Respiratory Monitoring in Anaesthesia MCQs

Most pulse oximeters illuminate tissue with two wavelengths of light: 660-nm red light and 940-nm infrared light. Because carboxyhemoglobin has an absorbance at 660 nm, very similar to O2 hemoglobin, it produces a falsely elevated Sao2 when present in the blood. Hemoglobin F, bilirubin, and fluorescein dye have no effect on pulse oximetry. Methylene blue, as well as indigo carmine and indocyanine green, lowers the Sao2 as measured by pulse oximetry. Methemoglobin absorbs red and infrared light equally well and gives saturation readings of 85%. Reference: Barash: Clinical Anesthesia, ed 7, pp 702-703; Miller: Basics of Anesthesia, ed 6, p 327.

Pain due to abdominal or thoracic incisions compromises pulmonary function because of guarding (splinting). Decreased movement of the chest wall reduces tidal volume and functional residual capacity, promoting atelectasis, intrapulmonary shunting, hypoxemia, and less commonly, hypoventilation. Reductions in vital capacity impair coughing and clearing of secretions. Reference: Morgan and mikhail's clinical anaesthesiology 6th edition pg 1062

It is important to remember that capnography will show a normal capnograph and end-tidal CO 2 immediately following endo-bronchial intubation. Anesthesia providers must be vigilant to always listen for bilateral breath sounds and observe bilateral chest rise to confirm tracheal intubation. Oesophagal intubation will show progressive decrease of etCO 2 with breaths. Bronchospasm will show steep upslope of the trace Sudden decrease in etCO 2 is seen in pulmonary embolism. Reference : dorsch and dorsch 5th edition pg 594.

Oximetry depends on the observation that oxygenated and reduced hemoglobin differ in their absorption of red and infrared light (Lambert–Beer law). Specifically, oxyhemoglobin (Hbo2) absorbs more infrared light (940nm), Whereas deoxyhemoglobin absorbs more red light (660nm) and thus appears blue, or cyanotic, to the naked eye. Reference: Morgan and Mikhail’s clinical anesthesiology 6th edition pg 120.

Many different clinical situations will cause pulse oximetry to read in characteristic patterns. Methemoglobinemia absorbs both wavelengths of light and tends to converge around a saturation of 85% Carboxyhemoglobin only absorbs red light, but not infrared light, and can vary widely in saturation readings Methylene blue, a common dye used during surgery, tends to cause saturations to converge around 65%. Reference: Morgan and Mikhail’s clinical anesthesiology 6th edition pg 121.