaO₂ is very low. Thus, slight changes in the PaO₂ will result in greater unloading of oxygen to the tissues.

    

    ---

    Left shift (affinity)*	Right shift (affinity)*
    [H+] (pH)	[H+] (pH)
    2,3-DPG	2,3-DPG
    temperature	temperature
    Fetal hemoglobin (HbF); P50 = 19 mm Hg	Pregnancy; P50 = 31 mm Hg
    Dyshemoglobins (MetHb, COHb)	Hemoglobinopathies (thalassemia, sickle cell hemoglobin), SulfHb
    Tobacco use	Chronic anemia
    	Volatile anesthetics

    *Affinity of Hgb for O₂ holds tightly to O₂; *Affinity of Hgb for O₂ released at the tissue level.

• P₅₀: Describes the PaO₂ at which the hemoglobin saturation is 50%. The binding and dissociation of oxygen to hemoglobin can be affected by several factors including: pH, PaCO₂, temperature, and 2,3 diphosphoglycerate (2,3 DPG). These changes result in right and left shifting of the oxy-hemoglobin curve.
  • Right shifting refers to a decreased oxygen affinity to hemoglobin; a greater PaO₂ is required to saturate the hemoglobin molecule. Physiologic conditions include acidosis, as well as, higher PaCO₂ levels, temperatures, and 2,3 DPG levels; such conditions are prevalent in peripheral tissues and facilitate the unloading of oxygen. The relatively flat upper portion of the curve is less affected, thus minimizing the impact on the loading of oxygen in the lungs.
  • Left shifting refers to an increased oxygen affinity to hemoglobin; a smaller PaO₂ is required to saturate the hemoglobin molecule. Physiologic conditions include alkalosis, as well as decreases in PaCO₂, temperatures, and 2,3 DPG.
• The principle of pulse oximetry is based upon the differential light absorption characteristics of oxygenated and deoxygenated hemoglobin.
  • Oxyhemoglobin absorbs more infrared light at wavelength of 840 nm.
  • Reduced deoxyhemoglobin absorbs more red light at a wavelength of 680 nm.
  • Transmission method: The emitter and photodetector are on opposite sides from each other. The transmitted red (R) and infrared (IR) signals are received at the photodetector, and the R/IR ratio is calculated and converted into a SpO₂% value.
  • Arterial pulsations result in a momentary increase in blood volume across the measuring site and enhance light absorption. An algorithm differentiates absorbance between static tissue components (e.g., venous flow, bone, skin, muscle, etc) and dynamic arterial flow.
• Limitations in pulse oximeters can result in false readings, false alarms, data drop outs, and missed desaturations from:
  • Motion artifacts
  • Irregular heart rhythms
  • Electrocautery interference
  • Intense ambient light interference
  • Poor signals during low perfusion or poor perfusion states
  • Vasopressor use
  • Cold periphery
  • Some types of nail varnishes

• The optically shielded probe is usually applied to finger tips or ear lobes. Occasionally, it can also be applied to the buccal mucosa or ala of the nares in situations of poor perfusion.
• These probes contain light emitting diodes applied on one side of the finger and a sensor on the opposite side. The probes may get significantly warmed and need to be changed in vulnerable populations: preemies or neonates.

• Decreased cardiac output: Results in decreased oxygen delivery to the tissues (causes include reduced cardiac preload, heart failure, and shock). With anemia, pulse oximeter SpO₂% may be normal; however, there may be decreased oxygen delivery to the tissues.
• Hypoxia: Mechanisms may be broadly categorized as due to low inspired oxygen concentration, hypoventilation, ventilation–perfusion mismatch, shunt, and low cardiac output. Various medical conditions such as pneumonia, pneumothorax, and pulmonary embolism cause hypoxia often by a combination of the above-mentioned mechanisms.
• Dyshemoglobins: Can affect the accuracy of the conventional two wavelength pulse oximeter readings.
  • Methemoglobin will read at 85% oxygen saturation regardless of actual levels. Pulse oximeters measure two wavelengths of light: 660 nm (red, oxyhemoglobin) and 940 nm (infrared, deoxyhemoglobin). Methemoglobin absorbs equal amounts of these wavelengths; a ratio of pulsatile and nonpulsatile absorbances equal to 1 corresponds to a hemoglobin saturation of 85%. Normal methemoglobin levels are <1%.
  • Carboxyhemoglobin and oxyhemoglobin have similar absorption characteristics. Its presence will give falsely elevated readings and therefore is clinically significant. Carboxyhemoglobin does not carry oxygen and also affects dissociation of oxygen to the tissues. Usual carboxyhemoglobin levels are <1.5% in non-smokers, and higher in city dwellers, and can be as high as 3–15% in smokers.
  • Fetal hemoglobin will not affect the accuracy of estimation of hemoglobin saturation. HgF, however, does cause a shift in the oxyhemoglobin dissociation curve to the left (binds oxygen more tightly resulting in higher saturations at lower PaO₂ levels). Consequently, there is decreased "off-loading" of oxygen at peripheral tissues.
  • Effects of sickle cell hemoglobin are controversial with some studies showing large errors and others minimal.

• Pulse oximetry allows for the immediate recognition of hypoxia and has contributed to the safety of modern anesthesia and intensive care management. It is a routinely used monitor in most clinics, every operating room, and perioperative and intensive care unit across the US and the western world.
  • Shows excellent correlation with the measured co-oximeter hemoglobin SpO₂ above 75%.
  • Used during patient transport as well as in procedure rooms where sedation is administered.
  • Used during intrapartum monitoring of the fetus, neonatal resuscitation, and premature neonates to limit pulmonary oxygen toxicity and retrolental fibroplasia.
• Specialty sensors
  • Congenital heart disease: Sensors have been developed that show greater correlation with saturations as low as 60%.
  • Parameters such as the pleth variability index (PVI), have been derived from the variation in the plethysmographic waveform during ventilation. This may be helpful in the goal-directed administration of intravenous fluids during perioperative management.
  • Newborn sensors have been developed that can measure the high pulse rate, as well as provide maximum sensitivity that allows for quick application of monitor information.
  • Specialized monitors exist to detect carbon monoxide poisoning in the field by firefighters and emergency medical technicians (EMT), as well as diagnosis and monitoring of treatment in the emergency rooms.

Delivery of O₂/min = CO (mL/min) × (hemoglobin concentration in gm/dL × 1.34 × SpO₂) + (0.003 × PaO₂ in mm Hg).

• Methemoglobinemia
• Oxygen-Carrying Capacity
• Intraoperative Hypoxia
• Mixed Venous Oxygen Saturation

Gundappa Neelakanta , MD