CRACKCast E005 – Patient Monitoring

In CRACKCast, Podcast by Adam Thomas2 Comments

This episode of CRACKCast covers Rosen’s Chapter 005, Patient Monitoring. Proper management of any acutely ill patient requires accurate monitoring. This involves understanding of the strengths and limitations to the monitoring modalities at the emergency physician’s disposal.

Shownotes – PDF Link


1) List 6 situations when pulse oximetry is not useful

Pulse oximetry basics:

  • pulse oximetry uses LED lights to assess blood oxygenation by assessing the fractional difference between the wavelengths of oxygenated and deoxygenated blood
  • calculates the percent of hemoglobin in the oxyhemoglobin state (not Pa02)
    • pulse oximeters are accurate between 80-100%
    • below that range large changes in Sa02 can occur with small changes in Pa02

Limitations to pulse oximetry:

  • pulse oximeters are unable to distinguish oxy/deoxyhemoglobin from MetHb and COHb
    • Methemoglobin (MetHb)
      • caused by exposure to an oxidizing agent which changes hemoglobin to its ferric form that is unable to bind 02
      • classically the cyanotic patient who doesn’t respond to O2 therapy
    • Carboxyhemoglobin (COHb)
  • carbon monoxide poisoning can occur from smoke inhalation, automotive exhaust, propane heaters, wood stoves, gasoline motors etc.
  • classically the patient is one with “hypoxia, lactic acidosis, and hypotension”
  • often have headache and altered LOC

With both MetHb and COHb the SpO2 will falsely read as high

So when is pulse oximetry not useful?

Three settings:

1) Methemoglobinemia

  • cyanide poisoning
  • antimalarials
  • benzocaine
  • post methylene blue infusion

2) Carboxyhemoglobinemia

  • CO poisoning

3) Poor light penetration

  • dark nail polish
  • low perfusion states
  • dark skinned patients

2) List 10 situations when capnography is useful

Capnography can be qualitative or quantitative:

End-tidal CO2 measurement

Colorimetry (qualitative)

Uses a breath by breath assessment

purple = <4 mmhg CO2

tan = 4-15 mmhg CO2

yellow = >20 mmhg CO2

  • used mainly for confirming endotracheal tube placement post-intubation
  • quantitative waveform capnography is gold standard

Waveform capnography (quantitative)


1) confirms ventilation/respiration and tube placement (gold standard)

2) a sudden rise during cardiac arrest may indicate ROSC

3) the most sensitive way of detecting apnea during procedural sedation

4) useful in the postictal/intoxicated/overdose patient to determine if they have adequate ventilations

5) acidotic patients develop a compensatory resp. alkalosis and therefore often have a dropping ETc02.

6) to roughly correlate between alveolar CO2 and arterial CO2 in people with normal lung physiology

7) Capnography waveforms can help diagnose lung pathology for mechanically vented patients


3) Describe the ETCO2 curve

Capnography Waveforms

A, Four phases of a normal capnogram. 1-2, The carbon dioxide–free portion of the respiratory cycle. 2-3, The rapid upstroke of the curve, representing the transition from inspiration to expiration and the mixing of dead space and alveolar gas. 3-4, The alveolar plateau, representing the alveolar gas rich in carbon dioxide and tending to slope gently upward with the uneven emptying of the alveoli. 4-5, The respiratory downstroke, which is a nearly vertical drop to baseline. B, C,and D, See text for full explanation.

The shape of the capnogram can give you information about obstructive airway disease (shark-finning), spontaneous respiratory efforts, or ET cuff leaks.

Absolutely fascinated by ETCO2 and cant get enough?  Check out this fantastic lecture on everything you might want to know about the ETCO2 waveform from the MarylandCCProject.


4) List four indications for invasive blood pressure monitoring

Intra-arterial catheter is the most accurate

Indicated when:

  • hemodynamic instability is anticipated
  • when dynamic monitoring of the patient’s condition or treatment effects is needed in real time (volume shifts)
  • frequent arterial sampling
  • inaccurate BP due to obesity or dysrythmias

Wisecracks: False Pulse Ox Readings

Another way to think of the causes for a false pulse ox reading is use the letters “SPO2”

  • S = structural change to the hemoglobin molecule due to a dyshemoglobinemia
    • methhemoglobinemia or carboxyhemoglobinemia
  • P = post-methylene blue
  • O2
    • pOlish
    • lOw perfusion states


This podcast was uploaded by Ross Prager (@ross_prager)

Adam Thomas

CRACKCast Co-founder and newly minted FRCPC emergency physician from the University of British Columbia. Currently spending his days between a fellowship in critical care and making sure his toddler survives past age 5.
Chris Lipp is one of the founding Fathers for CrackCast. He currently divides his time as an EM Physician in Calgary (SHC/FMC) and in Sports Medicine (Innovative Sport Medicine Calgary). His interests are in paediatrics, endurance sports, exercise as medicine, and wilderness medical education. When he isn’t outdoors with his family, he's brewing a coffee or dreaming up an adventure…..