Ch. 8: Oxygen and Carbon Dioxide Monitoring

Question 1

Pulse Oximetry

Answer 1

• Oxygenated Hgb absorbs more IR and less red light than deoxygenated.
• Pulse ox can detect pulsatile flow and thus arterial rather than venous.
• Less accurate below SBP 80 or Saturation < 75%
• CO-Hgb can give false high Saturation. Met-Hgb variable.
• 4-20 sec delay. Less on forehead reflectance probes.

Question 2

End-Tidal CO2 Monitoring

Answer 2

Quantity of expired CO2 depends on:

1. Metabolism
2. Venous return to lungs
3. Alveolar ventilation

Question 3

Colorimetric CO2 detectors

Answer 3

• Impregnated with pH sensitive metacresol purple
• <4mm Hg Purple
• 5-15 mmHg Tan
• >20 mmHg Yellow
• 25% false negative for correct ETT placement during cardiac arrest due to inadequate venous return.

Question 4

Quantitative CO2 Monitors

Answer 4

• Measures amount of IR light absorbed by CO2
• Sidestream or inline.
• Normally ETCO2 is 2 to 5 mmHg less than PaCO2
• With impaired ventilation or perfusion this breaks down.
• Normal wave starts at zero rises rapidly then slowly increasing plateau. Does not return to zero if rebreathing CO2/hypoventilation.
• Slow upstroke if kinked ETT or bronchospasm.

Question 5

Abnormal ET CO2 value

Answer 5

Zero:

• Esophageal intubation, extubation, apnea
• Cardiac arrest

Low CO2

• Hyperventilation, Hypopnea
• Pulmonary Embolism
• Hypothermia

Increased CO2

• Hypoventilation
• ROSC
• Fever and seizure w increased metabolism.

Question 6

Capnography during CPR

Answer 6

• ROSC unlikely to be present if ET CO2 less than 10
• ROSC often heralded by rapid increase in ETCO2
• CPR is ineffective if ETCO2 less than 10
• Note Bicarb will liberate CO2 so don’t misinterpret rise after bicarb as a sign of ROSC.

Question 7

Capnography during procedural sedation

Answer 7

• Oxygen desaturation is a late sign of hypoventilation.
• ETCO2 is the most sensitive indicator.
• Hypoventilation can cause drop OR rise in CO2. So look for changes.
• Slow hypoventilation will cause higher ETCO2 and longer waveform.
• Shallow hypoventilation will cause lower ETCO2 and stubby short waveforms.

Question 8

Capnography during Mechanical Ventilation

Answer 8

• Get an ABG initially to correlate with ETCO2
• As long as metabolism and perfusion don’t change then ETCO2 changes can correlate with PaCO2 changes.
• Important in ICHTN and brain injury patients.