Anesthesia monitoring: Part II (Proceedings)


Anesthesia monitoring: Part II (Proceedings)

Apr 01, 2009

• Anesthesia monitors are only as good as the people who use them. It is imperative to know what is normal so that you can recognize when something is outside of normal.

Common anesthesia monitors

• Pulse-oximeter (pulse-ox), capnograph, non-invasive blood pressure monitor (NIBP), invasive blood pressure monitor (IBP), electrocardiography (ECG)


• Estimates oxygen saturation of hemoglobin by measuring pulsatile signals across perfused tissues

• Main function

o To detect low hemoglobin oxygen saturation which alerts us to hypoxemia, the final pathway for many life-threatening events

• How it works

o Utilizes wavelengths of light

• Deoxygenated blood absorbs red light

• Oxygenated blood absorbs infrared light

→ The ratio of red to infrared light provides us with an SpO2 value

• Terminology

o PaO2 is the measure of oxygen dissolved in plasma

• Determined by arterial blood gas measurement

• Tells us the efficiency of the lungs to deliver oxygen to the blood

• Measured in mmHg (millimeters of mercury)

• Normal PaO2 depends on inspired O2 concentration

→ Multiply inspired concentration by 4-5

→ Room air is ~ 21% O2 (normal PaO2 is 80-110 mm Hg)

o SpO2 is peripheral oxygen saturation of hemoglobin

• Measured as a percent

• Estimates PaO2

→ Numbers of concern are different

o PaO2 of 100 mm Hg = an SpO2 of 98%

o PaO2 of 80 mm Hg = an SpO2 of 95%

o PaO2 of 60 mm Hg = an SpO2 of 90%

o PaO2 of 40 mm Hg = an SpO2 of 75%

→ These numbers are based on a normal oxyhemoglobin dissociation curve

• Oxyhemoglobin dissociation curve and hypoxemia

o Shows the relationship between PaO2 and SpO2

o SpO2 of 95% and above represents adequate oxygenation

o Precipitous drop in PaO2 at SpO2 of 94% and below

• Indicated early hypoxemia

• SpO2 of 90% indicates a PaO2 of 60 mm Hg and warrants treatment

• Five reasons for hypoxemia

o Low inspired oxygen, Hypoventilation, V/Q mismatch, Diffusion impairment, Shunt

• Low inspired oxygen

→ Usually not an issue when using 100% O2

→ Check O2 tank and flowmeter

→ If using N2O, check ratios or discontinue mixture

• Hypoventilation